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ptjw7uk
01-29-2008, 02:12 PM
Hi All
Had a bad day in the shop . . .
It all started so well, machining another ball screw end end fittings when I dont know why I checked but my unknown make digital vernier just seemed wrong checked it with a ordinary vernier and it was reading low. So got the trusty Moore and Wright metric micrometer out and looked at it and at zero it was showing 0.02mm ah I thought a little tweak and all should be well. Welll - no the more I adjusted it the further the error got put it back in its case - need to read up on setting them up.
Back to the digital vernier checked it against a 8mm ground silver steel rod and it reads 7.9mm at a loss as to what could be wrong I ended up with my mitutoyo 12" vernier ok to use but a bit on the long side for use on the lathe.
So anyone know why a digital verier should go belly up, I think I may check the battery it was replaced not long ago but some of these batteries are suspect.
Also anywhere on the web on the sequence to setting a micrometer.
Peter

Evan
01-29-2008, 03:27 PM
Is there a particular reason that the tool steel rod should be exactly 8mm? 0.1mm isn't much and well within the tolerance expected for tool steel stock. That's only .004" in imperial measure.

ptjw7uk
01-29-2008, 03:46 PM
AS the silver steel is ground rod I always assumed it to be accurate and when measured against drills end pushing into 8mm id bearings it is a good 8mm diameter.
Although you have agood point in that I have never considered the tolerances of ground stock and in the light of my poor machining knowledge I always assumed it to be correct.
That reminds me I also have some bearings I bought with supposed 8mm bore that are a very sloppy fit I am unable to measure their bore as I do not have anything to measure them with as vernier calipers are a bit off for true diameter measuring, just another seldom used tool to acquire.
Peter

Allan Waterfall
01-29-2008, 04:23 PM
I just put my vernier on a piece of 8mm silver steel and it measure 7.99mm.
O.1mm would be well out of tolerance for ground silver steel.

Have you wiped the jaws of your vernier,closed it and then zeroed it ? Cheap digital verniers seem to need zeroing everytime you switch them on.

Allan

SGW
01-29-2008, 04:35 PM
Here's how to adjust a Starrett micrometer: http://catalog.starrett.com/catalog/images/objects/2900/2856.pdf

Google "How to adjust a micrometer" for more links.

JCHannum
01-29-2008, 05:36 PM
Not to seem picky, but a vernier is not a measuring instrument. I assume you are referring to a slide caliper that is digital reading. It is doubtful that it would read incorrectly in a small increment, they usually go completely dead, read some error code or the reading bounces around.

Drill rod is not a particularly good standard, and will vary some amount but 0.1mm is excessive. A tolerance of +- 0.013mm is typical for that size range.

There is no need to zero a 0-1" or 0-25mm micrometer for an accurate reading, make sure the anvil and spindle are clean, turn the spindle in until it stops, note the reading. If it is not 0, note the reading, take your measurement and add or subtract that reading as appropriate.

pcarpenter
01-29-2008, 05:49 PM
If by vernier you meant caliper, then the digital variety needs to be zeroed at its closed position every time its used to insure its accuracy. I find that sometimes its most accurate to re-zero it after its been on for a few minutes and come up to temp.

A digital caliper is by definition, not a vernier caliper. Rather than having a sliding set of scales of vernier design, it has an electronic position reader (by capacitance or mechanism) and a digital display of the length value. In short, vernier describes the mechanism used to indicate the measured value.

Here's a blurb about Pierre Vernier: http://en.wikipedia.org/wiki/Pierre_Vernier

I too have found that usually when my battery goes low, the display bounces from reading to reading. When it measures consistently off, its because it needs to be zeroed...something that should be the first thing you do before using it, as standard practice.

paul

Evan
01-30-2008, 12:08 AM
Heh. Electronic calipers actually use the vernier principle internally and so are digital readout vernier calipers. They contain two grids of slightly different spacing that produce a difference signal that is counted, much the same as the visual non electronic variety. In fact, in principle an electronic optical counter could be fitted to regular mechanical vernier calipers to provide a digital readout.

gmatov
01-30-2008, 03:09 AM
Of course, HM that I am now, the cheap ones I use do not "bounce around" when the batts get low. They either tell you the batt is dead or do not light up.

Whether the "good" ones give erratic readings with low batt, I don't know.

Paranoid that I am, I still like my micrometers for true measurements. That 7 inch or so "handle" on digital calipers seems to, along with the narrow jaws, give me a little insecurity as to whether I am getting a true dimension.

I like them for getting "close", but to finish, I generally grab the mics.

Cheers,

George

huntinguy
01-30-2008, 05:14 AM
I have found that when grinding graphite digital calipers seem to get lost. I would imagine that other materials will do the same to them.

ptjw7uk
01-30-2008, 06:07 AM
Hi All
Thanks for the communications - I have just looked up the specs on Silver steel http://www.silver-steel.co.uk/SilverSteelBS1407.html
and for 8mm it should be between 7.085 and 8.015mm diameter so 7.9 is way off and as the digital calipers show the same on an 8mm drill I think the calipers are shot or the battery....

Peter

Evan
01-30-2008, 06:57 AM
The specs for tool steel (silver steel) are always plus zero, minus something. The quoted specs for the less than 25mm size is "+0/-0.015mm". 7.9 is indeed well outside of that range. However, before you toss the calipers make sure they are closing all the way. A slight burr or other obstruction can prevent the jaws from closing totally but not interfere when measuring a piece of round stock.

John Stevenson
01-30-2008, 06:59 AM
Hi All
I have just looked up the specs on Silver steel
and for 8mm it should be between 7.085 and 8.015mm diameter

Peter

Change your supplier if it's that far out :D

.

Your Old Dog
01-30-2008, 08:22 AM
This is what many think of when talking about a vernior caliper, note the extra scale.

http://toolandsupply.ca/images/Precision_Measuring/yf-6v1.jpg

ptjw7uk
01-30-2008, 12:48 PM
Had a go at the micrometer and in the cold light of day it is now ok.
The digital guessing stick is a no no, tried a new battery with the same result.
Zero the calipers in the fully closed state and then measure 8mm silver steel it still reads 7.9, I then tried 6mm silver steel and it reads 5.9 it is as if something is wrong inside it. Thinking back on it I cannot sau for sure that I have ever tested it.
So consigned to the back of a drawer and then probably the bin as it cannot even be used as a distance measure on the drilling machine as the 0.1 error would appear to be constant!!

Peter

pcarpenter
01-30-2008, 01:10 PM
You may find a cheap set of gauge blocks to be a useful tool anyway. I sure wouldn't use presumed dimensions of raw materials as any sort of reference for my measuring tools.

Another alternative is to measure the standard that would (or should) have been included with your micrometer and see
what it reads.

Edit-- I am not saying it couldn't be the fault of your inexpensive calipers, but don't throw them away. BTW-- do you figure on using a drill to drill to more than .1mm precision? I have a very heavy drill press and I don't figure its that precise at best.

Paul

Doozer
01-30-2008, 01:23 PM
Here is what may be going on. Drill rod is centerless ground. Centerless grinding can develop a "chatter" condition, making the rods not round, but tri-lobal. (3 sided). So measuring it with a caliper or mic with 2 points it will seem to be at size, but yet will not fit a ring gage or tight bore such as a bearing.

--Doozer

JCHannum
01-30-2008, 03:19 PM
Drill rod is ground, and it is manufactured to a +- dimension as any product is. This is a link to the Starrett catalog with tolerances for their product. Other manufacturers are in the same range.

http://catalog.starrett.com/catalog/catalog/groupf.asp?GrpTab=Feature&GroupID=695

Use of drill rod as a standard is not a good idea as the actual dimension is not known. Comparing the measurement of two instruments to each other simply reveals they do not agree, not which one is correct. Your micrometer is probably the more accurate of the two, but no real conclusion can be drawn unless they are compared against a known standard.

I would doubt the electronics of the caliper are in error, but it is possible the jaws are not parallel. Hold the closed caliper up to a light source and see if they close to dark out at 0.