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
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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