How do you read a dial vernier caliper?

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mlteenie
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How do you read a dial vernier caliper?

Postby mlteenie » 28 Feb 2007, 9:43pm

I recently bought a used Draper dial caliper (PDV150) from eBay and I am wondering if I am reading it correctly. The reason I ask is because I was attempting to measure a seatpost diameter and the dial didn't seem to be giving me the reading it was supposed to.
I presume that one must calibrate the dial to zero with the jaws fully closed, yet when I do this the known seatpost diameter measurement does not match. The dial reads in 0.02mm. Can anyone help?

Thanks.

Russell
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Location: Farnborough

Postby Russell » 1 Mar 2007, 9:10am

I would be surprised if it does read down to 0.02mm, the one I have in front of me has minimum calibration marks of 0.1mm. That is, a full rotation of the dial is 10mm which is divided into 100 graduation marks (each one being 0.1mm). My dial is about 32mm diameter, so is your dial 5 times bigger than that (to give 5x greater accuracy)?

thirdcrank
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Postby thirdcrank » 1 Mar 2007, 3:49pm

mlteenie

I hate to see a query disappear unanswered and if I do not know the answer myself, I like to think by keeping something like this at the top, it may smoke out somebody who knows the answer. (I fancy jb may be the man here but he has been absent recently.)

My son is an apprentice trained fitter but he is working in Holland and all his kit of this type is in my garage. Based on what I have got from him by the jungle drums, I have found this (I quote from Wikepedia):-

Calibrated calipers may be mishandled, leading to loss of zero. When a calipers' jaws are fully closed, it should of course indicate zero. If it does not, it must be recalibrated or discarded. It might seem that a vernier caliper cannot get out of calibration but a drop or knock can be enough. Sometimes a careful tap is enough to restore zero.

THe following link includes a description of what a properly zeroed vernier scale should look like:-

http://phoenix.phys.clemson.edu/labs/cupol/vernier/index.html

I hope it is not a duff link. I hope yours are not duff callipers.

Russell
Posts: 184
Joined: 13 Jan 2007, 2:42pm
Location: Farnborough

Postby Russell » 1 Mar 2007, 4:12pm

Third

Your link refers to a vernier caliper, not a dial guage vernier, which as the name suggests has a dial for the minor units. The major units, which in the case of mine are 10mm, are read off as if it was a rule, the minor units, mms and tenths of mms are read off the dial. Therefore 1 rotation of the dial equals 10mm.

For mlteenie's to read 0.02mm the major units must be 1mm, therefore a single rotation of the dial must equal 1mm. This is then subdivided into tenths and hundreths of a mm.

Russell

Bob H

Postby Bob H » 1 Mar 2007, 4:27pm

I agree with Russel's comments ...........THIRD.

However my dial calliper reads in gradations of 0.02mm.

Most dial gauges rely on a finely engineered rack and pinion to convert the directional movement of the jaws/probe to give a rotary dial read out. By nature they are therefore very delicate. I suspect that yours has suffered some damage, which is unfortunately likely to be beyond economical repair.

Ebay relies on good will and condition description honesty on the part of its sellers and buyers. First off I would say you have been sold a pup; go back to the seller.

thirdcrank
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Postby thirdcrank » 1 Mar 2007, 7:40pm

mlteenie

Sorry it sounds like bad news, but al least I managed to interest somebody who knows what they are talking about.

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robbo
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Postby robbo » 2 Mar 2007, 10:12am

As a time served turner I wouldn't trust a dial caliper to .02 mm unless it was a reliable make (for instance Starrett, the 150mm is about £100) and had been properly calibrated which again is expensive, and an inaccurate sliding or rack caliper is notoriously difficult (read impossible) to correct. I always look on this sort of caliper as a second choice if a "G clamp" mike. is not available.

mlteenie
Posts: 327
Joined: 31 Jan 2007, 11:19pm
Location: London

Postby mlteenie » 2 Mar 2007, 11:56pm

OK - thanks for that. Not convinced it's knackered yet....

I dunno, looking at it I may be a little thick but I reckon it has to do with the fact that it reads in increments of 0.02 on the dial. ie. one lap of the dial appears to correspond to 2mm on the large scale (100 units on the dial).

This seems a bit weird - at the risk of sounding a little Spinal Tap, why not make the dial correspond to half that amount and make THAT 1mm! Surely it is a little unnecessary, not to mention potentially fraught with error to be dividing your valuable scientific measuring answers by two all the time?

What say you?

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archy sturmer
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Location: St Albans, Herts

Postby archy sturmer » 4 Mar 2007, 12:51pm

I see you can get guages with digital read outs very cheap on ebay. Does anyone lnow if these are any good? I want to be able to accurately measure, for instance, ball bearings, seat posts etc.
_Archy

thirdcrank
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Postby thirdcrank » 4 Mar 2007, 1:39pm

I have no idea about the gauge but I think you are on dangerous ground measuring ball bearings. The quality of ball bearing is largely decided by the accuracy of their sorting, but that exact similarity of size is only applied within a batch, not between them; mixing between batches, even of the highest quality negates the whole sorting process. These differences are not routinely measurable with the standard of calibration available on a hand held gauge.

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archy sturmer
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Postby archy sturmer » 5 Mar 2007, 8:38pm

Thanks TC -
I wasn't planning to compare ball bearings. I just want to be able to measure the (nominal) size of bb's so I can replace them with the right size when rebuilding a cup and cone bearing. I suppose there must be some way of doing this...
- Archy

thirdcrank
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Postby thirdcrank » 5 Mar 2007, 9:28pm

Sorry I misunderstood.

I still cannot help about what is best to buy to measure anything. I suppose when I buy ball bearings - which seems to be rare these days with sealed bearings etc, I usually know what size fits and just buy that size. Any doubt and a decent bike shop will know what you need and stock them. It was only relatively recently that I learnt the importance of not mixing ball bearings from different batches.

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andrew_s
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Postby andrew_s » 5 Mar 2007, 10:33pm

I've got a Draper dial caliper marked down to 0.02mm scale divisions, and the division spacing is approx 1mm so guesstimating to 0.005 isn't too difficult (2mm per dial revolution).
May be the same as mlteenie's - no model number visible, came in blue tin case with foam lining.

I've had no questions about the accuracy of mine.

By way of a quick check, the 27.2m mm seatposts on my 2 bikes both came in at 27.15mm, which is good enough to tell me that they are indeed 27.2 seatposts and not 27.0.


Take care to wipe the jaws clean before checking the zero adjustment. Unclamp the dial and position zero under the pointer to adjust. I don't remember adjusting mine any time in the last 4 or 5 years, and it's OK now.

Maybe there's a bit of a burr on the edge of the jaws?
You could be setting zero to the size of the burr, but measuring on a clean part of the jaw.
Can you see any daylight between the jaws when it's at zero?

mlteenie
Posts: 327
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Location: London

Postby mlteenie » 7 Mar 2007, 11:20am

hi andrew_s,

My draper gauge has the name engraved on the end of the handle.
Thanks for your entry - I think I now have a good idea of how to measure. I am probably being too literal with the measurements of things. I am not used to such precision measurements. The tool seems to be free of visible burrs.

Thanks to everyone for your help!

noel JJ

Postby noel JJ » 21 Mar 2007, 4:08pm

just a thought .02mm is about .0008 of one inch or one quarter the thickness of ahuman hair . A mote of dust between the jaws is suficient to cause your zeroing discrepancy . Very acurate for vernier calipers