Rotary Encoder for DRO? anybody done it.

TheBigLebowski · Dec 11, 2018

Has Anybody used a rotary encoder for a DRO?

Currently I have an old acurite DRO on my bridgeport, over the weekend the x axis scale bit the dust.

Instead of buying a new DRO setup, I am tempted to buy a good ground ballscrew set that wouldn't cost much more than a dro, and add a rotory encoder to it to keep track of position. The added benefit of course would be that I can easily add a couple of servos for a cnc setup in the future.

The kind of work I use the mill for would not be affected by the couple tenths of backlash in a ballscrew.

Anybody done anything like this?

EPAIII · Dec 11, 2018

There is no reason why that couldn't be made to work. The problem, as I see it, is that the non-functional scale and the rotary encoder are not likely to be talking the same language. So some kind of conversion board will be needed to convert the rotary code into whatever that scale output. One of those PIC style chips would be a good candidate: either a plain PIC device or one of the higher level ones like Arduino. Of course, it will require programming and to do that you will need detailed information about how your existing, non-functioning scale communicates.

HWElecRepair · Dec 12, 2018

As long as your Acu-Rite is a TTL signal box (anything by the MillMate) you can definitely add a rotary encoder to it. It might not be as simple as plug and play, but yet you can definitely get it hooked up.

+1 to EPAIII

DDoug · Dec 12, 2018

TheBigLebowski said:
The added benefit of course would be that I can easily add a couple of servos for a cnc setup in the future.

The kind of work I use the mill for would not be affected by the couple tenths of backlash in a ballscrew.

Anybody done anything like this?

Yup, over at "The cnc zone dot com"..... :toetap:

Billtodd · Dec 12, 2018

That's pretty much how every cnc machine works (a few have additional scales) , The SWI prototrak system on my knee mill works exactly that way .

One word of caution though: ball screws will back drive so you may have to lock the stationary axes while making cuts (the extra drag of motors and belts is usually enough on my machine).

Bill

9100 · Dec 12, 2018

A friend installed a ball screw kit on a BP and belted a rotary encoder to it. The readout works fine but the ball screw sucks. You can read the backlash on the original dials. It is definitely in the screw. He asked the manufacturer why it had .002" backlash and was told that was normal. A good ballscrew setup uses a second ballnut with a spring between them, removing almost all the backlash. My 1977 Boston Digital CNC mill has about .0004" backlash on the Y axis after 41 years of use. They did it right. It turns out that the BP replacement unit had only single ballnuts that have the balls selected to reduce backlash and naturally they will wear. If you can't get a proper ballscrew, don't bother.

Bill

steve-l · Dec 12, 2018

I'm not a CNC guy, but I wouldn't do it because it is poor engineering with what is available today. If the encoder is connected to the screw, the reading is indirect. There is simply too much between the encoder and the work that is invisible to the electronics, like backlash and temperature. I feel feedback must come from table movement only. Then you only have to deal with linearity between the scale and the table.

JST · Dec 12, 2018

steve-l said:
I'm not a CNC guy, but I wouldn't do it because it is poor engineering with what is available today. If the encoder is connected to the screw, the reading is indirect. There is simply too much between the encoder and the work that is invisible to the electronics, like backlash and temperature. I feel feedback must come from table movement only. Then you only have to deal with linearity between the scale and the table.

The right answer ^^^^^^^^^^^^^

Miguels244 · Dec 12, 2018

Yes it will work.
It may be a pain to communicate.
It’s a suboptimal solution, but no worse than using the hand dials.

adama · Dec 12, 2018

Check out the price of the chinese scales, its kinda not worth it and there going to be way more accurate than trying to translate screw angular position to actual movement, way better to just measure the movement directly :-)

CarbideBob · Dec 12, 2018

steve-l said:
I'm not a CNC guy, but I wouldn't do it because it is poor engineering with what is available today. If the encoder is connected to the screw, the reading is indirect. ....

I wonder how cnc machine tools without dual loop linear scales work?
Seems there are lots of those in the field making parts.......
Better maybe but once you mount the scale you sort of blindly trust you get caught up in Abbe errors and all that so its not a perfect world either.
If you think mounting a scale to the side of a table will produce stuff to its resolution as a dead nuts number on the table you are not understanding how this works.
Bob

Miguels244 · Dec 12, 2018

CarbideBob said:
I wonder how cnc machine tools without dual loop linear scales work?
Seems there are lots of those in the field making parts.......
Bob

Really?
I’m surprised to hear that.
Who is selling machine tools without direct reading feedback?
As I said, it’s no worse than using the dials on the crank, but why bother.

TheBigLebowski · Dec 12, 2018

Lots of the lower end VMCs come without glass scales standard. only one of the six fadals I used to work on had glass scales. Same story with haas. So long as the machine is kept tight (biggest issue on many fadals being thrust bearings) they are acceptably accurate for most parts. This is also how most of the later protrak type mills operate.

That being said, anybody have recommendations for an ok cheapo dro.

CarbideBob · Dec 12, 2018

Miguels244 said:
Really?
I’m surprised to hear that.
Who is selling machine tools without direct reading feedback?

I can only do "duh" here.
Do you even know how dual feedback loops work and the problems involved?
In holding size part after part in many cases adding a linear scale can actually be worse than running a single motor encoder off the screw.
Any servo system has direct feedback, scale and motor become two feedbacks which disagree and the working solution becomes much harder.
Bob

steve-l · Dec 13, 2018

Bob,
You are not the only person on this forum that understands servo systems and feedback schemes. Yes, in years past rotary encoders were common, but not anymore and for very good reason. In those days thermal compensation sensors were also very common along with other band-aids all to compensate for those accuracy factors not visible to the electronics. Today everybody uses direct feedback simply because it is better and cheaper. Please note that I did not say optical scale feedback. Feedback can also be provided by magnetics as well at much greater cost.

adama · Dec 13, 2018

TheBigLebowski said:
That being said, anybody have recommendations for an ok cheapo dro.

SINO there cheap as chips, a std bridgport size setup is sub $400 its a no brainer, there also pretty dang reliable too!

steve-l · Dec 13, 2018

Adama is correct the SINO DROs are very good. Just as good as any I have used. I bought a BP clone in China 12 years ago or more and the SINO 3 axis DRO was factory installed. It has been flawless and accurate. You will also find that the SINO DROs are also sold under several other brand names.

Miguels244 · Dec 13, 2018

CarbideBob said:
I can only do "duh" here.
Do you even know how dual feedback loops work and the problems involved?
In holding size part after part in many cases adding a linear scale can actually be worse than running a single motor encoder off the screw.
Any servo system has direct feedback, scale and motor become two feedbacks which disagree and the working solution becomes much harder.
Bob

I build automated machinery for a living.
For anything except rough positioning we used direct read from the axis wer were controlling.
Screw variance, vibration and loading made that generally necessary.
Never built a machine tool though, I guess the loads and precision are lower than I would guess off the top of my head.

CarbideBob · Dec 13, 2018

steve-l said:
Bob,
You are not the only person on this forum that understands servo systems and feedback schemes. Yes, in years past rotary encoders were common, but not anymore and for very good reason. In those days thermal compensation sensors were also very common along with other band-aids all to compensate for those accuracy factors not visible to the electronics. Today everybody uses direct feedback simply because it is better and cheaper. Please note that I did not say optical scale feedback. Feedback can also be provided by magnetics as well at much greater cost.

Ok, so I'm wrong.
Guessing that since you know such you also know pitch,yaw,roll and abbe errors due to reader head placement.
Can you provide me a link to magnetic capable at 1/10 micron? I'm sort of stuck with optical or laser scale to achieve this.
Bob

Miguels244 · Dec 13, 2018

CarbideBob said:
Ok, so I'm wrong.
Guessing that since you know such you also know pitch,yaw,roll and abbe errors due to reader head placement.
Can you provide me a link to magnetic capable at 1/10 micron? I'm sort of stuck with optical or laser scale to achieve this.
Bob

Have you looked at the Sony scales?
Micro Epsilon makes some interesting s nsors for narrow range high resolution work too.
MTS magnetorestrictive sensors can give high resolution but you need to build the correction map.

Rotary Encoder for DRO? anybody done it.

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