I have had a 12" dial caliper for quite some time. It has the "guarded rack" design, where the depth rod on the right covers the rack. Well, last night I went to use it, and it bound up, and I could tell that something was jamming the rack-pinion engagement. I managed to get it apart and cleaned the rack as well as I could, and now it works fine from 0-7", but from 7 -
12" there are many spots where it binds and then the pointer jumps. I've looked at the rack with a microscope and cleaned all the little bits of debris out of the rack, but it still binds and jumps. I ordered another one, but anybody have any suggestions? I have studied the rack teeth with the microscope, and I am really STUMPED! I can hardly tell any difference between the good 7" and the bad 5" portions!
A shiny metal background isn't always best for visual inspection. Get some clay-like stuff (I like HandiTak from SuperGlue/Pacer) and mold it into the suspect spot, then pull it off and inspect it with the microscope.
Worst case, it tells you nothing. Best case, it sticks to a bit of transparent stuff and fixes the issue.
Have you used a straightedge on the rack to see if it was widened in the slot? It doesn't take much on tiny teeth like that. A grain of sand between the teeth, if it doesn't break off a tooth, might wedge the space open, either vertically or horizontally. Also check for any missing guides/damper discs from the disassembly.
Thanks for all the suggestions, but I finally fixed it. I was real afraid to try to get into the clockwork part, as I wrecked another caliper some years ago, the pointer would NOT come off, and I finally broke the shaft.
But, this one came apart fairly easily, and I was able to fix it. What was wrong is when the pinion jumped, it bound up the anti-backlash gearing. This is one of those 0.1" per turn dials, so it has a fair bit of gearing behind the dial.
So, the procedure on THIS particular unit, but I suspect many of the Chinese knockoffs are similar: First, pry off the knurled bezel around the crystal with jeweler's screwdrivers. Pry up one side a bit, then pry up the other side, pretty soon the bezel comes off. There is a spacer ring behind the crystal, and the dial will be loose. Now, this is the tricky part, you use two jeweler's screwdrivers at the same time to evenly pry off the pointer. Hold your finger over the pointer so it doesn't fly across the room. Now, the dial can be lifted off. There are 3 screws that hold the clockwork to the slider. Remove the screws and you can lift that off and examine the two pinions. Clean and apply a speck of watch lube to the pinion shafts. Place the clockwork back onto the slider and put the rightmost screw in, loosely. Now, with the jeweler's screwdriver, wind the visible gear about
1/4 turn such that you can feel the spring taking up a little load, and then press the clockwork against the rack and insert the other 2 screws and tighten. Slide the slider back and forth and see how it feels.
This was the thing that I didn't understand. When the pinion jumped due to a chip in the rack, it unloaded the anti-backlash spring, causing the gear train to bind. One way it is spring-loaded, the other way it is a set of gears with no compliance. So, slight variations in the rack and pinion teeth cause it to jump rack teeth. As soon as I reestablished the correct loading of the anti-backlash spring, it was smooth as silk.
Well, now that I know how to take these apart, I can fix it the next time such a thing happens.
The smaller calipers that have the .2" travel per turn don't have this complicated mechanism on them, and you can just run the pinion off the end of the rack with no issues.
The two tools that I have for the purpose are somewhat different.
One has a handle, two bars pivoted from that, two more pivoted from those, which come together with a bar a bit back from the ends. The ends are claws with rounded backs, and those backs rest on the dial, as the claws come up (as you push down the handle) under the back of the pointer to pop it off, and the bar catches the pointer.
The other has two hooks which fit under the back of the pointer's hub, a turret with four different diameter points (you select the one which is just a little smaller than the pin on which the needle fits). It also has two bowed flat springs along the sides. You hook the hooks under the head, put the selected pin on the end of the pin in the mechanism, press the two bows together, and the hooks draw up as the pin presses down to pop the hand from the mechansim's pin.
Either of these can be purchased from watchmaker's supply houses, though I lucked into both used.
The kind of hub which has a blind tube on the back to push onto the pointer spindle.
The other one which I described (the one without the turret of pins), the one with rocker arms, pushes on the dial and hooks on the underside of the hub, sos that one would work.
The first try to find it found the other one instead.
URL: search down for:
bergeon #7 Watch Tool Presto #7 Hand Remover $68.50
O.K. On eBay:
auction # 201793151501
no center pusher on this one, so it would work.
Or this one has a clearer image, and is eBay item # 172355658816 It appears to have a spring-loaded plunger added to keep the hand from popping off and getting lost. However, it does not need to push on the center pin. It levers with the two claws. Lots of photos showing how it moves.