Q: Hey Aaron, I need to make a 1" hole in the top armor of my bot. The top is 1/16" 7075 aluminum. Thanks.

A: You can go buy a 1" hole saw, but for a small job like this I'd score the outline of the hole onto the aluminum and then drill a series of small holes just inside of the scored line and use a small file to break thru from one hole to the next. A curved file can then smooth out the hole to the scored line.

A: Mark J. here: a few thoughts:

• You haven't calculated weight. Titanium is lighter than steel, but multi-inch thick pieces are still very heavy. I figure your front scoop will weigh 150 pounds by itself.
• With the current damage/aggression scoring, you're gonna have to do better than push opponents around to win matches.
• That's way too much horsepower for a robot that only goes 8 mph. You need horsepower for acceleration and speed. The amount of pushing power is limited by the weight on the drive wheels and the available traction. Horsepower excess to that requirement just spins the wheels. Half that much horsepower would push just as hard.

A: All common questions that we have previously answered in detail. Click those green buttons near the top of the page to access the archives and get reading, pilgrim. Start with the .

Some short answers to get you started:

• all the money you can spare, and then some;
• yes -- check our book reviews and section 629.892 at your local library;
• if you don't have a rich uncle you're not gonna get a sponsor;
• one thing at a time, with a lot of mistakes along the way.

A: I don't wander thru the pits with a caliper in my hand measuring armor, but as a guess: 'Shovelhead'.

A: The rear chassis uprights on 'Ice Cube 3' are bolted in place rather than welded. They can be removed along with the rear armor to make it easy to change the motors and gearboxes. Sometimes it is worthwhile to sacrifice a little strength to make between-match repairs go more smoothly.

A: You'll need a gearbox! A gearbox reduces the speed of the motor and increases the torque. Without a gearbox, the motor would not have enough torque to give the robot any pushing power at all, it would use way too much amperage, and it would melt very quickly from the stress.

The heavier your robot is and the larger the wheels are, the more gear reduction your motors will need. The Team Tentacle Torque & Amp-Hour Calculator shows that a pair of the 'Small Johnson' motors in a 12 pound robot with 3" wheels would do well with a gear reduction somewhere around 12:1.

A: Mark J. here: I've seen many beautifully crafted, titanium armored, mega-powered combat robots that could not fight their way out of a paper bag. Why not? Because getting the basic design principles correct is way more important than all the exotic materials and CNC machining in the world.

Team Tiki got the basics very right with 'The Brown Note'. The robot was low, powerful, and very controllable. The steel wedge was so nasty to start with that it didn't really show any additional damage. Once 'K2's nasty spinner got hold of the plywood 'The Brown Note' was just so many splinters, but the lesson to be remembered is that you must spend the time and energy to design your robot around the functions it must perform well to be successful before you get to the less important aspects.

You may wish to examine the career of heavyweight 'Evelyn, a Modified Dog' from Team K.I.S.S. for additional support of this theory. As the builder of another plywood covered combat robot once said, "Complex design is easy -- simple takes work."

A: We've answered questions just like this before, so dig thru the archives, starting with the . Building a robot to 'just move around' is very different than building one for competition -- meeting the competition rules takes planning and added expense, so decide before you start to build. Read thru the archives, see our recommendations on books, and dig in!

A: There are advantages to both designs, but I'm a believer in the chains and sprockets approach.

• With a motor on each wheel, if one end of the 'bot is lifted off the ground you lose the power available from the motors on that end.

• With chains and sprockets, all the motor power on each side remains available to any wheel in contact with the ground.

A: Mark J. here: If you want similar performance to Vladiator you'll need a similar power to weight ratio. Vladiator is a 340 pound superheavyweight powered by twin Etek motors producing a combined 30 horsepower: 340 / 30 = 11.33 pounds per horsepower. To get that same power ratio in a 120 pound middleweight you'd need: 120 / 11.33 = 10.6 horsepower.

The NPC-02446 motor that comes with the 'build your own gearbox' kit puts out about 0.75 horsepower. You'd need 14 of them to get close to the power you're looking for!

Three horsepower from four of the NPC motors will give more than adequate performance in a middleweight -- the Team Tentacle Torque & Amp-Hour Calculator shows a top speed across a 36 foot arena of 14 MPH in 2.7 seconds. You just aren't going to get the rubber-burning-mad-gerbil-in-a-popcorn-popper action that the high-end powerbots can display.

A: Sounds very good -- might even be overkill. Armor material and thickness choice depends on the design of your robot, how the armor is supported, and the combat tactics you plan to use. You're in the right ballpark.

A: I'm sure your design concept is very clear to you, but I'm going to need more information before I can make recommendations on motors. For a start:

• What weight class are you designing for?
• What type of weapon will you be powering?
• Tell me more about rotating the weapon around the robot.
• What balance do you want between mobility and weapon power?

Q: I'm thinking of making it a middleweight, and the weapon will be a pneumatic arm similar to that of the Judge's, and it will have a vertical spinner attachment, or else it can be swapped out for a flipper or a hammer. It will be mounted on a small housing that is held by and arm that will rotate it around the robot's body. Since the weapon will take up a a lot of weight, along with the armor the drive doesn't need to be fast or have a lot of pushing power. Also since the weapon will have some big stresses, what do you recommend for the arm, and the arm that moves the weapon around the body. Thank You.

A: A couple of suggestions:

• Robots with multiple weapons do less well than robots with a single weapon. Multi-weapon robots were fairly common at Robot Wars, but none of them did well. I'd suggest sticking with a single powerful weapon.

• Have you ever played around with a gyroscope? They are very difficult to point in a different direction, exerting plenty of force at a right-angle to the direction you try to turn them. A vertical spinner acts like a gyroscope that resists being re-pointed. Putting one on the end of an arm and trying to move it quickly in an arc would flip your robot over! No matter how powerful and heavy the arm holding it, you couldn't move it any faster than you could if it were just mounted to the robot.

A: Seems unlikely. Once flipped over it would be pretty stable resting on the rotor -- the 'bot body would just spin freely. You'd need a fairly complicated SRiMech to put it back upright. However, gyroscopic forces make a 'bot with a big spinning rotor difficult to flip over in the first place.

A: Mark J. here: clever thinking, but weight and mass are different properties. Mass is a measure of an object's resistance to change in velocity, while weight is a measure of force exerted on an object by gravity. The formula for kinetic energy is:

Magnetic downforce only increases the apparent weight of a robot, not the mass. The kinetic energy of your example robot can only be increased by increasing its velocity.

A: My best advice about tracks on combat robots is "don't do it". Tracks are way more trouble than they are worth on a smooth surface.

Most tracked 'bots are about 'square': the tracks are nearly the same length as the width of the robot. It helps maneuverability to have a tread support sprocket near the center of the track that is just a little lower than the front and rear support sprockets -- similar to the layout for a six-wheeled robot. Avoid the pain and go with wheels.

A: Set screws suck. They come loose at absolutely the worst times. You can bolt thru the pulley and use the radial prop mount holes on the motor case, or drill all the way thru the pulley hub and the motor shaft and drive in a small hardened pin.

If you have to use setscrews, file a deep flat spot on the motor shaft, use Loctite, and check it for tightness before every match.

Q: To attach a pulley to my Axi, could I just screw down the prop adapter around it really tight, and not use any set screws?

A: I don't have an Axi 5330 here to look at, but isn't the prop adapter held on with set screws? Set screws are not a good method of securing a mechanical linkage to a shaft. What is adequate for a propeller spinning in air is not adequate for a pulley that will encounter much greater forces.

You want a mounting method that relies on something other than friction to prevent rotation of the pulley and which will not fail if a threaded connection loosens a bit -- and a threaded connection being directly stressed will loosen!

A: A wide wheel track results in a stable and easily controlled 'bot with a slow spin rate when turning. This is desireable in ramming 'bots that must be carefully positioned for an attack.

A 'bot with a narrow track is more difficult to control in turns and has a higher spin rate. The higher spin rate is desireable in thwackbots that rely on a high spin rate for offense.

A: Since when are moustrap cars combat robots?

Larger wheels have lower rolling resistance than smaller wheels. They also make it easier to obtain a high ratio between the action of the trap spring and forward movement without efficiency-robbing gears. A really good mousetrap car will creep forward very, very slowly -- speed is not efficient!

I've seen mousetrap cars use old LP record albums (search your local thrift shop) as wheels. They work great!

A: Mark J. here: Beats me. Chassis strength depends on design, triangulation of members, gusseting, armor type, method of armor attachment, size, and construction technique as well as the amount and type of material used. Without knowing a whole lot more about your design and building skill level, I can't begin to answer your question.

A: No limit in the rules, but the larger the 'bot the thinner the armor has to be to make weight. Most builders keep them compact.

Q: How heavy is a typical featherweight?

A: The typical 'bot in any weight class is very close to the weight limit. The RFL featherweight limit (North America) is 30 pounds. UK featherweights have a 12 kilogram limit.

Q: What is the size of a normal featherweight?

A: A typical featherweight might be 16" square and about 4" high. There is a lot of variation and there are no size limits -- as long as it can fit into the arena.

Q: How light can a featherweight be?

A: There is no minimum weight specified in the RFL rules, but if you're at or under 12 pounds you qualify as a hobbyweight. Lighter 'bots are at a disadvantage in combat.

If you're serious about building a combat robot, you really must make the effort to attend a combat tournament and see a real competition. You'll get answers to questions that you didn't even know you should be asking!

Q: Can a DeWalt14.4v old style drill motor be good to power a wheel for a featherweight?

A: A pair of DeWalt 14.4 volt motors/gearboxes would be a good choice for a featherweight. With 3 inch wheels and the gearbox locked in 'high', the 'bot would have a theoretical top speed of 12 MPH at 18 volts. The motors could spin the wheels while drawing only 22 amps. All very good!

You can 'test drive' a selection of motor/gearing/wheel/weight combinations at the Team Tentacle Torque & Amp-Hour Calculator.

A: Mark J. here: I have no idea what you're talking about. Perhaps if you explained your question in detail and included links I could help.

A: According to the archived Stinger website, their top speed is only 9 mph -- not at all fast for Robot Wars. Power came from twin Bosch GBA 24 volt motors -- about 1.25 horsepower each. The motors were geared for acceleration and pushing power. I think it's their quickness you remember, not their speed.

A: Mark J. here: torque isn't the issue -- total power is the issue. Clamping force and speed will depend on the geometry of the clamp as much as the torque of the actuator. You usually aim for a minimum three times as much clamping force as the weightclass you compete in, so about 200 pounds of force at the clamping point for a lightweight.

A: If the arena you compete in has a smooth enough floor for drag wedges to be sucessful, you can probably get away with a hinge on your wedge that will allow it to drag as well. Put a limit on how far the wedge hinge can move -- you don't want it to fold back under the 'bot, or flip upward.

A: Read down about seven questions for information on 'hubs'. Attaching a wheel directly to a motor is a very poor idea -- without gear reduction, performance will be awful.

A: Different arenas have different problems with floor seams. Even the same arena can have tight seams one time and problems the next because of minor changes in how and where it gets assembled.

I can't recommend a robot design that depends on the skill of the arena assembly crew. I also don't like the idea of a wedge that can fold back under pressure and lift the front of the 'bot up off the arena floor.

A: Possible, yes. Good, no. An overhead thwackbot like Toe-Crusher needs to have the mass of the robot nearly balanced on the drive axle in order for the acceleration/deceleration torque of the drive to be able to throw the weapon 'over the top'. The overhang behind the drive axle has to be able to clear the ground as the 'bot swings over for weapon impact. With six-inch wheels, you would have less than three inches of rear overhang -- that's probably not enough to pack batteries and other heavy components to counterbalance an effective weapon hanging off the other end.

Q: What's the best way to distribute weight in a thwackbot?

A: As mentioned above, an overhead (torque reaction) thwacker needs weight positioned out behind the drive axle to help balance the weight of the weapon out on the end of the boom. Place your batteries, electronics, motors, everything you have space for out in back of the axle centerline. I'd aim for about 85% of the total robot weight to be on the drive wheels with the remaining 15% resting on the weapon. If you get too much weight on the weapon it will become difficult to swing the weapon over for impact. If you have too much weight on the drive axle the weapon impact force will be reduced. Some 'trial and error' tuning will be needed.

The approximate weight distribution applies to traditional non-overhead thwackers as well, but you'll have more space behind the axle for weapon weight offset.

A: Mark J. here: I don't know what your chassis design looks like, but I'd drill and tap multiple mounting holes across the top and bottom of each the two aluminum plates and both thru the top and bottom plates of the robot chassis. The plates will need cutouts for the wheels. Additionally, I would use the existing mounting holes on the plates to bolt thru chassis tubes or into bracing blocks or bulkheads perpendicular to the plates. You do not want the plates to distort in relation to each other or the gears will die!

A: Ramiro Mallari built 'Punjar' from exercise treadmill parts. The very wide wheels are converted treadmill rollers. Perhaps he believed that super-wide tires would give super traction?

Punjar had a long and successful career. Active from 1996 to 2001, Punjar racked up a 14 win / 8 loss record. That places Punjar 24^th in the all-time heavyweight rankings -- 10^th among those with 20 or more fights.

A: Not even close. Here's what the 2007 RFL rules say:

Pressure Drop's walking mechanism is cam actuated via a linkage from a continuous rotary source -- no weight bonus for that.

A: Go look around the Mechanical and Drive Components page at Robot Marketplace. While you're there, browse the rest of the site as well -- it will answer a lot of basic questions.

A: A wood baseplate! You don't see much of that anymore -- but I like it. A good quality plywood ('marine grade' is best) is really quite strong for its weight. Don't use woodscrews to fasten down your components; drill thru the baseplate and use a nut/washer/bolt to securely anchor everything.

There are a lot of choices for the rest of the chassis. Square steel tube can be cut with a hacksaw, drilled, and bolted together. Use nylock nuts or a liquid threadlocker to keep everything from vibrating loose. You'll want to gusset all the joints for strength. Armor can then be fastened directly to the square tube chassis.

I'd strongly recommend getting a copy of Grant Imahara's book, Kickin' Bot. It covers everything you could possibly need to know about chassis building, as well as every other aspect of combat robotics. It will save many times it's cost in time, materials, and performance.

A: It's usually a 'trial and error' process. Design and build your 4-bar lifter for its primary function as an effective weapon. Once the robot is working you can try extensions to the height and width of the top lifter arm until you hit a combination that will tip your inverted 'bot back onto it's wheels -- like the top 'claw' on the 'Pack Raptors' (pictured). Designs with greater lift will require smaller extensions.

General robot design can affect the ability to self-right as well. The original (1996) version of Biohazard was able to reliably self-right, but lost that ability when defensive side-skirts were added. A narrow or short 'bot is easier to self-right than a wide, long 'bot.

A: Robot Sumo! Great stuff. I'm guessing that this will be remote control sumo. Check the previous sumo tips in the Ask Aaron archives. A few specific suggestions:

• Go to your public or school library and find a copy of Robot Sumo: The Official Guide by Pete Miles. You'll get enough tips from Pete to stomp your opposition.

• Keep your design simple. Sumo matches are won by looking after the details, not with complicated design or exotic weapons.

• Yes, use a wedge -- they are useful for more than defense. Once one or more of your opponent's wheels are off the arena surface you have a strong pushing advantage. Your wedge should be razor-thin at the front, make even contact all the way across, and scrape the arena surface.

• If you absolutely must have a weapon other than your wedge, consider a simple servo-powered clamping arm on top of your 'bot to grab and hold your opponent after they climb your wedge. That should be simple, inexpensive, and effective -- but I'd just stick with the wedge.

• You've got a pretty big 'bot on a really small arena -- gear for pushing torque, not speed. Speed is not your friend on such a small surface.

• Superior traction beats superior power. Coat your tires with a thin layer of pure silicone rubber sealant and let cure completely (about a day). Rub the tires between matches with alcohol on a clean rag to remove any dust and contaminants.

• There's usually a weight limit in sumo. If there is no weight limit in your challenge, make the 'bot heavy! A heavier 'bot has more traction, can deliver greater pushing power, and is harder for a light 'bot to push.

• Get the 'bot built early enough that you can become very comfortable driving it in an area as small as the arena. Practice your driving skills!

A: We don't use Computer Aided Design software for our robots so I can't make a recommendation, but if you type 'free CAD' into Google, you'll find links to a ton of them.

A: Your problem isn't the weight of your armor, it's how the weight of your 'bot is distributed. I'm guessing that this is a two-wheel drive robot and that your drive wheels are too far away from the center of mass to get good traction. You need to either redistribute the heavy components of your 'bot to put more weight on the drive wheels, or move the drive wheels closer to the center of mass.

Q: That's a good answer, but I didn't tell you enough about my 'bot. I made it cheap -- it's a remote controlled car (worth £10 or $20) and the armor is what once looked like a dustpan but with a cover. It's covered in decorative foil and stickers. Is there anything I can do to improve the turning on the car?

A: I like your use of available parts! You've built a 'bot and you're out there having fun.

Your front tires are probably soft plastic and may not have enough grip to turn the 'bot. Their grip can be greatly improved by coating them with a thin layer of silicone rubber sealant, available at a hardware or auto supply store. Use the 'pure silicone', not 'siliconized' caulking or another type of sealant. Any color will do. Clean the tires with alcohol or another solvent and let dry completely before applying the silicone to get the best bond. The sealant is pretty thick and sticky to apply, but it doesn't need to be perfect. Let the silicone cure for a full day and give it a try.

Another possibility is that the weight on the front wheels is too great for the small steering servo to overcome. You can't replace it -- your radio isn't compatible with hobby-grade servos. If that's the problem, you're back to shifting weight toward the rear drive wheels to lighten the load on the front wheels. Best luck!

Q: I'm writing back to thank you for your help. The robot's steering has greatly improved and I owe all my thanks to you.

A: Glad I could help!

A: Rounding the edge armor of your 'bot could help resist hits from vertical spinners, but angled armor has to face all sorts of weapon attacks. Sharply sloped armor can be effective against many weapon types: vertical and horizontal spinners, bricks, and rammers. If you add drop skirts you have good protection against wedges, lifters, and flippers too. Rounding the armor as you suggest could retain some additional protection from vertical spinners when inverted, but would leave you very vulnerable to wedges and lifters. Are there really so many vertical spinners fighting at your local tournaments that you have to design specifically against them? Also consider how often you will get inverted if you have effective drop skirts.

If you are really worried about inverted protection, the best solution could be a V-profile plus a hinged drop skirt. The armor profile remains the same when inverted and you maintain protection from lifters and wedges. See the diagram.

A: The motors must be securely fastened to the chassis by mechanical means:

I don't know which 'Beetle Gearmotor' you're using, but they likely have some mounting holes that could be put to good use with some simple angled metal brackets. A pair of automotive steel hose clamps fastened around each motor and thru the chassis can be used in a pinch. Machined aluminum or UHMW polyethylene clamp mounts surrounding each motor and gearbox would be better. Look around the 'net and see how other builders do it.

A: The Robot Marketplace antweight battery packs are compact, light, and well made -- but they have only a 370 mAh capacity. Fine for an antweight, not nearly enough for a hobbyweight powered by RS-540s.

Pay attention to the 'estimated battery capacity required' that the Team Tentacle Torque Calculator provides. A hobbyweight powered by RS-540s plus a weapon motor is gonna want more than 1000 mAh.

A: See previous post on battery capacity.

A: I think you're asking about drop-skirts. They are armor panels hinged along the top edge that drop down at an angle and slide along the arena floor. Small 'bots can use strong tape to make the hinge, but larger 'bots need full-length mechanical hinges.

A: You rubber-mount armor by running the mounting bolts thru a large rubber grommet in the chassis. Be sure to use a 'fender washer' on the back side of the grommet, and self-locking nuts. Don't tighten the mounting nuts down very tight -- leave room for the rubber to flex and absorb impact. Rubber mounting is particularly useful for polycarbonate armor, which tends to crack at high-stress mounting points.

A: Mark J. here: A speed controller is one of the last components to consider when building a 'bot -- not the first. Your speed controller specs will be determined by the amperage requirements of the drivetrain. The process goes like this:

1. Decide what you want your 'bot to do in the way of fighting strategy.
2. Outline a design that will accomplish that strategy.
3. Find components that will meet the design requirements.

A: Robot sumo competitions have highly variable rules, and careful study of the rules is needed to figure out what designs will work well. Are you entering the autonomous or R/C category? What weight/size class? What material is the arena surface made of? Do the rules allow magnet traction aids? Do the rules allow vacuum traction aids? Are there rules about how 'sharp' the edge of your wedge can be? Are 'sticky' tires allowed, or do tires have to pass the 'paper drop' test?

Write back with details about your competition, and tell me more about your motors. In the mean time, try to find a copy of Robot Sumo: The Official Guide by Pete Miles in your local library.

Q: I am entering a R/C category. The maximum weight is 2.5 kg and the robot must fit within a 50cm X 50cm box with no height restriction. The arena floor is made of vinyl. The rules do not specify anything about traction aids (nor do I know what they are). There are no rules about how sharp the wedge can be. And, tires may not leave residue on the playing field. Thanks!

A: It's details that win R/C sumo competitions. A wedge is a winner only if it's better than your opponent's wedge. You need to make sure that it sets perfectly level all the way across it's width and that the leading edge is sharp and flush with the arena surface. Read thru those rules again -- there's usually some restriction on dangerous sharp edges. If not, make it razor sharp, but fit a cover to it between matches for safety. Side and rear drop-skirts are uncommon in sumo competitions and may be more trouble than they are worth.

Tires are another critical detail area. Super-tacky reusable lint rollers are very effective as sumo tires, and they leave no residue. See Dave Chu's Sumo Project for an example of their use. Clean and dry them before each match. Other tire compounds (polyurethane, silicone rubber) are useable if you're not willing to custom make your hubs and tires. See Pete Miles' book referenced above for details.

Traction aids are devices that make the apparent weight of the 'bot greater in order to get more traction. If the arena has a steel base, magnets can be used to pull the 'bot toward the surface with great force. A magnet-bot can climb right up the vertical face of a steel cabinet and even run upside-down on a metal ceiling! A similar effect can be achieved on non-magnetic surfaces with a vacuum fan system and sliding surface seals.

I'd suggest sticking with a conventional design for your first build. Finish it early enough to get plenty of driving practice. Keep things simple and sweat the details.

A: Mark J. here: Is that total stall torque from all motors at the axles, or stall torque available per driven wheel? I also need to know the wheel radius, the number of driven wheels, and the weight on the drive wheels to run the calculations.

'Punjar' was a scoop-fronted rambot with a lot of speed and pushing power. To duplicate that performance, you'll want to design for thrust available at each driven wheel to be about two times the weight normally on that wheel. Much more than that just smokes the tires and wastes weight (and money) on heavy drivetrains and batteries.

I'll guess that 788 inch-pounds is the total torque available per drive wheel for a two-wheel drive 'bot. Assuming a 3" wheel radius, that gives you (788/3) = 263 pounds of thrust and no more than 50 pounds of weight on the wheel. If my assumptions right, that's more than five times the weight in thrust -- way overkill. If that 788 in-pounds is the total of all your drive axles, you're pretty close to right.

A: It's simple to build a 'bot with a high spin rate, but it's very difficult to get it to spin and stop just where you want it. Your skill as a driver will be the determining factor. You also have to plan for unexpected situations where you aren't free to maneuver. Most builders armor-up all the way around.

A: Spinner killers are all about the scoop. For a great example, take a look at Jim Smentowski's middleweight actuated-scoop spinner killer Breaker Box. The scoop must start out at a very shallow angle - nearly horizontal - and curve up to around 60 degrees. Length and height are about equal, with a gentle and uniform curve. Jim's using 3/8" titanium for the scoop, with very heavy support arms. His scoop makes up almost half the weight of the 'bot! The simplest way to figure the surface area of a curved surface is to make a thin cardboard mock-up, then flatten it out and measure.

Another question about my spinner killer: do you think I should go with .25" S7 tool steel or 3/8" titanium 6Al-4V? I thought titanium would be better, but S7 tool steel is what impact teeth are made of, so it should be able to stop the spinners I guess. The tool steel would weigh about 3 pounds more for the size of my scoop, but would cost much, much less. What do you think?

A: Mark J. here: S7 tool steel is both hard and unyielding, which makes it perfect for transmitting energy from a rotary weapon to the target via small teeth. To resist the impact of spinning weapons you're looking for a material that can spread the force by deforming and then snapping back into position -- a property known as 'toughness'.

Although tough for a tool steel, S7 alloy is more brittle than titanium and in large sheet form may crack on heavy impact. You run the risk of a big weapon hit shattering the large plate like glass! If you want to use steel, consider a tough spring steel that can flex on impact, like EN47. Remember that you'll have to heat treat either metal after the scoop is assembled to obtain optimum performance, and this will add to your costs.

6Al-4V titanium is extremely tough and resilient. It will take enormous impact and come back for more. It's perfect for absorbing spinner attacks. It is expensive, but the experienced builders that use it wouldn't be spending their money if it weren't worth it.

A: Mark J. here: the conventional way to build a combat robot is to decide what the robot should do to give you a tactical advantage and then draw up a design for a robot that will do what you need. You've drawn up a design and now want to figure out what it will be able to do. I smell a train wreck.

• Three-wheeled omnibots are inefficient in utilization of available power: you'll never be able to use more than 2/3 of the total motor capability in linear motion.

• Omniwheels have a low contact area and poor traction. They were designed for low-speed, low-power applications. They are not suitable for putting a lot of power to good use.

• The only combat omnibot I can think of is Alcoholic Stepfather. It uses four mecanum wheels -- not omniwheels.

Q: Ok, I did a little thinking and the way you stressed mecanum wheels, I think I should use those. I had planned on making a Why-Not / Y-Pout style thwackbot with these wheels. What if I came up with a way to keep the impact away from the robot itself, maybe a hammer on a steel cord connected to each point on the triangle? In my mind, I think that would work. But then again, I don't have the experience you have, so what do you think?

A: I'm glad you're giving this project some thought. I have respect for builders that are willing to try something different. I did some research and found a lightweight 3-wheeled omnibot with a spinning undercutter blade weapon called Y-chromosome that fought at Battle Beach (video) and BotBash (video). You might want to talk to the builders at Team Radicus.

Here are a few more things to consider:

• Your omnimixer will not allow you to spin at high speed and simultaneously move in a controlled manner under radio control. The only omnibots that I've seen move while spinning did so at a very low spin rate along a pre-determined course under full computer control. The technology for a high spin rate while moving is called 'Melty Brain' or 'Tornado Drive' and has been custom implemented on only a very few 2-wheeled thwackbots.

• A mecanum wheeled omnibot requires four motors and wheels. A three-omniwheel 'bot would work better for a thwackbot, but I really don't see much advantage over a two-wheeled conventional thwacker.

• The current RFL judging guidelines are stacked against thwackbots. All points come from damage and aggression. If you can't move while in 'attack mode' you aren't going to get many aggression points. Sit and spin never wins.

A: The only active team I can think of is Zwolfpack Engineering. Their lightweight "1st Abe Lincoln on the Moon" has a successful implementation of Melty Brain. Note that the system got its name because thinking hard about how to get it to work causes such mental overload that it may actually melt your brain. Zwolfpack has obviously suffered significant cerebral damage as evidenced by the names they choose for their `bots.

The Zwolfpack website (www.zwolfpack.net) is not terribly helpful, and I don't have other contact info for them. You might try asking around at the RFL forum. My advice is to stay away from Melty Brain.

A: Search the Ask Aaron archive for 'hobbyweight', Peter. You'll find more than a dozen posts about motors, weaponry, radios, and battery capacity.

A: That depends on what you call a 'walker'. Shufflebots can be very fast -- Dave Hall's 'Drillzilla' claimed a top speed in excess of 30 MPH, but shufflebots are no longer considered to be 'true walkers' in the rules. In the British TV series 'Technogames' the walking sprint race winner 'Scuttle' covered the 25 meter course in just over 7 seconds, about a 7 MPH average speed. Scuttle doesn't qualify as a walker under the current rules either.

Building a true walking robot is very challenging. You might get some tips from looking at designs in Servo magazine.

A: I don't know of anyone currently building true walkers for combat. I'd suggest getting a copy of Servo magazine and looking thru the ads for walker kits. The makers of these kits may be able to supply parts and design ideas for your combat walker.

A: Instead of wheels, shufflebots have two or more long 'feet' on each side of the 'bot that are operated by crankshafts. Each 'foot' is lifted and set down again as the crankshaft turns to move it a little forward or back. The motion is bouncy and makes maneuvering tricky. Shufflebots once got a weight bonus as 'walkers', but a few years ago they changed the rules and shufflebots now don't get that extra weight.

A shufflebot requires a complex drivetrain and custom made parts to make a slower and less maneuverable 'bot than you'd have if you used wheels. Don't bother!

Note: Dear Aaron, the 2006 draft of RFL rules offer a weight bonus to non-wheeled robots including shufflers. [Ted J.]

Not true, Ted. Section 2 of the RFL 2006 rules says, "There is no weight bonus for shufflers or other forms of locomotion which are predicated on rolling." Shufflebots are dead.

A: Mark J. here: I could use a little more info on what you're building! The selection of a proper bearing or bushing depends on how long the support needs to last, how precise the alignment needs to be, the type and magnitude of the force that will be applied, the speed of operation, and the depth of your wallet.

Since you're powering this mystery device with a servo, I'm guessing that the force and speed are both pretty low. Since it's in a combat robot, I'll assume it doesn't need to last forever and that extremely precise alignment is not an issue.

You can likely get away with just drilling an appropriately sized hole in a block of strong, slick plastic -- like UHMW polyethylene, polycarbonate, or nylon. If you need to absorb high shock loading, you might want to go with a bronze 'oilite' bushing. Oilite bushings are cheap, durable, and widely available in a variety of sizes.

A: Sounds like somebody wants me to do their homework for them.

An automobile gets better gas mileage when it travels slower, right? That's because friction and drag increase quickly with greater speed. It's much more efficient to move along at a slow and steady pace than to sprint forward and coast to a stop. A well designed mousetrap car can travel dozens of yards if it keeps speed under control.

Hard, large diameter wheels reduce rolling resistance, and when combined with a low gear ratio will reduce the torque available to accelerate the car and will keep speed down. A typical mousetrap car has one end of a thread wrapped around the drive axle, with the other end pulled by an extension bar from the mousetrap. The number of turns the rear axle makes while the mousetrap unwinds is the equivalent of a 'gear ratio'. A longer extension bar on the trap lowers the effective gear ratio.

Your goal is to keep the car just barely moving along for the entire time the trap unwinds. Tinker with the length of your trap bar extension to get a steady forward crawl -- lengthen it if you're going too fast, and shorten it if the car stalls.

Hey, this question isn't even about combat robots! Why am I answering it?

A: See the Wikipedia differential article.

A: Mark J. here: some machine shops just don't accept small one-off custom jobs, but a clear explanation of what you want and a friendly smile will improve your chances. Machine shop time isn't cheap -- do as much prep work as you can and listen to their questions and suggestions. Let them know that you appreciate their taking time to work with you.

A: Mark J. here: There are a variety of large-hole making devices that will fit in standard-size drill chucks. Here are some options:

• The proper way to do this is with a Hole Saw. A 3/4" hole saw with arbor and pilot drill bit should run about $15 at your local tool supply shop. Get one with a central 'pilot drill' to guide the saw. You can also buy a hole saw set with several diameters for around $30 - they come in handy!

• You might be tempted to use a wood boring Spade Bit. This is not recommended. Although they are inexpensive (about $3) and will crudely bore thru Lexan, a spade bit is difficult to position and control precisely with a hand drill. If you insist on using one, drill a small (1/8") pilot hole, securely clamp the plastic to scrap wood, and securely clamp the wood to your workbench. Be prepared to ruin a few pieces.

• With a soft material like Lexan, you can always scratch a 3/4" circle in the surface (a penny is 3/4" diameter), drill several small holes to 'rough out' the interior, and finish with a curved hand file. This will get you 'reasonably round' if you have the patience.

A: You make something better by standing on the shoulders of giants.

A: Mark J. here: simple answer first: most hobbyweights I see are built around a hypothetical top speed of around 12 to 15 miles per hour. Hypothetical top speed is calculated by the formula:

Example: a 6000 RPM motor with a 6:1 gear reduction and a 12" circumference tire gives:

For help with motor, gearing, tire, and battery selection, check the Team Tentacle Torque Calculator. Be sure to investigate the 'Acceleration" button.

A: Mark J. here: in designing a combat robot, you have to think about how all the components of the machine will work together. A chassis design that would work well for one design could be a disaster in another design. Since I know nothing about your design, I can't comment on your frame.

You have a number of choices for a heavyweight 'bot. You can use welded tubular steel, bolted steel angle, interlocking aluminum flat panels, a stiffened composite pan, or bonded polycarbonate. You can integrate the armor into the chassis as a stressed component, or have a separate armor shell. Be sure to consider your skill level and experience in working with the materials involved, as well as your budget.

A: Mark J. here: That's over two pounds worth of motors in a six pound 'bot! How do you plan to use that much power in a small arena? By the time you add in batteries (lots of 'em for those motors), gearboxes, wheels, and a chassis you're not gonna have much weight left over for armor or a weapon. Keep your design elements in balance - go with smaller motors.

A: Mark J. here: Taps are available in diameters up to 4 inches, but they are expensive. You may also consider thread milling for large diameter holes. Unless you're going to be doing a lot of large diameter tapping, give the job to a machine shop.

A: Mark J. here: pocketing removes material from low-stress areas of a panel or component by milling away some of the thickness while leaving a border of thicker material. If properly done this results in reduced weight while retaining the majority of the strength of the original panel. The location and depth of pocketing requires extensive stress analysis of the component.

The exterior panels of combat robots don't really have any 'low-stress' areas. They can be exposed large forces from any angle and at any point. I do not recommend pocketing external 'bot panels -- although it looks really cool.

A: Building a successful combat robot requires experience in mechanical design, construction techniques, material properties, and control systems. See my FAQ post on where to get help in these areas.

Robot builders use a variety of methods to design their robots. Some use computer aided design tools, some build detailed cardboard models, and some just design as they build. We like to sketch out the overall design, make a complete parts list - with prices and weights, and jump right in to the build. Our sketches wouldn't be much use to anyone else building a combat robot, and copying someone's exact design isn't as rewarding as designing your own 'bot, anyhow.

Keep your first 'bot simple and make sure the mechanical basics are well covered.

A: Combat robots don't have axle hooks. See web articles on mousetrap cars.

A: Technical question - Mark J. here: that depends on how many and what type of motors you use, the gearing, tire diameter, and driving style as well as the weapon motor type, how much you use it, and how heavy the spinning mass is.

Look at 'bots that have a similar set-up to the one you plan and use their experience to help you decide on a battery pack. You can also get some help from the Team Tentacle Torque Calculator which provides an estimate of required battery capacity based on motors and various design factors.

A: Hi, Nick! Since you're asking about windshield wiper motors, I'm guessing you're thinking about a hobbyweight or featherweight class robot? We built a hobbyweight with windshield wiper motors almost five years ago. Even though we were running the 12 volt motors at 24 volts, the gear reduction was so large that they were really slow! They were also very heavy for the power they provided. I'd stay away from them.

Keep your first 'bot simple. Wedges win more matches than 'bots with active weapons, so get some experience before you start showing off with fancy weapons. Cordless drill motors are fast, powerful, simple to mount, and pretty cheap. Fasten them down to a simple flat panel base, armor up, and bolt on a sturdy wedge. Spend some money to buy good speed controllers -- that's the one place you shouldn't scrimp. Search the archive for some other tips and book references. Best luck!

A: Try: www.shender4.com/thread_chart.htm for pilot and clearance hole sizes.

Once you have the hole, the tool used to put threads on the inside is called a 'tap'. The tool for putting threads on the outside of a rod is called a 'die'. You can purchase an inexpensive Tap and Die set, or you can purchase individual taps and dies at your local tool store. A brief guide to tapping holes with hand tools can be found here.

A: Mark J. here: Countersinking forms an angled cut-away around a hole for a flat-top fastener that leaves the fastener top flush with the surface to provide a smooth surface.

Counter boring forms a flat-bottomed recess around a hole to provide fastener clearance and/or offer a flat surface for the fastener on a curved, sloped, or irregular surface.

A: If you don't want to buy hubs, then spend the money for a mask so nobody will know who that guy was that had his wheels fall off. Read my post about hubs in the Ants, Beetles, and Fairies section.

A: Mark J. here: There are three broad categories of screws: wood, self-tapping, and machine. A 4-40 screw is a machine screw that is should be threaded into a pre-tapped hole. Drill a pilot hole with a #43 drill bit at low speed, then use a 4-40 tap to create the threads. Alternately, you may use self-tapping screws with Lexan. They will also require pilot holes, but will not require pre-tapping the threads. Cramming a machine screw into an unthreaded hole is a weak bodge - don't do it.

A: Mark J. here: Motor mount design depends on powertrain layout. If a large motor is being held in critical alignment for an open gear or chain reduction, it must be held very precisely with no wobble or the gear/chain system may fail. Such designs should secure both ends of the motor. Mounts for small motors with attached gearboxes (gearmotors) that do not rely on the mount for alignment of the gear reduction may allow or even encourage a little shock-absorbing 'give' in the mounting design and material.

A popular design for insect-class robots with small gearmotors is the wide circular clamp: a machined hole on a block of aluminum or plastic (UHMW polyethylene or Lexan) with a gap along one side that can be closed down with screws/bolts to clamp the gearmotor in place. The wider the clamp, the more secure the mounting. Remember, 'bots with exposed wheels put more strain on the mount.

I've also seen low-budget mounts made from wood blocks and steel automotive hose clamps that were surprisingly functional. You don't need a full machine shop to build a workable 'bot.

Mark J. here: They aren't thwackbots -- triangular 'bots with an omni wheel at each apex are called 'omnibots'. Yes, they can rotate, but the really cool thing is that by differentially powering the three wheels, they have the ability to move in any direction without turning -- that's called 'holonomic motion'. There's a cool video of a four-wheel holonomic omnibot (same principle as a 3-wheel version): holonomic at www.charmedlabs.com. An R/C omnibot requires a computerized transmitter with programmable mixing to properly do it's tricks.

Autonomous omnibots can be programmed to move (slowly) while spinning (slowly): Frisbee at www.charmedlabs.com, but doing it at variable high spin rates and variable directions under radio control is a REAL challenge. It's called 'Melty Brain', 'Tornado Drive' or 'Cyclone Drive' and many builders have spent years trying to get it to work. It requires on-board processors, motion sensors, sophisticated programming, and it still doesn't work reliably. Don't even try to cram all that into an antweight!

You need a 'hub' to connect a thin weapon to a small diameter shaft -- a precision machined connector that will hold the weapon in correct alignment and provide sufficient depth to allow a stable and snug fit onto the motor shaft. This hub will be the most highly stressed part of your weapon system, so don't try to bodge this.

Note that it's generally not a good idea to connect a weapon directly to a small motor. It is difficult to attach a hub securely enough to a small shaft to be able to transmit large weapon loads. Search the Ask Aaron archive for tips on belt drives that can give better weapon performance. Hubs and belt drives smaller than 3mm bore are hard to find.

A: Thanks!

Omnibots are way cool to watch! The can move forward and back and turn like a regular 'bot, but they can also move sideways without turning and even rotate while moving in a straight line! Check out the videos of the flame throwing superheavyweight omnibot Alcoholic Stepfather.

Omnibots use some variant of an omniwheel or mecanum wheel with built-in rollers that allow it to provide thrust in one axis and slip without resistance in another direction. By applying power selectively to different wheels you can get the 'bot to perform its tricks. You can view a detailed description of a robot using mecanum wheels here.

Omnibots may have three or four wheels. Each wheel needs it's own speed controller and dedicated radio channel. Check the earlier post on omnibots for more videos, and search the web for 'mecanum'. I think you'll pick up the idea after you see a few examples.

You actually have two challenges in running an omnibot: building the drivetrain, and programming the R/C transmitter to make the 'bot controllable. You'll need a computerized R/C system with multiple user programmable mixes or some double-fancy helicopter swashplate settings. If you get the 'bot built and need help with the programming, write back and we'll put our heads together!

Q: Could I make an omni-bot in the 'Robot Arena' simulation game?

A: Hmmm... There aren't any omniwheels or macanum wheels available in the Robot Arena parts box, and the control options don't offer channel mixing for proper omnibot control. If you did manage to cobble one together, it would be a poor representation of the real thing.

A: Small 'bots usually have a skid of slick plastic -- like polyethylene. Bigger 'bots may have a ball caster or an omni wheel. Our heavyweight 'bot 'The Gap' has a wide roller machined out of Teflon in the front so it can slide sideways and roll forward.

A: Tech question, Mark J. here: the smaller timing belts (MXL series) are rated up to 20,000 RPM, and can transmit as much as 400 watts of power. They're much more effective at power transmission than other belt types. For a full engineering summary of timing belt selection and performance, see: www.roymech.co.uk/Useful_Tables/Drive/Timing_belts.html.

A: Tech question, Mark J. here: The wider the belt, the more power it can transmit. Wider belts also stay in place a little better.

A: There are plenty of websites that give detailed information on autonomous sumo 'bots -- try Dave's Sumo Robot Project Page for a start.

A: A tank-steer 'bot steers the same way no matter how many wheels it has: all the drive wheels on one side of the 'bot turn at a different speed and/or direction than the wheels on the other side. Robots with four or more wheels drag their wheels sideways a bit as they turn. That takes a little more power, but it works just fine.

A: If tracks were a real advantage, lots of winning robots would use them. They don't! I'd go with wheels.

Wheel/tire selection depends on many design factors with which the wheels will have to fit. If your wheels will be exposed to attack, use a solid or foam-filled tire that can't go flat.

For general use, I like the Colson wheels. They have been used on combat robots for many years. They're pretty light, durable, have good traction on arena floors, and are inexpensive to replace when damaged. Team Delta sells Colson wheels in different sizes and will even make wheel hubs to fit your drive shaft.

A: Thanks! How long it takes to build a robot depends on how complicated the design is and how much experience you have working with the tools and materials you need. Getting some friends who have skills you don't have will help a lot! I've seen experienced teams build a heavyweight 'bot in a week, but our larger 'bots take us about two months to put together. That includes time to design, get parts, build, and test.

A: Thanks, Coleman! Before you start building a 'bot there are a few things to take care of. First, read the rules for the competition you plan to enter very carefully. You don't want to build your 'bot only to find out you aren't legal for the competition! Next, take time to find as many websites from competitors who have been to competitions like the one you want to enter. Read everything they have to say. Finally, draw up a design that you can actually build. The coolest design won't do you any good if you don't have the skills (or money) to build it.

The words 'best' and 'cheap' don't go together in robot building -- especially with speed controllers. You don't want cheap parts that will put you out of the competition if they fail. See what other builders that have been successful use in their 'bots. It won't be cheap speed controllers! For reliable parts, try Team Delta (www.teamdelta.com), and the marketplace at www.robotcombat.com.