Linear Encoder Question

[Verboten]

Hello,

I have the opportunity to buy a used Coordinate Measuring Machine for parts (it's in rough shape and I don't really need two tons of granite in my shop) and I was wondering if somebody could help me out with a couple of questions.

It has 3 linear encoders (x~4',y~3',z~1.5'). The scales seem to be etched on stainless strips (~1/2") that are mounted on steel bars that are about 1/4" x 1". The etched lines run repeatedly for about a 1/4 inch and then seem to pause for 3/8 inch and then repeat. I have never seen a linear encoder before - what type of resolution would you guess these have (it was a nice machine at one time)? Would they be suitable for a DRO system on a lathe?

It also seems to have what I would call pneumatic bearings - little ~ 3x3" brass pads that use air pressure to reduce friction. They seem pretty neat. Has anyone here used them on a project? Ideas? Anyone know of a place online that sells something similar? I'm currious as to what they would cost new.

Thanks for your time.
-Jason

[HuFlungDung]

Hi Verboten,

What you've got there is the coordinate measuring machine from Fred Flintstone's time, when tape measure scales were in vogue

[Verboten]

Of course it's old - I probably wouldn't be able to buy it or even think about tearing the thing apart if it weren't.

It looks just like this machine does:
http://cmmxyz.com/70CC/404.html
- if you mailed it USPS

If that offended anyone, it was just a joke. (you couldn't send that USPS)

[vencnczone]

I just coiped and pasted some info from my site for you. Sure there are all kinds of encoders available in the market. The price vary anywhere from 50$ to 10,000$. Here is place that may have used encoder for a reasonable price oracle encoder service Toll Free: 888-994-2001

http://www.drmachines.com/DeskTopDef...dex=2&tabid=53
Positional Feedback

Means the feedback to the control to maintain the position (In case of digital there could be just one unit acting as both velocity and positional- This device could be either going to the control first and the drive next, and vice versa). This device is used to achieve the repeatability. In case of axis it repeats the position linearly and in case of spindle it reaches the same angle in a circle thereby able to produce a thread.

Resolver:

This is the one of the earlier devices. This is an analog device. It can be checked with a scope. Still, it is hard to tell whether it is OK or not. Best way is to switch it, if it is suspected. One has to keep an eye for the bearing in the unit, which tends to go bad.

Encoder:

This is a digital device, which is used as a positional feedback device. It has a photocell and three receiving cells, one for home position and two for incremental position. This can measured with a scope. Look for two square waves. One has to keep an eye for the bearing in the unit, which tends to go bad.

Pulse Coder:

This is very similar in construction to the Encoder. This is used to provide both velocity and positional feedback. The output from this unit can be configured to go to drive first and control next or vice versa. When there is problem, any one of the three areas, drive, control or pulse coder could be causing it and it is difficult to say which one.

Scale:

Though there are a few kinds of scales available, Linear glass scale is the widely used one. The scale has to be kept clean. It has a reader moving on it linearly. This has a photocell and three receiving cells, exactly as in the case of encoder. There is home pulse every so often. Also there is a unit inside the scale, which can be moved to assign a home position pulse. The output of the head is connected to a Preamp.

[HuFlungDung]

Verboten, were all the electronics stripped off the machine? What did the scales interface with originally?

[Verboten]

vencnczone,

Thanks for the info. Nice site, neat idea.

HuFlungDung,

Each scale has a reading head ( I'm unsure of the proper name for this part) that is bolted to each axis. The brand is Dr. something or other (there was some info that I should have written down). The encoder then hooks into a box that has a pneumatic valve for each axis and just a few components mounted to a simple board that has a couple of what I would call ribbon-type connectors that must have led to external controls (that are not with it). I didn't see where any parts were stripped out - but I could have missed it.