DRO magnetic scale installation question

[dml66]

I'm about to replace the DRO glass scales on my lathe with magnetic scales. The X and Z axis glass scales are fixed positionally; the read heads travel with the cross-slide and carriage respectively, because glass scales integrate the read head with the scale, this arrangement made the most sense. Magnetic scales, at least the one's I've selected, separate the magnetic scale from the read head, they only require proximity tolerances to be respected.

Looking over the cross-slide installation today, it would be a lot easier, and more elegant, to fix the read head and let the magnetic tape travel. I've verified that the uninstalled scale works either way; it doesn't seem to care which part moves. But, before I make a dumb mistake, is there a compelling reason to have the read head travel, and fix the magnetic scale?

Thanks!

 

[mksj]

On the lathe, the head is fixed and the scale moves for the X axis, that is the typical installation either for glass scale or magnetic. The magnetic scale if moving needs to be rigid enough to not flex or move. There is a specification for clearance between the magnetic tape and the head, some supply a plastic feeler gauge. My experience has been that the clearance does not need to be very accurate, but depends on the scale manufacturer.

 

[Beckerkumm]

Is there a reason to replace the Z scale? I like magnetic but prefer glass for the longer Z axis. Scale is stiff, easier to support, and more adjustment.

Dave

 

[dml66]

Is there a reason to replace the Z scale? I like magnetic but prefer glass for the longer Z axis. Scale is stiff, easier to support, and more adjustment.

Dave

I have the X-axis scale in-hand and will install it, I'm not 100% convinced I need to replace the Z-axis glass scale. I made my own DRO software which incudes Z-axis power feed using a BLDC motor, this way I can program run operations like feed-in, stop, feed-out, stop; sort of CNC-ish. Whereas these glass scales have hysteresis issues, I'm not sure tightening up the Z-axis will do much considering the inertia and momentum baked into the power feed cake.

I'll most likely leave well enough alone with Z.

 

[rwm]

My cross slide has a fixed read head and moving scale. Works great!

 

[dml66]

Got the new magnetic DRO scale more or less finished today, figured I'd share a few pictures and some thoughts.

First, I know I broke the cardinal rule by installing this on the spindle side of the carriage but, since the read head and the scale are fully protected, and not physically connected to each other, they are more swarf-proof than integrated glass scales. We'll see, I may live to regret the decision .

The read head is an M-DRO unit with 1 micron resolution: https://www.sra-shops.com/magnetic-linear-encoder-reading-head-1-micron-resolution

The magnetic tape: https://www.sra-shops.com/machine-dro-1/digital-readout-systems/high-accuracy-magnetic-linear-tape

The magnetic tape support: ttps://www.sra-shops.com/machine-dro-1/digital-readout-systems/two-part-magnetic-tape-support-profile-300mm-length

Here's a picture showing the read head and scale installed on the carriage. The head is tucked into an OEM slot and secured in place with a piece of 1/16" thick aluminum and 4 button head screws. The scale is secured to the cross-slide with a steel cross member.



This picture shows shows the magnetic tape on-end, inside it's protective cover:



Last but not least, this one shows the gap between the read head and the scale:



The DB9 pinout on the read head is identical to the Eaason scale (DRO Pros, now discontinued) it replaced. The signaling is the same, TTL quadrature, it snapped right into my computer interface,

So far, the results are quite good. My main beef with the glass unit concerned hysteresis; the scale appeared to report position movement which was really some internal movement of the device's physical connector rather than actual cross-slide movement. Maybe I just got what I paid for, which wasn't much since the company had them on closeout. My second beef, was giving up some real estate on the tailstock side, and limiting compound angles; those obviously go away.

For the price, about $250 shipped, and some shop time, I'm hoping this gets me closer to where I want to be.

 

[rwm]

I thought about mounting mine on the spindle side of the carriage. Then I realized that when the chuck jaws were out more than usual I would crash into the scale! I know for sure I would do it. Hoping you have better luck!

 

[dml66]

I thought about mounting mine on the spindle side of the carriage. Then I realized that when the chuck jaws were out more than usual I would crash into the scale! I know for sure I would do it. Hoping you have better luck!

That's a valid concern for certain; crashing a tool into the chuck never ends well, I always set a carriage stop to prevent such an occurrence. This particular installation projects the front of the sensor assembly by about 3/8" or so; it sits on the carriage, not in front of it. Put another way, the projection is the magnetized assembly, the cheap part. If the head gets hit, that means the entire carriage got hit, all bets are off at that point.

 

[Nesse1]

That Machine-DRO scale is very interesting. And unique, I think, in sandwiching the magnetic strip inside aluminum extrusions.

 

[rwm]

BTW - M DRO also sells the tape separately. That could be stuck to the cross slide itself for a very low profile installation.