Author Topic: First project on my new 4th axis, making a micrometer dial  (Read 705 times)

Offline JayMcClellan

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First project on my new 4th axis, making a micrometer dial
« on: April 18, 2017, 10:28:40 AM »
As the first real test of the new harmonic drive 4th axis that I built (see http://madmodder.net/index.php/topic,12033.0.html), I used it to engrave a micrometer dial for an adjustable carriage stop for my metal lathe. The first video is a tutorial on using Fusion 360 to generate the toolpaths:
<a href="http://www.youtube.com/watch?v=94ns-bt-XSs" target="_blank">http://www.youtube.com/watch?v=94ns-bt-XSs</a>
The second video shows the machining:
<a href="http://www.youtube.com/watch?v=3g9IFnVDOTo" target="_blank">http://www.youtube.com/watch?v=3g9IFnVDOTo</a>
I'm very happy with how the 4th axis unit worked, although I ran into some other minor problems as shown in the video. In the end it came out pretty good. I posted more info on my web site at http://www.brainright.com/Projects/CNC/MicrometerDial/.

Offline philf

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Re: First project on my new 4th axis, making a micrometer dial
« Reply #1 on: April 18, 2017, 12:06:49 PM »
Jay,

Have a look at GCode Ripper :

http://www.scorchworks.com/Gcoderipper/gcoderipper.html

It is a free programme which can swap either X or Y axes for A and reverse the direction if necessary. You just load your GCode with X, Y & Z and save it as X, A & Z (or A, Y & Z).

It saves having to swap connectors over and changing direction (easy to forget to switch back).

I've used this for engraving trophies on my 4th axis.

My 4th axis also uses a Harmonic Drive but it's of the true Harmonic Drive type with zero backlash. The problem with mine is that it's a 200:1 reduction so very slow.

Phil.
Phil Fern
Location: Marple, Cheshire

Offline JayMcClellan

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Re: First project on my new 4th axis, making a micrometer dial
« Reply #2 on: April 18, 2017, 02:09:16 PM »
Thanks Phil, I'm using my own post-processor that I wrote for Fusion 360 so it would be easy to enhance that to optionally swap axes when it generates the G-code. I thought about doing that up front, but I wanted to make my video work for folks that might be using different post-processors without the ability to modify them. It looks like GCode Ripper would be a good option for them.

The Harmonic Drive gearboxes are advertised as "zero backlash" but of course it's not quite zero. The type that I have is sold in two grades, specified to have backlash under 3 arc-min or under 1 arc-min. I can't figure out which mine is from the part number on it, but I presume it's the cheaper 3 arc-min grade which means backlash up to 0.05 degree. I plan to measure the actual backlash of my system, including that of the 3-jaw chuck since its jaws may permit some movement that is backlash-like. Measuring that and the holding torque might make an interesting video.

I was worried that the 45:1 ratio of my gearbox would be too slow but it actually seems pretty snappy at least when rotating with no load. I probably can't rotate it as fast under load (when cutting) without losing steps. I was lucky to find one with this ratio, which seems about optimal for my needs. With a 200-step motor and 1/8 microstepping being roughly the practical limit for positional accuracy of a stepper, the 45:1 ratio gives a resolution of 0.005 degree. Assuming that the physical backlash of the system is roughly 10x that as noted above, the positional accuracy is not significantly limited by stepping resolution and mainly limited by the backlash. I will try to measure it but I'm guessing that with all things combined, and some opposing torque applied, I should get a positional accuracy of about 0.1 degree total or +/- 0.05 degree. That would give me an error up to about +/- 0.001" / 0.02mm on the circumference of a 2" / 50mm diameter part.

Offline philf

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Re: First project on my new 4th axis, making a micrometer dial
« Reply #3 on: April 18, 2017, 03:14:53 PM »
Hi Jay,

The type I have is certainly advertised as zero backlash achieved by preloading the tooth engagement. Up to 30% of the gear teeth are engaged at any one time.

Whatever the truth - I can't feel or see any effects of backlash when engraving with a fine drag engraver.

There's a good description of the non-planetary geared Harmonic Drive here:

<a href="http://www.youtube.com/watch?v=3mWemlMEzFk" target="_blank">http://www.youtube.com/watch?v=3mWemlMEzFk</a>

When I say mine's slow it is really slow. Yours looks very quick by comparison. I'm using Mach3 and can't get a fast enough pulse rate. Beggars can't be choosers though because my Harmonic Drive came for free out of a scrap silicon dicing machine from work. I may experiment with a pulse generator to see how fast I can make it go and prove one way or another if my limitation is Mach3.

Cheers.

Phil.

Phil Fern
Location: Marple, Cheshire

Offline nrml

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Re: First project on my new 4th axis, making a micrometer dial
« Reply #4 on: April 19, 2017, 03:59:46 PM »
Thanks for posting that video. I really enjoyed it. The best bit was showing the problems and not editing out the fact that you had to do re-do it more than twice to get it right. Nice website by the way. It is amazing how many of your projects are stuff I have on the go or intend to do in future. I will certainly be visiting it frequently for ideas and inspiration.

Offline JayMcClellan

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Re: First project on my new 4th axis, making a micrometer dial
« Reply #5 on: April 19, 2017, 05:43:04 PM »

When I say mine's slow it is really slow. Yours looks very quick by comparison. I'm using Mach3 and can't get a fast enough pulse rate. Beggars can't be choosers though because my Harmonic Drive came for free out of a scrap silicon dicing machine from work. I may experiment with a pulse generator to see how fast I can make it go and prove one way or another if my limitation is Mach3.

Cheers.

Phil.

Phil, just a suggestion - if your stepper driver is configured for microstepping you can turn that off. With your gear ratio there's not much benefit to microstepping, i.e. it won't significantly improve positional accuracy, but it reduces the maximum speed. With very little load you should be able to run the stepper up to about 1000 RPM and I would think Mach3 should have no problem giving the required 3KHz pulse rate. That should get you 5 RPM output, not zippy but not terrible. Yours will still be 4X slower than mine, but sounds like it was infinitely more affordable.  :clap:

Jay

Offline JayMcClellan

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Re: First project on my new 4th axis, making a micrometer dial
« Reply #6 on: April 19, 2017, 05:49:58 PM »
Thanks for posting that video. I really enjoyed it. The best bit was showing the problems and not editing out the fact that you had to do re-do it more than twice to get it right. Nice website by the way. It is amazing how many of your projects are stuff I have on the go or intend to do in future. I will certainly be visiting it frequently for ideas and inspiration.
Glad you enjoyed it. I debated about whether to include the problems or edit them out, not because I'm embarrassed but just to keep the video from getting too long. I decided to leave them in, figuring people might get some benefit (or at least enjoyment) from seeing things go wrong and how I fixed them. After all the best mistakes to learn from are someone else's!

Offline SwarfnStuff

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Re: First project on my new 4th axis, making a micrometer dial
« Reply #7 on: April 20, 2017, 02:24:46 AM »
Jay, it is a truism (I think) that if we don't learn from our mistakes we are doomed to repeat them.
        As you said, "After all the best mistakes to learn from are someone else's!" I too much prefer to learn from others mistakes .  Not that it stops me making the same or similar errors of judgement though.  :scratch:  :zap:
  So thanks for posting your errors of judgement as they are not mistakes, just opportunities to learn?

John B
Converting good metal into swarf sometimes ending up with something useful. ;-)

Offline Fredbare

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Re: First project on my new 4th axis, making a micrometer dial
« Reply #8 on: April 20, 2017, 03:57:54 PM »
Thanks Jay for sharing, very interesting.

John