Thursday, 11 June 2020

4th axis CAD - and "milestones" in Fusion (WTF??)

WTF indeed!
Perhaps seemed a bit arse about face to be buying bits for a build before actually designing anything. But in fact I did most of the design up front and wanted to get stuff on the way before documenting the design. Of course, it actually arrived pretty quickly, before I'd finished the design and managed to clear up a misunderstanding on my part about exactly what "milestones" are within Fusion 360. 

"Milestone saves" within Fusion 360:
Yes, the bottom line is that 2D drawings won't update past the last "milestone save" you made in the Design space. Once you create a milestone, you have to make all subsequent saves also as milestones - or remember to do a milestone save before you go into drawings. If you don't know this, you end up tearing your hair out trying to understand why the effing drawings are stuck at an old version of the design. And it seems that once you've created a milestone, you can't delete it. Leave that damned tickbox unticked!

Rant over. Anyway, in terms of the design, this is what happened and to some extent why it happened that way.

Benchmarking:
First, take a look around at what's been done so I can learn from others. There are several examples of 4th axis builds on Pootube to learn from. 

Firstly, there's a Swedish Viking called Metal Musings who documented his progress in the LinuxCNC forum.He used the SHG version of the harmonic drive. That was a good choice in my view, although I'm not sure why he didn't make use of the hollow shaft to allow stock through, rather like a lathe headstock. The upside is some simplicity, as it's possible to drive the input shaft directly. He's clearly a nice guy with a dry sense of humour. I like fishheads!

Then there's P Lopez who is using LinuxCNC and is actually building a 4th & 5th axis.  Using integrated motor / gearbox model - very neat and doubtless very expensive. Most of the used examples seem to start around the $500-800 mark. Like MetalMusings, he's gone for a direct drive (after all, the motor IS integrated into the drive), with the motor behind the drive unit. Unlike him, he's preserved the large bore through hole. Nice - but not the way I can afford to go.

This guy K Kim FinaxMusic is Korean and he's used a belt drive to offset the servo motor and also provide a reduction ratio. I like that. He's also made use of the through bore. Looks as if he's gone for a smaller (20 or 25) model. Pretty neat but rather short on details.

Over the pond there's Guy with Dickhead Haircut. Nothing personal of course - it appears there's no law against it there and of course it's possible he may be doing it for a bet or a forfeit, so who am I to criticise? Either way, this is another direct drive concept and is clearly a smaller drive unit - based on a different family (solid shaft input - SHF?). Doesn't feel like something I'd like to emulate - either the unit or the haircut.

Design principles:
So, given that inspiration, he's my concept:
  • SHG model harmonic drive, "32" size. The smaller 20 and 25 sizes are 1/4 and 1/2 the torque rating, yet aren't significantly smaller or cheaper. And the next size up (40?) is getting a bit silly in size.
  • Toothed belt reduction drive. The most common ratio seems to be 50:1 reduction and the unloaded torque required to turn the input is not insignificant, so a direct ratio may be pushing things. I can't be arsed to go through any sums here but I did look into it, honest.
  • Reduction ratio. I need a motor pulley with a "taper lock" fitting. Well strictly speaking I would prefer one. In the 5mm belt pitch, there's a min tooth count of 34T or so. For the driven pulley, I need a boss type pulley, with a boss that is large enough to mate with the input shaft. I also want to maximise the reduction ratio while keeping the pulley small enough to fit the assembled drive / pulley through the hole in the housing. That makes it a 60T then.
  • I plan to open out the bore of the driven pulley to the same diameter as the bore of the input shaft. So it makes sense to also bore out the chuck. The typical bore of a 125mm dia 3-jaw chuck is less than 36mm but there's plenty of meat in the body of the chuck to allow it to be bored out to that size.
  • 200W Yaskawa SigmaII Servopack and 200W SGMAH servomotor. The peak torque rating should be sufficient to overcome the unloaded drag torque and leave margin for machining and rapid movement.
  • 15mm loominum plate for the base and front (vertical) plate, with 10mm for the less structural side and rear plates. I should just be able to use M4 fixings for those, without the heads breaking through.
CAD design:
Here's what I've designed in Fusion. I haven't finished the top cover yet, so it hasn't got any fixings. The other stuff is 99% done, although I can't be arsed to add the myriad fixings to the assembly simply for cosmetic reasons.
Outside view:

Exploded views (from within the "Animation" space within Fusion):

Bottom view:
With the covers removed:
Can't be arsed to try to "fit" the timing belt, as Fusion doesn't really support that yet. I've convinced myself the belt length is correct and I can tension it without the motor bracket emerging through the side of the housing.

Right, enough talk - let's make the fucker....

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