Cheap digital callipers - observations.

I have cheap (~ £25) digital callipers which I thought pretty much OK. When I bought them I checked them out and even wrote a positive review on Amazon claiming that they were within spec (+/- 0.02mm). I can't remember how exactly I tested, but likely it was on parallel faced objects which I could either measure more accurately by other means or had certified dimensions.

I've just turned some nominally 17 x 8 mm brass discs which I wanted to be +0/-0.01mm on diameter, and according to my Sunday best Mitutoyo mic they were OK - 16.995 to 16.998 mm. But imagine my horror when I idly applied the callipers - 16.78 - 16.84 mm! Way out of spec!

I checked the callipers on the three Jo-blocks I have - 10, 20 and 25mmm so 10, 20, 25, 30, 35, 45, 55mm. The callipers consistently measured 0.01 or 0.02mm over size. Which is fine. The micrometer was at worst 0.002 mm out on the 10, 20, 25mm blocks.

I can only think that the discrepancies are due to the calliper jaws 'racking' when measuring a round object beause the jaws are in point contact with the object. Maybe that's the reason why Mitutoyo callipers are so expensive? More to do with the mechanical engineering than the measuring technology, which seems common to all these devices - even expensive Mitutoyo callipers quote +/- 0.02mm?

I'd be interested to hear opinions, especially from users of high-end callipers. Is it really worth the investment? And if so why?

Robin.

Edited By Robin Graham on 12/08/2023 23:41:54

Edited By Robin Graham on 12/08/2023 23:44:24

Edited By Robin Graham on 12/08/2023 23:45:29

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

That's odd, if there is contamination on the jaws when zero'd and clean when measuring something, I would have thought they would read undersize, not over, but I'm sure you clean them before use.

I've a variety of digital calipers, from Lidl & Aldi which I use for general rough measurements as it's not a major problem if I drop them; I've have never found them to be just a little bit out like that.
Either they broadly read the same measurement as my Mitutoyo, or Insize ones, or they fail completely and give a bizarre reading, though sometimes one that fools me.
One set suddenly started counting 10-11-12-13-13-15-16 etc, which had me foxed for a while.

With the cheap ones, I do find I need to keep the gib strip quite tightly adjusted to prevent the sliding jaw cockling a bit, since that also leads to under reading, I wonder if that's what's happening.
When you close them to zero, the jaws fully support each other, but when you then use them on a round bar, the moving jaw slants a bit.

If I need to use a caliper, as opposed to a micrometer, for a critical reading, I either use the Mitutoyo or Insize digital ones, a conventional vernier, or a dial caliper.
With either of the mechanical ones, I also pre-check with a gauge block(s) of about the expected final reading.

Bill

Edited By peak4 on 13/08/2023 01:31:18

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.

This old thread might be worth revisiting: **LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=122223&p=2

MichaelG.

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

I have never liked digital callipers. I always find that they give inconsistent readings for me. Change where the pressure on the jaws is, change how much pressure you apply with the thumb wheel, accidently reset the zero and not notice. Too many ways for them to give odd readings. Never have that problem with my vernier callipers with thumb wheel screw adjustment and they are always within tolerance when cross checked with a micrometer or with gauge blocks.

Martin C

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

I use Mitutoyo calipers - mainly because as you say, they are well-engineered - they perform (and perform consistently) for years, use hardly any batteries, rarely need adjustment, and will stand being dropped occasionally.

The one I have now has been my sole workshop caliper for maybe 5 years, can't remember if I've ever changed the battery (?maybe?), I've tweaked the gib screws once and reset the origin once (after dropping it). I'm in the workshop at least once a week, was more over 2020 / 21. It's a farm workshop.

Original purchase price was £75, I expect to get at least another 5 years out of it (dependent on local gravitational anomalies), so hopefully will cost ~7 per year for an excellent caliper offering reliable and trustworthy measurement.

Have you tried adjusting the moving jaw gib strip ?

Under the top edge of the sliding jaw is a copper or brass gib strip that is adjusted with 2 very small slot head grub screws that require a small jewellers screwdriver to adjust. Lightly snugging the screws will lock the sliding jaw, then very small backing off adjustments will provide a light, shake free, drag.

When my former collegues brought me digital calipers (inexpensive Insize brand) that were "misreading", a good clean, lightl stoneing off any nicks or bumps and re-adjusting the gib screws almost always brought them back to normal levels of accuracy & repeatability.

Nigel B.

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

Soime time ago, MEW reviwed a number of digital callipers.

Some of the cheapies had shorter batteryb lives because they kept the memory alive when switched off

The LIDL and ALDI ones fared pretty well, and as esxpected the £80 Mitytoyo came out best.

Apparently, there been instances of fake Mitutoyos being sold, which, unsurprisingly, do not live up to the reputation of the genuine article!

What seemed to be the best compromise between price and overall performance appeared to be the M&W one sold by Machine DRO. (About £24 at the time)

My old and now battered (Thumbwheel missing, battery cover retained by an O ring ) LIDL one is still the one in everyday use.

But the jaws are wiped and the Zero checked and reset almost everytime. II is reasonably, but not ABSOLUTELY repeatable, needs to used with care, and an overcheck with a micrometer or bore set., for final precision.

As so often, you get what you pay for.

Howard

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.

I have a used DIAL Caliper that has seen a lot of industrial usage by non-Machinists. It is Mitutoyo. However, when measuring anything, I always "close" the Jaws to the Workpiece with my Thumb & Index Finger on the Jaws - I "squeeze" the Jaws across the Diameter being measured, or across parallel surfaces being measured. I find that gives good Repeatability when cross-checked against Gauge Blocks.

That being said, I always use a Micrometer if it can be used and I have the size.

Thanks for replies. It's not dust, grease or whatever on calliper jaws, micrometer anvils or gauge block faces - all scrupulously clean. Measurements are repeatable to within 0.001mm using the micrometer and 0.01mm using the callipers.

SoD makes good points about technique - however, even putting insane pressure on the calliper thumbwheel I can't shift the reading more than 0.02 mm.

I didn't know that the gib was adjustable - thanks Nigel. Having adjusted it the discrepancy is down to 0.1 mm, the callipers measuring undersize. Still a lot though. I have a Mitutoyo vernier (ie not digital!) calliper in a drawer somewhere. Not used it in a while (failing eyesight) but I remember it having that sort of silky smooth feel characteristic of quality instruments. I'll dig it out and see what it says.

My reply: Not at all! The reason I posted was that I wondered why both my micrometer and callipers read within spec (+/- 0.002mm for the mic, +/- 0.02mm for the callipers) when measuring parallel faced objects (gauge blocks) but differed when measuring round objects of similar size. Of course the mic is more reliable - measuring a 1/2 inch edge finder (round obviously) with the mic gives 12.698mm, the callipers give 12.58.

I realise that they don't really matter in the great scheme of things, but little problems like this bug me!

Robin