Q: Where can I buy a bearing block for a 6mm shaft with a 14mm center height?

A: It's really common to get your whole robot design laid out only to discover that you need some very specific part that's difficult or impossible to find. If you have machining skills and a proper shop you can often make the required piece. If not, you either change the design or learn to 'bodge'. You'll probably do best to get a short 6mm pillow block and put a spacer under it to bring it up to the correct height.

A: Mark J. here: you might as well use spit and a piece of string. For best strength, Lexan must be bonded with a solvent cement designed for plastics. A solvent cement chemically welds the joined surfaces together into a single piece as strong as the surrounding material. Your local hobby shop may have a suitable cement, but don't let them sell you cyanoacrylate (super glue) or epoxy for this purpose. Craftics is a good on-line source for plastic solvent cement.

Watch the video on plastic bonding on YouTube.

A: 6AL-4V titanium is TIG weldable.

A: We've said this often: structural performance depends on factors other than just material and thickness. The size and shape of the piece, mounting method, and support from other chassis elements all count toward performance. Just being given the material and length of a piece is too little information. You're not even telling me what you want to make out of the UHMW!

A: I wrote to Dan Danknick and asked about the additional suports shown in some of the images:

So, no -- the long bolts and end plate supporting the motor in some of the images is not included in the current kit and is no longer available.

A: Mark J. here: the various tool steels are not generally suitable for armor. A tool steel is designed to be harder than other steels, but hardness should not be confused with strength. In order to obtain high hardness, other properties are sacrificed. Tool steels tend to be britile -- not a characteristic you want in armor. You are also going to have dificulty finding a plate of tool steel large enough for the bottom plate of a superheavy.

A tool steel bottom plate would be highly resistant to damage from a 'killsaw' style arena hazard. If you really did want to use a tool steel for this purpose, 02 would be a good choice.

A: I don't know of any builders that use metallic tungsten. Tungsten does have high tensile strength, but there are many other properties to consider in material selection. Tungsten is brittle -- it does not hold up well to impact. It also weighs two and a half times as much as steel, which makes it much too heavy for structural members in a combat robot.

A: Mark J. here: Nylon 6 has high tensile and flexural strength, but it performs poorly under heavy impact: it breaks. Polypropylene is a little better, but still far inferior to the more commonly used plastic or composite armors. Try polycarbonate, UHMW polyethylene, or Garolyte instead.

A: Mark J. here: the 2024 and 7075 'aircraft' aluminum alloys are good choices for armor or wedges. Both alloys are strong, hard, and fatigue resistant. The difference between the T3 and T6 tempers will not be noticeable for your purpose. A good summary of aluminum alloy properties can be found at OnlineMetals.com.

You're in the right ballpark on thickness, but just how thick will depend on how well the wedge is supported by the mount and the overall size of the wedge. When in doubt, go thicker.

Other materials you might consider:

• garolite;
• 6AL-4V titanium;
• low carbon spring tempered steel.

Q: How about 0.05" 7075-T6 Aluminum for a 4 centimeter long 10 centimeter wide wedge supported by 8 screws on the top of the robot? (the wedge being bent down and dragging against the floor). Thanks, you're a great help.

A: Hmmmm... that's not what I would consider to be good mounting support. The force applied to the wedge is going to be at a point farthest away from the mount, and the aluminum has already been bent at an angle below the mounting point. You're going to bend the wedge further downward with a good hit. Good engineering practice calls for more support closer to the point where the force will be applied. A full-width brace between the chassis and the wedge would greatly strengthen structure. With that brace, your .05" 7075 wedge should be great.

Q: Do you think that side skirts 2.5 centimeters long and 13 centimeters wide, made out of 0.05" 7075 T6 Aluminum and supported at the top would need extra support near the bottom? Is it a good idea to hinge side skirts or not? Would that tape that you use for your skirts be strong enough for antweight side skirts with the same dimensions as above?

A: Hinged side skirts aren't going to get the same type of stress that might be applied to the fixed front wedge, so no extra support should be needed there. Side skirts should be hinged -- you want them to drop down and ride along the arena floor with zero clearance. We use 3M #396 Super Bond Film Tape for our insect-class skirt hinges. It would be plenty strong enough for your purpose, but it is difficult to find. Aircraft control surface tape (also called hinge tape) from your local hobby shop is very similar.

A: A web search for 'surplus neodymium magnets' will turn up a large number of sources for very strong rare-earth magnets. I've had good luck with K&J Magnetics. Why not go with wheel magnets?

A: Mark J. here: CAUTION -- full-pressure CO₂ systems are dangerous and definitely not recommended for builders without extensive experience.

For those builders with pneumatic system experience, the usual choice for a high-pressure actuator is a hydraulic cylinder. Hydraulic components are typically rated 1500 to 2500 PSI, well above the maximum pressure available from unregulated CO₂. Hydraulic cylinders are available from industrial suppliers, such as McMaster-Carr.

I would generally discourage anyone from attempting to construct their own high-pressure pneumatic components.The RFL rules call for high-pressure pneumatic components to be rated and/or certified for use at the maximum system operating pressure plus 20%. I don't believe you'd have much success convincing an event organizer that your homebrew parts have an industrial certification.

[MATERIALS] Q: I've heard that paintball CO₂ regulators from Palmer-Pursuit work in combat robots, but which one should I choose? I also need a suggestion for a solenoid valve -- it doesn't need to be anything wonderful since I'm using a buffer tank.

A: Mark J. here: Suppose you were working in a pizza parlor and a customer came in and asked, "Which pizza do I need to feed my family?" They haven't given you enough information to answer their question. How many people are in the familly? How old / large / hungry are each of them? How do they feel about anchovy?

Same thing here. I don't know what weightclass your robot is in, the size of the actuator, the geometry of your weapon linkage, the flow rating for the other components in the system, or your expectations on performance. I cannot answer your question.

Note: a buffer tank will not make up for a poor-flowing actuator valve. A buffer tank allows the accumulation of a volume of gas at full actuation pressure between the pressure regulator and the actuator valve. This allows use of a regulator that may not have the flow capacity to keep up with your actuator clyinder, but the critical importance of the actuator valve in flowing enough gas fast enough to the actuator is unchanged. A pneumatic system will perform no better than the poorest performing component in the system.

A: Some of their wire choices specifically say they have silicone insulation. If it doesn't say, it's a good bet that the insulation is standard PVC.

A: Mark J. here: no!

V-belts have internal cords that prevent stretching and provide structural strength. Cutting a belt and heat welding it back together would give you a belt that would fail immediately under load.

The smallest conventional A-size belt I can find is 16" at www.mainsupplies.com, but it wouldn't hurt to check your local auto parts store or lawnmower repair shop.

Your best bet may be a resizable PowerTwist belt. These are made up of individual linked sections and can be assembled by hand to any reasonable size. I have never used them, but they are reported to be as strong as conventional belts.

A: Mark J. here: I've never seen S7 flat stock wider than 6". Flat Stock Express and onlinemetals.com have 6" widths up to 1/2" thick. Both suppliers offer other grades of tool steels in widths to 12" and thicknesses to 4".

Q: In a previous question regarding a spinner-killer's scoop, Mark J. said that S7 tool steel comes hardened. In another question about ramming spikes, he said that it doesn't. Which is true?

A: Raw tool steel stock from a metal dealer is shipped in a relatively soft annealed form so that it may be machined, shaped, and formed. It will require hardening before use. If purchased in a finished tool form, the steel is hardened. If in doubt, ask your supplier.

If you buy from a metal dealer you'll know what you're getting, but I have access to an exotic scrap metal yard where the material may be either annealed or hardened. Sorry if you found my comments regarding the spinner-killer scoop misleading -- I've edited that answer.

Q: Which other grades of tool steel would you recommend for a featherweight spinner with a 8in disk?

A: I wouldn't recommend making your whole disk out of tool steel. Take a look at disk weapons from other builders; they tend to be aluminum or titanium with hardened steel teeth. You use tool steel for impact surfaces where hardness counts in delivering damage, not for the main body of the weapon where toughness is much more important. Also, a piece of tool steel that large may warp significantly when hardened.

If you decide to ignore my advice, go with an oil quenched tool steel like 'O1' that forms a relatively thin hard surface and still retains interior toughness, or a warp-resistant air quenched alloy like 'A2'.

A: The press-fit BaneBots hub for 3" to 5" Colson wheels is only 0.59" in diameter. If you drill out the bore to 0.50" there won't be enough material left for a keyway.

The best solution is to order your Colson wheels fitted with with custom hubs from Team Delta. They will modify the wheel and mount a bolt-on hub that will fit perfectly and give no trouble.

Q: Hm... Are there any Colson Hub solutions adaptable to either the 36mm or 42mm Banebots motors for the 2" wheels? Could I Loctite in the keyway included from either the Robot Marketplace or Banebots and broach a keyway?

A: The Colson wheels are easy to modify. The 13mm bore of the 2" wheel can be enlarged to accept the 15mm BaneBots hub for the 36mm gearmotor. Alternately, you can broach a keyway into the wheel and mate it directly to the 1/2" shaft of the 42mm gearmotor.

Don't bodge the hub with Loctite, tape, or glue. Hold the wheel in place with proper mechanical methods: the banebots gearbox shaft is tapped on the end for a bolt/washer combo, and the inner limit can be set with a locking collar or spacer. I've seen more robots break down from bodged hubs than any other single cause. Do the hubs right or you'll wish you had.

A: The Du-Bro wheels are made for R/C airplanes -- your local hobby shop probably stocks these. The wheels are not solid aluminum chunks, so the 1/8" axle hole can only be drilled out a little. They are entirely not suitable for a featherweight combat robot.

A: Mark J. here: you haven't given me enough information about your weapon for me to make a specific recommendation. In general, ABEC grade 3 steel ball bearings are adequate. Higher grades are are more expensive and are made to higher precision, but precision isn't critical in this application. A 'shielded' bearing design will help keep grit and debris out of the bearing.

A: Aluminum is a popular material for bot frames, but you need to know that there are various grades of aluminum: 6061-T6, 2024-T3, and 7075-T6 are strong and durable alloys well suited to bot construction. Other alloys may be very soft and relatively weak. Browse thru this archive for information on different aluminum alloys.

A: Yes -- but Laminite is very heavy and somewhat brittle in thin sheets. There are better armor materials.

A: An abrasive cutting disk works very well. Wear eye protection and a dust mask. A fine-toothed hacksaw will also do an acceptable job. Square up the cuts with a sanding block.

A: Yes, I've seen many antweights with recycled kitchen components for armor. I'm always thinking about robot applications when I wander thru a dollar store or thriftshop.

A: This is the first complaint I've ever heard about traction with Colson wheels. The rubber is pretty grippy on smooth, clean surfaces. Clean the tires with rubbing alcohol between matches and let dry. Move heavy components around on your chassis to concentrate weight on the drive axle. If you're still getting excessive wheelspin it's just because you have more torque than weight.

A: In combat robotics a linear actuator is a device which converts the rotational motion of an electric motor into the extension/retraction of a rod to provide a pushing and/or pulling force. Their speed is not nearly as great as pneumatic actuators with the same force, but they require only electric power and simple controls to operate.

Typical uses in combat robotics include lifter weapons (BioHazard), clamping jaws (Jawbreaker), and self-righting mechanisms.

Q: What linear actuator would be best for a Lightweight clamper? How much power would it need, and what speed controller should be used?

A: Mark J. here: I'm not sure that I can recommend any electric linear actuator for a lightweight clamper. There are dozens of manufacturers offering hundreds of models, but it's still unlikely that you'll find an off-the-shelf unit that will meet your needs. Tell me about your design and I'll see if I can match an actuator to it, but effective combat clampers larger than insect class use custom actuators or pneumatic power.

A linear actuator is typically controlled by an R/C Dual Ended Switch, not a speed controller.

A: Mark J. here: I'll take the metallurgy question. The alloy of choice for impact weapons is 'S7 tool steel' for a good balance of hardness and impact resistance. The best material source I have is McMaster Carr: search for 'S7 tool steel'. The 'unpolished mill finish' rods are fine for your application and are much less expensive than other options.

Tool steel is supplied in an annealed form -- soft enough to be machined to a point or chisel profile. After you have cut and shaped the spikes the steel must be heat treated to produce the hardness needed to penetrate armor. Check your local phone book for companies that heat treat metal. Tell them what the spikes are made of and how they will be used.

Too much trouble? Your local tool store has heat-treated chisels and punches that would be a reasonable substitute.

Now you have some spikes that will take enormous punishment and punch thru armor like cardboard. The next problem is that spikes don't 'absorb' impact, they just transfer it back to their mounting points. Make those mounts strong and well braced. The judges won't be impressed if your spikes fall off or your chassis buckles under impact.

Q: Okay then, what would be the best way to mount the spikes? Could they be welded after heat treatment, or could the chisels be welded out of the package?

A: Yes, you can weld heat treated steel, as long as you're careful not to heat up the whole mass and ruin the treatment. Chisels are generally heat treated only on the pointed end, so they retain ductility at the 'hammering' end. I'd rather make a mount for the spikes from which they could be removed and replaced quickly if needed. Maybe a steel tube welded to the chassis that the spike could slide into? Run a A bolt thru both the tube and spike to hold it in place. Drill the hole in the spike before you heat treat it, and weld a plug at the back end of the tube to take the force -- the bolt only holds the spike 'in'.

A: Mark J. here: the HF NiCad battery packs are not 'combat quality': they are heavy and bulky for their capacity. You can use them if you're on a really tight budget, but don't try to use the battery chargers that came with the drills. They are far too slow.

Each pack has a capacity of about 1.2 amp-hours, so a single pack should be plenty to run two HF drill motors in a light 'bot for a 3 to 5 minute match. Check the Team Tentacle Torque Calculator for an estimate of the battery capacity your 'bot will need.

A: Really bad idea! You're gonna be in combat and suddenly have your 'bot shut down to prevent possible battery damage? How about the damage your opponent is gonna do to you while you're sitting there immobile?

Best solution is to make sure your battery pack has enough capacity to last the entire match. If you're still concerned, there are devices available that flash a light and/or beep loudly when the Li-poly pack voltage drops too low (example). That gives YOU the option of shutting down.

A: Mark J. here: it's very ambitious of you to build your own wheels. Any particular reason you don't want to use the wheels everybody else is having success with?

Getting a high-grip material to bond to the plastic wheels you're making isn't going to be easy. Industrial plastic wheels have their tires bonded in place in high-pressure molds that aren't practical for a small-scale builder to duplicate. Some sumo robot builders mold a multi-part urethane resin around their wheels for their special high-traction needs -- but this requires a custom mold and their wheels don't have to stand up to spinner attacks. I've also seen sumo builders glue a wide rubber band around the outside of a plastic wheel with contact adhesive.

You could try a layer of RTV silicone sealant applied to the outside of your wheel. You can get silicone sealant at any hardware store. It isn't going to stick very well to the Lexan, and getting it on in an even layer without a mold is going to be tricky.

A reasonable solution might be to machine a groove around the outside edge of the wheel and glue a rubber O-ring into the groove. Industrial suppliers like McMaster-Carr can supply O-rings in a wide range of sizes and specific materials.

My suggestion: don't make it hard on yourself. There are lots of reasons Colson wheels are so popular. Use them and you won't regret it.

A: Mark J. here: I wish you'd mentioned how big and long the tube is, how much material you need to remove, and how precise you need to be! If the tube is big enough, you can use a 'flap sander'. It's a bunch of abrasive flaps attached to a shaft that fits in your drill. With a shaft extender, you can work down a pretty long tube with one. They come in several sizes - check your local hardware store.

For a smaller tube, you could use sandpaper glued around a wood dowel a little smaller than the inside of the tube and work it by hand.

If the resulting tube has to be very precise, a machine shop could put it on a lathe and bore or shave the inside diameter. Best luck!

A: Mark J. here: Radicon makes a variety of industrial gearboxes with different designs and capacities. I don't know which one you want, plus I don't have their catalog. Industrial gearboxes are made for durability, not light weight. A typical industrial 20:1 worm gearbox with a 200 inch-pounds output capacity weighs about 10 pounds.

A: Mark J. here:

1. See the earlier post on garolite.
2. Thickness will depend on the dimensions of the baseplate, the distance between load points, the location and number of stiffening elements, and many other design factors. The Totalinsanity chassis design tutorial my provide you with some help. Note that I've never seen a garolite chassis for a 'bot bigger than a beetleweight.
3. Garolite weighs about 1.1 ounce per cubic inch.

Q: My bots footprint is 12x12 inches. The garolite would have to cover the bottom, so a 12x12 piece with it being screwed into .25 titanium at each edge. So the maximum span would be 11.5. The motors would be as far as possible to the corners to get maximum benifit from wheels on the outside. From center of front left motor to center of front right motor is 6.128", and center from front right motor to back right motor is 6.044. Could .125" garolite work for this setup? I've heard that it is less bendy than an equal thickness of aluminum, and people do use garolite in 30's, maybe more but 30's are the largest I've seen. Thanks.

A: My best advice is to ask a builder of one of those featherweights you know about how thick their plates are. As an educated guess, I think you could get away with 1/8" garolite -- if you have it supported on all edges and can add a stiffening vertical center support up to the top cover. Going to 3/16" would add about 9 ounces to the weight. I'd go for that.

Like carbon fiber, when garolite fails it fails big. Aluminum dents, garolite comes all apart. Use metal washers to spread the load at equipment mounting points, and have a spare baseplate in the pits.

A: Polycarbonate cuts, drills, and machines very much like other soft plastics. It is a good armor material only because it can absorb a large impact by deforming. A normal sharp drill bit bores through it just fine, a fine-toothed hacksaw cuts it easilly, and sandpaper will smooth the edges.

A: Carbon fiber should not be bent - it seriously weakens or breaks. Angled or curved carbon fiber was formed to shape when made, not bent afterward. The whole point of CF is how stiff it is.

A: The antweight-sized micro pneumatic parts were made by Inertia Labs, but they are no longer listed at either thier site or at the Robot Marketplace. There is contact info at the Inertia Labs site -- you could drop them a note and see if they still have a few pieces around, but it looks like they no longer make them. Try a 'parts wanted' post to the RFL Forum to see if anyone has spares in their parts bin.

A: Mark J. here:

Advantages:

• The Lite Flites would certainly save weight. A 2.5" Lite Flite wheel / tire weighs less than half an ounce, versus 2.25 ounces for a Colson.

Q: My wheels would extend only a .25" from the top and bottom.

A: The protection sounds great, but there may be another problem. The Lite Flites are pretty 'squishy'. With a few pounds of weight on them, your .25" clearance might squish down to almost nothing. You can always go to the 2.75" size if you need to. Keep a few spares in your toolkit in case things get rough.

Q: What about these wheels? Foam-filled pneuamatic tires, 4"x2.1", only 90 grams each including foam insert, and they appear to have good traction qualities for $17 a pair.

A: Those tires are a fairly common offroad R/C item made by HPI. You can match them to a variety of R/C wheels. Drawback: 90 grams for the tire plus 30 grams for the wheel ($12.95 a pair) equals 120 grams -- more than 4 ounces. The 2.5" Colsons you rejected as too heavy only weigh 73 grams, have great traction, and are combat proven. Colsons are popular for a reason -- use them.

I plan on using 2 IFI victors with the Robot logic IMX-1 mixer, and that mixer has the ability to adjust when inverted. If I use the invert feature on the Robot logic mixer, will that avoid the 'death spin', and what do you mean 'death spin'? Thanks.

A: Mark J. here: when your invertible tank steer 'bot flips over, the forward/backward response to your control inputs is reversed -- a forward command will back the 'bot up and vice-versa. However, the turning response is not reversed -- a left turn command still turns the front of the 'bot to the left because both the direction of the wheel rotation and the side of the robot the wheel is on have reversed and the two reversals cancel each other out.

Unfortunately, an upside-down gyro reads left as right and right as left. When your 'bot drifts a little left, the inverted gyro reads that as drifting right and dials in some left steering. This causes more left drift, to which the gyro responds by dialing in even more left steering. It's a 'positive feedback loop' that sends the 'bot into an uncontrollable 'death spin'.

A mixer with an invert feature will correct the forward/reverse response issue on an inverted robot by reversing the interpretation of the throttle input, but that won't avoid the death spin. The gyro itself must be shut down. Note that this isn't a problem in the aircraft that the gyros were designed to work with because they do reverse directional control when inverted.

I've added an explanation to the gyro article to make it a little clearer.

A: Mark J. here: Multi-layer armor like you suggest is tricky and often less effective than single-layer armor of the same weight. Lexan is effective at energy absorption only if it is allowed to flex. By backing Lexan with rigid aluminum you will greatly reduce the desirable energy absorbing properties and impact will be transmitted directly to your relatively thin underlayer. You can vary the hardness of armor in layers, but the layer stiffness should remain fairly constant to allow the load to be spread evenly. Search the Ask Aaron archive for 'composite armor' for more comments.

I'm not a big fan of aluminum armor, but T6 tempered 7075 is generally the best aluminum alloy for that purpose. It's harder and has higher shear and yield strength than 2024 or 6061. It's price is comparable to 2024, and about twice that of 6061. Note that both 7075 and 2024 are very difficult to weld.

A: Yes, titanium is expensive -- but thousands of dollars?? Check with Titanium Joe. Aluminum is not a good match against the tool-steel teeth of a serious spinner. With scoring based so heavily on visual damage, you can't afford big gashes in your scoop. I'd rather see you go with a thinner, hardened or tempered steel scoop reinforced across the back as needed.

A: Mark J. here: Tensile strength is measured by placing an increasing linear pulling force on a material sample until it breaks. The unit of measure is 'pounds per square inch', and is calculated by dividing the maximum resistance provided by the material sample by the cross-sectional area of the sample in square inches.

Tensile strength is not a very useful measure in evaluating the suitability of a material for most combat robot uses. More interesting for applications such as armor are measures of impact resistance, toughness, elasticity, and hardness.

Thank you for your answer.

A: All variable-speed dual-channel robot motor controllers on a robot with differential steering will allow you to:

• move straight forward and backward;
• rotate in place clockwise and counterclockwise;
• move forward while turning left or right; and
• move backward while turning left or right.

A: Polycarbonate and aluminum can be bonded with a 'Goop' type adhesive, but mixing materials to make a composite armor is tricky. You're trying to get the best properties of both materials, but it's just as easy to end up with the worst properties of each! I don't recommend it.

Q: Would 1/8" polycarbonate plus 1/8" aluminum be adequate heavyweight armor?

A: Heavens, no! You'll need much thicker armor for a heavyweight. Current design favors at least 3/4" high-strength alloy aluminum or 1/2" titanium for protection from heavyweight spinners. Quarter-inch plastic or aluminum is purely sub-light class.

Q: Titanium is expensive! How thick should polycarbonate armor be for a heavyweight robot?

A: Mark J. here: good armor is less expensive than having your opponent punch thru your poor armor, destroy your 'bot, and blow you out of a tournament. Heavyweight 'bots are expensive. If there was a less expensive alternative to exotic metal armor, builders would be using it. The current field of heavyweight spinner 'bots are brutal!

No matter how thick you make the plastic, polycarbonate is not a direct replacement for titanium. They have different strengths and weaknesses. Take a look at the Team Stupid materials page for a listing of suggested uses for different materials and alloys.

A: I'm not sure what you mean by 'trim'. If you're reducing the outer diameter of the bushing, it should ideally be done on a small lathe. Small adjustments may be made with a jeweler's file. Bushings should be a snug fit -- they should not rotate in their mounting.

A: There are a lot of different types of steel. The properties of the metal vary widely according to the elements alloyed into the mixture, the process that is used to form the steel, and various heat-treating processes that may be applied to the metal after it is formed. A 'high carbon' steel that has been specially heat tempered takes on properties that allow it to bend and 'spring back' rather than permanently deform is called 'spring steel'. The process often leaves the surface of the steel darkened with a blue tint. Spring steel is more resilient than structural steel, but it is not as tough -- it will break rather than dent.

A: Mark J. here: Let me pose another question first -- Is it a good idea to combine Carbon Fiber and UHMW Polyethylene to make composite armor?

Consider the properties of the two materials: carbon fiber is extremely stiff with a fairly hard surface, while UHMW is soft and so pliable that it is considered 'unbreakable'. If the proposed composite was placed under stress, all the force would be placed on the stiff CF with the UHMW simply bending out of the way with little resistance. Only when the CF fails would the properties of the UHMW become useful. Bad combination!

Back to your original question: if you decide to do this anyway, what glue should you use? Polyethylene is almost as slick as Teflon and very difficult to bond. Epoxy, silicone, and cyanoacrylate adhesives won't work. The surface has to be chemically modified to allow an adhesive the chance to get a grip! 3M makes a couple of two-part adhesives that will do this: Scotch-Weld Structural Plastic Adhesive DP8005 and DP8010. They are difficult to find, expensive, and tricky to work with. You can view the product data sheet for DP8005 here: DP8005.pdf.

If you really want to bond CF to something for effective composite armor, try Garolite and an epoxy adhesive. UHMW, like polycarbonate, needs to flex to absorb energy and be useful as armor. Adding a stiff CF layer doesn't make good sense.

Q: My armor is also the chassis side plates. I need to tap holes into the side plates to mount the top and bottom chassis plates. Garolite (like CF) doesn't hold edge taps well. Can I glue CF to Lexan?

A: Lexan is much easier to glue than UHMW polyethylene, but you'll still have the problem of the Lexan flexing a lot more than the CF. You can use a gel cyanoacrylate 'Super Glue' to bond CF to Lexan, but I think I'd use 'Goop' adhesive or silicone sealant for a little 'give' with a countersunk screw thru the CF into the Lexan in each corner for a little mechanical insurance.

A: Mark J. here: there are a HUGE number of specialized steel alloys, and several different standards for referring to them. There are entire college courses devoted to steel alloys and library shelved filled with books on the topic. Add to this the various heat treatments, casting and milling options available and you're quickly in deep water!

To narrow your search I suggest that you first determine which alloys are available from your supply sources and research their properties. It won't do you much good to find an alloy you want that you simply cannot get in the shapes or quantities you need!

• A good introductory article on steel and aluminum alloy designations: ESPRIT Mechanical Design.

• An overview of the different designation types for carbon steel: Key to Steel.

• General descriptions of commonly available stainless steel alloys: Small Parts, Inc. - Stainless Steel.

• ASTM, ISO and IEC Test Methods: IDES property descriptions.

• Physical and mechanical properties of specific metal alloys and plastics: MatWeb Material Search.

A: Mark J. here: there are a lot of different measures of material strength, some of which are overlapping. Each measure may have several different test procedures that can be difficult to directly compare. Some useful material physical properties to examine include:

• Tensile Strength is a measure of how much pulling force a material can withstand. It is commonly measured as the peak pulling force a material can withstand before it begins to fail (ultimate tensile strength).

• Yield Strength is a measure of a specific attribute of tensile strength: the amount of pulling force required to permanently deform the material. Materials with low yield strengths tend to `smear' when hit with a high energy weapon.

• Impact Strength is a measure of the amount of energy a material absorbs when breaking by impact. This is also known as 'toughness'. Some materials that are very strong are also brittle and can shatter under sharp impact. These make poor armor!

• Modulus of Elasticity is a measure of material stiffness -- now much the material flexes in response to a specific amount of force. Materials with a higher elastic modulus flex less than materials with lower values.

• Hardness is a measure of the resistance of a material to surface penetration. Hard materials do not scratch easily, but may be more brittle than softer material.

• Density is a measure of the weight per unit volume of the material. Tests of material strength are conducted on samples of a specific size, not of a specific weight. A one inch diameter rod of 4130 steel may have twice the tensile strength of a one inch diameter rod of 6061 T6 aluminum, but it weighs three times as much! With a combat robot, you're generally looking for strength per unit weight.

The www.matweb.com site has information on the physical properties of a wide range of metals, plastics, and related materials.

A: I really like recycling useable parts to make something new. You can also learn a few things about mechanical engineering by taking apart an old VCR. You probably won't find anything very useful in the way of drivetrain or electronic components but there are plenty of gears, belts, pulleys, and maybe even motors that could be used for a weapon system on an insect-class 'bot. Be aware that some of the motors in a VCR are likely brushless motors that require special electronic driver circuits to operate.

One problem with recycled parts is replacement spares. Unless you have another identical scrap VCR, it may be difficult and expensive to replace a recycled component damaged in combat.

A: Mark J. here: Your Li-Poly battery is toast! The usual cause of this 'outgassing' is overheating brought on by too high a discharge rate -- you're asking for more amps from the LiPoly than it can provide. Replace it with a larger capacity battery. It's also possible that your charger is providing the wrong charge rate. Never charge Lithium cells with a charger not designed for the purpose.

Whatever the cause, *** IT IS NOT SAFE TO USE THE BATTERY ***. Do not attempt to charge or discharge the battery. Drop the battery into a plastic bucket with 1/2 cup of salt dissolved in a gallon of water, cover the bucket, and let it set for at least two weeks. It will then be safe to toss out with the garbage.

Li-Polys are not nearly as robust as NiCad or NiMHd cells, and can be dangerous if abused. Electrifly.com has a good article on Li-Poly care.

A: Mark J. here: A thin layer of silicone rubber does improve the traction of many types of tires and treads. Pure silicone rubber can be found at auto supply stores. It's used for making automotive gaskets ('RTV silicone gasket maker'). Pure silicone kitchen and bathroom caulking from a hardware store is also OK, but do not try to substitute similar products like 'Goop' or 'siliconized latex caulking'. Silicone rubber comes in assorted colors, but they are all about the same for grip.

The trick to application is in getting a good bond so the silicone rubber doesn't peel off. Clean the surface of the tire very thoroughly with rubbing alcohol and let dry. Uncured silicone rubber is sticky and messy to work with, so spread out some newspaper and have paper towels handy. Use a popsicle stick or old knife to spread a very thin layer of the gooey silicone evenly onto the tire surface. Apply enough pressure to get the rubber down into the tire pores, Wipe excess off the edges with a towel. Let 'cure' for at least a full day before use. Curing RTV silicone smells like vinegar.

Clean silicone grips better than dirty silicone, so clean your tires before each match. I use a little lighter fluid on a towel, but be very careful with any flammable liquid!

Remember: better traction means less wheel spin, and less wheel spin means higher amperage consumption by the motors. Make sure your speed controllers can take the extra load.

A: Mark J. here: Judging the relative strengths of metals is difficult and sometimes misleading. The tensile and yield strengths of magnesium are more than twice as great as common aluminum alloys, but the shear modulus is four times less, which means that magnesium is brittle; it tends to break rather than bend. This restricts its use in combat robots to internal braces and mounts that are not exposed to high impact. It would be really embarrassing to have your armor shatter and fall off!

Also, magnesium is not that light! Common alloys are about 67% the density of aluminum. You'll save a little weight, but it may not be worth the trouble for small parts.

One last thing - magnesium is flammable! If exposed to a high-energy weapon impact it could ignite, and water or CO² fire extinguishers won't put it out! Your event organizer wouldn't like that.

I know that Biohazard uses some magnesium in its internal components, but in general people use more common materials. My metal suppliers don't stock magnesium, so you're on your own for a source.

Thank you.

A: Mark J. here: The preferred method of cutting carbon fiber is an abrasive cutting disk, but you can use a fine-toothed hacksaw.

Safety precautions when cutting or sanding carbon fiber include a dust mask (always!), safety goggles (always!), and gloves (always!). CF dust is very irritating, and jamming a loose carbon fiber into your flesh is painful, easy to do, and difficult to remove. Don't take shortcuts!

A: Unobtainium:

1. an item so expensive, rare, or otherwise difficult to get that it is effectively unobtainable.

2. a reference to a material with physical properties that do not exist, as in "I broke the titanium hub, so I'll build the next one out of unobtainium to make it stronger and lighter."

A: Making custom gears requires machining skills and special equipment. It's much better to find gears that will work, but that can be difficult as well. Try a local hobby shop, or sift thru a surplus supply store. Old toys can yield valuable parts, too. Try a web search for 'slot car gears'.

A: Mark J. here: I did some research. A free-flowing powder placed inside a hollow cylinder can improve rotating balance. There's a product available for balancing automotive tires called 'Equal' that might be what you saw. Drum weapons are difficult to build precisely enough to be well balanced and can use all the help they can get!

A: They don't buy them, they build them. See the post on flamethrowers. They're ineffective and dangerous -- not for novice builders!

A: Mark J. here: One last time: 'Cheap' and 'Robot Combat' don't mix! If you're going to spend your time and effort to design and construct a robot and take it to to a competition, you don't want some crappy component to fail and flush the whole project.

A: Mark J. here: Delrin is an Acetal plastic developed more than 50 years ago by Dupont. It is a little heavier and stiffer than Lexan polycarbonate, but it is not nearly as impact resistant. Delrin makes good machined components (gears, bolts, actuators), but polycarbonate makes better armor.

A: LiPoly batteries are a good choice for any weight class -- if you can afford them and can maintain them properly. They provide a lot of power for their weight, but they do have limits on their safe output amperage. Be sure to allow for that in your calculations.

Take a look at my dad's previous post about calculating battery capacity. For a starting point, I'd try about 2000 mAH for a hobbyweight with an active weapon.

Most builders use a single large capacity battery rather than multiple smaller batteries for each ESC. Multiple dedicated batteries could leave you with no drive power but plenty of weapon power still available. The single battery will also be better able to survive large amperage surges.

A: I don't know 'Cobra'. The official name database at BotRank.com does not show a 'bot named 'Cobra' in any weight class, and neither does the RFL name search. Sorry.

Most hobbyweight (12 pound) and featherweight (30 pound) 'bots use cordless drill motors and gearboxes for their drive train. Top of the line is the DeWalt 18 volt drill motor and transmission, but a DeWalt with all drive components will run more than $150. Many builders in these classes use motors and gearboxes from less expensive, lower voltage drills. The 'Handiworks' 7.2 volt drill was once a favorite for this purpose, but they are no longer widely available. Look around your local discount tool store and you should be able to find something for $30.

You might want to reconsider your desire for 'really fast'. Robot combat is not a race and trying to drive a fast 'bot can be a real handful.

Q: Aaron, this is more of an answer than a question: in an earlier post someone asked for information about a 'bot named 'Cobra'. I have been privileged to see Cobra in action. It's a fast wedge bot from Sydney, Australia -- powered by modified DeWalt drills.

A: Thanks for the info! DeWalt has powered more top flight 'bots than I could possibly name here. Robot Marketplace offers a wide range of DeWalt motors and gearboxes, and Team Delta has the 18 volt DeWalt motors and gearboxes plus special combat mounts and driveshafts.

A: 'Scrap metal' could be almost any type of material, Chris. Some could be great, some would be useless. I live in an area where I can get exotic scrap titanium from the aerospace industry that makes tremendous armor, but you may not be as lucky. Check my dad's Armor Guide to get some ideas.

A: Soldering is a very good skill to develop for making all sorts of things. A soldered electrical connection is strong and unlikely to fail in combat. Some speed controllers do come with either pre-soldered or screw-type connectors, so you can build a 'bot without soldering. It would limit your choices of components.

Maybe you could find a team-mate who can solder?

A: Garolite is a branded composite fiber/resin material similar in properties but much less expensive than carbon fiber composite. Stick with the woven glass cloth varieties like G10 or G11 for general robotics construction for their high impact resistance. There's a good discussion of Garolite at the Spambutcher Robotics site.

A: Mark J. here: "Best" depends on lots of factors. Various metals, plastics, and composites are all useful for different designs, applications, budgets, and builder skill levels. Commonly used materials include:

• Aluminum: Different alloys vary widely in strength. Don't use the stuff you can buy off the rack at the local home center - it is soft and weak. 6061-T6 alloy is nearly as strong as mild steel and is widely available thru metal dealers. Aircraft alloys like 2024-T3 and 7075-T6 are harder and nearly twice as strong, but more expensive and difficult to find. Aluminum armor has a tendency to `smear' and show damage.

• Steel: There is a huge array of steel types: hot rolled, cold rolled, tool, stainless, high carbon, low carbon, plain carbon, mild, nickel, invar, chromium, tempered, annealed, quenched, ferritic, pearlitic, martensitic, killed, capped, spring, and chrome moly - to name a few. If you want to use steel, do your homework and pick out something suitable. Mild steel (called `1018') and Chrome Moly (`4130') are commonly available. Other materials offer greater strength for the same weight, but mild steel is cheap and you can find it in sheets, angles, and rods at your local hardware store or home center. See also other posts about steel alloys and terminology.

• Titanium: Top of the line `bot armor, titanium is nearly as light as aluminum and nearly as strong as alloy steel. It's also very expensive, tricky to work with, and makes pretty white sparks when hit hard with a rotary weapon. A common alloy used for `bots is 6AL-4V.

• Lexan: Also known as polycarbonate, Lexan is a clear, lightweight plastic with an enormous capacity to absorb impact and pop back to its original shape. Leave room for the material to flex when designing - fastening points are common failure sites. Lexan can also be chemically welded for very strong joints. You can find Lexan at your local glass shop or home center, but make sure you're getting polycarbonate and not clear acrylic plastic - acrylic is useless for `bot armor.

• Polyethylene: Ultra-high molecular weight (UHMW) polyethylene plastic is lightweight, cut resistant, unbreakable, and very easy to machine. It takes huge abuse and bounces back for more. Thin sheets are flexible, but thick plates can be used for motor mounts and main chassis components.

• Carbon Fiber: Amazingly stiff, strong and light -- but when it fails it comes all apart! Great for small `bot chassis plates and top armor. Expensive!

• Fiberglass: A blend of woven glass fiber and a resin media, there are lots of fiberglass formulations. A high performance type called Garolite is popular with some insect-class builders for chassis plates and armor. It is heavier, less strong, and less stiff than carbon fiber composite but much less expensive.

Remember, you don't have to use the most exotic materials to make a winning `bot. I've built two champion robots with wood armor!

A: OK, everyone say it at once: EBay! You should be able to buy standard servos for $10 or less. Your local hobby shop may cut you a deal on some and save you shipping costs.

A: Clear plastic 'Package Tape' is available pretty much anywhere. It might be good enough for your purpose.

If you really want the good stuff, 3M makes a specialty tape that's super strong, very flexible, and so sticky it will even bond to polyethylene. I use it to hinge the side skirts on my beetleweight. It's called 3M #396 Super Bond Film Tape. It's hard to find and it isn't cheap. You can get it here: www.hillas.com.

A: Check with local sources that sell window glass. They often sell polycarbonate as burglar-proof window material. Your local home center may also have polycarbonate, but make sure it's the real stuff -- they may also sell clear acrylic plastic that is not nearly as strong. Check the yellow pages under 'Plastic' as well.

If you can't find what you need locally, www.robotmarketplace.com sells a variety of materials suitable for 'bot armor (Lexan. carbon fiber, titanium). Check their Metals & Materials section.

A: You don't mention how large your robot is going to be. For smaller 'bots, take a look at the 'Rubber Treads' section at www.classictintoy.com. For a medium sized 'bot automotive V-belts or timing belts might do. Big 'bots can use treads from light equipment like snow blowers.

Remember that treads are heavier than wheels, often fail in battle, and are easy to attack. They're also expensive.

A: We get parts from many sources: our local hobby shop, pieces scavenged from toys, stuff we make ourselves, and web stores. Places you might try:

Team Delta
Microbotparts.com
Sozbots
Robot Marketplace