Stressing over numbers

I cant do this any more, im getting so streesed over measurements. Yesterday I had to bore out a coupling to fit a shaft. Measure the shaft. Need to take 1.2mm out of coupling. Set it up in the lathe. Take a cut of 0.9mm(diameter) on DRO out the inside of the coupling. Just offered the shaft up to check and the thing just slips on arrgggh.

Measure a shaft with digital micrometer. 10.05mm Measure again later 10.01mm measure again later 10.06mm. Whats going on in my life.

If I use a telescope bore gauge i try to get the lightest touch possible when offered up to the micrometer but it can be .1/.5/.7/.3

This is driving me crazy. Then if I work in imperial as theres more digits after the decimal point it gets even worse.

What am I doing wrong? Ive tried to buy the best measuring equipment I can afford.

Edited By Michael Edwards 1 on 26/05/2020 09:41:56

Edited By Michael Edwards 1 on 26/05/2020 09:42:25

I get days like that too...

Are you sure the DRO movement was diameter? If you went in 0.9mm with the toolbit then you got the expected result.

As to telescopic gauges..known problem beasts even in the best hands and in my shed things oten get worse in summer when it slowly heats up in there or winter when the woodburner finally starts kicking out too muh heat and not everythng has come to the same temp. If it;s a critical measure then I pick my time, recheck mics against standards etc or make up a plug gauge.

I'd guess I'm not the only one with a bin full of bad fit scrap.

pgk

It's all practice and experience

"measure twice cut once"

But getting there is a pita

I multi-measure a lot because those digi calipers can be a bit wayward

I also judge the cut manually by eye and "feel" because sometimes the gadgets just don't give you the bigger picture

Edited By Ady1 on 26/05/2020 10:07:10

[This posting has been removed]

It sounds as though you are clamping down hard when you take your measurements with a Mic, there is a little thumb screw on the end of the mic, use this instead of the larger diameter, also when you are on you diameter as you are turning I always give a gentle rock to make sure it is sitting completely square and on the parallel.

Its also prudent to know where your taking your measurement from as if you are already doing the above it could be that you are in fact turning a taper on your lathe. If you take a light skim (0.1 or 0.2mm), then measurements from the far end of a piece of metal 25mm thick and about 100mm protuding from the chuck, then measure the end furthest from the chuck, and the end closest to the chuck, and see what the difference in reading is on your turned piece.

The telescope gauge again could indicate that you are getting a taper turned on your lathe. Either that or there is something going on with your mic. They are sensitive to heat and will expand and contract a little. Try the test cut and some pics may help.

One other thing to note, if you need to take off 1mm cut on OD, dial in for 0.5mm then measure again, if correct take a cut of 2.5mm, check again, you may find that you need to adjust the amount you take off on subsequent cuts as you may remove 5.6mm or 2.55mm on the above cuts but make sure your removing half the material you need to remove on each cut to get to a precision fit.

Jon

[This posting has been removed]

I sympathise! Accurate measurement is hard, and so is accurate cutting.

The answer to the measurement problem may be simply one of practice. There's a bit of a knack to it, and some shapes (like shaft and hole diameters) are more prone to error than others. Not everyone is talented: I'm clumsy! Try sitting down at a dining table with a selection of objects and measuring them over and over again. The purpose is to develop a feel for instruments and remember it. Start with parallel flats and look for consistency. Don't force instruments to get expected results - the goal is to have the instrument produce consistent answers, not for the operator to push it to the 'right' answer.

Cutting is another problem; my main issue with boring is the tool bends and springs, add that to an 'off' measurement and the error can take you either side of what's wanted. For me, practice and taking care sort of fixed it.

A third possibility when driving on dials is failing to eliminate backlash before trusting the dial again.

Stay entirely in metric or imperial rather than mixing them wherever possible. Whilst a DRO eliminates most of the dangerous mental arithmetic, I tend to think in millimeters whatever the readout says. Pretty sure chaps who think in thou suffer the same problem - the readout says 0.01mm, their sub-concious mind is thinking 0.001 and turns the controls accordingly.

Learning is a tricky one. I like to practice without distractions, but it soon gets tedious. Others do better by plunging in at the deep end. Nothing like the prospect of ruining an expensive part to concentrate the mind! Before trying anything new, I like to do a few dress-rehearsals first. Doesn't always help - I'm about 5 years into the hobby and still make lots of mistakes...

Dave

Thanks for the reply gents. Its been really getting to me.

I can only remember once in three years that I cut 10.00mm on the DRO on the lathe and measured 10.00mm exactly on the micrometer. Since then it has been downhill all the way.

I do need to run a taper test actually on the lathe not done that in a while. Thanks for remonding me of that one.

The telescope gauges I dont use the ratchet it is litrally just touchy feely that I use with the mic. And it does tend to take me some time to get the feel of it as I end up backing off and redoing as I keep doubting myself lol. Straight jacket just been ordered lol.

Yes I am one of those people that keeps going in the house to check ive turned everything off after ive already locked up.

Edited By Michael Edwards 1 on 26/05/2020 10:28:56

I,m 20 years arsing about and I still make cock-ups, sometimes bigger ones, so don't feel too bad about it.

Just join the club, remember, you don't have to be nuts, but it helps !

It's probably safe to assume that the material is not changing dimension, so that leaves measuring equipment and technique. Looking at external measurements first I suspect the equipment. I don't tend to use the ratchet on my micrometers but go by feel. Even so the difference between barely touching and acting as a G-clamp is normally less than the differences seen by the OP. The alarm bell for me is the phrase 'digital micrometer'. Even worse if it's based on electronics. A cheap mechanical micrometer may not be accurate, but is still likely to be repeatable. With cheap electronics anything goes. It would be helpful to see a picture of the micrometer in question.

Measuring holes is a whole different ball game. Personally I've never got on with telescopic bore gauges. For holes up to 13mm I use these Mitutoyo gauges in conjunction with a micrometer:

Above 1" I use internal micrometers. For the 'here be dragons' area between 13mm and 1" I use the mating part, or a home made plug gauge, and machine to fit if tolerances are important. If not I just drill/ream and leave it at that.

Andew

Tool spring.

If you don't remove all spring before you take your last measure the DRO reads your next cut + the spring, which means you end up undersize by the amount of spring you had on the previous cut.

Consistent measurement down to very close limits is a fairly easy skill to pick up for anyone with mechanical sympathy.

Developing measurement set-ups using off the shelf equipment to do things that the makers never envisaged was a significant part of my work so I got pretty familiar with what is needed to coerce the right sort of behaviour out of things.

First, and most important, is consistency. Same result every time within the inherent accuracy and errors of the instrument. Consistently wrong can be sorted out later, fudge factor if need be.

Secondly, and closely related, is understanding what the reasonable expectations of accuracy are. Only way to deal with statistical variations is to make lots of measurements, which is no great help here. Even when I got paid for it I never liked it, try 500 measurements each on 1,000 data points with a reset needed for each one. In a dark lab! Computers have made things easier but there are subtle traps.

Thirdly, and the bit which most folk miss, is being able to see small variations. Which is where analogue instruments score. xxxx and half a line is a valid measurement so long as it is half a line width. xxxx and just over half a division is valid too, usually you can see the difference between 1/2 and 3/4. Digital devices are pants at this sort of thing. Flashing number rather than misaligned lines and no guarantee of equal hysteresis going both ways so error going up can be greater going down. Contrary to received wisdom digital devices do have backlash needing a certain movement to get all their numbers sorted out. With high end equipment you can ignore it because its all below the sensitivity threshold. More affordable stuff maybe not, something has to give when hitting a price point. Some of the early digital calipers were beyond terrible. Approaching 1/4" movement to get their counts in order. Great devices if you just wanted to close them down onto something in one smooth hit, not so good if needing to fiddle about. I still bought some and modded them tho'. There are good reasons why Newall stick with their fundamentallty analoglue ball scales.

However that said I'm inclined to think your troubles are more cut related. Itty, bitty, teeny weeny cuts are difficult. Unless the tool is uber sharp and dead on centre when cutting it will rub to greater or lesser degree in unpredictable ways. Slender boring tools need to spring slightly to generate sufficient pressure to start cutting. Which really doesn't help.

Best technique is to learn to trust your dials and make the last cut a cut. I usually reckon 10 or 20 thou is minimum. Take a cut, measure, set dial to read zero at final diameter and work down to it in reasonable stages. On a got to be dead nuts first time job there is much to be said for measuring two finishing cuts out make a finishing depth cut and verifying that yo actually got that cut. Adjust if need be. One sping cut should do the deed if its a touch small.

Clive

Thanks all. Here is a pic of the Mics and bore gauges that I use they are not expensive just what I could afford at the time.

I agree with Clive's assessment that the problem may be not taking consistent depths of cut. My experience of trying to get an accurately sized hole is that it is far easier with a big bore than a small bore. If you can use a big fat 25mm boring bar it is easier to take fine cuts than with a springy 10mm one.

Martin C

I'm not that experienced Michael, so take my advice with a pinch of salt, but could it be at this stage you need a mentor, someone to keep you right? I know clubs/societies and meeting people is difficult right now, but might it be better to do something a bit less demanding, and leave the fiddly stuff until you can get a hand with it?

Just a thought anyway. Don't beat yourself up about it man.

Take care

G.

Hi Gaunless, yep already a member of an engineering society but sometimes it is nice to have someone on your shoulder just saying yep thats fine.

I think it may just be me shooting for perfection but its not always possible, i need to get over myself and just get things done and stop scrapping things if its not bang on.

There are two separate issues; one machining to an accurate diameter, and two making consistent measurements of the diameter. What happens if you measure with a mechanical micrometer, and how does it compare with the digital micrometer. Of course we assume that both the work and micrometer anvils are clean and free of fine particles of swarf.

Andrew

I watched a video by Stefan Gotteswinter recently about how to turn precision diameters, it is worth watching **LINK**

Adrian

[This posting has been removed]

Don’t get too disheartened. There are other ways to avoid the disappointment.

Making a plug from a (scrap?) piece of material might be one way of avoidance. Another is to knurl the undersized piece, if not so much undersized, before loctiting together - if that is an option

Remember it is easier to fit the shaft to the internal diameter than the other way round.

Remember, too, that temperature can make small amounts of difference with both tools and work pieces - I (now) always give time for parts to reach room temp before measuring - been caught out by that one more than once, before now.

Sharp cutters and spring cuts, along with the stoutest boring bar you can muster, and snugging up gibs (or even locking the stationary slides). I jam the compound against its stop to help, too