Deathblades/RazErBlades Structural Redux

Alright, so the last test of the RazErblades ended in a mild disappointment as my shoddy metallurgy brittle-failed under load. Personally, I don’t blame it – I might as well have put the metal together with center punch dots the way I speed-brazed the thing together. No matter – a chance to rebuild the frame is also a chance to optimize the frame geometry for better component placement. As I was entering a dead week of waiting for parts, I took a few hours to think about the new frame design.

I decided to go back to using my classic T-nuts and slots method of building, just because everything goes together so quickly using it. For well-constrained structures with lots of fasteners, it’s also fairly strong.

This is the updated rendering of the frame after a few hours of speedCADing, with few structural T-nuts designed in. Most of the changes are to minor aspects of the design that will have a substantial impact on the way the electronics sit inside the frame and the integrity of the motor mounting.

  • The bottom of the frame is now flat. Previously, there was a slight (less than 5 degree) slope downwards to the wheels. I’m not really sure why this was ever put in, but it impeded the sanding of the brazed frame surfaces and made machining setups much harder.
  • It also screwed me out of about 1/4″ of “cavity space”. This was 1/4″ that could have been (will now be) used to make sure I wasn’t stepping on the radios.
  • I changed the angle of the motor shaft slot such that it wasn’t directly tangential to the load path the wheel sees. There’s now a bit of downward “hook” to lessen the risk of ditching a whole wheel.
  • The boot mounting plate has become a mounting block. Instead of using the Weird Shady Nut, the center hole of 1/2″ worth of aluminum will be threaded to accept the M6 x 1 screw. This new mounting interface is two 1/4″ aluminum plates screwed to eachother, the bottom having a solid slot interface with the frame’s side plates. In other words, the mounting should no longer be the weak point.
  • The two notches in the middle don’t serve any useful purpose (wire passing, perhaps?) but I thought it made the whole rig look more like its namesake.

Now with more T-nuts! As mentioned above, the internal cavity is much taller. In fact, it’s almost twice as tall as the 6S battery now.

Oh, yes, one more thing. I’ve made the cavity 1 millimeter wider to let the battery slip in without requiring… you know, additional machining. The ends of the wheel shafts will be shimmed slightly to account for this.

There is now certainly the possibility of switching to an even bigger battery – 22.2v and 2.6Ah is already quite substantial to stuff inside, but I’ve been shopping for 3AH and larger packs on Hobbyking. Unfortunately, they all seem to be much wider than the 40-42mm range, which isn’t helpful. Laying the cells sideways would increase my “width capacity” to 46mm or thereabouts, so that’s a possible solution.

design for manufacturing

A day and sheet of 1/8″ and 1/4″ aluminum later.

Not everyone has abrasive waterjets on call at the tap of an ID card. Fortunately, optimizing of designs for waterjet machining (for myself, mostly out of laziness) is complemented by new online manufacturing shops that will take your flat parts drawings and lets you purchase the parts cut in different materials.  It’s one version of the PCB-fab-but-for-MechEs that I want to see more of, and believe will be key in aiding the United States’ manufacturing industry while encouraging the DIY technical arts, the TE part of STEM, and makerdom as a whole.

But enough soapboxing. I keep plugging Simon’s abrasive waterjet (and LASER!) service Big Blue Saw because he and I go way back and are homies. Isn’t that right, Simon?

The way I fasten all this stuff together is with square machine nuts. Überclocker and Cold Arbor are both chock full of 6-32 and 1/4″-20 square nuts.

1/8″ thick material is a difficult one to size up square nuts for. Even a 6-32 nut is far too wide to be useful – it can be done, but it’s excessive because of the amount of overhang at 5/16″ wide each. A 4-40 square nut is generally 1/4″ wide, which is far more palatable. But there was a problem: McMaster, my go-to guys for hardware, don’t stock any kind of 4-40 square nut. I was concerned, because that meant I was potentially spec’ing out hardware which didn’t exist. I mean, if McMaster doesn’t carry it, why should it exist?

A search through the Intergoogles took me to Amazon of all places (seriously, did you know they have a whole industrial components division?), but unfortunately all the 4-40 square nuts came in packs of 5000.

That’s alot of nuts. In fact, it’s 7 and a half pounds of nuts. I decided to press forward and bought a pack, figuring I would never, ever need to buy these things again.

hey mcmaster, you should carry 4-40 square nuts in packs of smaller than 5000.

The 4-40 square nuts are standardized to be 0.25″ wide and0.098″ thick, for reference.

And here are the new plates all loaded with square nuts, ready for fastening. All the screws are standardized to be 4-40 x 3/8″ long socket head cap screws.

And the “version 2″ frames assembled, but without hardware. I still need to modify the top 1/4” aluminum mounting blocks slightly – they need clearance holes for 4-40 cap screws to go all the way through to the second layer. Both layers also need the center hole tapped for a M6 x 1 thread.

Hopefully, my custom magnets from Super(custom)MagnetGeorge will arrive soon, so I can get to some 4 wheel drive testing. Adding 100% more motor will make a world of difference in terms of handling.

5 thoughts on “Deathblades/RazErBlades Structural Redux”

  1. Instead of constructing boxes from aluminum plate, why not just put two blocks of plastic (PE or nylon) between the side panels? A few fastener stacks through the panels and blocks would give stronger points to mount the skate boot to, and it’s possibly lighter. Just using nut-against-T-slot friction to hold in the skate boot attachment plates (the ones at the top) looks like an extremely tenuous connection.

  2. Because it would mean I have to actually machine something. We can’t have that :(

    (You are correct, however.)

    In terms of boot attachment, that’s what the bottom attachment plates are for. They have a mechanical interlock with the side plates.

    The boot bolt through both of them.

  3. hey i’m very interested to get 4 of el wheelmotor, if may i ask will u willing to make me 4 of el wheelmotor that is a bit thicker than the wheel in the video and of course i will pay you for it. If your interested making it for me please send me an e-mail to and we will discuss about the price.Thank you

  4. Maybe time to consider investing in a MIG and spool gun setup? If you watch craigslist and ebay a while you could probably put together something you can weld aluminum with for less than a grand.

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