Thread: How to measure plus 12" accurately

I have mics up to 12" but say I wanted to measure something 36" to within .01 with out spending hundreds of dollars on calipers, are there any secret machinist tricks? Thanks.

If you have a lathe or mill with a decent DRO, use that. Anything from hard stops to a small scope with a cross hair can be used to locate the ends of what you're trying to measure.

What are you trying to measure? There are lots of other approaches.

A pie tape maybe?They work well and read just like verniers,dunno about price and part must be round,it will not measure egg shape.
Other than that unless you can get it on a mill table with a DRO your pretty much stuck with having to drop some doe to have the right tool.

Do it like the old days with a P&W jig borer. Get a series of rods that are less than 12" that you can measure with your mike. Make a tray that the rod to be measured and the calibrated rods can fit in, you will need some way to make a square end on each end. Use an inside rod mike to finish out the measurement.

Tom

I am trying to measure the length of a few shafts. I am making several and would like them to be somewhat close. No dro on lathe but I do have an old Bport. I could cut rods at different lengths say 24", 12", 6" and finish with depth mic or caliper. I don't know how accurate my lead screw would be at that length but I guess I might find out. Thanks Guys.

any secret machinist tricks?

Since you don't mind keeping secrets, you could take a surplus micrometer, cut in half and install it on a frame of your choice. You would keep this a secret to avoid embarrassing yourself, perhaps.

Something like a calipers or perhaps a series of extensions between ends.

You would need a series of calibration rods, and patience.

In a lot of things, it's either spending money, or time, or not getting the desired result.

If it passes a cert, or satisfies your need, why should anyone take offense?

On edit:

I am trying to measure the length of a few shafts

In that case, your frame would be quite simple, perhaps as simple as a beefy piece of threaded rod. Braze/TIG the micrometer pieces onto coupling nuts, and have at it. I do suggest dismantling the micrometers before application of heat You would still need calibration rods.

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Originally Posted by lowCountryCamo

I have mics up to 12" but say I wanted to measure something 36" to within .01 with out spending hundreds of dollars on calipers, are there any secret machinist tricks? Thanks.

Originally Posted by lowCountryCamo

I am trying to measure the length of a few shafts. I am making several and would like them to be somewhat close. No dro on lathe but I do have an old Bport.

Other than wear, your BP's lead screw will give you the accuracy you have mentioned. And, if you are willing to make an assumption, my "secret machinist trick" is that you can correct your measurements for the wear to within the accuracy you are asking for.

The assumption you have to make in order to use the leadscrew as your "micrometer" is that the wear on it is symmetric. For simplicity of this discussion, assume the backlash at one far end of travel of the table is 0. Let's say that after you've moved the table by 36" from that point you find the the backlash there is, say, 0.020". If you assume the wear on the lead screw is symmetric, that means 0.010" was worn from each side. What this in turn means is that when the dial on the lead screw tells you you've translated the table by 36.000" you've actually only translated it by 35.990" (if 0.010" had not worn away, it would have pushed the table just that much further). So, in this example, move the table so it reads 36.010" and it actually will have been translated by precisely the 36" you want.

If the wear is symmetric, this "machinist trick" will serve as the measurement stick you need to within the 0.01" accuracy you're looking for (you also have to pay attention to differential thermal expansion effects between the material of the mill and that of whatever it is you're measuring). I've only done this once, when I installed a DRO on a used mill I bought. The screw had 0.002" backlash at one end of travel, 0.000" at the other, and a max. of 0.023" near the middle. The measurements from the DRO showed the wear on each face of the screw were symmetric to within 0.001" everywhere along its length. Again, though, although this is what I would expect to be the case, I only have made these measurements on one mill.

You say you want to measure accurately to within .01 inch? And do it without breaking the bank. Go buy a 36 inch vernier caliper, you will be able to measure accurately within .005 inch. Probably even closer than that.

Lord Byron

Originally Posted by Bruce Nelson

You say you want to measure accurately to within .01 inch? And do it without breaking the bank. Go buy a 36 inch vernier caliper, you will be able to measure accurately within .005 inch. Probably even closer than that.

Lord Byron

I agree. A vernier 36" (1,000mm) vernier caliper will cost around from $400 and the same length in digital from around $600. Both will easily measure the accuracy you want. The digital almost certainly to 0.002".

Once you have a 36" caliper you can use it for many other things.

Gordon

Unless you work for free anything you come up with and have to make special will take more time than you realise Sometimes biting the bullet is best

Originally Posted by lowCountryCamo

...I wanted to measure something 36" to within .01 with out spending hundreds of dollars on calipers, ...

Originally Posted by Gordon B. Clarke

I agree. A vernier 36" (1,000mm) vernier caliper will cost around from $400...

I took him at his word that he didn't want to spend hundreds of dollars to buy calipers and gave him an answer that would let him do it for free and without requiring very much of his time. With an indicator on the spindle he could locate both ends of the part and read the length directly from the Bridgeport's dial (after correcting for wear) to within the accuracy he requires. For free.

Don't overlook the simple approach.
A decent quality 36" steel rule with 16R grads(1/100ths) and a magnifying glass will get you to within .010.

If you are making 36 inch parts you should have one of these anyways for layout and such.
Bob

Originally Posted by opscimc

I took him at his word that he didn't want to spend hundreds of dollars to buy calipers and gave him an answer that would let him do it for free and without requiring very much of his time. With an indicator on the spindle he could locate both ends of the part and read the length directly from the Bridgeport's dial (after correcting for wear) to within the accuracy he requires. For free.

I for one am not disagreeing with you. My not so obvious point was that rather than look at a caliper as an expense it could be thought of as an investment.

OK I'll disagree on one thing - nothing is "free" unless "time is money" is ignored. It took time to start the thread and all of us that have posted, as well as those that have used time to read the posts, have used time

I would check ebay or some used equipment dealer for used "Hoke" blocks. Even those that are in poor condition would be within .0002. Just assemble with rods and transfer your reading.

If you do go the custom measuring device instead of destroying a mic consider spending a smalleer amount. Starrett makes a micrometer head such as the #463 which 0.5 inch thousands including ratchet stop and lock nut. I like the cause you can have multiple custom devices and just install the head to read.

Originally Posted by CarbideBob

Don't overlook the simple approach.
A decent quality 36" steel rule with 16R grads(1/100ths) and a magnifying glass will get you to within .010.

If you are making 36 inch parts you should have one of these anyways for layout and such.
Bob

I agree use a Starrett rule/scale to measure 36" to 0.01". I have found not all rulers are the same. I saw a cheap 12" 1/32" graduated ruler was off about 0.010" per foot. When I used a digital caliper to measure only Starrett scales they measured all within 0.002". Use a headband magnifying glass or strong reading glasses to enhance your vision. With strong magnifiers a person can easily read 0.005". The other thing is being careful holding the scale straight and flat and edge even with part edge. With extremely strong magnifiers most can easily measure 0.002" or less. Basically it is like wearing a telescope or microscope to see better. You need to see straight on to avoid parallax error and hold ruler tight to part.

Originally Posted by Gordon B. Clarke

OK I'll disagree on one thing - nothing is "free" unless "time is money" is ignored.

I'll pursue this further, since your comment implies that use of a 36" caliper would take significantly less time than use of the table of a mill. I'm not so sure this is true. The longest vernier caliper I own is 18", and even it takes some time to use it in a way that the reading is meaningful. Both ends of the caliper have to be supported at the same height (which is possible to do by hand with 18"; not with 36"), and the middle probably also has to be supported with the 36" caliper to keep it level. Then time has to be spent trying to figure out how much tension to apply to get repeatable readings without springing the jaws apart. Even finding a place to store a 36" caliper, and bring it back out of storage, adds time to the measurements. Anyway, putting this all together, I don't think using 36" calipers to make the kind of measurements the OP describes would be significantly faster, or significantly more accurate, than use of the mill.

As for thinking of the $400 purchase as an investment, that consideration is background to every equipment purchase, not just this one. "Investing" $400 in a very specialized instrument should be considered in the context of the present need (and whether there are reasonable alternatives), as well as the likelihood it ever will be used again. Again, the OP wrote that he needed to measure "a few" shafts, he knew the utility of 36" calipers, but was looking for possible alternatives so he wouldn't have to spend hundreds of dollars.

get a 100ths rule

This would be awfully easy with a DRO on the Bridgeport.

You could get a used DRO for about the $400 of the caliper, and have a good usable machine for other purposes.

- Leigh

Originally Posted by DMF_TomB

I agree use a Starrett rule/scale to measure 36" to 0.01". I have found not all rulers are the same. I saw a cheap 12" 1/32" graduated ruler was off about 0.010" per foot. When I used a digital caliper to measure only Starrett scales they measured all within 0.002". Use a headband magnifying glass or strong reading glasses to enhance your vision. With strong magnifiers a person can easily read 0.005". The other thing is being careful holding the scale straight and flat and edge even with part edge. With extremely strong magnifiers most can easily measure 0.002" or less. Basically it is like wearing a telescope or microscope to see better. You need to see straight on to avoid parallax error and hold ruler tight to part.

Make sure that you buy a Starrett steel rule, they are one of the few (if only) manufacturers that use machine engraving to graduate their rules. Other manufacturers use photo etching to graduate their rules. You can tell if a rule is machine engraved by looking at the inch numbers with a magnifying glass, you will be able to see the engraving tool marks. If you look at the inch numbers where the engraving is done by photo etching, you will not see engraving tool marks. With a magnifying glass, you should be able to guarantee your measurement within less than .005.

Lord Byron

Originally Posted by The real Leigh

This would be awfully easy with a DRO on the Bridgeport.

You could get a used DRO for about the $400 of the caliper, and have a good usable machine for other purposes.

- Leigh

The cost of the DRO would not be the only investment. Consider the cost of the spindle mounted scope with cross-hairs. Also consider the compensation for parallax errors when trying to focus the scope on the ends of the workpiece. The same goes for purchasing the caliper, you would have added that measuring instrument to your arsenal.

Lord Byron