Bridgeport Spindle Encoder Design

[tblough]

I do have a question regarding your test on the lathe. At what rpm did you test it ? Also, is the reason you chose 64 bit "06B part designation" to achieve 24,000 rpm ? Is this a worry it could be too coarse, or is this more than adequate for the spindle and likely rigid tapping ?

When I had the encoder connected to my lathe Oak board, I ran the spindle through the complete speed range in both gear ranges, 450-6000 and 0-400rpm. There were no differential or quadrature errors during this process.

I chose the relatively low 32-bit "06B" interpolation setting for a couple of reasons. My max spindle speed is 6000 RPM. Centroid recommends a minimum 1024 line encoder (4096 cps) for spindle control. You don't really need much more than that since tapping is usually done at fairly low spindle speeds (400-800)rpm so synchronization with the Z drive does not need a lot of pulses per rev.

One other thing also needs to be considered. The equipment receiving the encoder information will have a maximum data rate that it can handle. Based on previous posts from Centroid this is 1MHz (at least for the Acorn - I'm assuming the OAK/ALLIN1DC will this or better).

The ACORN can take an extremely high encoder frequency. However, there may be filtering added in the future to reduce the frequency to 1 - 5MHz. To maintain future compatibility, it would be best to consider the maximum frequency 1MHz.

So, based on my design speed and a requirement of 1024 lines per rev, I looked for the widest pulse width that would meet these specs (pulse width = 1/frequency. So, 1MHz frequency = 1us pulse width).

Based on RLS' published max speed tables (https://www.rls.si/eng/fileuploader/dow ... les_01.pdf) for the MR050 axial magnetic ring, "06B" gives 1152 lines per rev (greater than 1024 Centroid minimum), and the "1/C" edge separation gives me a minimum 1us pulse width and a max rated speed of 6073 RPM, both meeting my design requirements.

I probably could have gone for a little higher resolution and narrower pulse width but there would have been no performance benefit, and the risk was higher frequencies are more susceptible to noise and computational issues. Keeping things as simple as possible improves reliability.

[cut2cut]

Thank you for the explanation Tom. One thing that confuses me is why the magnet rings maximum rpm specifications [edit] *increase* when there are less poles ?

For my application I'm trying to decide between 1µs edge separation (1Mhz) and .5µs edge separation (2Mhz). I'm "borderline" limited by the roughly 6800 maximum rpm of the 1µs edge separation. My motor runs at 3500 rpm at 120hz, with the pulley/belt its roughly double ( .5085 ratio ) so 7000 rpm at the spindle at 120hz. I can also drive the motor faster than 120hz although torque drops off, the benefit being tiny end mills require faster speeds and also do not need high amounts of torque. The motors ( see enclosed picture ) max "safe" rpm limit is 5400 rpm which, with the "gear ratio", maxes out at over 10k rpm. It may be the runout in my spindle won't allow me to use super small end mills at high rpm but it would be nice to be able to if possible. Anyway, its a tough choice. I'm leaning a bit towards gambling and going with 2Mhz which allows me my max rpm with significant headroom. Any opinions are welcome.

Best,

Jake

[tblough]

Jake,

I wouldn't worry too much about going to 0.5ms pulse width. Centroid said it "may" limit bandwidth in the "future" to "1-5Mhz. With all of those qualifiers, you should be fine with 2MHz.

I'm not sure why the max speeds are lower with fewer poles but it was curious that some larger interpolation factors actually had faster max speeds that some smaller ones. I can only guess that the little microprocessor in the read head doesn't have enough Hp to do some of the calculations for the "odd" speeds, and the ones that are powers of 2 are easier to calculate by using bit shifts instead of multiply and divide.

[tblough]

As a final post in this thread, here's a video of rigid tapping in action using this spindle encoder

https://youtu.be/ILQIeC_NvRw

Multiple passes and perfect threads. Encoder works like a charm!

[martyscncgarage]

As a final post in this thread, here's a video of rigid tapping in action using this spindle encoder

Multiple passes and perfect threads. Encoder works like a charm!

Great job Tom. Thanks for sharing the information and the work you went through to get it working.
Your support on the forums is appreciated as well!
Marty

[Reedfirst]

Im so sick of manual tapping. Your video inspired me to give this a shot.

I've got a good used bearing carrier, and a worn worm gear coming so I can machine the bearing carrier, encoder mount, and split collar. That way I can mock everything up before I take apart my head.

It appears RLS no longer has magnetic rings with the reference up for sale on their online store. I've sent them an RFQ for one and I'm waiting to see what they say. Hopefully the lead time isn't too long.

I've got a question on the cable you used. Did you have to remove the m12 x-code connector to feed it through the bridgeport head? Was it a special bulkhead type connector or just a standard M12-Xcode to rj-45 "patch" cable. I am assuming the other cable that connects to the head is a standard 90degree m12 Xcode cable.

I'm planning on removing the rj-45 from that and crimping on a db9 to connect to my acorn.

Thank you

-Adam

[tblough]

Adam,

RLS was having issues with the equipment needed to laser the index mark and was having to temporarily send out the magnetic rings to have this done. I don't think the delay was excessive.

As far as the cable was concerned, I used a stock right angle M12-Xcode to rj-45 patch cable and cut off the RJ-45 same as you plan to do. I purchased a M12 X-code bulkhead connector from Digikey and cut off an extra 10" of the patch cable to use inside the head to connect between the bulkhead connector and the encoder reader. I soldered the shield wire to shell of the bulkhead connector as well as the DE9 connector on the far end to make sure the shield ran all the way to the encoder. Shield was not connected at the encoder itself.

Here are the Digikey part numbers:
Cable 1849-1301-ND‎ Metz 5 meter M12 X-Code
Connector 1849-1149-ND Metz female 8-position X-Code solder cup

[Reedfirst]

Thanks Tom, I really appreciate your help.

I looked at other encoder options and this is by far the most elegant solution I've seen.

While I'm waiting on my encoder I'm wondering if I should go ahead and convert to a fixed pulley setup while I have the head torn apart.

Thanks again,

-Adam

[tblough]

You do loose low end torque by going with the fixed pulleys, but it does make VFD control much easier. And, with this encoder mounting, you still can use back gear with the encoder working so you do have access to really, really low end torque. I decided the benefits were worth it and went with the fixed pulleys.

[Reedfirst]

I think I am going to go ahead and do this too. My Vari-speed assembly is starting to make some noises so I might as well.

Do you happen to have the dimensions of the keyways I have to make? I'd like to get them made before I tear my machine apart.

-Adam