Caliper Calibration





Introduction: Caliper Calibration

Calipers can need calibrated to maintained accuracy. The main cause of calipers becoming uncalibrated are chips getting in the rack and causing the pinion to skip teeth on the rack. This can be seen if the dial is not returning to zero when closed and the dial will also vary about 0-5 thousands. This is due to the spring on one of the pinions not having tension. Having no tension will give the dial the play and not give accurate readings.The dial could also be reading accurately, but not be directly up.

Step 1: Tools for Recalibration

-small precision screw driver set

-small pin

Step 2: Remove the Crystal

First step is to remove the crystal from the caliper to allow access to the dial. This can be difficult to not scratch or crack the crystal. It may be advisable to purchase a replacement as they are difficult to remove without damaging. After removing the crystal remove the dial by carefully using two small flat screwdrivers. The screwdrivers will be placed under the dial and used to carefully lift the dial from the shaft. The dial is press fit onto the shaft and requires a small amount of force to remove.

Step 3: Remove the Housing

The housing can be lifted with flat bladed screw driver near the left
and right of the housing. Apply pressure lightly and work around the housing until it comes off. Posts underneath the housing hold the cover in place so displace evenly. This will expose the caliper movement.

Step 4: Remove the Rack for Cleaning

At the end of the calipers is a plastic stop that can be removed. This will allow one jaw of the caliper to be removed. after removing the jaw turn the caliper over and undo four small screws attaching the rack using micro precision screw driver. Carefully slide the rack out. a brass sliding plate may also be removed from the with the jaw if it comes loose. Unfasten the movement screws and pull the movement from the assembly and then clean the rack and remove any chips. Replace the rack and fasten with the four screws. The brass sliding plate can be put back and the removable jaw can be put back in.

Step 5: Removing the Play in the Gears

The movement has two pinion gears that engage on the rack. One pinion has a spring that takes the slack out gears to give accurate measurement. The movement has a green gear on the front and a white disk on the back. These both have holes that need to be aligned using a pin. The pin will need to be held in place and then the movement will be placed back into the rack

Step 6: Tensioning the Spring

The most difficult part of the re calibration is getting the right amount of tension. The tension is adjusted by how far the green gear and white disk are rotated when both pinions are engaged. moving the green gear and white disk counter clockwise increases the tension. Not having enough tension leaves too much play in measurements taken. Too much tension will not allow the gears to rotate correctly and will be either very stiff or will bind. This will be more apparent once the housing, dial face and dial are re-attached.

Step 7: Re Assemble

Put the caliper back together by placing the movement back so it engages with both pinions. fasten three screws to hold the movement in place. The housing can then be placed back over the movement. Once this is done you can replace the dial face and carefully press the dial back onto the shaft. Now check that the caliper is measuring correctly and has the right amount of tension. If it does not remove remove all parts to get to the movement and re adjust the tension and try again. If you have the correct tension place the crystal lens back in place and your done.

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    8 Discussions

    I worked with an older mechanic who used a standard caliper without the dial. He accurately and easily read to one thousandth of an inch and in metric as well. He had done it for years and was second nature to him. I can do it as well but it takes a bit longer and I have to think about it as I do not have to use calipers very often. I kind of like learning old school tricks and ways. We are so reliant on technology. I do have to admit that reading a dial caliper is much easier.

    3 replies

    The ones with no dial or digital readout a Vernier Calipers. Some dial calipers used to also havd a vernier scale on them, IIRC.

    I find it really annoying when companies advertise 'Vernier Caliper' for sale but it doesn't have a Vernier scale - i.e. it's either a dial or digital caliper.

    'Point it out to them and they reply 'Everyone calls them Verniers', grrr!

    I agree - Vernier calipers work like a slide rule, and it takes
    experience to read them well. I like dial calipers the best - the
    digital ones give a false impression of accuracy (Yes, I know the
    difference between precision and accuracy - which most people don't)

    check the crystal size carefully...

    instead of ordering a replacement when I cracked my last one (3rd party knocked it over, onto the lathe bed, crystal first), for about half the price, i got a whole new harbor freight special that used the exact same size dial.

    best part of that bargain is, i also now have a disposable, fairly accurate, dial caliper to use on the drill press, loan out, etc.

    1 reply

    I had not considered getting a crystal from a another caliper for a replacement. That is a good idea. Thanks

    Be aware that some dial calipers have a knurled knob that, when loosened, allows the entire dial to rotate. Close the caliper, loosen the knob, rotate the dial so that the pointer is on zero. Tighten the knob. Of course, as you point out, this leaves the zero in a position other than 12 o'clock...

    1 reply

    This is exactly what mine was doing. It was not at 12 o'clock when zeroed and would stop about 3 inches open. knew it was time to pull it apart and fit it.