Dirt on the face of the jaws or on the part being measured would be my first suspicion. I always try to remember to wipe the jaw faces and close to check true zero before making most measurements, especially critical ones. Another cause could be that you inadvertently ZEROED the display when using the callipers or ZEROED them with some contamination between the faces. It's easy to, don't ask how I know.

HTH

Martin

PS

I don't have expensive callipers so all mine are relatively low priced units. If critical then final measurements are made using a micrometer.

Edited By Martin W on 13/08/2023 00:43:07

Some time ago I went to a local agri shop to buy a caliper. The Geodore one repeated to zero out of the box, while the other two of same manufacturer did not repeat even if I wipe them with fingers, I showed to store owner and biought the Geodore one at double the price which was still not the price of a good quality one like Mitutyo. Its still ok but sometimes in use I have to wipe it.

My comment; You use calipers, not a micrometer, to take precie, accurate measurements?

I tend to find that if I use a caliper 10 times on an object, I can get slightly different readings probably 8 x. Particularly when leaning over the lathe often at an arkward angle. Slightest difference on finger pressure closing the jaws, slight 'rocking' of the caliper from right angles to the work, narrowness of the jaws etc all contribute. OK so many may fairly say to improve my technique – I have tried, but with little success.

Put a mic on the job and its done, consistently repeatable and accurate.

I use the callipers for handy, quick and easy readings & they are excellent – but when it comes down to getting measurements spot on its the micrometer for me every time.

regards

That's what you should get! Although digital calipers have a resolution of 0.01mm, they are only accurate to ±0.02mm. Even the expensive models!

Assuming it's physically in good order – gibs adjusted, no wear or damage etc, the fundamental accuracy of a digital caliper depends on the granularity of the track and the spacing of the sensor, These are easy to ensure during manufacture.

In action, much depends on the operator. First he must ensure the battery is OK, the correct buttons pressed, and the instrument is clean, especially the jaws. Second, he must align the jaws correctly with the object being measured. Alignment is easier said than done especially on curved items, where a rocking motion is needed. Don't forget round items may not be circular and/or could be tapered. Thirdly, the reading depends on thumb pressure, which is an acquired skill requiring a lot of practice and is still easy to get wrong on the job.

With my clumsy paws it's hard to apply thumb pressure consistently; you may be more talented! To get the feel, I practised with a set of precision ground parallels until the caliper always agreed with each parallel's nominal size. This way of learning is a bit suspicious because though my parallels are parallel, they're only specified within ±0.02mm. The feel I've developed could be 'off' because my caliper and training parallels are both up to ±0.02mm 'out'.

Test how good your feel really is! You need a mixed set of precision sized blocks and cylinders, a blindfold, and an honest assistant to read the display. The assistant hands blocks out randomly, you measure them by feel only, and, without comment, the assistant writes down what's on the display. After 15 minutes take off the blindfold and look at the written record. The readings should be slightly off as Mike describes.

In my experience accuracy isn't the main difference between expensive and cheap calipers. Rather, the expensive instruments are smoother (making it easier to apply consistent thumb pressure), and presumably better-made: less likely that the jaws move sideways; more resistant to wear; and sealed to keep moisture and dirt out of the electronics. The feel is better and results more trustworthy, making it less necessary to take repeat readings, and the batteries last longer!

When accuracy matters, switch to a micrometer. For best results micrometers also require a 'feel' to be practised, but the ratchet gets much better results than a caliper. Round stock is still harder to measure accurately than square.

Dave

Just want to add something now that I thought about it. My 1st no name digital calipers, I mounted to the mill to measure quil up/down movements. Now here I found it to be very nicely repeatable. THe difference here is that it just slide up and down connected to quil and mill head, there are no forces on it as if when you measure the outside or inside diameters of something, so maybe how it behaves when you clamp it for a reading is where the cheap ones fails the most noticeable.
With this one if I reposition the quil to zero mark as example the cutter hardly skims anything, so it seems to work ok using it this way. How its mounted the backlash does not matter.