# Thread: Measuring large items accurately i.e. +-0.1mm

1. Hi,

OK I'm sure there's an easy answer to this: I would like to measure some items in the 80-100cm range to an accuracy of say 0.1mm.

I have a nice micrometer that will measure say up to 150mm to maybe 0.01mm...

So...

TIA,

Alan

2. You will most probably find that your 150mm micrometer will only measure 125 to 150mm, but within that range, the reading will be spot on.

Almost any cheapo digital vernier would be able to measure what you want to the tolerances you require. Aldi very regularly have them on special offer for well under a tenner. They are accurate to around 0.05mm (0.002").

Bogs

3. Do you mean 80 to 100 MM or 80 to 100 CM which is 800 to 1,000 mm?
You can get 1 metre long verniers but they are expensive.

BTW only the BBC and dressmakers use Centimeters

4. I missed that John.

Must get myself to Specsavers.

Bogs

5. I don't think that there is an easy answer to your question. It also depends whether you mean you want absolute accuracy down to 0.1mm over a metre (that's .01%) or whether you want repeatability to within 0.1mm. Perhaps you want to transfer that measurement to something else within the 0.1mm.

Depending on the physical circumstances, 0.1mm repeatability doesn't seem too hard. I reckon I do that all the time in woodworking just with suitable stops and jigs, but the absolute accuracy is probably no better than 0.5mm 'cos that's what my eyes can see (and see next paragraph). And wood bends and stretches anyway.

Now to get absolute measurements right, that's another kettle of fish. It's going to depend on you having an accurate reference measure more than anything. You can go out and buy a 1000mm steel rule for under a tenner, but I bet you can't find a statement of how accurate it is. And then there are loads of other factors. Take thermal expansion as just an example. A 10 degree change in temperature results in roughly a 0.1mm change in length of a 1000mm steel rule. (Linear expansion coefficient of steel is about 11x10^-6). Now if you are measuring steel against steel then that's fine and they will change length roughly in line with each other, but if you are measuring aluminium then the expansion coefficient of aluminium is roughly twice that of steel, and so a 10 degree temperature change will break your accuracy requirement. (All that depends on which alloys of course).

But enough of that BS. It would seem possible to buy a 1000mm rule and then take the vernier scale off a small set of calipers and use that in the usual vernier method against the rule. It might work but it would need steady hand and eye.

6. Well good points, well made, particularly about expansion. To be more specific:

I have some folded steel parts being made to a target size of 799.5mm and they need to fit within a 'slot' of 800mm. I want to get a feel of the accuracy and variation of the delivered parts to check there will be no problems. So I want to measure a representative set of parts and I want to measure them to a fraction of a mm. I getting the feeling that a custom measuring jig (also in steel) which a micrometer engages with is the way to go. Chicken and egg problem of course, how do I check the jig :) I guess I'll get it carefully laser cut which in theory gives me a 0.1mm or better start?

Alan

7. If you do a few calcs to get the exact circumference of the part you want to measure, then make up a measuring rule out of say some steel pallet banding to exact length.
Then it would be a matter of wrapping it around the part and see if there is an overlap or a gap. Or mark up an over long piece and see if the loose end matches up to the mark. Or even wrap it around one end of one part, and mark it. That can then be used as a comparison on all the other parts, to see how close they are.

You can buy super flexible engineering rules that do exactly the same thing, for checking out large diameters, but I have forgotten the exact name for them. Not cheap though.

Bogs

8. Originally Posted by alanambrose

I have some folded steel parts being made to a target size of 799.5mm and they need to fit within a 'slot' of 800mm.

Alan
Impressive tolerances for folded steel parts. I wish you success :whistling:

9. I am still interested in what is being measured and that "slot". Is the folded bit relevant? Are you measuring across the folds or is it still a length that is being measured? Without knowing the physical aspects of what is being measured it is quite hard to see how to measure it.

I think that the variability is going to be quite easy. Assuming we are measuring a length, get a reference length a bit short of the target, say about 790mm. You don't have to know exactly what it is. Then use an electronic caliper or micrometer to measure the difference between it and your sample with them laid side by side with ends against a stop block (flat, machined...). Put the answers in a spreadsheet and normalise them or calculate the standard deviation.

The absolute is still hard, but can you not turn the absolute problem into a relative one. After all you want the parts to be 0.5 mm less than the slot. So, who made the slot? Does it exist and do you have it? If it doesn't exist yet, who is going to make it and can they provide you with a reference length. If you really do have to get a grip on the absolute measure then it is a pretty common technique, not just in metrology, to get an independent reference. I just suggest that it be referenced back to the slot.

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