One way to eliminate operator / equipment error is to test with a pointer in the spindle against a steel rule laid on the the table. With the DRO zeroed at the end of the rule, crank the table at least 150mm along the rule as indicated by the DRO. Check the spindle pointer is at 150mm on the rule. Then, again using the DRO, return back to zero and check the pointer really is at the start point.

It the test passes, the DRO is OK and you need to check how you're driving it. More likely is that the DRO is misbehaving. Nearly flat batteries cause odd jumping, and I once had trouble with a loose USB connector. Also, make sure that the head moves smoothly along the track and that the track is parallel and true to the table. Mine shifted after a while and the resulting binding caused readings to jump when the direction was reversed.

Dave

If you are moving 15mm based on the scale on the handwheel then moving back 15mm based on the handwheel scale what you have is backlash. The point of the DRO is to allow you to make movements using the DRO not the handwheel scale. Backlash is not usually a problem on a lathe as you only work from one direction.

Is this what you are doing?

Martin C

Sounds like backlash because the DRO tracks the worm, not the table

With a really cheap system like the one below backlash is "irrelevant" because the DRO tracks only the saddle movement, not the worm drive

Edited By Ady1 on 02/05/2017 00:30:54

All this talk of "worms" has me totally confused. Where's the worm on a WM16?

The two component halves of the scale – glass or otherwise – should be fitted one half rigidly attached to the table, the other half rigidly attached to to the machine part that the table is sliding in. Provided this is true and the two parts of the scale are correctly aligned to each other, then the DRO reading is dependent only on the relative displacement of the scale halves.

Any error is due either to a mounting that allows one part of the scale to move or deflection of the mount. In some cases you can get jumps in DRO reading due to poor electrical connections or grounding problems but those are usually not repeatable.

(Of course if you are comparing the change in the DRO reading to the change in the feedscrew dials, then just about anything goes unless you make corrections for backlash. But why bother? The DRO is supposed to replace the dials. I probably haven't looked at the dials on mine in 7 or 8 years).

Edited By Bandersnatch on 02/05/2017 01:48:24

Sounds like a loose sensor bracket to me.

Not that it;s ever happened to me. Oh no sir, not to me…

Neil

Ady1

I have never seen so much swarf between the shears, must have been a very big and long job!

I found with my cheapo, add-on Chinese scales that it was easy to move the mill table too fast leading to false measurements being shown on the LCD. Not when actually milling but rather moving the table say to a new position for the next cut.There seems to be a maximum "speed" which these scales can tolerate without errors.

OK ~ the OP is using glass scales which are undoubtedly better in that regard yet the reproducibility with my cheap scales is surprisingly good if the hand wheels are turned at a steady, slow rate.

Re: "Bandersnatch & fast traverse with Chinese scales"

After fitting those scales searching online kept recommending this website:-

**LINK**

It's detail is impressive but being "electronically challenged" I couldn't understand much therein. However it does refer to such scales being able to operate at a fast or slow sampling speed. Further searching found another website where the user of a Dore-Westbury mill reported "One problem was that the scale unit update was not very fast – only about 3 times a second. This was OK for co-ordinate drilling but was a problem when taking a continuous cut. The update was simply not quick enough to give a real-time position. However it was discovered that the scales could be switched into a faster mode by shorting out the clock signal with the +V signal. However this had problems too because in this mode the scale would not zero. In the end 2 momentary contact buttons were added to the remote readout to enable it to switch the scales between fast and 'zeroable' modes." Link here>

**LINK**

I don't have the courage to install his "momentary contact buttons" especially as he wrote "The original system eventually got damage and so some replacement units were purchased ensuring that they looked the same and had the same contacts. These were wired in and worked OK, but unfortunately the additional fast/slow modification no longer worked."

So I just rely on slow & steady handwheel turns instead. My Chinese scales must be about 20 years old anyway !

I have though installed a stabilised power supply for the 3 scales to replace the batteries and this improved things a lot. Again to quote from the 2nd url "Other engineers have been in touch to report problems obtaining a stable reading from these systems, particularly when batteries were anything but fully charged".

I have a couple of the small LCD-display scales with the USB type plug and recently found that if the cables connecting the sensors to the readout are touching the 'mains' cable at 90° to it, the scale will adjust erratically and go out of calibration when power is switched on to the spindle. So I think they are susceptible to electrical interference and may require some shielding (or better shielding).

Ed.

Edited By Ed Duffner on 11/05/2017 00:42:55