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DRO descends into madness

trlvn

trlvn

Ultra Member
OK, it is me that is going mad; not the DRO.

My mill/drill came with a Shooting Star DRO which I should probably just replace. But I'm stubborn. And cheap.

Brief background. Shooting Star used a mechanical system with a rack and gear to track position. I've read that the tracking is basically like an old computer mouse. On my X axis, the read head has always made a little clicking sound when I'm moving the table. Sometimes, the DRO readout would jump by 12-15 thous at the click sound (not always). I had thought there might be a piece of swarf in the gear. Last Friday, I decided to take it apart and a little cleaning would eliminate the jumps. (Foreshadowing.)

It went to hell immediately after I took it apart. The gear is catching on the metal housing of the read head. The guts of the read head are mounted on a leaf spring so that the gear always maintains contact with the rack. Otherwise, it would have to be manufactured to extremely close tolerances to keep the gear and rack engaged with minimal backlash. These are the main parts after disassembly:

X read head IMG_5352.jpg

AFAICT, there is a single #4 screw holding the guts of the read head into the housing. I'm afraid I'm going to break the screw (or the little Allen wrench). I tried heating the head of the screw with a soldering iron but haven't had any success. A soldering iron will get hot enough to break regular Loctite, right?

I really have no idea why this is giving me so much grief now. When I got the machine, the X axis rack (which is basically a 1/4" diameter steel rod) was damaged. Some goon had thought he could drag the mill around using it as a handle so it was badly bent. I got a replacement rack from Shooting Star a year or two before it went out of business. I don't recall having any significant trouble with the read head at that time.

Thoughts, suggestions? If I really can't make it work again, I'll probably move the Z axis read head to read X and maybe adapt a digital calliper for Z. Or suck it up and buy a modern replacement.

Craig
 
don't think a soldering iron is going to work, unless you have a very large one. There is a lot of metal there that will act as a heat sink. Do you have a small torch, like the little pencil style ones? Or even a jet lighter? If you have a plumbing propane torch you can heat a throw away allen key to red hot then put it in to the screw to transfer the heat. Wrap the surrounding area with a tamp rag. You need to be careful and take your time. Don't try to turn the hot allen key, remove it after an amount of time and use a good key.

At one time I had to recover some encoders that we had on a retrofit and the guy running the job did not want to buy new ones. New was in the thousands. Some one at some time had locktited the covers. I'm gonna say the screws were m3. So i used a pencil torch with a damp rag around the housing and tried to heat just the cover screws one at a time. It worked ok but it destroyed the integrated gasket on the back side of the cover. Lucky I didn't damage the encoder. I then used the red hot allen key method and it work. We had to pull the encoders apart to add new cabling as the original ones were way to short and damaged. This was on a draw works so lots of vibration and in a very messy area.

Good luck and post if you get it to move.
 
djberta said:
don't think a soldering iron is going to work, unless you have a very large one.
Click to expand...
I have an old Weller 140 watt gun-type soldering iron. It will melt solder after just a few seconds so I was hoping it would get hot enough to make the Loctite release. I suspect the screw is only about 1/4" long but you're right that the aluminum case will be conducting the heat away pretty quickly.

I like the idea of a red-hot Allen key. I'm sure I have one in my stash that could be sacrificed!

Craig
 
Who knows what temp, there are several flavours of Loctite and an OEM could have use some other make. Loctite Red for example, according to Henkel needs a temp over 550F (that is blue hot!) for removal. imo, with all the material acting as a heat sink around it, it will be hard to get it that hot with an allen key or soldering iron.

What to do? Some quite moderate tapping with a hand impact driver? Going in both directions can sometimes free them. I'd put my best hopes on pecking on the mill or using progressively larger left hand drills. When there is not much left, the thin remaining section of material can get blue hot from the drilling.

Worst, worst case, mill it out, thread a plug in the lathe and install it in the bore useing cylindrical retaining compound. I did that recently with a broken tap (oh, the shame of it!) and it worked like a charm.

No guarantees, just throwing some ideas on the pile.
 
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Are there any rubber parts or temperature sensitive parts around this screw ? if not - If you have small Bunsen Burner or gas burner with pointed flame and heat this screw until it will budge. Soldering iron is not effective enough for such operations. Just few days ago I removed set screw from flexible coupling on my older step motor using small gas torch. No damage was done.
 
Gennady said:
Are there any rubber parts or temperature sensitive parts around this screw?
Click to expand...
Until I get it apart, I really can't see what may be there. But since it isn't working now, more extreme methods won't make it worse!

Craig
(BTW, it is a pain that the search function won't accept the term "DRO"!)
 
trlvn said:
Until I get it apart, I really can't see what may be there. But since it isn't working now, more extreme methods won't make it worse!

Craig
(BTW, it is a pain that the search function won't accept the term "DRO"!)
Click to expand...
Try searching again with three characters.
 
trlvn said:
(BTW, it is a pain that the search function won't accept the term "DRO"!)
Click to expand...

Yup. But it saves overhead on the server that needs to be busy with other things.

Try entering this (or just copy it) into the Google search field as a one line entry. The site portion restricts the search to our forum and the quotes ensure that you only get hits for DRO.

Site:canadianhobbymetalworkers.com "DRO"
 
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As soon as I read "rack and gear" I thought "how can this possibly not have the same issues of wear and backlash that an old school dial on the lead screw would have?"

Just me, but I would not waste time on it. Either get a modern (glass or magnetic) *real* DRO, or go back to using the dials.
 
If there are no parts (on that tab) that can not be submerged and you have a large enough ultrasonic cleaner, you might be able to try placing that tab into an ultrasonic cleaner for a shakedown.

I have also found a heat gun can get pretty hot and you can localize the heat to a fine point.
 
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