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Thread: Precision tape measure
05-07-2010, 01:15 AM #1
Precision tape measure
I was wondering if anyone has a brand of tape measure that is accurate? I know the little hook on the end screws things up, but i am having issues with the scale on the tape, independant of the hook being wrong (.5 over 600mm). I am needing one that reads within .2 over no more than 4 meter. The hook doesnt need to move.
Or maybe a way of measuring shafts that long accurately. General tolerance is + - .3 for most of them. No DRO or scale on lathe bed available.
Oh and i did search but nothing much came up. If there is one search needs the sack coz it aint workin hard enuff.
05-07-2010, 02:01 AM #2
Correct answer here is a proper pull wire sensor system with a digital read out unit. Not cheap.
Alternative is to have have a root round the tapes with digital display. You might find one just about good enough if you set-up carefully.
Had a similar need round about year 2000 and found an expensive (Bosch?) tape with a digital display driven via an encoder and a strip of holes punched in the tape itself. Repeatable to the sort of level you are after but I did calibrate the thing first to be sure what was going on at the sub mm level. Also made up some special end hooks to clamp over the standard one for the accurate jobs. I set-up to work from an off-set start by zeroing the digital display with a bit of tape (6 inches or so) pulled out. Most of the error and inconsistency seemed to be at the beginning whilst the tape was starting out and tensioning into position. For long extensions you need to support the weight of the tape. Doesn't take much droop on along span to get 0.2 mm error.
I'm darn glad the firm bought it for me, something over £300 from memory. I had to lie on the purchase order to get it past the bean counters. However it was a lot cheaper than the proper pull-wire sensor set-up which I really wanted and, just barely, up for the job with a bit of care.
One rather naughty trick to squeeze a bit more out of a digital system is to add a very short screw adjustment system to pull it over to the next digit after setting up for the basic measurement. Subtract the movement from the display reading. You are relying on the repeatability of the encoder and processing system hysteresis curves but it does give a better idea of where you are between the digits so you can trust the comparative reading rather than accept ± 1 of the smallest digit variation. You gotta be desperate tho'.
05-07-2010, 04:27 AM #3
Dreaming, almost certainly...... The scales are printed.........
I have a Browne and Sharpe machinist's rule, on which the graduations have an error of about 0.02 inch in 6 inches...... i.e. 0.5mm in 150mm. It is an engraved/etched flexible rule, and is not a new B&S import type, but made in USA..
If Browne and Sharpe cannot get it right, you can't trust a printed scale tape measure, not without paying a goodish bit for it.
05-07-2010, 04:40 AM #4
I had feared it was a dream. The toledo steel rules we have all match up within a hair of a vernier scale. I use the rule on the vernier as a standard thinkin its pretty well spot on.
The electronic method with pull wire etc is far more complex than we are after. Its really only jobbing shop work. The + - .3 tolerance is the general spec for this clients parts, however tapes have been used for years and years and years, and none have come back so they must be acceptable. Its more the smaller stuff thats too big for verniers (+1000mm), and for the longer stuff being closer would be better. We do have 1500mm verniers but i dont fancy using them in a lathe measuring lengths. they are not ergonomic by any means.
My brain is still ticking over on this one, but thanks for the replies so far.
05-07-2010, 04:51 AM #5
I do believe that starrett sells a tape that is made on a thin stainless steel that should be accurate enough. they already make a PI tape for measuring diameters that is very accurate so I dont see how they couldnt make one for length.
Of course temperature will change the way the tape reads but if you measure things at nearly the same temp then you should get consistant readings. If you are outdoors then winter to summer there will be some variation.
05-07-2010, 04:53 AM #6
05-07-2010, 05:33 AM #7
05-07-2010, 06:24 AM #8
You're dreaming. steel tapes are pretty good but seldom reliable enough for accurate lay-out in machine work. You need a ruled scale - that is a spring steel rule whose gradiations are the product of a ruling engine.
If you're made out of money, furnish your machine tools with DROs then you can measure 600mm to 0.01mm.
Your next best bet is a 600mm steel scale as from a combination square set.
How about a cheap vernier caliper? http://cgi.ebay.com/40-HEAVY-DUTY-VE...item27af770791
I do some fancy work using a tape measure but if it has to satisfy a third party I use more accurate technique.
Last edited by Forrest Addy; 05-07-2010 at 06:49 PM.
05-07-2010, 07:35 AM #9
The classic question in metrology: how is your customer measuring them??
05-07-2010, 07:47 AM #10
Does anybody know how accurate those punched holes in a steel band drive "belts" are?
If they are good enough perhaps butchering a cheap vernier or digital caliper so you could hook a length on in place of the fixed jaw and arrange suitable abutment through a hole for the other end. Or possibly several of different nominal length so the caliper covers the difference. Obviously you need to calibrate somehow to start with but once you have a number its down to stability and technique. Had a 20 ft roll of the stuff in the too good to throw away drawer at work for ages which I believe was originally purchased with that sort of application in mind.
That high quality digital tape measure I mentioned seems to be off the market. Everyone now has decided that 1 mm is good enuf.
Or you could get creative with a ordinary drive chain as your basic measuring stick. Again with something suitable for the final, accurate readout. Quite a few precision machines have been made using a chain in this manner. Such as the modified Adcock 1ES mill we used to use to cut the tooling for lens making. Far easier than cutting a 3 ft long rack on an elliptical curve.
05-07-2010, 07:47 AM #11
05-07-2010, 06:33 PM #12
How is the customer measuring them? i dont think they would bother with lengths.
These are mostly pump shafts so im thinking that + - a 1.mm would possibly go unoticed. We have a 1 meter rule, i was thinking a longer one with a vernier jaw on the end might mork out pretty close. Shafts are measured as unroughed stock so bowing isnt an issue.
And a DRO on our lathes? O how nice it would be, its something ive wanted for Z axis forever. I use a dial for the X axis i think its easier. But not going to happen i dont think.
On that topic a bed regrind would be nice to. .2 taper over 600mm gets old quick...
05-07-2010, 09:52 PM #13
It sounds to me like you need to make up a couple of custom "standards" to measure against. The idea is similar to the rods that are used to calibrate micrometers and other measuring tools. Picture a steel rod with a hook block on one end and a ground face on the other. The length is within an inch of your nominal shaft length. Along the length of the rod are several insulating handles made from wood or plastic.
The key is that you take the standard to someone who has a long lathe with a DRO or some other means of getting a good measurement of it. It can be any length, as long as you know what that length is. The actual length, of course, will vary with temperature. So, you need to know the length at a particular temperature. Ideally, you want to come back with a chart showing the actual length of the standard over a range of temperatures that you would likely see in your shop. Now, you store the standard carefully so it won't get damaged, and so it stays around the same temperature as the environment the shafts are in.
When you need to measure a shaft, you put the standard against the shaft, being careful not to warm it up from holding it. Measure the difference between the standard and the shaft using a depth micrometer, vernier, or steel rule. Then check your room temperature, look at your chart, and do a little math to calculate the shaft's actual length. The final result can be very accurate if you're careful with technique.
05-08-2010, 03:15 AM #14
precision measuring tapes Invar / Lovar
precision measuring tapes made from Invar or Lovar metal cost over $30,000 in 1980 $ and we had calibration check by NIST which printed out a correction book.
.........Depending on tape measure supported by ends (2point) , 3 points, 4 points or continuous table showed force required to pull tape (stretch) usually 10 - 14 lbs for correct reading. We used 4 people for measuring
1) hold one end of tape
2) hold other end of tape with spring scale.
3) look at zero end of tape with magnifying glass or other optics
4) look at measuring end with magnifying glass or other optics
using Invar which is a metal that does not expand or contract much with temperature changes can cause problems. If measuring steel or concrete it will change length when temperature changes. using a steel tape measure to measure a steel object is sometimes better as they will change length at roughly the same length when temperature changes. We often measured daily length changes in a building as the exterior walls were effected by temperature changes from the heat of sunlight. Had a few discussions with engineers about the laws of the universe and they even apply to engineers whether they want to believe in them or not. Often see bimetallic effect when an expanding object is held on ends (bolted to cooler object) it can bow out of straightness. In my opinion any experience field machinist who does not immediately mention these basic things and produce alignment documentation going back decades proving these laws of the universe is not that reliable although I have seen mechanics work on million $ equipment and never double check readings so were not aware of measurement variations.
now a days a laser interferometer and some are built into electronic theodolites can track a precision golf ball (reflector) within sometimes 0.2mm over 10+ meter distances although different air temperature variation can cause air shimmering / waves and distort readings (desert mirage effect)
05-08-2010, 03:50 AM #15
Having said that, I'd like to see anyone reliably and repeatedly read a tape to 0.3 mm/0.01" over 4 metres/13 feet! Maybe if it was a whole mm and you just needed to confirm, but something like 3297.6 +/- 0.3 ... I'd reckon you have the wrong tool for the job. I'm happy with mine at 0.5 and I would have thought that would be about the limit but maybe my eyesight is just crap
05-08-2010, 04:30 AM #16
05-08-2010, 10:41 PM #17
05-08-2010, 10:43 PM #18
05-08-2010, 11:04 PM #19
Not helpful to the OP, but here's a curiosity worth bringing up. About ten years ago at an antique tool show outside Washington DC I saw a vernier rule graduated in thousandths of an inch. If I recall correctly, it was 18 or 20 inches long, and graduated only in whole inches. Left of zero (rule was graduated left to right) was the vernier mechanism, compete with magnifying glass. The edge was, of course, very thin to minimize parallax. It doesn't seem super useful, which may explain why I've never seen another, but someone must have thought it was worth the effort to make.
05-08-2010, 11:27 PM #20
To answer your original question, the brand I use and have been very impressed with is Fisco FISCO Precision Tools I have a few tapes and a laser measure but try to look after my Fisco as it's clearly much better quality than the rest. They're available in Australia but recall they aren't cheap.
I still reckon it's the wrong tool for the job, but if you're going out 4m guess you're not exactly spoiled for choice Personally anything over about 2-3m I tend to use the Fluke laser but they're not guaranteed for the accuracy you're after. But at those distances I'm constructing, not engineering