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Thread: A Basic Guide on How to Work with HDPE

  1. #1
    So I've been working fairly non-stop on beetleweights and featherweights for the past two years, and each of my builds have involved robots almost entirely constructed out of HDPE. I have a lot to update still in my build diaries, but since I was working on cutting some new bulkheads and weapon mechs for Barróg Doom, I figured that this would be a good time to document how I work with cutting thicker pieces of HDPE, as well as sharing many of the tips I have picked up over this time. Note that this is not a complete guide; I've used a single-speed bandsaw for all my builds and cuts may be improved by changing the cutting rate, I do not have access to a mill (though I have outsourced some pieces to others for milling), and I do not know how much of this could carry over to UMHW. I do feel that this guide will be ultimately useful however, and I would be happy to share what I know as I feel that the material has not been used to it's fullest potential. If the possibilities of HDPE can be explored by a larger field of samples and data and utilised by talented builders, I believe that it could seriously alter the current metas of robot combat.

    Some samples of work I've done in the past.

    HDPE is a high strength plastic, commonly most well known for being used in chopping boards, but the material has many advantages for robot combat. Firstly it's light; HDPE is only 0.97g/cm^3, this is a fair bit lighter than materials used for internal support or baseplates such as Aluminium (2.7g/cm^3), and considerably lighter than more common forms of armour such as steel/hardox (7.85g/cm^3). Secondly, it's far more easy to work with than metals; the added thickness compared to metals allows for threaded inserts or barrel nuts to be used as opposed to nutstrips or welding (you can even use wood screws!), and the material can be worked with using any tools typically used with wood, so many tools a builder may already have can be easily used, as well as being able to carry over many skills from working with wood. As such, the material is usually recommended for beginners, as it encourages experimentation and can be adapted quite easily for common weapon types (wedges, lifters, axes etc.)

    The material has advantages other than that however, advantages that I believe make it viable at the highest levels of most weight classes in robot combat. The lower density allows for the construction of larger robots, which can allow for more space to keep internals safe, usually by making it harder for opponents to get to where the electronics of a machine are. This density can also be useful for allowing for thicker bulkheads for high kinetic weapons, which can reduce the likelihood of distorting during combat (two of the deadliest FW spinners in the UK, Binky and Get Shrekt, both use HDPE bulkheads). Most interestingly, HDPE has the ability to flex, allowing it to absorb hits a lot more effectively than metals which rely primarily on deflecting as much energy as possible. If a machine is designed right, accounting for weight distribution, geometry, and overall construction, HDPE can allow a bot to take hits from the deadliest of weapons with little but a scratch. I've gone over some of my theories in my Barróg build diary, but a more famous example would be of Gabriel, who survived the deadliest horizontal spinner the UK had ever seen, Carbide, at full power for over three minutes (an American example would be the success of Jonathan Schultz's "Huge" family of bots)

    So today I was working on getting the second weapon bulkhead and lifter fork cut for my new FW Barróg Doom. They are both made out of 20mm HDPE, bought from DirectPlastics. These are parts that I would prefer to be mm accurate, particularly for the holes for the lifter transmission. Previously I would have used tape, a ruler, and a set square to measure out the distance from a point I knew the measurements from such as the corner of the piece, but recently I've taken to using a method used by Sam Price of Get Shrekt. I take the pieces I want to cut from Solidworks (any CAD program should be fine) and print them out to a 1:1 scale. Then, I trim off any excess paper and use a glue stick to stick the drawing to the HDPE sheet. I've found in the past that the paper tends to peel and rip off while cutting, so to reduce the risk of this happening I put some tape over any areas I intend to cut or drill over. You will probably need to use extra tape during the cutting process depending on the glue you use, especially if your saws end up catching on the paper/tape.

    One sheet glued and taped, ready for cutting.

    For long cuts, I prefer to use a circular/skill saw, as it gives the best finish from all cutting methods I've used so far, as well as being straight and perfectly level. For this sheet I have the bottom of the bulkhead sharing the base of the sheet where the cut is at it's finest, and only go in with my cuts as far as possible, allowing for space for structural curves later on. As with any cutting, safety glasses and ear protection is highly recommended, swarf flies far, and these saws can be loud.

    Lining myself up for the final of the four cuts I can use the circular saw for.

    For any curved cuts however or any cuts that can optimise the amount of leftover material, I use a jigsaw. The jigsaw has a lot more flexibility than a circular saw, but there are certainly disadvantages to deal with as well. Pieces need to be clamped down, which can often be tricky when keeping the saw clear and free to move with smaller parts. Without a guide, cuts are less likely to be straight, and the thinner and shorter your blade is the more likely it is that the blade will bend and the cut will not be square (this was a big issue with Barróg when I didn't know how to work with a jigsaw like I do now). On top of all this, turning the blade in thicker materials causes the blade to heat up rapidly, which can melt and almost re-solder your swarf to the part.

    Getting the lighting to show this was tricky, but you can sort of make out the melted swarf solidified back to the bot.

    To account for these issues I've adjusted a few things. First, I make a first pass around the outline I want cut, leaving almost the space of the thickness of the blade between the line and where the material is being cut. This allows the blade more freedom when doing curved cuts as it doesn't have to put up with the extra friction of the outside wall of material, lowering the time the blade overheats. It also gives a better visual of if the blade is bending or not, allowing you to adjust if need be and increasing the likelihood of having a square cut.

    A good example of what a first pass cut typically looks like. The above cut is roughly 30mm long, quite the distance for any curved cut.

    Even when doing all this however, the blade will still overheat rapidly. With 20mm, I find myself only able to cut roughly 20-30mm (as can be seen above). into the plastic before the blade becomes too hot to touch and the swarf starts to become a sticky problem. To speed up the cooling process, I typically keep a cup of water handy to run the blade in after a cut. Plastic cups work best for this as you could easily chip ceramic mugs while running the jigsaw. Just two or three seconds are all that are necessary for this 2mm thick 8mm long blades to cool down back to being cool to the touch. Then, I run some kitchen towel along the flat of the jigsaw, keeping clear of both the trigger and the jagged edge of the blade, to absorb any spare droplets that may have splashed onto the blade, preventing damage to the paper/tape.

    A common setup for me. Sheet clamped down, towel underneath where any piece may fall to save the floor, and a cup of water on hand to cool any blades down.

    Once a piece has been cut, all that's left to do is to drill holes into the material. Centrepunch your holes (very easy to do with plastic), and just drill. I'd highly recommend using a drill press for all of this, as it increases the likelihood that all your holes will line up, but it isn't essential if you're patient (Barróg was built entirely without a drill press). I would highly recommend using barrel nuts for fastening pieces to each other, but this post is already getting long, and would be better suited to a follow-up post. Your end result after all of these steps should be a highly accurate, strong part ready to be used in any key component for your bot! Hopefully this guide has stood as a useful introduction, and I intend to update it with any other info I find in the future!

    The finished pieces, ready for holes to be drilled.

    The weapon mechanism, still awaiting the front claw, weight saving holes, and mounting to the base.
    Last edited by Shooty; 17th June 2019 at 07:19.

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  3. #3
    Great stuff! Thanks for taking the time to write all this up. I'll have to try gluing some plans onto the raw material in future, hopefully that'll reduce levels of wonkiness a bit!

    We've been on a bit of a learning curve building EWC from HDPE. Most of the cutting has been done with a bandsaw which is a good jack-of-all trades, as you can cut perfectly straight and square with a fence but also do some curves and mitred cuts. I've also used a sliding compound mitre saw for mitre cuts and that worked pretty well, although it was awkward to clamp - partly due to the relatively small parts, partly because HDPE is a bit slippery.

    What're your preferences for drill bits? I'm leaning towards brad point wood drills being better than metal drills as they seem to be better at scooping all the swarf. Holesaws cut really well but the resulting holes are nothing like the size they're meant to be. OK for rough cuts but not for accurate holes.

    I did a little bit of work with HDPE using a CNC router and that came out quite well. Although it needed quite a lot of cleaning up (both the pieces and the surrounding area...) the material was very forgiving to work with despite my lack of CNC experience (it would be good to practice on even if you're not planning on using it as it's so soft the chances of breaking tools are reduced compared to aluminium, say).

    Another thing that's worth highlighting is that press-fitting things like bearings into HDPE is really easy due to it being so soft, you can make your holes quite a lot on the too-small side and still get things into place.

    Barróg Doom is looking awesome so far!

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