There are various more expensive approaches, like inside micrometers, but unless you are doing a lot of small bores you probably don't want to go that way. You can make your own go/nogo gauge by turning the ends of an offcut of metal, one to the exact size and one to a thou under. You can also make a lap, and take the last thou or so out using that. With a lap, you can get a feel for how parallel the bore is too. It also pays in the smaller sizes of bore to make the hole first and then make whatever goes in it afterwards to fit.

The other approach is to use a reamer to take the last cut out of the bore. That means owning a reamer the size you want, which can be expensive, but you can get pretty consistent sizes that way. The size a reamer will leave does depend a little on how much you leave for it to cut out.

John

There are other ways, but better telescopic gauges might help, as well as practice – for both consistency and on known diameter holes.

The link supplied by Thor is only good for consistent diameters – not always good for machined items where the end section may be ‘bell-mouthed’ while machining.

This Old Tony made a video on improving a set of fairly rough (as in ‘cheap&rsquo gauges. Might be worth watching?

John’s suggestions are good alternatives, with likely better outcomes.

Hi Guys,

I've used inside friction callipers for years, but if I want a precision size I make a plug gauge 1 thou under and lap as needed. Very often a spring cut gets you very close.

With practice a decent quality telescoping gauge will easily get you within .001"

Tony

OP doesn't identify the make or quality of the telescopic gauges.

With good quality (M&W, Starrett, Mitutoyo) gauges and practice you can get sub 'thou repeatability, then measuring this with an external micrometer introduces its own error. Overall repeatability with practice and developing a feel for the technique, expect to get repeatability of measurement say +/- 1 thou.

Cheap and clonky telescopic gauges won't let you develop the "feel" successfully.

There is a definite knack to setting the gauge in the bore – refer to Adam Booth's technique on Youtube. But you do need to practice.

edit – Emgee and Tony type quicker'n me.

Edited By Simon Williams 3 on 31/10/2020 09:21:06

Where possible, prefer to use three point mic, or ball gauges, rather than telescopic gauges. (Frightened that the setting will change during clamping )

For slightly larger bores prefer to use a Bore Set, if possible, or a Bore mic. if the bore is large enough.

Am lucky to have acquired a Compaq bore set for 0.8 to 2" bores, with a few missing extensions / contacts. Spares are not available and the complete unit is excruciatingly expensive USD 5+K, with a six month wait for a new one. But it is industrial Swiss quality.

So finished up making the fittings with Tap and Die for the 0.5 mm pitch threads.

Ultimate test has to be the fit of the mating part, (Turned after having measured the bore )

Howard

Hi Guys,

This is how I measure bores ! Use callipers to get the bore diameter and then measure with a micrometer.

In my last post I said friction callipers, I meant to say spring callipers. Sorry.

In this picture I'm measuring the bore in a Norman tool holder block. Its 35 mm diameter.

This shows the technique used.

I use a set of three Moore & Wright teliscopic gauges down to 3/8" and. then some of their ball gauges to 3/16". They are good enough for 0.0005" with practice.

Before all the fancy bore gauges etc were available in the early part of the 20th century, and before,,turners used calipers and a rule ,their calipers were the firm jointed type,they would not use spring type calipers with screw adjustment,they would get a close fit in a bore by adjusting the calipers by hand and then adjust to get a very fit by just tapping the arms of the caliper usually on the lathe toolpost and apparently they were very accurate,if large numbers of one size were required then hardened go/no go plug gauges would be used,a lot of shops only had a few micrometrs usually in the care of the foreman. During my apprenticeship our boss used to tell us how jobs were done when he started work. For small bores the M & W bore gauges are very accurate, for larger work I have M&W plus some Mitutoyo telescope gauges plus an internal mike with with a range of 2 to 6 inches, I find the M & W tele gauges are better than the oriental ones,they have a better feel. when I can get at the open end of a bore I tend to use a Swiss Rochalon vernier where the outside of the jaws is machined to a radius, and then use these jaws as a bore gauge I find knife type jaws are not accurate enough, by using a mike to measure across the jaws and using the same mike to measure the mating piston or shaft only one measuring instrument is used,for better accuracy ,if the verniers reading is used then it introduces a possible secondary error.with all these methods it does require a lot of practice .