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Tracked Cat delta skidsteer

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I've been converting one of these Bruder Cat delta skidsteer toys to RC for some time now.  It's been on the back burner for a long time because I've been learning how to use the printer, draw in 3D, and upgrading my Chinese clone Prusa I3 into a usable machine that gives me predictable results - not just pretty looking junk.

I'm working on something similar to this:
I'm using a similar gearbox to what SmallHaul used, but I've changed out the motors from 3 volt motors to 6 volt motors.  I had originally planned on a 6 volt ni-cad pack but now I'm wondering if I shouldn't upgrade them to 9 volt motors so that I could run a 2s LiPo battery pack. The drive motors will be controlled by a T-Rex controller and I am planning on printing the drive sprockets.  I've printed a test section of the hex shaped drive shafts coming out of the transmission and that fits just fine.  I've also printed up a 9 tooth drive sprocket for the track belts but I've still got some clearance issues there.  0.2mm is apparently not enough when 3D printing, at least on my machine it's not.  When I've got a usable prototype part I'll take a picture for Show and Tell.

I don't plan on using servos for the bucket lift/lower or the bucket tilt.  Or is that bucket curl?  I don't know - I'm just a lowly EE, you construction types can correct me.  My plan is to use screw drives for these functions with the motors controlled by a T-Rex Jr. controller.

This little project is one of the reasons that I got my 3D printer, to make parts for this.  I spent most of Labor Day weekend drawing this thing in 3D.  So I guess maybe I better get on the stick and get busy making some usable parts.


Well I printed out a bunch of junk prototype parts over the weekend, even took some pictures.  Unfortunately I didn't remember to download those pictures from my camera, which is still sitting at home on my desk.

I printed the parts for the track drive sprockets.  They look real pretty and they're even a nice snug fit on the 2.6mm hex drive axles from the Tamiya gear box - unfortunately they don't fit the treads.  The treads are a flexible rubber band with hard plastic tread plates bonded to the inside of the band.  I measured the width of the plate at 6mm, I couldn't get an accurate measurement of the gaps between the plates so I measured the length of 5 plates and spaces by measuring from the leading edge of the 1st plate to the leading of the 6th plate, this measured 50mm.  So I've got a 10mm pitch between the links.  When I wrap the tread around the existing Bruder "drive sprocket" it looks like 9 "links" will fit.

So, 9 links at 10mm per link gives me a circumference of 90mm.  Dividing this value by Pi gave me the outside diameter of the "drive sprocket" with 9 teeth.  I used that value when 3D modeling the sprocket.  But when printed, the first version didn't fit.  I didn't allow enough clearance at the bottom of the pocket for the guide tabs and they bottomed out holding the track plate clear of the side plates of the drive sprocket.  I redesigned the pockets and reprinted the sprocket, it fit the track plate just like it was designed that way.  When I tried wrapping the tread around the sprocket to check the spacing it didn't fit.

WHAT THE F----!!!!  I re-measured everything and came up with just EXACTLY the numbers that I was expecting, so why doesn't this stinkin' thing fit.

Late yesterday I had an AHA moment.  I realized that the 4mm gap between the tread plates was not located where I was originally thinking it should be.  I was thinking that it was on the side of the plate that contacts the drive sprocket, it's not.  The contact point between the tread plate and the drive sprocket is at least 1mm closer, the thickness of the plastic plate, to the sprocket axis.  I modeled it again, this time using the brute force method.  This time the diameter to the contact point was about 28.5mm, as opposed to the 30mm-ish that I was using before. 

This time I may just have a winner, I'm going to reprint this tonight and find out.  I'll take pictures of the before and after versions.


Since nobody believes you if you don't gots pictures, I GOTS PICTURES!

The first attachment shows the 1st, 2nd and 3rd failed attempts at making a usable drive sprocket for the tracks.  The group in the lower left was the 1st attempt.  The "sprocket" was SUPPOSED to fit into a pocket.  My printer doesn't make neat and reliable bridges - yet.  The group in the lower right is the 2nd attempt which eliminated the large pocket.  That part was a success but it didn't fit the tracks properly - not enough clearance at the bottom of the openings for the track plates.  In the 3rd attempt, that blue part on the gearbox axle I went from a 2 piece assembly that would be glued together to a 4 piece assembly that is bolted and glued together - still didn't fit as shown in the 2nd attachment.

The 4th try was the charm, not the 3rd, as shown in the 3rd attachment.  When I wrapped the track around the drive sprocket and it fit just like it was designed for it, I couldn't resist.  I had to put it on the skidsteer and see what it looked like.  I've got some clearance issues, 4th attachment, but that 's just a matter of getting rid of a little plastic.

Next step will be to get rid of those fake rollers on the track frames and make them working.


Well I've accounted for the clearance problem between the drive sprocket and the existing skid steer frame.  The clearance problem is STILL there but at least I now know why it's these when the model showed plenty of clearance.

It would seem that some dummy drew the track frames 4mm wider than they actually are.  Discovered that one when I printed the frames for the left track rollers and they didn't fit - not even close.  Since I copied, pasted, and modified the left side parts to get the right side parts I compounded my error.  In my defense most of the original 3D modeling for this was done over the Labor day weekend and there were a couple of LATE night sessions in there - maybe I should have quit and gone to bed a little earlier?

Anyway, I'm in the process of fixing my Boo-Boo's and will soon print test parts for the left side track roller frames.


Forgot to mention, both the track roller frames prints failed about 3/4 of the way thru the prints.  I'm working on the reasons why they failed, but with what I have discovered so far I believe that I can safely assign most of the blame for the failures to myself.

Tried some stuff that didn't work, tried other stuff to fix it and just generally mucked things up.  You know how it goes.

Last night I think I figured out the root cause/s and started fixing the blunder/s.

More to follow,


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