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DRO errors, or are they??

Problems with dimensions

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John Baguley25/10/2020 11:46:42
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504 forum posts
54 photos

I had a similar problem after fitting a DRO to my mill. I decided to check the DRO readings against the movement set by the leadscrew and dial and there were significant differences. Naturally, it being a cheap DRO from China I thought the errors were with the DRO. The first thing I checked was the alignment of the scale but that was fine. Well within spec.I decided to check the DRO readings against a 12" stack of gauge blocks and the readings were spot on. The original errors that I got were due to errors in the leadscrew and not the DRO.

John

Sam Longley 125/10/2020 13:02:59
939 forum posts
34 photos
Posted by John Baguley on 25/10/2020 11:46:42:

I had a similar problem after fitting a DRO to my mill. I decided to check the DRO readings against the movement set by the leadscrew and dial and there were significant differences. Naturally, it being a cheap DRO from China I thought the errors were with the DRO. The first thing I checked was the alignment of the scale but that was fine. Well within spec.I decided to check the DRO readings against a 12" stack of gauge blocks and the readings were spot on. The original errors that I got were due to errors in the leadscrew and not the DRO.

John

Problem against that is that if I machine a diameter of 13.5mm & set the DRO then machine a step to 12mm ( In stages) the difference should be 1.5mm regardless of the dials. Currently it is not.

I have , this morning machined some steel pieces as smooth as I can get them, each with a step of a diameter recorded on the DRO. Tomorrow I am taking them round to a pattern making friend & asking him to verify the dimensions with his micrometer. I want to make sure that I am measuring the diameters correctly. . I have also used a dial gauge on the end of the cross slide & that varies with the DRO depending where along the slide I am reading (ie near the centre or well out from the centre) .

I am confident that the scale is parallel. It is only 150mm long & It showed no error on a dial guage

Although the lathe is 6 years old it has had little use.

 

From your comment above I put the dial guage on my mill table & over 20mm it matches the m machines DRO perfectly on both X & Y dimensions

Edited By Sam Longley 1 on 25/10/2020 13:04:36

Dave Halford25/10/2020 14:18:25
2015 forum posts
23 photos
Posted by Sam Longley 1 on 25/10/2020 13:02:59:
 

I have , this morning machined some steel pieces as smooth as I can get them, each with a step of a diameter recorded on the DRO. Tomorrow I am taking them round to a pattern making friend & asking him to verify the dimensions with his micrometer. I want to make sure that I am measuring the diameters correctly. . I have also used a dial gauge on the end of the cross slide & that varies with the DRO depending where along the slide I am reading (ie near the centre or well out from the centre) .

 

Edited By Sam Longley 1 on 25/10/2020 13:04:36

It's very easy to force a reading a few thou smaller than it really is.

So the DRO and dial gauge both agree, do the machine dials also agree?

It's possible that cross slide screw varies along it's length, or your centre height is not spot on so the tool digs in a little. 

Edited By Dave Halford on 25/10/2020 14:27:25

Sam Longley 125/10/2020 15:40:10
939 forum posts
34 photos
Posted by Dave Halford on 25/10/2020 14:18:25:
Posted by Sam Longley 1 on 25/10/2020 13:02:59:

I have , this morning machined some steel pieces as smooth as I can get them, each with a step of a diameter recorded on the DRO. Tomorrow I am taking them round to a pattern making friend & asking him to verify the dimensions with his micrometer. I want to make sure that I am measuring the diameters correctly. . I have also used a dial gauge on the end of the cross slide & that varies with the DRO depending where along the slide I am reading (ie near the centre or well out from the centre) .

Edited By Sam Longley 1 on 25/10/2020 13:04:36

It's very easy to force a reading a few thou smaller than it really is.

So the DRO and dial gauge both agree, do the machine dials also agree?

It's possible that cross slide screw varies along it's length, or your centre height is not spot on so the tool digs in a little.

Edited By Dave Halford on 25/10/2020 14:27:25

No!! I said that the DRO & dial gauge do not agree. The difference depends on how far i am out from the centre

To be honest in the end the dials do not matter. They are just a method of checking. If i can measuer a diameter with the micrometer then set the dro to that measurement I would expect to just turn the dial until the DRO gives the reading i want. That way all backlash etc is eliminated. Trouble is it is not happening & it is not happening in a linear fashion so I cannot simply re calibrate the scale & i do not know why.

I just wondered if others had a similar problem & how they solved it. It may be that they have not actually noticed it or if they have they have not bothered & just adjusted for it as they went.Seems a waste of a DRO if they have though.

John Baron25/10/2020 16:04:22
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520 forum posts
194 photos

Hi Guys,

Any digital device can miss a step, usually reading speed related.

Dave Halford25/10/2020 16:26:23
2015 forum posts
23 photos

Have you swapped the cables around to see if the fault moves to the other axis display?

Have you tried zeroing the readings and using it as a basic measuring tool cutting out the maths?

There have been issues with scales changing reading when the motor is switched on, that was a lack of AC earth.

Edited By Dave Halford on 25/10/2020 16:43:10

Edited By Dave Halford on 25/10/2020 16:49:58

Sam Longley 125/10/2020 17:45:03
939 forum posts
34 photos
Posted by Dave Halford on 25/10/2020 16:26:23:

Have you swapped the cables around to see if the fault moves to the other axis display?

Have you tried zeroing the readings and using it as a basic measuring tool cutting out the maths?

There have been issues with scales changing reading when the motor is switched on, that was a lack of AC earth.

I had not thought of swapping the cables but will give anything a go.

I do have a wiring issue. Ever since I had the machine from new I have had to press the start/stop switch sometimes up to 5 times to get it to start. I have found that wedging a piece of wood against one of the components to push it up against the copper strips does help, as the clips that hold it in place are naff. Perhaps there is an issue there, but I tend not to stop the motor if poss, because I have found that having pressed the start switch & it does not start, if I then forget to press the stop switch it sometimes starts on its own whilst i am handling it. No fun with the chuck key in place. It has rapped my knuckles twice, but i gave it a good swearing atsurprise

However the DRO is wired through a different 13 amp socket & it is earthed to the lathe.

Dave Halford25/10/2020 18:06:17
2015 forum posts
23 photos

The DRO doesn't like being the only lathe earth . There's a certain amount of earth leakage on power units that's noisy.

Edited By Dave Halford on 25/10/2020 18:11:21

Edited By Dave Halford on 25/10/2020 18:11:54

Sam Longley 125/10/2020 18:10:20
939 forum posts
34 photos
Posted by Dave Halford on 25/10/2020 18:06:17:

The DRO doesn't like being the only lathe earth .

I am confident that the electrician who wired my workshop did it properly. Whilst the lathe itself is poor the earthing in the workshop is Ok. The mill DRO works perfectly well. I could disconnect the earth from the lathe, but i cannot see how this alters the readings. I will try it though.I will give any suggestion a go, thanks

not done it yet25/10/2020 18:14:52
6745 forum posts
20 photos

Dave H has indicated so many possibilities that have not been considered. If the resolution of the readout is only 0.01mm, do look up the specified system accuracy - it is likely rather more than you think. That alone could account for some of the apparent discrepancy suspected.

At present, posters are simply going round in circles.

Dave Halford25/10/2020 18:16:04
2015 forum posts
23 photos
Posted by Sam Longley 1 on 25/10/2020 18:10:20:
Posted by Dave Halford on 25/10/2020 18:06:17:

The DRO doesn't like being the only lathe earth .

I am confident that the electrician who wired my workshop did it properly. Whilst the lathe itself is poor the earthing in the workshop is Ok. The mill DRO works perfectly well. I could disconnect the earth from the lathe, but i cannot see how this alters the readings. I will try it though.I will give any suggestion a go, thanks

I'm sure he did to, but the issue can be in the lathe check the mains cord earth is terminated on the earth spade properly, no paint in the way of a good contact on the mounting stud etc.

Nicholas Farr25/10/2020 18:23:48
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3330 forum posts
1531 photos

Hi Sam, if your lathe and DRO are earthed via their supply leads and then you connect an earth between the DRO and the lathe, you could have an earth loop. Earth loops on audio equipment connections produce an annoying hum, so an earth loop between lathe and DRO may cause electrical noise which may give odd readings.

Regards Nick.

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