Adam Harris | 10/05/2019 09:29:46 | ||
533 forum posts 26 photos | When looking at used bore micrometers, analogue dial with a variety of tips each having a specific range, should the small revolution counter dial (not the main outer dial) be at zero when at rest with nothing touching the tip? If not at zero at rest, how would one get an absolute reading for a bore? It seems on ebay the used ones are often far from zero at rest... | ||
Emgee | 10/05/2019 09:53:19 | ||
2610 forum posts 312 photos | Adam, the bore micrometer or vernier are set on a known size gauge ring before use to confirm the reading, the bore mic should be adjusted if needed and the external bezel on the vernier can be rotatad and locked to suit the dimension. Emgee | ||
mgnbuk | 10/05/2019 09:55:06 | ||
1394 forum posts 103 photos | Are these not "comparators", rather than "micrometers" ? To use them, a ring gauge of known size is used to set the zero on the dial gauge on the comparator, then the deviations from the setting gauge in the bore to be checked are read off the dial gauge. The comparator in itself is not an "absolute" device. Nigel B | ||
Adam Harris | 10/05/2019 10:01:47 | ||
533 forum posts 26 photos | Oh I see thank you both. So without a gauge ring, I could take its reading against my known other (digital) micrometer set reading, then insert in the bore and measure the difference in readings and add/substract from set reading of my (digital) micrometer to get an absolute bore measurement? Edited By Adam Harris on 10/05/2019 10:02:20 | ||
Adam Harris | 10/05/2019 10:05:01 | ||
533 forum posts 26 photos | Would it be less fiddly (better) or more fiddly to use a high quality telescopic bore gauge (eg Mitutoyo) with the aim of measuring down to 0.001"? Edited By Adam Harris on 10/05/2019 10:05:28 | ||
Watford | 10/05/2019 11:21:58 | ||
![]() 142 forum posts 11 photos | Posted by mgnbuk on 10/05/2019 09:55:06:
Are these not "comparators", rather than "micrometers" ? To use them, a ring gauge of known size is used to set the zero on the dial gauge on the comparator, then the deviations from the setting gauge in the bore to be checked are read off the dial gauge. The comparator in itself is not an "absolute" device.
Is there much difference when checking an outside micrometer against a slip gauge. Some mic's even come with their own standard gauge in the case. Or am I being stupid? Mike.
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Kiwi Bloke | 10/05/2019 11:35:40 | ||
912 forum posts 3 photos | Adam, You asked: 'Would it be less fiddly (better) or more fiddly to use a high quality telescopic bore gauge (eg Mitutoyo) with the aim of measuring down to 0.001"?' Well, have you ever tried? Telescopic gauges need a bit of practice and a fine feel to get consistent readings, and, of course, are just transfer devices - the micrometer does the reading, so multiple error mechanisms. Certainly more fiddly. A decent bore comparator should indicate 0.0001" and can be set, as suggested, against a micrometer, slip gauges, etc. It will also directly indicate out-of-roundness and taper. Also need a bit of practice and finesse, but easier than a telescopic gauge (at least in my hands). If you want a bore micrometer (of the three-leg variety) get a mortgage. If the bore is large enough, however, an internal micrometer is fine, and about as fiddly as a telescopic gauge, but at least reads directly. | ||
Nick Hughes | 10/05/2019 11:48:22 | ||
![]() 307 forum posts 150 photos | If it helps, there is an example of use in my Hemingway Dynamic Toolpost Grinder thread here:- **LINK** (Scroll down to around the 13th photo) Nick. Edited By Nick Hughes on 10/05/2019 11:53:06 | ||
Adam Harris | 10/05/2019 11:53:05 | ||
533 forum posts 26 photos | Ok great - I will look for a dial comparator. Many thanks | ||
Simon Williams 3 | 10/05/2019 12:11:48 | ||
728 forum posts 90 photos | I've done some experimenting with telescopic gauges to see how accurate I am with them, and although it's not terribly scientific (I'm not working in a metrology lab with the resources to arbitrate on measurements to tenths of a thou) I would offer the following. Firstly I measured a bore of about three inches with an internal micrometer. I could do this to a repeatability of about +/- 2 - 3 thou, as my ability to apply the same force to the micrometer barrel consistently was the limiting factor, though getting the micrometer straight across a diameter of the bore was also fiddly and variable. Taking successive measurements just taught me that I was a hopeless case and wasn't getting better. I then tried telescopic gauges, in my case the Moore and Wright ones, though I do have some Starrett ones. I found my measurement was immediately at least as repeatable, and with practice I got better in that the measurement I made with an external micrometer was also about +/- 2 thou repeatability initially, but I got better with practice which I didn't with the internal micrometer.. As the process of making a measurement with a telescopic gauge means making two measurements - one to set the gauge to the bore and another to measure the distance across that gauge - there is scope for more variation than a direct measurement. But using the telescopic gauge was more repeatable than the internal micrometer - at least in my hands. After some practice (I was boring a hole for a bearing) I was fairly confident my bore measurement was repeatable to about +/- 0.5 thou. Some of my gauges are in better nick than others, and it also depends to some extent on being well within the working range of the gauge, but that just taught me to be careful that my telescopic gauge was working properly. Measuring the same bore with a digital caliper is revealing - the caliper may have a resolution of 1 thou but it sure ain't that repeatable. I also have tried the same experiment with smaller holes within the range of a Starrett internal micrometer of the caliper kind (model 700) and find this is more repeatable, typically +/0.5 thou or may be a bit better, but of course this only measures the mouth of the hole and says nothing about the bore. I've been careful to talk about repeatability, as I don't have anything I could actually say was calibrated to give an absolute accuracy.
HTH Simon
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Adam Harris | 10/05/2019 12:29:17 | ||
533 forum posts 26 photos | Thanks Simon. Oh dear , what to do? What to do?....And I was thinking that with a dial comparator all i need to do is take the peak dial reading as I twiddle it carefully about in the bore , and adding/subtracting difference to the preset reading, I am done for the actual bore... Edited By Adam Harris on 10/05/2019 12:33:05 | ||
Kiwi Bloke | 10/05/2019 12:46:33 | ||
912 forum posts 3 photos | Well, turn up a go, no-go plug gauge. Multiple steps, if you like. But a bore comparator is so much nicer - but pricey... Make sure it has all the anvil extension pieces! It's certainly possible to do better than 0.001" with telescopic gauges, but fiddly. The bore comp's self-centring facility is a great bonus. You only need to wiggle it about in one plane, to get a minimum reading. | ||
Adam Harris | 10/05/2019 12:53:59 | ||
533 forum posts 26 photos | Ah yes quite right - register a Maximum reading in the finger twiddle rotation plane, but a MINIMUM reading in the wrist dipping plane - not so easy to get both right simultaneously I suspect! Well I have now got my dial comparator and have plenty of time allocated to practice. Edited By Adam Harris on 10/05/2019 12:56:36 Edited By Adam Harris on 10/05/2019 13:03:32 | ||
Fowlers Fury | 10/05/2019 13:05:42 | ||
![]() 446 forum posts 88 photos | Valuable comments above ! Kiwi Bloke ! makes observations I can endorse from experience.
I suppose, given my ignorance in such matters, that the least subjective method might be to turn down a diameter of steel on the lathe until it "just" fits and then measure its OD with a micrometer (allowing for temp equilibration). But that's fraught with problems of surface fnish etc and gives no indication of bore ovality.
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Adam Harris | 10/05/2019 13:07:59 | ||
533 forum posts 26 photos |
My investment - a large sensitive dial that will no doubt go whizzing round and around erratically as i tear my hair out finding the "best" reading! Edited By Adam Harris on 10/05/2019 13:09:50 | ||
Adam Harris | 10/05/2019 13:12:03 | ||
533 forum posts 26 photos | I have indeed up until now used the spring bow internal calipers wiggle feel method, but not good for aiming at press fit accuracy. Edited By Adam Harris on 10/05/2019 13:12:29 Edited By Adam Harris on 10/05/2019 13:13:11 | ||
Fowlers Fury | 10/05/2019 14:09:55 | ||
![]() 446 forum posts 88 photos | 'had written before" take maximum reading as it's moved around". This was it in use:- | ||
Adam Harris | 10/05/2019 14:15:45 | ||
533 forum posts 26 photos | Using the lathe to hold two of the axis steady is good Edited By Adam Harris on 10/05/2019 14:19:43 | ||
Simon Williams 3 | 10/05/2019 15:55:24 | ||
728 forum posts 90 photos | This seems to substantiate the Law of Ladies Underwear. "The less you want the more it costs" Simon | ||
Fowlers Fury | 10/05/2019 15:55:33 | ||
![]() 446 forum posts 88 photos | Adam, I don't disagree in principle about holding two of the axes steady but I didn't do so for 2 reasons.
For reasons to do with the special piston rings that were to be used, the bore had to be 1.500 + or - 0.001" throughout the length. I was using a between centres boring bar and that had to keep being removed & replaced for the DBG as the required ID got closer - a PITA.
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