DRO installation - a salutary lesson

I bought a Digital Readout kit from a reputable seller to install on my new milling machine. The y-axis assembly went on fine, but I had a terrible time getting a stable reading from the x-axis one. The reading fluctuated wildly even when the bed was stationary. At first I thought it was due to some sort of misalignment and spent ages tweeking and adjusting the mountings but to no avail. Then I got to wondering about the way the measuring head was screwed down. For the most rigid method I changed the screws which were supplied and used my own countersunk screws. I had already made sure that they did not foul the sliding scale, but I discovered that if I released them by a couple of threads the problem went away. So the solution was to shorten them by two mm, and bingo everything was hunky dory.
I’m not an electronics expert, but I’m most intrigued to know the technology and how a piece of steel in close proximity could cause such interference? Any reply from a knowledgeable guy would be most appreciated.
Bob

Could it be that the screws used transmitted an electronic circuit or magnetic field ? I await the experts opinions as this has intrigued me.

There have been other posts on this subject recently that have highlighted potential earthing issues with the machines, that has resulted in earthing currents flowing through the DRO cables. This is a potential safety issue that needs to be addressed ASAP.
1) Check all exposed metalwork on the mill has continuity to mains earth. I would disconnect the DRO scales from the console for the duration of the test ready to eliminate this path
2) Your DRO console should have an earthing point, make sure that that this is connected to the mill metalwork at one point.

Magnetic or optical scales ?

At a guess, magnetic.

Nigel B.

I wonder if the screws were distorting the internals in any way and causing a bad connection on the electronics?

Bob most scales have to be installed to a tolerance that is sometimes quite a narrow window.

Your mystery scales might be just on the border line or there's a dry joint or dicky cable. if the metal was important they would have supplied ally or brass screws.

I think this is the most likely reason too.

From the OP's original information it is nigh on impossible to give a meaningful answer, all we know is one axis worked and one didn't, but we have no idea of what type of DRO this is and even less idea of the actual mounting details. The fact that loosening the fixings changed the behaviour is a clear indication (to me) that something was being distorted even if the ends of the screws were not fouling on anything.

Although the DRO is now working, if it were mine I would want to know the actual cause of the problem.

Ian P

He said that he shortened the fixings by 2mm and it's all OK. Surely we can assume that they still aren't still flapping about 2 threads loose.

Dave, but is was the 'loosening' that cleared the fault and we have no idea what the term meant. 'If' the loosening was just a quarter turn the its unlikely that is was the protruding threads making contact with anything.

Ian P

A couple of threads was mentioned in the OP Ian, which to me meant the read head would wobble if left like it.

FWIW I fitted an M-DRO kit of mag scales, and I did have one problem that wasn't mentioned in the instructions.

I mounted the sensors on the small ali blocks provided and found that the strain relief where the cable entered the sensor body was slightly larger than the body itself, so that if you tried to tighten the screws the connection would be put under strain. To obviate this I put a washer or two between the sensor and the mount for packing, to allow the bolts to be fully tightened without straining the connection. The installation mainly went swimmingly, I had no problems and it performs very well. The third place of decimals on the display is rock solid to 5 microns (the resolution), it only changes slightly, sometimes, when you tighten the slide lock.

I bought a M250 with a industry quality DRO fitted when new (optical). Had a readout problem after fitting a VFD, this that was completely cured by earthing the readout.

Richard

Thanks for all your replies, although no-one has explained how these things actually work.

#Ian P - not sure if I was allowed to mention brands, but what the heck - it was bought from Arceurotrade. There is no info on their website as to the type.. The part no is 100-061-00500.

#Stuart Bridger - there is no facility to earth the unit. In fact the display unit is supplied with a plastic mounting plate with keyhole slots to slide it in. It is battery operated, so I'm not sure whether an earth is necessary.

#mgnbuk - Looking inside I cant see any light ?!? so I suspect it is magnetic

#Bryan Ceder - I could see the screws when the scale bar is removed and it did not touch or distort anything.

#Dave Halford - absolutely correct. I tightened the screws after I shortened them.

I could vent my spleen about the fact that the documentation provided only described how to use the thing and not how install it. Although it was titled "Digital Scale with Steel Deam Instruction". Your guess is as good as mine as to what that means. Not to mention the four other spelling mistakes. I would have thought any self respecting supplier would proof read with due diligence anything coming from abroad where English is not the native tongue. But that is a digression.

Bob

PS New restrictions tonight in Kirklees means I shall have more time for the man cave.

Capacitive

New restrictions tonight in Kirklees means I shall have more time for the man cave

You are affected by the antics of the residents of Ravensthorpe & Dewsbury as well then Bob. No wonder there are calls from Huddersfield residents that those areas are are separtely administered as "North Kirklees". No coincidence whatsoever that the lockdown was announced just before the start of Eid.

Can't help with capacitive scales - not much experience. But maybe the screws were bottoming before the read head was fully secured, allowing it to float slightly & shortening the screws fully secured it ?

Nigel B.

Ian P

interesting lesson Bob., So capacitive slides are as fussy as optical? I might start saving for the posh ones.