How to be accurate!

[John OlsenParticipant @johnolsen79199]

Another approach that has not been mentioned is using a parallel reamer. You bore the hole to just under size and take the last bit out with a parallel reamer. The big disadvantage to this is that you need a reamer in the particular size that you want. (There are expanding reamers, but that gets you back to the problem of accurately measuring the hole.) It also only applies to through holes. For really fine work, there is the problem that the size a reamer cuts to will vary with the amount left to be taken off and the sharpness of the reamer, but for our sort of work they will get you closer than you can measure.

You can also finish a hole with an expanding lap, this is the usual method used for model Diesel and glow plug engines for amateur construction. This can give a very parallel bore and the fit of a piston can be good enough to retain compression.

Where possible, I would usually make a bore first, then make the item to fit afterwards. I agree with the guys above who suggest a plug gauge for checking.

John

[Martin ConnellyParticipant @martinconnelly55370]

The effect of less than razor sharp edges of a calliper being used to measure bores has been raised twice. I looked at the error due to a 0.25mm (generous compared to what I have on mine) width on the contact edges of the calliper. For a Ø16 bore as is being discussed here it is 0.001953mm. Effectively ignorable in a home workshop.

Simple Pythagoras used to get the value and CAD used to double check it.

Martin C

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Martin Connelly on 09/11/2020 08:56:06:

[…]

For a Ø16 bore as is being discussed here it is 0.001953mm. Effectively ignorable in a home workshop.

[…]

.

Certainly less significant than your equation of 16mm with 0.6”

… but seriously : I did suggest that it was part of a conspiracy
[ i.e.one of several contributing factors ]

MichaelG.

[Ramon WilsonParticipant @ramonwilson3]

Martin

I don't think anyone is suggesting that a digi caliper isn't capable of measuring accurately to get within the near .002mm (.0008in) difference you state but as someone well versed to working to 0.002mm at work a caliper just wouldn't be given consideration to work to. The flats are there of course but their effect is I agree very small. That they are accurate is not at dispute but they are not an instrument that will give consistent readings and particularly so when used transverse to the lathe. The problem arises to me (and yes I use Mitutoyo calipers on a daily basis) as to which result is chosen if working to fine limits. If an accurate measurement of anything – lathe or mill – is required then it is not the instrument I would chose if a micrometer is available. Obviously if it's the only instrument available then choice is not an option.

There remains of course the inability to measure any further than the length of the internal jaws and of course the impossiblilty of measuring a bore inside a crankcase for instance. A plug gauge will tell you two things – if it's fit is correct then not only the size sought but any degree of taper too – something a digi caliper in even the most careful of hands cannot do

As always it's 'horses for courses' and it's down to indvidual choice – the original poster was obviously taken by surprise at his extra 4 thou being removed which in all probability on aluminium was due to a wrong movement of the dial, possibly based on an incorrect reading and not push off and then dig in as can be on some materials.

As John above aludes lapping is an option but not something that should be neccesary on such an op as first described.

All I've said on the matter is with the best of intentions to those with limited knowledge of the subject – not to teach Granny etc

Regards Tug

[Neil WyattModerator @neilwyatt]

Digital calipers (aside from the cheapest) have a gip strip and typically two adjusting screws.

Quality ones come with these adjusted, on cheaper ones they are often loose – sometimes to compensate for a lack of parallelism of the main beam.

It's worth checking as if they are adjusted correctly this can improve repeatability by several thou. Gentle attention with fine wet and dry can also ease up tight spots on cheap calipers potentially bringing more benefit than any loss of accuracy.

Neil

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