DRO errors, or are they??

having a Machine Dro on my mill & being very happy with it I decided to buy a DRO for my lathe. I baulked at the cost so bought a Sino Dro at £299-00.

Not so nice to read & already I wish I had gone to the same as I had bought before, but too late now.

So my first task was to machine 5 bronze bushes for my boiler. Each one 12mm with a 13.5 mm shoulder. A simple operation. I expected to take a cut on the bronze , measure it with the micrometer, set the dro at whatever size ( say 14mm) then simply machine to 12mm.

No such luck. On all 5 bushes, Whatever I tried I never achieved the 13.5mm or the 12mm. Each time I came out under my required size.

I made sure that I locked the cross slide for the final cuts,So I began to suspect something wrong wth the DRO.

I decided to wind the cross slide in and out the full width & see what readings I got at 40mm intevals. 40mm being 10 turns of the dial & easier to read zero after each set of 10 turns. I turned in one direction only to avoid backlash

Below are the readings I obtained. The red figs are the differences. One will see that moving out from the centre the reading differences are different from those going in towards the centre. In only one case do they read 40mm. Also the overall dimension one way is 160.05 & the other way is 159.9. That makes it difficult to decide whether to calibrate the scale ( I think there is a facility to do this)

So why am I getting these differences? I can accept that I can get a bit of a difference if I do not read the dial perfectly ( Surprised at that though) but is it the screw on my cross slide being out or the scale on the DRO being imperfect?

is this sort of thing normal with cheap DRO's & can I ever expect to machine anything acurately on my elastic chinese Warco & my cheapo DRO

Edited By Sam Longley 1 on 25/10/2020 09:03:20

I'd use a drop clock and check against that,. At least then you establish where the error lies.

Jim

Could you mount one of the scales from the lathe on the mill and compare the two?

This would eliminate any errors from reading dials or tolerances in the screws.

Regards.

David.

Hello Sam,

I know nothing of DROs - although I've been interested in watching Joe Pie use his to great effect.

However, my immediate reaction to your boring undersize, was spring in the boring tool. Did you take at least one spring pass on arriving at the measurement - because that may be the problem. Sorry if I'm teaching you to suck eggs here but just a thought....

Regards,

IanT

Edited By IanT on 25/10/2020 09:33:29

Chuck up a piece of bar (say25mm) take a small cut and measure the result. Set the DRO to zero and then take a second (small) cut. Measure again and compare the measured size to the DRO size. carry on in this manner (without resetting the zero on the DRO) See how the sizes compare. This will show if the DRO has a problem and eliminates any leadscrew or backlash issues. Remember to lock the cross slide each cut. Watch to see if the DRO changes size before/during/after each cut as this indicates a locking problem.

Martin

You are working with 2 possibly questionable components [cross slide screw & or DRO], I would use a decent DTI & slip gauges to check things out but you may not have them? You need to use some thing of an accurate size to measure the DRO readings against.

Tony

1. Bronze is more awkward to cut, steel would be a better test.

2. It's better to check movement against a dial gauge not the lathe dials.

3. It could be mounting bracket slack or the scale may not be dead parallel to the cross slide travel.

4. Does the reading change when you lock the cross slide?

Edited By Dave Halford on 25/10/2020 09:46:16

Strange! Inconsistent errors. This is where slip gauges are a help

1. Carefully check all the mountings fir movement also the x slide screw, thrust bearing and the nut ( unlikely but it’s for elimination while you are at it, check the gibs) check there is no movement on a dti while the x slide is being locked

2 clamp a block of known size say a 321 block with a bar of flat clamped to it extend beyond say the headstock end So giving two test faces in the same plane
use a dti to set it parallel then feed in to zero on tthe block with the dti then zero the x slide dial n DRO. Check on the clamped bar. After a small movement to clear with the saddle
this will check the DRO and the feed screw

Ha! Some similar comments while I was thinking about this n typing!

Edited By Zan on 25/10/2020 09:53:17

Zero DRO, grab hold of the toolpost and then as hard as possible push it in, record reading, pull it out, record reading.

Seems to me that the screw or nut are loose somewhere?

Turning outside diameter

I did try steel when I first fitted the DRO but thought it was just a learning issue.

My dial gauge will only give  max 25mm of movement & I would not be able to use the full 25mm, which is not a lot but I will try that- thanks

The scale is fitted to the cross slide so must be parallel & no way would it move, I am confident of that !!!

I lock the cross slide at the reading I want. it does tend to move whilst doing that, but there again there is always backlash in the cross slide, so one might expect the tiniest movement- or would one? I just make sure it is locked on the DRO reading i want. I also check that it does not move thereafter.

What I do not understand is why the measurement reads consistently 159.90  one way & 160.05 the other way.

I have done more readings than shown on the image & all are similar.

Edited By Sam Longley 1 on 25/10/2020 10:16:17

If you ignore the DRO and only use the cross slide dial, do you then get repeatable diameters that match the cut you have applied?

Martin.

The fact you locked the slide on the final pass will have altered the amount the tool was able to deflect so you got a different cut. If your finish cut was also shallower than the measured cut again deflection will have been different.

To check the scale put a DTI against the slide and compare that to the DRO not the handwheel.

I made sure that I locked the cross slide for the final cuts,So I began to suspect something wrong wth the DRO.

I expect this is where you went wrong (my underlining).

Hi Sam, you are also comparing two sides of your screw/nut combination and there is the possibility of there being slightly more wear in one direction than the other, other factors to consider are the thrust bearing parts of to screw and drag on the dovetail slides can vary in one direction to the other by very slight movement in the gib of even due to wear/bedding in. As has been said, the only way is to check against a good DTI or a known good readout, mounted rigidly.

Regards Nick.

If the unlocked cross slide was moving whilst taking a cut, I would expect the DRO reading to fluctuate too.

Martin.

Agree with everyone who said don't compare the DRO with the dials.

Generally even cheap DRO's measure accurately (within their design limits). One thing that might cause Sam's symptoms is the scale and head not aligning properly. Although the fit isn't super-critical, if the head presses against the scale at any point as it traverses, the scale bends slightly and readings pop to and fro as it flexes. Make sure the scale and head are fitted to the lathe so the head runs freely.

As fitting DRO scales can be awkward due to obstacles, their alignment can be compromised, and tiny movements show up. Don't ask how I know...

Dave

.

There are many possibilities for error being caused by installation and/or use [and some excellent suggestions made here] ... but :

For the sake of sanity; my first step would be a side-by-side, or end-to-end, comparison of the two systems.

... Could you easily fix the Sino scale on the table of the mill ?

MichaelG..

I have had a DRO on my lathe for a long time and CNC retrofitted it after the DRO was on it. The X axis DRO perfectly matches the CNC inputs, set a 10mm move in CNC and DRO shows a 10mm move. However I found that when using the DRO for manual work it was always a case of get close to target size and measure the workpiece then reset the DRO to match. There are too many variables when using a lathe to get consistent results over more than a few millimetres. Examples are workpiece gets slimmer so flexes more or the diameter changes so the cutting speed at the tool changes and so changes the forces on it. Any small error in tool height will also cause apparent errors. So on a lathe I think the X axis DRO is an aid to manual machining but should not be expected to allow hitting a size perfectly after taking off substantial material. The Z axis DRO works great for making things to size because surfacing and facing are such different processes.

My experience, YMMV

Martin C

I will say that if the scale is not absolutely parallel to the movement of the slide then there is a (very) strong possibility of an error in your readings.

We had a Dro fitted to a large lathe and the big shafts we were machining were giving problems with errors. On checking the cross-slide scale we found a 0.05mm error on the alignment the DTI from one end to the other. The supplier assured us that that amount would not matter but when the service engineer came along we were able to prove to him that it did. He said that most crossslides were much shorter and any error would not show up.

So very careful alignment is uaually the answer!