The mostest weird DRO issue!!

[Ian PParticipant @ianp]

I have called it an issue because I don’t really know what category this fault/problem falls into.

I have a Sino SDS2L display with a magnetic scale on the cross slide and an optical scale along the back of the bed. This setup has been in use for several years without any problems, today however something really inexplicable has reared its head whilst I was turning an aluminium part 60mm diameter and 60mm long. My normal technique is to clean up the stock surface then measure the diameter and (not having moved the cross slide handwheel) enter that value into the DRO X axis. In theory the DRO is ‘calibrated’ for that toolbit.

My lathe (Harrison M250) has power feed and after a few failed attempts to get the DRO set, discovered that as the tool traversed the 60mm length of the part, the X axis display was slowly decrementing so that nearest the chuck it was indicating about 0.10 mm less than I had set it to. The part was parallel however and it was only the display that was wrong.

If I manually traverse the carriage (even slowly) over the same 60mm length the display does not change a jot!, if I engage the feed though, the decrementing X axis problem syndrome is absolutely repeatable. Once the tool gets to the chuck-end of the cut and the DRO value has/is reduced and I then wind the carriage back to the start the displayed value stays at its smaller value. The part diameter is close to, but not exactly, at what I had aimed for.

I have tried to diagnose the problem, locked the slide and checked the read head position etc but I am baffled.

After some further thought I can only think that there must be something amiss inside the DRO box itself.

Ian P

 

[DC31kParticipant @dc31k]

Unplug both scales and and swap them when you plug back in (so X on the display is reading Z on the machine).

Try a similar length cut under power feed at the tailstock end of the bed.

[Robert Atkinson 2Participant @robertatkinson2]

Sounds like electrical interference. Possibly the screen or a ground wire has broken over time.
Depending on the construction of the read head (assuming this is moving it’s the most likely location for damage) This may or may not be an easy fix.

Could also be a new source of interference so check grounding on the lathe power circuits.
Also try connecting a tempary earth lead from the read head mounting to the lathe bed.

Robert.

[Ian PParticipant @ianp]

I’ve just got back in the workshop to further investigate what is happening. Unlike last night, the first longitudinal pass without removing any metal showed no change of DRO diameter value whatsoever so all back to normal.

One thing I realised is that when the fault was present the X axis display was slightly jittery, today it is rock steady. Its normal for the least significant digit (0.005mm) to toggle a bit but it was particularly tetchy yesterday when I was setting diameters.

I’m letting the machine warm up and will see if I can induce the fault to return (I will then follow the advice of Robert and DC31k)

Ian P

[Dave HalfordParticipant @davehalford22513]

According to the Sino KA scale installation PDF the read head to scale clearance should be 3mm if yours is in spec at each end of the scale and both parts are solidly mounted then you may have to remove an end, slide the head out and carefully clean both units. Sometimes fine dust gets passed the seals.

[Ian PParticipant @ianp]

Not sure what a Sino KA scale is but the errant (or was errant) scale on my lathe is a Machine-DRO magnetic type. It has the self adhesive magnetic strip set in ali extrusion on the edge of the cross slide and the read head fixed to the carriage (0.50mm +/-0.50mm ‘gap’).

So far I have not been able to reproduce the fault since it appeared a couple of days ago.

Ian P

[KEITH BEAUMONTParticipant @keithbeaumont45476]

I had some jittery moments with my magnetic tape DRO, a few weeks ago. It was installed about 4 years ago and has been perfect until then. I removed the Ali extrusion protector and was surprised at the amount of fine, oily swarf that was on the tape. A thorough clean of tape and gap of read head and it is back to working correctly.

Keith

[KEITH BEAUMONT]

On KEITH BEAUMONT Said:

I had some jittery moments with my magnetic tape DRO, a few weeks ago. It was installed about 4 years ago and has been perfect until then. I removed the Ali extrusion protector and was surprised at the amount of fine, oily swarf that was on the tape. A thorough clean of tape and gap of read head and it is back to working correctly.

Keith

Yes I found exactly the same when discovered the decrementing digits problem, but cleaning made no difference.

Ian P

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