Integrating Encoder Into Gearbox

[asid61]

Originally Posted by InFlight

The the encoder is adjacent two 2.5 Inch OD CIM Motors that protrude 4.5 inches out the rear of the gearbox. Minimizing the encoder profile outside the gearbox provides no advantage, and is counter to having a more reliable gearbox system.

The mechanical specs for this encoder are on the website. Please take note of the hole tolerances. CAD is one thing; student repeatable machining of center holes to a 0.0003 inch tolerance on center is quite another.

According to the product page (link's broken) the maximum TIR allowed is 0.006", so there's a bit more breathing room than that. Still not much though. I think the 0.0003" is the encoder shaft relative to the encoder, otherwise anything less than an ABEC-5 bearing could be out of tolerances.

[scottandme]

Originally Posted by InFlight

The the encoder is adjacent two 2.5 Inch OD CIM Motors that protrude 4.5 inches out the rear of the gearbox. Minimizing the encoder profile outside the gearbox provides no advantage, and is counter to having a more reliable gearbox system.

The mechanical specs for this encoder are on the website. Please take note of the hole tolerances. CAD is one thing; student repeatable machining of center holes to a 0.0003 inch tolerance on center is quite another.

971 Encoder Mount Example Here

It's a pretty simple operation in the lathe - just drill undersize and then ream (as suggested by Austin). The doc you linked just establishes that the encoder shafts are made 2 to 6 tenths undersize, just means that you can probably get away with a 0.2500" reamer if that's all you have on hand.

You want a mount that prevents the encoder from rotating, and doesn't create any angular or parallel misalignment. The bent lexan design will allow the encoder to "float" a bit, which solves the parallel/angular problem.

[InFlight]

There seems to be an assumption that a basic lathe comes from the factory, and continuously maintains perfect alignment between the tail stock ram axis and the spindle axis; to allow perfectly on-center holes to be drilled.

The reality is it takes a somewhat experienced machinist with a test bar and a dial indicator to adjust the tail stock to be aligned perfectly laterally and vertically. The average FRC lathe is use is likely only accurate to 0.01 inch on-center.

Drilling center holes to the stated -0.0000/+0.0006 on center tolerance would be a non-trivial challenge for many FRC teams. To do it consistently you would need to use a collet holder in the spindle.

[techhelpbb]

Originally Posted by tr6scott

We saw the same issue last year on our elevator, as we pushed the pid to get more speed out of the elevator, we had trouble tuning the system at higher speeds. The issue was not surgical tubing slipping, it was some allowed twisting when elevator changed directions when stacking at high speeds.

We replaced the surgical tubing with 1000psi 1/4" fuel line from autozone, and was able to tune system just fine at high velocity change rates.

I did not believe it that a ball bearing encoder could flex the surgical tube, but the system controls was significantly less stable than with the fuel line.

Did the same thing on all my test robots.
Works adequately and allows me to use a wider range of encoders.
Less picky than encoders with the encoding wheel on the assembly shaft as well.

[scottandme]

Originally Posted by InFlight

There seems to be an assumption that a basic lathe comes from the factory, and continuously maintains perfect alignment between the tail stock ram axis and the spindle axis; to allow perfectly on-center holes to be drilled.

The reality is it takes a somewhat experienced machinist with a test bar and a dial indicator to adjust the tail stock to be aligned perfectly laterally and vertically. The average FRC lathe is use is likely only accurate to 0.01 inch on-center.

Drilling center holes to the stated -0.0000/+0.0006 on center tolerance would be a non-trivial challenge for many FRC teams. To do it consistently you would need to use a collet holder in the spindle.

You're confusing the shaft diameter tolerance with TIR. The H1 encoder has a maximum TIR of 0.006", and the shaft itself is between 2 and 6 tenths undersize (-0.0002 to -0.0006).

So the shaft diameter has a tight tolerance, but the encoder might have up to 6 thou of runout, an order of a magnitude larger than the shaft diameter tolerance.

In any case - using a reamer would give you a nice clearance fit, and then use a set screw/retaining compound/whatever to hold.

If your lathe is poorly setup, and doesn't drill holes on center:

1) Fix that
2) It doesn't matter - since the 1/32" piece of lexan holding the encoder will happily flex/float if the encoder has any axial/parallel misalignment. Same idea as using a helical beam coupler/flexible coupling that can deal with axial/parallel/radial misalignment.

[Owen Busler]

Thank you everyone for your replies! I really like the robustness of this design in addition to to how it doesn't require much sacrifice in terms of material in the load bearing part of the shaft. I used some 1 inch leg .75 inch tall 1/16" thick Z 6061 to support it and I think this will work very well thanks again!

https://drive.google.com/open?id=0B6...VpFZ05IY0lNLUE

[Gdeaver]

Many on the forum may think we are crazy, but for 2 years now we have been using the CUI AMT 10X series encoders set to max cpr. We have mounted them to Andymark, vex pro and custom gear boxes. The kit comes with every thing needed to mount them to several different shaft diameters. yes, the bottom housing is fragile if you want to disassemble the encoder. Digikey sells replacements. I was in a rush and had to replace 2. They have been rock solid.

[asid61]

Originally Posted by Owen Busler

Thank you everyone for your replies! I really like the robustness of this design in addition to to how it doesn't require much sacrifice in terms of material in the load bearing part of the shaft. I used some 1 inch leg .75 inch tall 1/16" thick Z 6061 to support it and I think this will work very well thanks again!

https://drive.google.com/open?id=0B6...VpFZ05IY0lNLUE

I would go for plastic instead due to the flexibility and ease of fabrication. 6061 doesn't bend well either, so you'll need to have a large radius on it or risk cracking.

[AustinSchuh]

Originally Posted by asid61

I would go for plastic instead due to the flexibility and ease of fabrication. 6061 doesn't bend well either, so you'll need to have a large radius on it or risk cracking.

To add to that, we like lexan/plastic because you avoid putting much stress on the encoder.

[RoboChair]

Originally Posted by asid61

I would go for plastic instead due to the flexibility and ease of fabrication. 6061 doesn't bend well either, so you'll need to have a large radius on it or risk cracking.

He was referring to the extrusion profile that you can buy, not bending your own sheet. You can use really thin aluminum to get the same flex as the plastic but the plastic is so much easier to work with.

[asid61]

Originally Posted by AustinSchuh

To add to that, we like lexan/plastic because you avoid putting much stress on the encoder.

Adding onto that, we sheared the shafts off two encoders last year because we used 1/8" aluminum mounts bolted on with 10-32 screws. When push came to shove, the encoders lost.

Originally Posted by RoboChair

He was referring to the extrusion profile that you can buy, not bending your own sheet. You can use really thin aluminum to get the same flex as the plastic but the plastic is so much easier to work with.

I didn't know you could buy aluminum like that, that's pretty neat!

[InFlight]

Originally Posted by scottandme

You're confusing the shaft diameter tolerance with TIR. The H1 encoder has a maximum TIR of 0.006", and the shaft itself is between 2 and 6 tenths undersize (-0.0002 to -0.0006).

So the shaft diameter has a tight tolerance, but the encoder might have up to 6 thou of runout, an order of a magnitude larger than the shaft diameter tolerance.

In any case - using a reamer would give you a nice clearance fit, and then use a set screw/retaining compound/whatever to hold.

If your lathe is poorly setup, and doesn't drill holes on center:

1) Fix that
2) It doesn't matter - since the 1/32" piece of lexan holding the encoder will happily flex/float if the encoder has any axial/parallel misalignment. Same idea as using a helical beam coupler/flexible coupling that can deal with axial/parallel/radial misalignment.

If you measured the current runout of your own lathe you might be surprised. Most will be over the 0.006 value which was my point.

If your drilled off center; your applying a bending load into the resolver shaft every rotation with direct mounting. At 100 rpm your going to be challenging a floating mount, and getting higher loads in the resolver bearings.

This is the reason many of us prefer alternate non-direct mounting. For close mounting a no contact magnetic rotation sensor would make far more sense.

[asid61]

Originally Posted by InFlight

If you measured the current runout of your own lathe you might be surprised. Most will be over the 0.006 value which was my point.

If your drilled off center; your applying a bending load into the resolver shaft every rotation with direct mounting. At 100 rpm your going to be challenging a floating mount, and getting higher loads in the resolver bearings.

This is the reason many of us prefer alternate non-direct mounting. For close mounting a no contact magnetic rotation sensor would make far more sense.

We have two lathes, one that is so terrible it cannot make straight cuts of any depth (and I mean any depth) and another smaller one that belongs to a teacher. The teacher one was just bought this year and has runout of well under 0.001" with the stock 3-jaw, which I bored out slightly for accuracy. The larger one, for all it's junk, has only .003" TIR on the 3-jaw. I've gotten the tailstock inline with it once before to within 0.001", but the machine didn't hold up very well over time. I've never experienced a failure with plastic encoder mounts regardless of mounting, just because they are so flexible. I'm all for magnetic encoders, but as far as the COTS options go they are pricey compared to AMTs or similar.
If our lathe can do it, any lathe can do it. Seriously though, getting stuff within 0.006" is not difficult if you check it every once in a while. Grinding the chuck jaws and using a good indicator help immensely.

[scottandme]

Originally Posted by InFlight

If you measured the current runout of your own lathe you might be surprised. Most will be over the 0.006 value which was my point.

If your drilled off center; your applying a bending load into the resolver shaft every rotation with direct mounting. At 100 rpm your going to be challenging a floating mount, and getting higher loads in the resolver bearings.

This is the reason many of us prefer alternate non-direct mounting. For close mounting a no contact magnetic rotation sensor would make far more sense.

Assuming you're talking about tailstock alignment now? This is one of those "fixing the wrong problem" situations. Just align your tailstock, it's really not that hard. Alternately, get a tiny boring bar and solve the problem in a different way.

Originally Posted by AustinSchuh

To add to that, we like lexan/plastic because you avoid putting much stress on the encoder.

Sounds like it works perfectly well for 971. I've always used helical beam couplers, or through bore encoders, no issue with either.

[jfish872]

If you mount everything tighyly, and try to make the assembly as simple as possible it should work just fine.

Are you trying to mount the encoders inside of the gearbox or on the outside?