DRO drifting

SamI

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Hi all,

I was in the workshop yesterday happily turning away when I noticed that my DRO had drifted. Going back over the parts I was making it suddenly shifted by 1.3mm diameter after about 2 hours of machining. The drift was sudden, i.e. all dimensions were fine then the last three diameters were all out by the same amount (approx. 1.3mm). Thankfully I noticed as I must admit I have become quite reliant on the DRO as up until today it has never been out by a significant amount.

I do have a theory that it was the magnetic base which I put on the cross slide next to the scales. It's a glass optical system so I would have thought that the mag base shouldn't cause an issue? The timing does coincide with the drift however I couldn't replicate the drift by placing the mag base in various locations on and around the scales afterwards which has left me rather puzzled.

One thing I have also noticed is that it can drift when the machine is not in use. Because of this I always check before each use but I'm still puzzled as to why it has been doing this. In this instance there wouldn't be a mag base anywhere near the scales.

So I'm wondering if anyone else can suggest a reason as to why my DRO drifts?

Thanks,

Sam
 
Thank you for the suggestion. That's certainly a possibility and although I didn't notice anything at the time it is very possible even if I had seen the lights dim I wouldn't have thought anything of it. Thinking about it, it would also be worth me looking where the leads hook up to the display unit as I was raking behind the lathe yesterday too.
 
I'm assuming that its drifting on just one axis. Swap the cables around on the back of the readout. If the problem stays on the same axis, you know its the readout itself and therefore an electrical/electronic problem. If the problem changes axis then you know that its an issue with the scale on the machine. This will at least get you looking in the right place for the problem.
 
Check the simplest stuff first -- the DRO mount points. I had a similar problem and was blaming the scale for a troublesome glitch in the displayed position. It turned out to be a loose mount for the scale. It has very small machine screws and I was overly concerned about stripping the threads, so they weren't nearly tight enough. They eventually worked loose and the problem began.
 
Thank you all for your suggestions. Certainly a few things for me to junk about!

I’ve only noticed it on the X axis however I zero the Y axis so often that I probably wouldn’t notice if it drifted overnight.

The voltage drop on lathe startup is a good suggestion. Certainly it is enough to din the lights although the DRO beeps when powered off and t never does that upon starting the lathe. Still, I suppose it could cause it to get confused.

I have checked all of the mounting points for movement and there is nothing as far as I can tell. Certainly if I try to love the scale by hand I get nothing until the cross slide moves to take up backlash.

One thing I did notice today was that the earth cable wasn’t connected at the back of the DRO unit. I remeber now that when I installed it the earth cable supplied was only a few inches long and therefore utterly useless. I had intended to fit a longer one but clearly it slipped my mind. I’m not sure if this could cause any issues such as this?

I’ll have a go at measuring voltage drop over the next few days. I would do it this evening but I have the furnace running at the moment and the supply is only rated at 16 amps to that side of the workshop which means it trips easily when the lathe fires up. I should probably run a spur from a different circuit to power the furnace!

Thanks again for all of the suggestions! It’s given me a few things to investigate and without your helpnid probably still be scratching my head!
 
Are you talking drifting over night or drifting while in use? If over night, then temperature change will do that.
 
Both. It has always drifted overnight but that has never been of a huge concern until Sunday when it drifted whilst in use. I had wondered whether the two issues could be related. I couldn't tell you exactly how much it can drift overnight but it is often fairly significant (i.e. more than a few mm) so I'm not sure different rates of expansion between the cross slide and the scales would be the cause of that.

That got me thinking though – how much will temperature changes affect the DRO?

As an exercise I calculated the change in length over a 10°C change in temperature using the formula L1 = L0 x α x ΔT

Where L1 is the change in length, L0 is the original length, α is the coefficient of thermal expansion for a given material and ΔT is the change in temperature.

A quick google and I found the coefficient of thermal expansion of cast iron (the cross slide) to be 10.8 x 10-6 m/m K (rounded to 11 x 10-6 m/m K) and aluminium (the scale housing) to be 21-24 x 10-6 m/m K (where I took 24 x 10-6 m/m K as a worst case scenario).

So to plug the numbers in we have for the scale

L1 = 0.2 x 0.000024 x 10 = 0.000048m = 0.048mm

And for the cross slide

L1 =0.2 x 0.000011 x 10 = 0.000022m = 0.022mm

Which gives us a worst case difference of 0.026mm (0.052mm across the diameter).

This wouldn’t be enough to cause me serious issue on the parts that I am making although I think that it is something that I will check periodically throughout the day going forwards to account for this.
 
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Get that earth ground connected!! As a former automation engineer and having taught electronics for 32 years, I can tell you first hand of a number of problems associated with poor grounding, especially in an electrically 'noisy' environment like a shop. I'm not saying that this is the source of your problem, however, especially since you'd said the drift will occur overnight when most machines would be turned off. And it would be unusual for a grounding problem to affect just one axis as well. What I'm saying is that this is the source of a potential problem and is easily eliminated. Have you swapped the cables around on the back of the DRO yet?
 
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