Magnetic DRO Scale question (not for squeamish!)

[graham-xrf]

In a recent Clough42 video, he was mounting DRO that worked entirely by a implied "magnetic" scale principle.
The brand name on the 0.004" stainless strip was "ELECTRA MAGNETIC SCALE".
Google's algorithm relentlessly substitutes "electro" for "electra", so my purpose is thwarted.

Most Chinesium scales, including the ones I bought, use a etched glass grating, and an optical principle.
This one is different. Apparently, it can be saw-cut to any convenient size without going through glass.
Can anyone help with even a limited qualitative explanation of the working principle?

The Clough42 video (if squeamish, move on, I will understand).
--> Shop Accident Caught on Camera - Lathe DRO Install, Part 2

I found a PDF of one version, but it does not say how it works.

 

[WobblyHand]

From what I understand they are basically a magnetic tape with a precise pattern on them. A magnetic tape reader is used to detect the 1's and 0's. (Or analog pattern.) Think tape recorder. The magnetic tape is usually bonded to a stainless steel strip. The strip can be easily cut to any size. I think the mag DRO's are more compact than the glass ones, so they are easier to install. I have a magnetic DRO on my Grizzly G0752Z as a factory option.

 

[graham-xrf]

From what I understand they are basically a magnetic tape with a precise pattern on them. A magnetic tape reader is used to detect the 1's and 0's. (Or analog pattern.) Think tape recorder. The magnetic tape is usually bonded to a stainless steel strip. The strip can be easily cut to any size. I think the mag DRO's are more compact than the glass ones, so they are easier to install. I have a magnetic DRO on my Grizzly G0752Z as a factory option.

Thanks for that. They are clearly accurate enough. I may check out relative cost, compared to glass-grating types.

 

[RJSakowski]

When I fitted my G0602 lathe with a DRO, magnetic scales weren't available and glass scales were too large for the x axis so I went with the iGaging capacitive scales (similar to those used on the ubiquitous digital calipers) and used the TouchDRO. The capacitive scales have a resolution of 10 microns or .0004" which, when doubled to indicate diameter, is somewhat limiting but if I need to turn to sub thou precision, I use a micrometer anyway so it's not that big an issue. Were I to install a DRO on the lathe today, I would use 1 micron magnetic scales..

My x axis mount was very similar to Clough42's mount. I mounted the z axis scale on the back side of the lathe rather than the front as it was less obtrusive there. I mounted a DRO on the tailstock for the third axis. It has been in service for over seven years. Here are the whole gory details: https://www.hobby-machinist.com/threads/another-lathe-dro-install.34106/ (disclaimer; no real blood or gore).

Edit: corrected dropped digit

 

[Winegrower]

I too would like to understand the theory of operation of mag scales. I developed the scales to see the magnetic pattern, and found it’s surprisingly coarse. The resolution is determined by the read head, not the strip, so I imagine two hall effect sensors interpreting relative magnetic field strength and generating a quadrature signal for direction and distance.

Somebody, please offer a better explanation?

 

[WobblyHand]

I too would like to understand the theory of operation of mag scales. I developed the scales to see the magnetic pattern, and found it’s surprisingly coarse. The resolution is determined by the read head, not the strip, so I imagine two hall effect sensors interpreting relative magnetic field strength and generating a quadrature signal for direction and distance.

Somebody, please offer a better explanation?

Maybe not super technical, but here's a partial explanation: https://www.touchdro.com/resources/scales/magnetic/overview.html

 

[Eddyde]

This may be helpful:

https://atechauthority.com/pdf/Understanding_Scales.pdf

 

[RJSakowski]

Maybe not super technical, but here's a partial explanation: https://www.touchdro.com/resources/scales/magnetic/overview.html

This may be helpful:

https://atechauthority.com/pdf/Understanding_Scales.pdf

Thanks for the links. Magnetic scales are far more complicated than I had envisioned. I was expecting a technology similar to that used for computer hard drives.

The error analysis has given me pause for though. Particularly the difference between high end and the low cost scales. What would concern me is that there is no practical way of knowing what the performance of a magnetic scale is prior to installation and trusting the word of a vendor who most likely has no technical knowledge of the actual function of the product seems rather foolhardy.

As pointed out in the second article, there is a difference between accuracy and precision. Just because we can read to six decimal places doesn't mean that the reading is an accurate display of fact. Unfortunately, we have a tendency to accept a digital display as fact without really confirming that for the most part. There is no value in having a DRO that can resolve to 1 micron of the displayed value can't be trusted to better than a thou or two.

Aside from a manufacturing or installation defect, I would be concerned with issuws arising from use. The possibility of ferromagnetic dust altering the pattern of the scale in an environment where such dust is commonplace is disturbing. Another disturbing thought is magnetic domains shifting. In the early days of PC's, hard drives had to be reformatted on a regular basis to avoid CRC errors and video tapes would fail after a time due to magnetic domain migration. Magnetic media is generally not considered a good long term storage solution.

Verifying a DRO scale along its entire travel would be a tedious task, especially if looking at the resolution down to a few tenths of microns. Were I to attempt it, I would probably use a set of trusted gage blocks in concert with a .0001"/div. dial test indicator.

Considering all that, I think I will stay with my capacitive scales on the lathe for a while yet. They may not have the precision but at least I am aware of their limitations

 

[mksj]

The brand name is probably Electronica which make a wide range of precision instruments, including the DRO's like the EL400, 700, 750 the MX200, and probably labeled under other names. They are one of the mainstream DRO vendors, their scales come with calibration certificates for each scale/read head. I have their EL700 with their magnetic scales, as well as those from Machine DRO UK/SRA in the US. They all came with scale specific certs and at worse they were under 0.5 micron resolution. The magnetic type scales are used in some of the harshest machining environments in long lengths, some are absolute reading. I have checked my scales against quality 123 and 246 blocks, as well as tracking to the machine dials, and they are pretty much the same within the accuracy +/-0.0002" of the measurement. If you buy cheap scales, who knows what you are getting or if they are from the reject pile. Capacitive scales are far worse as to resolution, at least the ones I have used.

If magnetic scales were so susceptible to magnetic effects and/or ferrous materials, I doubt that they would be so commonly used. If anything they seem much more forgiving then glass scales and less prone to issues with contamination. They all get the job done, but these days I find they are so much easier install vs. the glass scales, and far less bulky. The magnetic scale is the same for the resolutions, the reader heads determine the resolution, and absolute type scales are just becoming available.

Magnetic Linear Encoders | Submerged Linear Encoder | Magnetic Linear Scales - EMS

EMS is leading manufacturer and Supplier of magnetic linear encoders, magnetic linear scales, Submerged Linear Encoder, encoder in oil for position and motion control.

www.electronicaems.com

 

[Eddyde]

Here's where I bought my Magnetic DRO for my lathe:

M-DRO - Machine DRO UK

A division of The Allendale Group, a family-run British company founded in 2004. Our roots are firmly based in the UK, with a head office, engineering and inspection department at our works in Hoddesdon, Hertfordshire. We pride ourselves on being measuring tool specialists offering tools...

www.machine-dro.co.uk

The sale was smooth and everything arrived safely. Unfortunately, I have not had time to install it yet...