DRO 1/2 Function not accurate

[darkzero]

Oh man, that sucks! Wow, how does something like even happen? At least you know you are not crazy. That would have confused the heck out of me too.

I'm out of ideas. I have no personal experience with mag scales except that I know you can buy just the magnetic tape. Not sure if DRO Pros sells just the tape. Hopefully you can buy just the scale part instead of the whole thing with the encoder. Maybe Mark @mksj has some ideas.

I just looked at your pics again now I'm at the PC instead of my phone. Now I see that it was the Y axis you are having trouble with.

 

[darkzero]

Ok, something did come to mind. Are those scratches or burrs & is it on bare metal? I'm not sure where the actual tape is. If it is a burr (high spots) I'm wondering if it damaged the encoder? If it didn't do any damage to the encoder & the burrs are just on bare metal, I wonder if stoning the high spots out will work? Like maybe the encoder is getting pushed out of alignment when passing over it? Well you did say it works fine in other areas so I guess the encoder is ok?

 

[Janderso]

I guess the encoder is the device that slides along the strip?
As you can see, I'm no authority on this subject.
It seems to work along the rest of it. I noticed one section, I was watching the dial indicator and the DRO and all of a sudden the DRO displayed some strange data. That's when I went in for a closer look.

 

[darkzero]

Yes, the encoder is the part that moves along the scale with the cable attached to it. I usually call it a read head but encoder is the proper name.

Can you feel high spot where those scratches are? And does movement feel funny when the read head slides over that area?

Ok just to be clear what I was referring to is the area circled in red. Are those high spots or is that just oil or something? The dings or whatever circled in blue don't seem to affect anything but might as well check.

 

[Winegrower]

Janderso, your experience and mine may be similar. I had very mysterious issues using the diameter function of my Ditron D80...it worked a lot of the time, but was occasionally just significantly wrong, maybe by 0.030” or worse.
I tried everything, and eventually tracked the error down to one particular repeatable region, with a big spreadsheet of movement of dial, DRO and 0.0001” DTI. Getting ready to show to DRO pros, I finished data collection, blew out the scale with shop air, thought I would for fun recheck, and the error was gone.

Todd and Dan at DROpros sent a replacement scale, which I installed and after linear compensation worked well, and has continued to do so for a few months now. To try to avoid a repeat, I used a thin run of clear caulk between the mag strip, the carrier tray and the stainless top cover. In addition, DROpros upgraded me to 1 micron resolution, and this fixed some precision (vs. accuracy) problems. They have been very supportive, and if I had purchased direct, I’d probably have pitched everything and started over with another system.

'So I am a bit skeptical of the magnetic scales...if they weren’t so tiny and cute, I would go glass I think.

 

[darkzero]

'So I am a bit skeptical of the magnetic scales...if they weren’t so tiny and cute, I would go glass I think.

If I'm understanding them correctly mag scales aren't actually magnetic to where they would attract ferrous chips but do the read heads have any sort of wiper on them? I always thought mag scales were not affected by dust, chips, or liquids.

 

[Janderso]

Is there an air gap? Setting.
Darkzero,
Those ain’t grease or oil marks, those are gouges and have been there since I bought this mill used. I’ll run a file over the aluminum strip with the damage.
Yes, I hope I can just buy the strip.

 

[Winegrower]

If I'm understanding them correctly mag scales aren't actually magnetic to where they would attract ferrous chips but do the read heads have any sort of wiper on them? I always thought mag scales were not affected by dust, chips, or liquids

Yes, there is a wiper on the read head that scrapes along the middle part of the stainless cover. I may be wrong here, but here is my reasoning. First, I looked at the actual mag strip with a “magnetic developer”, a thin sheet of Mylar that can be laid on the strip and shows the magnetized pattern. It seems very coarse. Second, when DROpros upgraded my system to a 1 micron resolution, it wasn’t the mag strip that changed, it was the read head. So I realized that the read head has to interpolate between domains on the strip. Third, the stainless strip is slightly narrower than the mag strip, and experientially, blowing out some chip accumulation along the edge changed the error pattern I was pursuing.

So based on that, and knowing the tiny distances that have to be resolved, I suggest that chips, possibly even magnetized chips, could get close enough to the read sensor, (undoubtedly magnetoresistive, because it’s not speed sensitive) to distort the field a bit and cause a transient error.

Liquids, dust, etc. don’t affect the mag pattern and wouldn’t cause problems.

A counter-argument is that mag fields fall off rapidly with distance, I wouldn’t expect this to be a problem a priori.

So who knows for sure? I would like to hear from a designer of these systems about the facts of how they work, and the immunity from field disturbances.

 

[Janderso]

I was able to order just the scale this morning from DRO Pros. Very helpful.
$145 plus tax and shipping.
Looking on their site, this is an expensive DRO!
Well, expensive to me. =$1,000 range. I guess that's cheap compared to some brands.

 

[Janderso]

Got the DRO scale last week from DRO pros.
I installed it today, everything seems to work fine.
The air gap or clearance is easily regulated by a piece of plastic/shim they provide.
I just mounted the head to the scale with the strip in between, tightened everything down, removed the shim and it works better than ever.