Magnetic DRO Problem

[houstonceng]

Hi All

A few years ago, I bought a 4 axis magnetic scale DRO, for my milling machine, from EMS-I and have been very pleased with it and the supplier's service and installation advice received when I purchased it. (Usual disclaimer).

Due to a number of personal reasons, I have done little work on the milling machine since 2018 until today Dialling in some dimensions on the Y axis, I noted that the DRO gave a false reading around 0.75 inches from the datum edge of the work. Visual check and then using a "clock" and the scale on the leadscrew confirmed it.

I noted a few bits of steel swarf stuck on the magnetic scale, so took off the reading head and cleaned it and the scale (no swarf was under the reading head as the plastic wipers were effective.)


After cleaning, accuracy was restored.

Hs anyone else seen this on their magnetic DRO or know why swarf would affect it ?

[timo]

Dear Andy,

Any swarf which sticks to the scale will be steel and hence become magnetised. This will confuse the reading head. If the reading head was optical and it counted black lines along the scale to sort out its position, drawing in black marker on the scale would obviously not be a good idea. For magnetic readers, adding lots of little magnets onto the scale does the same sort of thing. The sales blurb for magnetic scales usually says that they are not swarf and coolant proof and it is a good idea to both protect them from same and also give them a regular clean.

Hope this helps.

Best Regards

Tim

[houstonceng]

The reading heads and scales are IP67, so should be coolant proof, unlike optical scales, which is one reason why I chose magnetic over optical. Bits of steel stuck to the scale outside of the reading head ( at each end) shouldn’t affect the readings as they are relative not absolute, however, I can see that a small bit of swarf stuck on the scale either side of the reading head would provide a magnetic shunt between adjacent magnetic segments and would affect the reading. Must have been a little bit that was pushed sideways by the wipers.

[jem]

A lot of very useful information, thank you very much.

best wishes

Jem

[ettingtonliam]

Hm A precision measuring device intended for use in an environment where lots of ferrous swarf flies about, but it can easily get upset by - ferrous swarf.

[92220]

I've had my Machine-DRO optical scales on my mill since 2002 when I bought it, with no problem until last year. When I rang Allendale technical they just said to re-set the digital box, which they talked me through, and it has been fine ever since. The scales come with splash covers and while the outside is dirty, the inside is clean even though the bottom is open to the elements to allow the tracking arm to pass along it. When I bought my DRO I was advised to go optical because there is almost nothing that, in a normal working environment, will adversely effect the readings.

Bob.

[houstonceng]

Funnily enough, I talked to the Machine DRO guy at the MEEx Brooklands and he said that Optical would be affected by coolant and couldn’t be shortened (except with great care) so magnetic were the way to go. I had to use the open scales rather than hooded as I couldn’t get the shrouded ones in the correct orientation on my milling machine. They did come with bolt on covers. I fitted the x axis one, but the y, z and u axis can’t be fitted so far. Perhaps I’ll have another thought and, maybe, come up with an alternative solution.

[SteveW]

Guys,
Quite a while back I gave the DRO topic a good look-at. The magnetic types I found used a long tube per axis filled tightly with ball bearings (steel spheres butted together). The clever bit ran along the tube effectively sensing the equivalent of sine and cosine information from the magnetic field around the serial balls and converting this to a movement reading.

Swarf shunting the magnetic fields around the balls would stuff the readings. It may be that the clever bit's excitation field has magnetised your balls (should not have stood so close) thereby sucking up the swarf.

The digital calibers use a similar system but based on capacitance.

[92220]

I'm not sure why you can't fit the protective covers. I don't know what milling machine you have but I have a Tom Senior Universal, which is the same size as the Major, and all 3 of my scales are protected, from swarf and coolant, with the covers supplied with the DRO. Where are your scales fitted? Have you any photos of the machine?

Bob.

[houstonceng]

Sept 12, 2019 20:41:59 GMT SteveW said:

Guys,
Quite a while back I gave the DRO topic a good look-at. The magnetic types I found used a long tube per axis filled tightly with ball bearings (steel spheres butted together). The clever bit ran along the tube effectively sensing the equivalent of sine and cosine information from the magnetic field around the serial balls and converting this to a movement reading.

Swarf shunting the magnetic fields around the balls would stuff the readings. It may be that the clever bit's excitation field has magnetised your balls (should not have stood so close) thereby sucking up the swarf.

The digital calibers use a similar system but based on capacitance.

The magnetic scales that are generally available are similar to the optical ones being flat scales on which the reading head runs. Some have the reading head integrated with the scale and in others it is a separate unit.

The type to which you refer were considered, but were rejected on price.

[houstonceng]

Sept 13, 2019 10:35:36 GMT 92220 said:

I'm not sure why you can't fit the protective covers. I don't know what milling machine you have but I have a Tom Senior Universal, which is the same size as the Major, and all 3 of my scales are protected, from swarf and coolant, with the covers supplied with the DRO. Where are your scales fitted? Have you any photos of the machine?

Bob.

Don't have any way of posting pictures of my machine, but the pictures of an installation on an almost identical machine using DRO Pros supplied scales (identical to mine which were bought from EMS-i) are on the following pics.

web5.streamhoster.com/hvp/DROPROS/RHZ1.jpg shows Z axis.

web5.streamhoster.com/hvp/DROPROS/RHY1.jpg shows the Y axis with reading head fixed to underside of table and scale on knee.

web5.streamhoster.com/hvp/DROPROS/RHQ2.jpg shows U axis.

My X axis is fitted on the front of the table and gas the cover fitted. The Illustrated installation is on the back of the table thus web5.streamhoster.com/hvp/DROPROS/RHX1.jpg

[92220]

Hi Andy.

Now I see why you can't fit the covers. My mill is similar to yours. I fitted the scales with the slot pointing downwards to prevent liquids getting in, by making up angled bracketry. I then fitted the covers and made sure there were no clashes. The vertical scale had rather convoluted brackets but they were solidly made to prevent vibration, and it all works fine. I don't see why you couldn't do the same with your machine, from those web photos. I'll see if I can get some decent photos of my set-up but the mill is tucked away in a corner and not easy to get to to photograph the scales. I'll see what I can do.

Bob.