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.
Hit Facebook for M.R. Tool Repair.
Mark Ratkowski is the guys name, ask him about it and he can tell you
what to look for and what it takes to repair it. He repairs and
certifies pretty much any type of measuring gear.
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.
[ ... ]
Ouch! There are special tools for the purpose.
[ ... ]
The two tools that I have for the purpose are somewhat
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.
search down for:
bergeon #7 Watch Tool Presto #7 Hand Remover
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.