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Thread: Deathly Hallows - Beetle - Evil K'Weevil

  1. #11
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    Still working that design through. Getting under weight limit is killing me. Of course this is also all calculated as not drilled a hole or soldered a wire yet.

    The motors, wheels and hubs look good and allow flexibility in design.
    Weapon and it's motor is a problem as what I have in mind dictates chassis dimensions and will take 1/3rd the mass I predict
    I will have to size the battery (LiPo) last when I see how much weight is left I have a min and max option 700mAh to 1500mAh and weights
    The remainder I can spend on chassis and armour.

    Here I may need to get creative. From a simple box I am moving to more complex shapes to reduce surface area. Could look really weird! Cool evil weird but weird. I think the HDPE bulkheads made from that Bonzer Board is a no go, surprising heavy but at 12.7mm it soon adds up.

    I will try and generate a CAD image soon worst case next week. At the moment I have no been able to virtually assemble the parts other than the drive system.

  2. #12
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    Evil K'Weevil.png
    So not quite done -but this is a mock up of Evil K'Weevil.
    LiPo 3S
    RB-Sct-1060 geared drives (12V 730 RPM)
    Rotary interchangeable weapon in drill chuck
    Weapon design uses drill motor (but trying to save wight here)
    Armour is an area I can work on. Balancing weight and strength.
    Calculation at moment coming in at 1517g would like 50g comfort at least

  3. #13
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    Looks great! We need more drills in the lower weight classes, they can have a good impact on plastics and other materials. To save weight, a large MFA motor would do the trick, they're the same as drill motors but lighter. That is if you're able to modify the shaft to take drill chucks.

  4. #14
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    That would be a good idea Theo. I have searched for direct to Drill chucks but have not found anything that opens wide enough for the bits I would fit so far. What I have found are two possibilities:

    1 - a motor arbor looks to be used in RC planes for fitting props, this would take core cutters but not all weapons
    2 - a CNC collet grip adapter and collet grip could work, but could also be heavy

    I will keep looking, and if anyone has any ideas please let me know

    EDIT: This may work - ER11 6mm CNC extension but the chuck would have to be heated to expand before fitting a 6mm drive shaft. Not sure how I would do that though, as my soldering iron could not manage that load. I could heat in oven to around 250'C but that's as hot as I could go.

    EDIT: Arbor solution With this M10 arbor and this chuck I can use a smaller motor. But I need to find a <150g DC motor with 5mm shaft.
    Last edited by Deathly Hallows; 19th November 2017 at 20:40.

  5. #15
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    I have been quiet as away on business and despite the OnShape having an Android App, I couldn't get to grips with it. I managed to investigate a weapon motor alternative to the drill I had planned. I also think I will have to get the weapon machined for me rather than an off the shelf option when I was using a drill. Using a motor with lower rpm and more torque but smaller in size and weight allowed me to also change the shape slightly reducing surface area so less weight in the armour too. Here's a 'naked' snap i.e without the sidewalls. I must add drive mounts and the electrics but its coming together I think.
    Evil K'Weevil v0.3.png

  6. #16
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    So this is a shot with the side walls and the weapon I designed. I had to ask a friendly engineer to produce a 'step' drawing though as my CAD skills fell short and I imported it in the model. It will have to a be a custom piece which is a shame. Weapon and it's motor will consume just under a third of the mass I calculate. The chassis, drive, electrics and armour two thirds. Seems reasonable proportions. Is this typical?

    EKW v0.4.png

    I am wondering where the centre of gravity will be. But I have tried to ensure I balance the weapon by sitting it back inside the frame hence the shape at front and also by placing the battery behind the drive motors.

    This morning before work I designed the mounts for the drives. I made them a couple of mm shorter in height than needed to allow some adjustment to ensure the motor sits central. My idea is that with a few washers I can pad them out to compensate for small inaccuracies in the construction of the body.

    The skids (blue) will also allow me to alter the angle the robot sits at if needed. However I also reduced the weapon size slightly as originally there would have only been 0.7mm clearance if the robot sat flat and level. Which sounds great but I think in real world the arena surfaces would have been an issue, and that assuming the robot did not get a bit twisted after receiving some whacks.

    Still need to work out how most of it bolts together. May split the front plate so the sides and upper plate can be made into one piece. This could work really well. Welded together I could reduce internal fixings and it would be strong (for a beetle). A few fixing points to tie it to the lower floor plate (with internals)would be enough. What do you think?
    Last edited by Deathly Hallows; 28th November 2017 at 13:38.

  7. #17
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    Played with the idea of bending a single sheet to form the side walls all the way round the robot except for the front, where the weapon comes out. This would avoid welding and should create a very strong shape. Particularly if the floor and roof then form an internal structure by sitting inside the folded walls. Roof and walls could be welded together and mount on the floor in one piece with some angle brackets, easily bolted or unbolted.

    Question - I had originally planned for aluminium but would titanium be worth it? Max would be 1-1.5mm G2 Titanium due to weight. How does this compare to 2-3mm Aluminium. The hardness figures are one thing but real world experience is better, the supplier websites are that helpful when you want to know how it would stand up against a spinner.

  8. #18
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    I thought i would try and rework the design to reduce the volume of the design and therefore reduce the mass committed to the body/armour. In the process I lost the 'tail' at the back, and around 20mm from the width. This saved me about 100g.

    Checked my peak load calculations too. The drive motor stall is 4.9A and weapon only 2.19A surprisingly, so 12A total. Peak load is then low and easily managed by even a small 700 mAh rated just 20C, but I want more run time and a cushion.
    With the available mass I can fit a bigger battery. The Zippy Compact units have a low mass and a 1300mAh 3S should give me 11 minutes at full power, so in the real world much more and many battles. It's a bit of a squeeze to fit the battery, perhaps I will need to angle it to allow access to the leads. I don't think I will really know best way to squeeze it in until the bots assembled.

    Here's a inside pic. Space for the electrics on opposite side to the battery.

    Evil K'Weevil v0.4.png

  9. #19
    This looks awesome and pretty unique. Im looking forward to seeing how it turns out.

  10. #20
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    Quick update -

    So I have an order in with HobbyKing and RobotShop for the parts.

    I'd originally ordered the battery from a UK Ebay supplier but Royal Mail refused the parcel. Apparently they fly up post from Southampton to Scotland - perhaps its just the Christmas chaos but he had to refund me. HobbyKing had an offer on so no big issue.

    I have had to reduce the weapon in complexity and weight. Here is a render in OnShape. Still looks pretty mean.
    EKW Weapon Assembly.png

    Also been looking into getting the body made up for me. Bit beyond me at the moment, certainly until I get myself a bit of equipment and make a space in the garage to work.

    Need to get some applications in for some competitions. Due to my location I will be limited in the events I can attend.

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