Dial Caliper Repair
Dial calipers are both the easiest and the trickiest shop tools to repair particularly without proper tools. Here we present some hints, tips and instructions. We will withhold the essential "tricks of the trade" because that would end up costing us, and other repair shops, much needed business.
Jaws: if the jaws are no longer parallel or flat, they can be ground and lapped but that is the work for a specialist. If you try this yourself, you will probably end up grinding away so much that you will be left with a completely useless tool. Or, the jaws will be in worse shape than they were before. In either case, a useless tool. If the jaws need work, then by all means send the calipers to a shop willing to take on the work.
Mechanics: the movement of the dial caliper can be repaired by anyone with some time on their hands, a few common tools and not apt to cry over a failed attempt.
Tip #1: never disassemble more than absolutely necessary.
Common problems include the missing slider end stop. This is the plastic piece at the end of the beam that keeps the sliding elements in place. It may look like two screws are involved but in most cases the plastic piece has two pegs which pop into the metal beam. This is a no-brainer if you can get hold of the correct slider stop.
(Most Chinese dial calipers have metal slider stops which may be riveted in place. On Starrett 1202 they use screws to hold the metal slider stop in place.
Tip #2: unscrew the bezel clamp before prying the bezel off. Especially in Mitutoyo models where the plastic bezel clamp can easily break if you forget to do this.
Removing the bezel and crystal is step number one in a repair that involves the mechanical movement. On newer Mitutoyo and Starrett models, the bezel is held in place by a rubber o-ring. You pry the bezel off, gently, by slipping a large, flat-bladed screwdriver under the rim of the bezel. Crystals are inserted with a crystal press and the bezel can be pushed back on when the repair is done.
On Chinese-made Starrett 1202 calipers, the bezel can be pried off using a large flat blade screwdriver. Again, be sure to remove the bezel clamp screw first. The new crystal can be pressed in place with your thumb, from below, so that it forms a dome. Don't forget the white plastic spacing ring when you press the bezel back onto the calipers. Now here's the clincher: Starrett doesn't sell new crystals because they only come completely assembled, from China. You can substitute our #395 (see page 233) if you want.
On the Brown & Sharpe (Tesa) dial calipers, the bezel is part of the metal housing and can not be removed separately. You will pry off the metal housing by inserting a thin, flat-bladed screwdriver or other tool under the four corners and wedging it off gently, a little at a time in each corner. This will keep the plastic sockets intact (we'll get to that) and has the added benefit that the dial hand will also come off at the same time. You can then push the old crystal out from behind. New crystals will be installed as the last step of the repair.
More information on crystal insertion, crystal press, and replacement crystals can be found on page 233.
Tip #3: if there is a clicking or slight skip at regular intervals then the problem lies with the gears in the movement. If cleaning the movement does not solve the problem, then replacing the whole movement is the easiest solution.
If the clicking or the skip occurs at just one spot, chances are there is a broken or bent tooth in the rack. Fist, check to see it isn't a bit of dirt or a metal chip that has gotten stuck between the teeth. Clean the teeth of the rack with a stiff bristle toothbrush and use a thin pointy needle to get all the dirt out. Do not unscrew the rack (although this would seem to be an easy way to clean it) because doing so will destroy the hairspring tension in the movement. If the rack needs to be replaced, it's an easy matter on Mitutoyo dial calipers. You will have to remove the movement, of course, and then you can unscrew the rack (look for the small screws on the back side of the beam). Take note of the rack's orientation before screwing the new one in place.
The Starrett 12" rack consists of two separate 6" racks. Take special notice of the spot where these two join. If the movement clicks or jumps at that spot, then you have the racks too close together. Just loosen the screws on one of the racks and then slide it so that the teeth will mesh with the gears correctly. Naturally, check the calibration after such an adjustment.
Starrett dial calipers have unique, cleverly simple movements and backlash gears. Be careful when taking these apart because parts tend to come loose and fall all over the place. Keep your eye out and remember what goes where. Although deceptively simple, if you think you are "all thumbs" this repair attempt may prove it.
The Chinese-made Starrett 1202 models are actually better than the standard made-in-USA versions in some ways. The sturdier, nicely made movement is an improvement over the old calipers which probably haven't changed design since they were first invented. But, parts for the movement are not sold separately! You will have to buy the complete movement assembly, with the bezel and the crystal. This actually makes life easy assuming the movements are available. Starrett often doesn't have stock and just as often doesn't know if they will ever get stock. Tip: if you decide to take it apart, only unscrew the 3 larger philips head screws (leave the 5 smaller ones alone).
Repeatability (returning to zero when the jaws are closed) is due mainly to the spring tension in the movement. This is probably the trickiest of the tasks in a repair: getting this tension correct.
Brown & Sharpe dial calipers: remove the crystal and hand as described above or by using a hand puller. You may be able to get the crystal out with a small screw driver along the edge, but chances are that you will damage the crystal. This is something you should expect. Have a spare crystal on hand. Pry off the entire metal housing by wedging a flat blade screw driver under all four corners. Pry it slowly, a little at a time, so that you don't ruin the plastic sockets which hold it in place. Once it is off, you will see what this is about. If the sockets are damaged, you should replace them. These sockets insure that the case remains firmly attached to the caliper body, otherwise you may also notice fluctuations in readings.
Now you can remove the movement, held in place by three screws. Do not disassemble the movement. The white plastic gear will receive your focus of attention. There are two parts to this gear: the upper toothed gear and the lower plastic disk. Between them lies a small spring. Hold the lower disk steady and rotate the upper toothed gear counter clockwise until the two small holes line up. Insert a pin into the two holes so that they remain lined up. Now you can reattach the movement but don't tighten the screws just yet. You will have to engage the rack teeth with the center pinion's teeth. This is no easy task because you don't have a lot of room for error. If engaged too snuggly, the movement will be rough, or not move at all. If you back off too far, you risk losing the spring tension you just worked so hard to get right. When you finally do find the correct placement of the movement, you can tighten the screws and remove that pin from the gears. With luck, it will work but more than likely it will take a half-a-dozen tries before you get it right. This is the kind of frustration that will drive you to have another cup of coffee, if you can stand the trembling hands.
Press the metal casing back over the plastic sockets working from one corner to the next, a little at a time. We find that a vise comes in handy at this point but we'll let you figure out a convenient method on your own.
Brown & Sharpe 599-579-3 was an excellent dial caliper, the likes of which will probably never be seen again. Parts are mostly unavailable now but if you need to clean the movement be aware that it is a different process from the new models. To get at the movement you will have to remove the crystal, usually by destroying it. After you lift off the hand (with a hand lifter - an absolutely necessary tool) you can remove the spacer, the wavy washer and the dial. This reveals three screws which hold the movement cover in place and you will want to remove these to get at the gears.
Early Mitutoyo models such as 505-626-50 had a very simple movement without any backlash gears or springs. It kept remarkably close tolerances and good repeatability and offered very simple repairs. Given enough of a shock, the gear teeth could easily jump which put your dial hand out of the 12 o'clock position but that's why Mitutoyo offered the simple method of resetting the hand with a tool that was enclosed with the original calipers. Fortunately the teeth were almost never damaged. Only one gear and one gear clip made up the movement and both parts are replaceable. Even easier is the replacement of the entire movement assembly. Just two screws hold it in place. Give the gears just enough clearance so that everything moves smoothly and you are all set.
Replacing the dial hand on Mitutoyo calipers
The hand (pointer) for many older Mitutoyo calipers has been replaced with a new style. Unfortunately these hands have very small holes and the pinion (usually 0.7 mm in diameter) will probably not fit unless you ream the hole carefully to just the right size. Very small reamers are needed for this purpose. The hand should press-fit in place using a staking tool. Trying to force it onto the pinion will probably cause damage.