Author Topic: Bob, help! (caliper repair)  (Read 4448 times)

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 5423
  • Country: us
  • Republic of Vermont
    • www. sredmond.com
Bob, help! (caliper repair)
« on: January 08, 2015, 01:00:46 PM »
Bob, I have a cheap Chinese dial caliper that I've owned since the start of my metal working. So it's got sentimental value. Also it continues to work when my cheap digital calipers stop working, which they do for any of a hundred reasons. In fact it always works, and always seems accurate. It even works with no glass. It's easy to read in low light (white dial). I don't even know why I bother with the others.

The crystal broke (not my fault - gravity forced the wrench I was holding to fall out of my hand onto it......). So I thought I'd try your method of making a new plastic lens.

But I can't figure out how to get the bezel off!

« Last Edit: January 08, 2015, 03:13:42 PM by vtsteam »
I love it when a Plan B comes together!
Steve
www.sredmond.com

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 5423
  • Country: us
  • Republic of Vermont
    • www. sredmond.com
Re: Bob, help! (caliper repair)
« Reply #1 on: January 09, 2015, 10:08:44 PM »
Woohoo I finally got it -- but not before a long trip to the underworld and back!

If Bob, if you see this, I'm sure you're gonna see those pics and say, Uh oh, why did you take that slide out of the caliper! :doh:

Well I stupidly thought that maybe there would be a clue to getting the bezel off hidden in the works.

But taking the slide out of a caliper is a no-no. Y'see there's these little pinion gears that engage with the rack on the slide, and they are 2 in number on this caliper, and the reason for 2 is, it's an anti-backlash setup, naturally. Backlash in a caliper would make it useless.

So the 2 gears are tensioned against each other with a spring mechanism, set to just the right tension. And when you take the slide out, you release that tension. so now your caliper returns to maybe zero, or maybe sometimes minus two thou, or maybe plus 5 thou, depending on how hard you close the jaws. So now the problem isn't just a broken crystal, but a plus or minus 5 thou caliper.

And the bezel? Well it turns out you just pry it off. No hidden circlip or spring, no hole to stick a paperclip down. Just insert a flat head screwdriver and pry it off -- on this caliper, only.  Don't try it on yours......

Okay I've got the bezel off, and the dial and hand are still on, and I figure out something is amiss because of inconsistent readings when I put the slide back in. So, dig deeper.

I pry up on the hand, and, pop, naturally it flies across the room. It's about the size of a pin, and it's somewhere on the wooden living room floor. I stop all traffic. And crawl around the floor on hands and knees where I thought I heard it land. Luckily it wasn't carpeted, as I'd have had no idea in which direction to search. After a couple minutes I spied it right where I had heard the pin drop.

Okay put that on a white piece of paper and look at the brass case under the dial. I can see a little U shaped cut in the case that reveals a gear, and another smaller hole opposite that reveals part of a second gear. Nothing else of note. 3 screws hold the case in place.

Those come out, I flip over the movement case and I see for the first time the 2 pinions mentioned earlier. Gee, why two, I think. Gradually the anti-backlash idea begins to form. I fiddle with the two gears an see that they aren't directly connected, you can move one in relation to the other, but there does seem to be tension between the two if you wind one while holding the other still -- so they are spring connected. Okay I get it.

So now the problem is, how do I restore the anti-backlash tension? I do some research online, but while there is talk of Starrett and Mitutoyo calipers, I'm coming up blank with my cheapo "OEM" brand calpers. However, some of the other brands have you insert a pin somewhere while winding another gear, and I try that in the two gear openings I see.

No-go. Those gears are directly connected  to each other, not spring loaded. I puzzle all this out trying different things for a half hour. Eventually I realize both gears directly train to one of the pinions, and the other one is spring loaded to that whole train. So I think, what if I pin the train, reinsert the case in the caliper, but twist it slightly, so the spring gear engages with the rack, but the train gear does not, and then just slide the caliper a tiny bit to wind the spring, and then carefully twist back to engage the other pinion. If I don't breathe and hold everything together with one hand, I can maybe reach each one of the 3 attachment screws, and a screwdriver with the other hand, and ease those babies in.

And it worked!!!! :ddb:

I was used to reassembling things by now having taken them apart about a dozen times already -- you see, you can't test the anti-backlash without screwing the case back, adding the dial and hand to check it. Anyway, it went back together quick this last time, and no visible backlash!

Also I was able to set the hand to 12 o'clock with the jaws closed. It hadn't held that position for 10 years, I think! It was always a few degrees off to the right, which is bothersome, even though you can correct the zero position wth the bezel.

And now the calipers are as smooth as silk again after cleaning and adjustment. Very very happy -- I thought I'd wrecked it.

And the crystal?

Well actually I haven't made it yet....... :lol:
I love it when a Plan B comes together!
Steve
www.sredmond.com

Offline wheeltapper

  • Sr. Member
  • ****
  • Posts: 345
  • Country: gb
  • Spixworth Norfolk
Re: Bob, help! (caliper repair)
« Reply #2 on: January 10, 2015, 07:58:50 AM »
Hi
this post has come in handier than you realise.
some time ago I got a second hand mitutoyu dial caliper from fleabay and when it arrived it had 5 thou float on the needle.
I contacted the seller and he sent me another one so I had this useless one kicking around and, like you, I couldn't get into the damn thing.

after reading this I thought ' I cant mess it up any more ' so I attacked it with a screwdriver and BINGO the bezel popped off.
after removing the needle ( no I didn't lose it, forewarned is forearmed ) I got to the inside.

there was only one gear but it had a groove about one third from the end and a springy brass piece with a notch that pressed the gear against the rack.

it seemed that over time the steel spindle had worn the brass down a bit and the pressure had been reduced so I put a slight curve in the strip, reassembled it and wonder of wonders it now works perfectly.

I cleaned all the pieces in isopropanol first and you would'nt believe the cr*p that came out.

so thanks for trailblazing cos otherwise I wouldn't have attempted it.

Roy.


I used to be confused, now I just don't know.

Offline krv3000

  • Hero Member
  • *****
  • Posts: 2089
  • Country: gb
Re: Bob, help! (caliper repair)
« Reply #3 on: January 10, 2015, 11:32:18 AM »
hi sos for the delay in geting in tuch bin biz trying to fix a problem with me car  any way glad you got ther in the end

Offline vtsteam

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 5423
  • Country: us
  • Republic of Vermont
    • www. sredmond.com
Re: Bob, help! (caliper repair)
« Reply #4 on: January 10, 2015, 12:14:53 PM »
Thanks Bob, I never would have even tried something like this if you hadn't posted all of your DTI repairs!

Roy, great!! I had the same thought -- well it's no good as it is, I might as well try o fix it....

So the final bit here, Bob showed how he made new lenses for DTI's some time back from plastic sheet, heat formed on the bottom of a spray can, and that's what I wanted to do, too. But in my reading online about calipers, it said that you could just use flat sheet and spring it into place on many calipers -- that's how they come.

Well I tried that first with cardboard to get a pattern, and then a piece of vegetable container, but it never worked. I kept getting ridges and it would deform the bezel ring to oval. And that's probably true because my crystal was originally glass, and it was obviously pre-formed to convex shape and didn't put any pressure on the ring. Luckily it snapped back to circular when I took out the plastic.

So then I decided to do Bob's method. But he bent a whole sheet and then cut it out, while I already had a closely fitted circle to work with. So I needed some method of heating a small finished piece up that wouldn't deform it. I tried a hair dryer, but that was impossible to control re. heat.

So then I tried a cup of hot water heated to near boiling in the microwave oven. I kept putting the cup in the oven for a few more seconds, then dropping the lens in to see if it softened enough to be able to be formed on the bottom of the spray can. Eventually I got to a temperature that alowed the plastic to be worked, but not overheated. I pulled it out with tweezers, and just pressed out the wrinkles on the bottom of the can with my thumbs. This worked!

And the lens fit just right, with a very little bit of tension, into the bezel. The plastic actually got clearer, the first time I heated it with the hair dryer -- it had been a practice piece, because it was very slightly cloudy to start. But now it was perfect, and I just reassembled the caliper. And well, ahem......

Bob's yer uncle..... :beer:

I love it when a Plan B comes together!
Steve
www.sredmond.com

Offline krv3000

  • Hero Member
  • *****
  • Posts: 2089
  • Country: gb
Re: Bob, help! (caliper repair)
« Reply #5 on: January 11, 2015, 08:50:21 AM »
brill see it ant hard it may take a few gos to get it right