Thread: Metrology for Amateurs

Here a few pics. All of the same component, from an Aldi "Workforce" caliper (not made in Germany... your guess is as good as mine....)
On this caliper the depth gauge broke off, If you look at the spot weld you know why. Also note how the top beam guide is made. On other similar price calipers, they use bronze strips between the beam and the circuit board. It is an extreme case of inferior quality - I have not seen any worse digital caliper than the ones sold by Aldi. Beware. Chris


Workforce_Aldi_Caliper1.jpgWorkforce_Aldi_Caliper2.jpgWorkforce_Aldi_Caliper3.jpgWorkforce_Aldi_Caliper4.jpgWorkforce_Aldi_Caliper5.jpgWorkforce_Aldi_Caliper6.jpgWorkforce_Aldi_Caliper7.jpgWorkforce_Aldi_Caliper8.jpg

Yeah that's BAD! Apparently the earlier ones were better, so based on the recommendations of the forum, next time they came up I bought 3, with the intention of chopping them up and mounting them on things like my drill press, and one I'd keep primarily as a marking gauge. I even asked them to keep 3 aside especially for me. What a disappointment they are, I think the same as yours. 2 remain in their plastic, the one I did open was disgustingly built and the reading skips all over the place so is completely useless. You reminded me Chris to open up the others, salvage the battery from them, then introduce hammer to caliper and see who wins.

Pete

Originally Posted by Stustoys

Just a note for any lurkers who may not fully understand the "Marco" comments and may think I am poking fun at Marco......I'm in fact poking fun at myself for measuring to 1/20th of what some consider 2/10ths of SFA in the first place.

I generally put faith in what seasoned experienced people say..... Hence now why I run away screaming whenever I am offered a mars bar and a can of coke...

Originally Posted by .RC.

I generally put faith in what seasoned experienced people say..... Hence now why I run away screaming whenever I am offered a mars bar and a can of coke...

RC, I think you might mean it's a waste of time considering less than 5 microns. Anyway that's a view that comes up whenever metrology is discussed. To be honest I'm a bit tired of hearing it. I'm not claiming I can make anything to a micron, but the discussion was about calibrating instruments, and for that you're supposed to have ten times the resolution in question. I agree it's wise to listen to those more experienced, but I feel sure that Marko's position relates to making stuff, not calibration. Willing to be corrected on that.

PS: In particular the question was about testing second hand instruments to see if they're any good. I may not have been clear about that. Not that I'm against tangential discussion, as long as it doesn't turn septic.

Nah I was just saying use whatever decimal point you think you need... But be aware of the pitfalls when dealing with such fine measurements and calibration... My Tesa 0.001mm dial indicator has the calibration sheet with it... The accuracy of the device changes over the range of travel..

And using something to measure is different to using something to compare.... Repeatibility can be more important then accuracy.... Most of my measuring is comparing one part to another... Then using loctite....

And beware of people offering mars bars and cans of coke...

Originally Posted by .RC.

Hence now why I run away screaming whenever I am offered a mars bar and a can of coke...

Originally Posted by .RC.

And beware of people offering mars bars and cans of coke...

I thought it was a chocolate covered pretzel you should run from

I like your setup Bryan. I'll try and set it up one day soon and check my dials. I just bought a .0005" Tesatast for $20, add it to the list to check....

Cheers,
Ew

Originally Posted by Ueee

I thought it was a chocolate covered pretzel you should run from

Hi Ewan,

No, RC is referring to one of Marco's jokes.... I'll PM you rather than repeat it here.... then you can explain to me the chocolate covered pretzel?

Regards
Ray

Nice one....PM sent for the others it has to do with a movie called "Mallrats" in which a pretzel gets the Marco "what is 5 micron" treatment.

In an attempt to bring the thread back to the original topic, my question is... when do you need absolute accuracy from a dial test indicator?

I'd rate hysteresis and smoothness of operation ahead of accuracy. Most of the time it's relative measurements, checking runout or comparing nearly identical parts.

In a production sense it's a go or no-go indication, so you set limits, you generally don't care exactly where it is in the range as long as it's somewhere near the middle?

For a dial gage, I can see the need for accuracy, (positioning a cross slide etc... ) but not so much for a DTI... or have I got it all wrong?

Regards
Ray

G'Day fellas,
A quick question for those of you that have the gear and ability to test such things, how accurate are Mitutoyo micrometer standards ? , just wonderin.
Regards,
Martin

It is usually written on them isn't it?

It is on the ones I have...

Originally Posted by RayG

In an attempt to bring the thread back to the original topic, my question is... when do you need absolute accuracy from a dial test indicator?

For a dial gage, I can see the need for accuracy, (positioning a cross slide etc... ) but not so much for a DTI... or have I got it all wrong?

Regards
Ray

I agree Ray - the range of a typical DTI is usually too small to do anything but compare. Mine mainly gets used for tramming and centreing. For DTI precision and repeatability are far more important

A couple of non standard reliant tests

• Close your vernier type calipers. If you hold them up to the light there shouldn't be any coming through - reveals wear and non-squareness.
• Close the calipers on a piece of paper. A bit like using paper to check that something is even in a vice, the paper should not pull out more easily in one place or the other.

Michael

Originally Posted by RayG

In an attempt to bring the thread back to the original topic, my question is... when do you need absolute accuracy from a dial test indicator?

I'm all for bringing this thread back on topic. After all I've heard that joke 27 times.

I hear it time and time again, especially on P.M , always from American's, that a Dial test indicator, doesn't need to be accurate over its travel, as its only a comparative instrument. I.e you are only interested in zero - zero. Whilst that's true for dialling in a 4 jaw, or indicating a bore.

The second letter of D.T.I means Test. For testing things with a specification, you would hope that your DTI has an accuracy with-in a range of travel. That can be as simple as testing the run out on a disc brake. That's nothing you can adjust, it just needs a measurement. They may have a spec like +/- 0.05mm T.I.R. You most certainly need to know that your instrument is accurate at that range.

This if for things that are production manufactured. Journals on crankshafts will have a tolerance for run out between centres. Acceptance or rejection depends on the accuracy of the indicator.

I'm sure it was Nick Muellar, that once linked a Din Standard for Dial test indicators. It most certainly had an accuracy per travel specification.

Whilst I hear notable American metrology experts like the Rozen over at P.M discount it. Didn't one of the famous American firm's? They had a particular style of indicator point. That was a tear drop shape. Only reason they ever did that was to compensate cosine error's over the travel of the gauge. I'm sure it's on Long Island Indicators site. They would have never don that if it wasn't meant to measure over a range. A simple sphere / ball would suffice other wise.

You blokes will remind me.

Bearing spacer's. I need to get them better than two microns flat, parallel & too size. (The old tenth rule) Grasp / horror. I'll often grab a set of digital calipers, an instrument way past that 10:1 thing. Say I measure 11.8 mm with the calipers. I'll get the 10.00 slip and the 1.80 slip gauge for 11.80. Then I'll zero a DTI to that stack. I'll add / subtract what ever the DTI tells me, with in about 50 microns, using a 2 micron D.T.I. If it's more than that I might change the 1.8 slip for a 1.7 or 1.9

Any time I double check by pulling out the 0.0x and 0.00x slips. I'm normally within the same division on the D.T.I. Accumulated errors with using 4 slips instead of 2, account for some of that.

So I'm in the camp that a D.T.I is actually a measuring device. It has that nasty cosine thingy. But if you are aware of that and keep the stylus mostly flat / horizontal, (Unless you have one of those 12 degree thingy's). You can almost take its readings to the bank.

Dr. Schlesinger specified a lot of things. Every machine tool builder the world over, I've ever seen, uses D.T.I's over conventional indicators. You will often see a spec for 0.02 or 0.05 per metre.

Regards Phil.

Hi Ray,

I agree with the "jist" but

Originally Posted by RayG

In a production sense it's a go or no-go indication, so you set limits, you generally don't care exactly where it is in the range as long as it's somewhere near the middle?

But what do you set the limits with? you either need a dti that is "correct" or you'd have to set the limits with gage blocks(which could be ticky)

You arent going to make me check all my dial gages are you? hehe

Stuart