Deathly Hallows - Beetle - Evil K'Weevil

[Deathly Hallows]

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?

[Deathly Hallows]

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.

[Deathly Hallows]

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

[Scottie]

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

[Deathly Hallows]

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.

[Albear]

Does look mean indeed! My worry is that the prongs of the spinney thing will bend on contact with another robot, if you fancy something just as mean looking and a bit beefier how about one of these? https://www.ebay.com/itm/361201931762

[Deathly Hallows]

Nice! I had to give up on the chuck and interchangeable weapon. I found loads of tasty tools/bits I could fit, but the original drill motor and assembly was too heavy and using an arbor, adapters, chuck etc would also add too much mass. I want some spare capacity for some decent armour. I may return to the pure drill concept in a Featherweight, possibly multiple interchangeable weapons.
I personally liked core cutters and forstner bits as weapons, but these would look awesome! graduated hole cutter

The shape I chose is deliberately 'square edged' as I like the idea that they could flip as well as cut into opponents. I'd aim to get the tip to be as close to the ground as possible to get at wheels, or under the other bots. Through the use of skid plates I should be able to adjust the angle of the robot depending on the condition of the arena floor. If I was able to get the prongs fabricated from something tough say 3+mm Titanium or Hardox my hope is in this class they should be able to take most of what is thrown at them. Perhaps need to get the calculator out and do some calculations. Deformation and sheer I suppose I could calculate, but I'll have to do some research.

The motor powering the weapon will be this one from RobotShop which has 42kg-cm stall torque but is only spinning at around 100rpm. For me that seems plenty, unless I pin something to a wall I am never cutting/boring through it. I want to basically smack things about and if I come across something with weak armour I could in theory spike it and chew/spin it up. Not much in a 1.5kg class will stop that weapon moving with a radius of 3.5cm it will still flip or push/punch anything with ease (I hope). But I do have concerns about a fast spinner.

As a complete novice this is all based on estimates, instinct and supposition. Only building it and then competing will tell.

[Deathly Hallows]

OK time for me to show my naivety and show you how I think I should wire this little beast up.

So the components:
Turnigy brushed 30A ESC for weapon - motor has 2.19A stall
Sabretooth RC 2x12A ESC for drives - each motor has 4.9A stall
12V LED for power on indicator which draws a few milliamps

My goal is to add fuses to protect the equipment.
I was thinking I should limit the max load on the inbound of the ESC, which would protect the motors too. So based on the information above a 10A fuse should suffice for drives, and smallest I have is a 5A for the weapon see below.

IMG_20171220_172605.jpg

[edit: sketch shows two 5A fuses]
Or is this the wrong way of going about it? And I should place a larger say 25A fuse on the inbound of each and add individual fuses to motors. This would cost me 40g more as I am using blade fuses and covered fuse holders.

[Ocracoke]

Despite the rules not making it a requirement for LiFePO4 batteries, I fit a fuse directly after my batteries but before they split off into the ESC and drive motors (I'll add a diagram of this shortly). For LiPo batteries, this is a mandatory requirement:

7.8.3 Fusing
A fuse rated below the maximum burst discharge of the battery MUST be fitted.The maximum burst discharge current is calculated by multiplying the C rating by the capacity. E.g. 25C 2200mAh = 55 Amp

I would therefore think about adding that into your circuitry as well would be something to consider here. As for fuses for each part of the circuitry, the idea makes sense so I'd keep it.

[cosmin]

If your battery is a LiPo, my understanding is that you pretty much need a master fuse. Everything powered from it must be on a fused circuit, i.e. including the power light, and the power light shouldn't be on a separate fuse because then that fuse blowing would mean the light is off on an active robot.

The thing to keep in mind is that fuses are mainly intended to stop wiring or battery fires. I would personally just use one master fuse. The individual fuses as shown in the diagram wouldn't add much. Stalled motors won't draw enough current to blow them (which is good anyway), so they'd only blow if one of ESCs failed short (which should be quite unlikely). With both drive ESCs on the same fuse, either one failing would take out your drive and lead to a loss. Basically they'd only help if weapon ESC failed short (by allowing you to continue to drive), which I think is not that likely. Meanwhile, they add weight and potential failure points.

[Deathly Hallows]

OK thanks - what I take form this is:

1 - a 25A fuse on my 1300mAh LiPo battery should be enough
2- if I have the weight I could consider additional fuses (as using blades in enclosures quite rugged and the Sabertooth cost a few quid)
3 - I should read the rule book again

[Deathly Hallows]

Today was the day that the Robotshop was meant to get delivery of my motors. Lunch time still showed out of stock. After lunch they showed as out of stock and another month delay. So a wrote a restrained but firm email to customer support at 4pm.Got an answer by 7pm. I have to say they have great customer support, always answer really fast! Apparently that was an error and they have received my parts and they will be in the post tomorrow. Woohoo!

With my Christmas presents (tools) I finally look like being able to start work in earnest when the bits arrived. I also ordered some small aluminium sheets for making smaller custom parts and I will use one as a mock-up for the base, and tweak that design before making the base form something harder.

When I first saw the stock error I had another look for alternative parts and suppliers. As you saw in another thread I did see some on ebay with encoders but I also located a company in Italy - not sure if anyone has dealt with them before StepLab - shipping to UK is around €17.50 for express up to 5kg - English website is a bit unstable and incomplete but Italian looks good. Motori, Planetari, you get the idea. For slow post and no tracking its a lot less. I may order my next lot from there.

[Deathly Hallows]

Shipment confirmed, will be around a week I guess. Which is OK as I am away on business. However this weekend I can start work on a baseplate to fit the bits to when they arrive.

IMG_20180112_170240.jpg

The drawing is an export from the 3D cad in OnShape. We will see if I can cut it myself. I think some expletives will be heard coming from the garage.

Talking of the garage. It is nowhere near ready to be called a workshop. I ordered some workbench legs from Amazon as every table and workbench I found was either too low or overpriced or made from flimsy crap materials (5mm MDF). The legs look great, so will scour local kitchen show rooms for an offcut or damaged worktop this weekend to fit to them.

[Deathly Hallows]

A few steps forward today.

1 - Couldn't find any decent timber or solid wood worktops on Gumtree but found a bargain at Homebase £49 for a 2200mm x 600mm x 26mm wood worktop. That is now fitted to the legs. Will need a third central leg but for now supported by my portable workbench.

2 - Insulated some more of the garage which won't make much difference until the rest is done, but easier before everything gets in the way.

3 - Stated cutting out the base plate. Impressed with the Dremmel. I started with a hacksaw but it was slow and would seen become obstructed due to length of cut, switched to the angle grinder disk on the Dremmel and quite impressed. Ran out of light (another topic for improving the garage more lights).

Pictures tomorrow

[Deathly Hallows]

IMG_20180114_113511.jpg
I marked out basic dimensions on the backing foil of the aluminium

IMG_20180114_151628.jpg
Cutting with the Dremel was actually not the best. It can do it for small bits but found on longer cuts the disk jammed. Used the hacksaw for most of it and then polished it off.

IMG_20180114_153103.jpg
Loving the pillar drill I got for Christmas. Went through it like a hot knife through butter. Needed some holes to start sawing from. These slots will house the wheels ultimately. Wider than necessary but if I need more traction at least I have options.

IMG_20180114_161431.jpg

This is as far as I got today. Not the prettiest bit of metal, as my workmanship on first cuts was lacking some finesse. I need to saw out the other wheel slot. Then I will check dimensions once I have the motors etc. before drilling. All in CAD so far and reality may be different.

Hopefully parts arrive this week. So next weekend I can make that slot and then start laying it up. Moo chance this week as away on business trip.