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Thread: Experience needed regarding FW Beater and Armor

  1. #1
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    Hi there.

    So... i am finally in a stage of planning where i got most things sorted out.

    Still got one big and four minor questions where i just need some experience, and build diaries are either telling different stories or nothing about it at all:

    1. (big one)
    My Weapon will be in a somewhat beater-style, just two of them on one axis, with a 8cm "gap" of only axis between them and a bit asymmetric to give it a better bite, but yeah. At least roughly. but i am really struggling what material, thickness, and how wide the "spokes" should be.
    Dimensions will be (roughly) 30cm long (along the axis) and about 25cm diameter when rotating (those would be really hard to change if i had to).
    not sure about that yet (some parts still have to arrive), but i think i will have 4, maybe even 5kg to put into that. Somewhere in that range.
    But would like to know the "minimum safe solution" for the thickness, so i can try to do the actual designing on my own and just know when i am trying to do something too likely to blow up.

    2. Pulley/belt to drive the weapon. What diameter etc. do you guys use in your featherweights? When reading build diaries i get the feeling of "anything is possible", but long-term experiences are rarer in those. So... would like some advice on what actually worked for a featherweight vertical spinner for quiet a time. Else i'd just have to try, but since i can't make my pulleys myself and changing something on them would be some work... planning to get different diameters in case my spin-up-speed isn't what i imagined anyway, but to ask a friend for that, and a bit lter ask him to do it again just a little bit thinner/wider wouldn't be nice

    3. Top Armor sheet metal (most likely 3 or 4mm titanium), side armor 20mm HDPE.
    Now i want to screw both together.
    Considering i might have to get the top off quiet often (for charging, repairs and whatever), hwat would be a reasonable solution here?
    i've so far seen/read/heard of
    "just take some wood screws und punch them in there",
    "drill holes, cut threads in them, and use M5 (or M6, or M7, or... seems everybody got a favourite size) screws in those" and
    "drill holes, put inserts in, and then use Mx screws."
    Now... the last one seems to be the most professional one, but also the heaviest and most expensive (and possibly most time consuming) option. But also everybody seems to have different distances between those screw, and use different screws (regarding length and thickness), or even give such helpful remarks as "a lot" or "a bunch". I just can't see a pattern there, when you should use what and why and how much/long/wide the screws...
    Guess i could somehow put that together, but i am feeling likely to either not make it strong enough or putting so much metal in my HDPE, i could have taken Hardox right from the start. So would be great to have some experience like "i take 5 screws m5 for every 10cm HDPE and (don't) use inserts" or stuff like that, and if you ever had any problems with it.

    4.Now this was just an idea... do you know of any bushings coming as two halves? So i could put them on an axis without an "open" end? Would in this case be used to mount my beater to three bushings, two in the ends and one (almost) in the middle. So let's say take a bulkhead, cut it in half, screw it together again, drill a hole where the cut is, screws out again, put both halves of the bushing around the axis, screws in again, and got one mount in the middle of the axis.
    Would think that would give it a bit extra stability, but should also work without... at least i hope so.


    Now that's quiet a wall of text, hope someone takes the effort to read that stuff and possibly even answer. If you read this: thanks already

  2. #2
    Andy's Avatar
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    Pendulum uses a 20mm thick plate of toolox that is then watercut for the beater, with the ends turned to spin inside bearings. id say 20mm thickness its minimum, 250mm dia 300mm length is very big though, mines 145mm dia and 240mm side for comparison. Id avoid using multiple parts to make the beater weapon since itll just cause more weak points, id say 25mm thickness is safe if you do go with the size you want.

  3. #3
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    Regarding fixing things together I wasn't sure which approach either, so I'm doing them all!

    Using bolts where they will fit and screws everywhere else (lots and lots of screws). The HDPE I have doesn't take a thread very well, and I was worried that if I kept inserting and removing screws for maintenance they would get loose so I have a hinged trapdoor for access, going to try the inserts route for sealing it.

  4. #4
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    Quote Originally Posted by Andy View Post
    Pendulum uses a 20mm thick plate of toolox that is then watercut for the beater, with the ends turned to spin inside bearings. id say 20mm thickness its minimum, 250mm dia 300mm length is very big though, mines 145mm dia and 240mm side for comparison. Id avoid using multiple parts to make the beater weapon since itll just cause more weak points, id say 25mm thickness is safe if you do go with the size you want.
    Yeah, the scetches already look... well, like an ice cone dropped on the side, to be honest. wish i could say "scary".
    Problem is, i can't do the standard-beater-design. It will all be cut from one piece and stay as one piece, but in the middle will be an 8cm part where it is only axis, and not beater. can't avoid that... need that space for the weapon motor and to have a third point on the floor. the weapon got so big, and the only two wheels are so far on the other side of the bot, i think i'll need something to slide on somewhere in that region.
    Could slide on the sides, too, but that would need way more weight, is more likely to get hit, and i'd have four points on the ground. somehow i think three points seem to be more stable, even more given the expected gyroscopic effects.
    Also, trying to just move the weapon away from its motor would greatly increase the weight needed for armor, and even further increase the diameter of the weapon. And that one is already bigger than i wanted^^

    But 25mm as minimum is a number i can work with, sounds good. And how wide would you make the spokes? just 25mm, too because that looks good? Or maybe there's a better way to do those than just rectangular?

    Quote Originally Posted by lowndsy View Post
    Regarding fixing things together I wasn't sure which approach either, so I'm doing them all!

    Using bolts where they will fit and screws everywhere else (lots and lots of screws). The HDPE I have doesn't take a thread very well, and I was worried that if I kept inserting and removing screws for maintenance they would get loose so I have a hinged trapdoor for access, going to try the inserts route for sealing it.
    Trapdoor is a good idea, in my case i am going for an "inner shell" made from thin polycarbonate that can be taken out completely of the armor. "just" have to unscrew that and the weapon every time... not quiet sure if that is wise, but since i wanted to put that layer in there to save my esc etc. from at least some of the vibrations anyway, it seemed the easier way to do this. I'll see if it works.

    (and next bot will be somewhat box-shaped. Really got enough of all these stupid angles and having to work around those.)

    Anyway... so maybe just wood screws where i don't need to remove them often, and inserts where i do?
    But what size of screws, and how many? every screw is some precious weight^^
    Regarding that... are titanium screws a worthy idea, or doesn't the reduced weight count up for the (possibly?) reduced strenght?
    Last edited by Runsler; 17th February 2017 at 15:51.

  5. #5
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    my weapon spokes are more like 35-40mm just because i wanted to make the part as strong as possible but 30mm is probably ok, I do not recommend having the wheels at the back of the robot, it will drive badly as they dont have enough weight over them, try and have them in the middle or behind the beater, itll drive far better.

    Do you mean youre trying to drive the beater directly from the motor?

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    Yeah, i fear the driving might be a problem, but since it will be very slow anyway, i can't hunt anybody down even if i could drive it well. And with those gyroscopics... even turning will be interesting.
    Sadly, battery and ESCs are way bigger than the gearboxes, so i had to put them further to the "opening" of the cone where the beater is, and the wheels further to the tip. i still got about 5cm wedge from the wheels to the end of the bot, but yeah, that's not much.

    I know it might be stupid, but hey... it's my first bot, and since i don't expect much of it, i thought i could as well try to go for an interesting design not seen that often instead of building the typical box shape. There will most likely other bots follow, where i'll go more for function than form.

    And no, no direct drive (i may be stupid and ignore some advices, but not this one^^)
    That's why i asked about the pulleys (question 2). It's just... the Motor will basically sit "inside" the weapon diameter.
    Imagine a beater. now double it up, with the axis joined together. now in this middle space the motor will sit, and the pulley will go there. (that's why i can't just order one if it doesn't work, i need one made from two pieces with a rectangular cross section i could then screw together again. Possibly another stupid idea, but if i put all of them in one bot, i have less left for the next one!)


    and yeah, spokes as wide as possible to make them sturdy seems reasonable, only the weapon could store more energy with the weight further on the outside... still, "not under 30" again is something i could hopefully work with.


    (maybe after the weekend i could post some rough scetches/pictures of the parts aligned as in the bot to clarify the design a bit, if you are interested. but not much to see yet, since many materials will only arrive the next few days. mostly just having the "guts" of the bot by now and playing tetris in a paper box with them)
    Last edited by Runsler; 17th February 2017 at 16:23.

  7. #7
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    I need a sketch as I'm not sure I understand the design idea

  8. #8
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    From what you have said, this is what I am seeing...

    Runsler Weapon.JPG

    This is to scale, 250 OD, 300mm weapon length, from 20mm plate steel. The pulley in the middle is Ali. Rough weight is 5.6kg

    Is this correct?

  9. #9
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    That is looking quiet like what i have in mind/on paper. only the part with the pulley is 8cm wide. (just the "gap", not the pulley, that would stay close to one of the sides) so when the weapon rotates, the motor would fit into that gap and doesn't get hit. And the assymetrical part is done so one side really is a bit longer while the other is a bit thicker, effectively making it a one-tooth spinner. Well, if it works at all, didn't put in into CAD yet, and it seem to be a bit too thin or heavy... depending on how much pockets i could put into my side armor^^

    Imagine a typical wedge that could drive on both sides, wedge angle about 45°.
    now put a weapon motor in the middle of the back on it.
    make the sides longer.
    put the beater alex has pictured between those sides.
    so the distance from axis to mid of the "back" of the original wedge is just a little bit more than the diameter of the beater. The motor would get hit, so make a gap in the beater for it, with the pulley on it.
    now with the side walls continued at the 45°, the beater will also almost scraping the ground, no matter what side you drive on.
    And if it stops working -> wedge.
    Not a good wegde, given the position of the tires and about 60% of the total bot length is weapon ( o.0 ), but well...
    Driving-wise it just might behave like Nightmare from BattleBots, but well... i'll be a terrible driver anyway, so better make a slow bot with the main purpose on driving "point the right end at the enemy" and nothing else.

    For pulleys i have assumed a widht of 1cm, would that be good?

    Also... while sleeping i came to think of:
    since i hardly can change dimensions of the weapon without having to change everything (just making it smaller would lift it from the ground too fast, so i'd had to buld a completely different bot), would it be a legitimate idea to make the weapon from some different material (Ali? Titanium?), with a bit smaller diameter and thicker, and only put screws as teeth in there?
    Seen screw-teeth some times, but i am not quiet sure about the details and how that worked out. specially how i would try to balance it with screws added, and if placed in a "softer" material the screws won't damage the acctual beater on impact.
    Last edited by Runsler; 18th February 2017 at 07:52.

  10. #10
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    I know we have a "be nice" policy around here, but that beater design has some major problems. If its done on the cheap, it probably won't survive even one match and it will cost a small fortune to make a part that will work reliably:

    1) As described the beater is all one piece; how is the pulley going to be fitted to the central shaft? Cutting it in half and screwing it back together will add bulk and reduce its strength.

    2) Placing the motor close to the central shaft means that the pulleys will have to be fairly small and the belt between them will be short - far too short to fit over the beaters on either side. The shortest belt that will slip over the beaters is about 400mm and that will place the motor too far out. The only solution to that problem is to use custom length urethane V belts that are joined in place. Having used those belts on smaller & lighter beaters, I know they will not transmit the torque needed to spin up this beater design. This problem is the real show stopper - I don't see any good solution without a major design change.

    3) Fabricating the part is going to be difficult & expensive. Beaters are usually water-jetted from steel plate, then mounted between centres on a lathe so that the stub axles can be turned - no problem so far. Turning down the central portion from square to round will need a large lathe with at least a 250mm swing over the saddle. Getting a cutter in between the beaters will need over 100mm of tool stick-out which, together with the hardened steel, will create extreme tool chatter and a very poor surface finish. To do this operation successfully will take the sort of multi-axis CNC machinery normally used to make car crankshafts IE:

    Of course, if the central shaft isn't supported, then it can be left square and the two-part pulley can be made with a square hole .

    4) having a central support bearing is a good idea - the beater will very likely bend or snap without it. Two-part bearings are used on the crankshafts of cars and work well when properly lubed and the shaft running in them is well polished. Getting the central shaft polished is also going to need the sort of machinery used on crankshafts. The bearing itself is easy; just cut a plain bronze bearing in half. The mount for the bearing is much harder; it needs to be precisely made, probably requiring a CNC mill.

    5) The 80mm space between the beaters is really a bit tight to fit a motor powerful enough to spin up the beater. The closest beater design I can think of is Conker 3 and Alex needs a monster out-runner to spin it up. Once you add a motor mount and a pulley with a hub, the 80mm is more than used up. The pulley width for even small belts is 14mm plus around 8 to 10mm for a hub. The pulley on the beater can't be hard up against the beater arm as the motor pulley needs clearance from the beater as it swings past - add another 2 to 5mm. Then there is a motor mount; it has the support the motor rigidly and should be 10mm thick aluminium. That all adds up to at least 34mm, leaving just 46mm for the motor. Let's not forget some clearance between the back of the motor and the other beater, so around a 40mm long motor is the longest that will fit. Finding a motor with enough power in a 40mm length is basically impossible. I think this beater will need a motor at least 60mm long to spin it up to an effective speed - A scorpion 4530 might be a good starting point.

    ==============================

    I hope this isn't too disheartening and that you can change the design a bit to make it easier and cheaper to build.

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