Author Topic: Timing Belts vs Gear Drives - 4th Axis  (Read 1702 times)

Offline Bob La Londe

  • CNC Jedi
  • *****
  • Posts: 4074
  • ^ 8.5 pounds on my own hand poured bait.
    • View Profile
    • CNC Molds N Stuff
Re: Timing Belts vs Gear Drives - 4th Axis
« Reply #15 on: April 15, 2020, 18:13:12 pm »
Depending on some details I am looking at a max of maybe only 10 RPM and not likely more than 50 RPM using a stepper.  Of course I'll test to failure (step error trip) and then back off, but Its not going to be fast.  I still haven't decided on the primary work holding, but I am thinking maybe ER40 collet chuck.  I already have some ER32 stuff and a set of ER32 collets, but... 

I have been thinking of all kinds of "other" applications after its finished upto even making a 4 sided tombstone. 
« Last Edit: April 15, 2020, 18:23:16 pm by Bob La Londe »
Getting started on CNC?  In or passing through my area?
If I have the time I'll be glad to show you a little in my shop. 

Some Stuff I Make with CamBam
http://www.CNCMOLDS.com

Offline Bob La Londe

  • CNC Jedi
  • *****
  • Posts: 4074
  • ^ 8.5 pounds on my own hand poured bait.
    • View Profile
    • CNC Molds N Stuff
Re: Timing Belts vs Gear Drives - 4th Axis
« Reply #16 on: April 17, 2020, 14:57:06 pm »
A harmonic drive sounded good, but my first attempt in that arena went badly. Turned out to be missing parts.  Signs showed it was likely not new (grease was thin and milky like it had water contamination).  Worst it was mislabeled.  Somebody had penciled a part number across the box.  That was what they advertised.  The box actually had a label and part number that was correct.  That actually was the incomplete item in the box.  Sigh. 

Well back on the hunt for a solution.  A harmonic drive is not out of consideration, but most new from distribution or OEM are quite a bit of change. 

I've done a bit of playing with numbers and with the typical 1o micro stepping drives getting the number of steps to come out even I am looking at 4.5:1 as the lowest gear/pulley ratio.  That comes in at 25 steps per degree.  9:1 would be better just from a math point of view (5 steps per tenth degree), but pulley engagement with timing belt would be an issue.  I'm not fond of the idea of a 2 stage pulley arrangement. 

Oh, well.  back to the search for the "perfect" solution. 
« Last Edit: April 17, 2020, 15:04:53 pm by Bob La Londe »
Getting started on CNC?  In or passing through my area?
If I have the time I'll be glad to show you a little in my shop. 

Some Stuff I Make with CamBam
http://www.CNCMOLDS.com

Offline lloydsp

  • CNC Jedi
  • *****
  • Posts: 8409
    • View Profile
Re: Timing Belts vs Gear Drives - 4th Axis
« Reply #17 on: April 17, 2020, 15:39:53 pm »
"...but pulley engagement with timing belt would be an issue."
--------------
9:1 with an 'outside' idler near the smaller pulley (adjustable, but fixed, not spring-loaded) would solve the engagement issue on the smaller pulley.

It would only require a slightly-longer belt and a BB idler roller to accomplish.  If you're really particular about keeping 'stretch' equal on both belt segments, you could use two idlers, one on each side of the small pulley.

Lloyd
« Last Edit: April 17, 2020, 17:13:51 pm by lloydsp »
"Pyro for Fun and Profit for More Than Fifty Years"

Offline Bob La Londe

  • CNC Jedi
  • *****
  • Posts: 4074
  • ^ 8.5 pounds on my own hand poured bait.
    • View Profile
    • CNC Molds N Stuff
Re: Timing Belts vs Gear Drives - 4th Axis
« Reply #18 on: April 17, 2020, 17:46:53 pm »
I have been worrying about what to use as a spindle  The harmonic drives I've looked at could easily support a chuck adapter or faceplate bolted directly to them.  Machined in place for final fit up of chuck of course.  Since I opened my options back up to timing belts I was worrying about what to use as a spindle again.  The Taig Lathe 5C spindle is very price competitive at $599 with a  5C spindle bore and taper.  My concern being its dovetail mounting.  Oh, I could certainly cut a dove tail in a piece of plate.  I'm concerned about the rigidity of its actual clamping mechanism.  I'm pretty sure its aluminum and the head is aluminum as is all their other spindles.  They claim quite good TIR (0.0002"), but that goes out the window if anything flexes. 

Then it occurred to me.  I have a mini lathe with about .0002-.0003 (tested) that's been in a perpetual state of CNC conversion for several years.  (There is always something more important to do..  I'd like to be able to go 5C, but I have a 3C adapter for it that is still better than .0005.  3C only goes upto 0.500", but I could knock it out and use 3MT collets if needed... and I have made chuck adapter plates for mini lathes before.

I could just pull all the extraneous hardware (saddle, leads screw, gears,) off put a stepper on the headstock and have a complete ready to go 4th axis.  With only a 7" throw I might have to forgo my big dreams of making a dozen parts on a 4 sided tombstone, but it would make a perfectly acceptable 4th axis.  Its a little heavy to throw around, but I can lift it, and its all made of cast iron mitigating some of my other concerns. 

It would be nice to cut off the bottom of the bed, but I would be afraid it might not stay straight if I did that.  It might only be useable on the bigger machines in the shop as is. 

Just thinking out loud here. 
Getting started on CNC?  In or passing through my area?
If I have the time I'll be glad to show you a little in my shop. 

Some Stuff I Make with CamBam
http://www.CNCMOLDS.com

Offline Bubba

  • CNC Jedi
  • *****
  • Posts: 3108
    • View Profile
Re: Timing Belts vs Gear Drives - 4th Axis
« Reply #19 on: April 17, 2020, 17:57:19 pm »
Bob,
Please keep in mind this/mine 4th axis drive is only used for wood routing. If this help you that's what I use, i know the belt is narrow but so far did it's job well. I really don't think idler pulley is necessary as long the proper tension is set. An other thing to think about those Chinese pulleys won't run concentric, both pulleys ID holes need to be bored out for proper size so I fixed the run out this way. Thanks for the lathe. ;D

   https://www.ebay.com/itm/XL-60T-10T-Timing-Pulley-Belt-set-kit-Reduction-Ratio-6-1-For-Stepper-Motor-CNC/263366733306?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
My 2ยข

Win10 (64 bit, CB(1.0)rc 1(64 bit) Mach3, ESS, G540, Endurance Laser.

Offline lloydsp

  • CNC Jedi
  • *****
  • Posts: 8409
    • View Profile
Re: Timing Belts vs Gear Drives - 4th Axis
« Reply #20 on: April 17, 2020, 18:45:03 pm »
Yep, Bubba.

The torque requirements will be low.  It shouldn't need the idlers, but it's an option to guarantee 'proper' engagement of the belt on the small pulley.  A longer belt will provide more-full engagement on the small pulley.

Lloyd
"Pyro for Fun and Profit for More Than Fifty Years"

Offline Bob La Londe

  • CNC Jedi
  • *****
  • Posts: 4074
  • ^ 8.5 pounds on my own hand poured bait.
    • View Profile
    • CNC Molds N Stuff
Re: Timing Belts vs Gear Drives - 4th Axis
« Reply #21 on: April 17, 2020, 20:07:10 pm »
Thanks guys.  I would definitely have to machine the bore of atleast one pulley if I use the mini lathe to make a 4th axis.  I would want it to go around the outside of the spindle tube so the spindle bore remains open.  While the bore is not huge, the tube is larger than the ID of most inexpensive pullies I have seen.  I was considering getting sintered metal tapered bushing pullies though.  They are more expensive, but also made to much better tolerances. 
Getting started on CNC?  In or passing through my area?
If I have the time I'll be glad to show you a little in my shop. 

Some Stuff I Make with CamBam
http://www.CNCMOLDS.com