[QUOTE=350TSS;549389]
Guess what? I buggered it up – obviously with the last cut of the day.
Boring the 37mm hole for the inner bearings I was measuring using vernier calipers and forgot that their measurement of an internal surface has to account for the thickness of the blades.
[QUOTE].
Its always the last cut of the day.
Trouble is the winning post comes into sight makes you get complacent, just at the very point when the stakes are at their highest.
Done it myself loads of times.
I don't understand your explanation of how the mis-measurement occurred though.
Vernier calipers should be able to measure bores directly with no need to take anything into account.
However, this method of measuring a bore is intrinsically inaccurate, as it requires an extreme level of technique/feel to avoid getting an undersize reading.
This is made worse (near impossible) when the workpiece is still in the chuck.
The problem arises because the caliper blades essentially form (and measure between) two parallel lines which are difficult to align with the bore at its widest point.
The internal gauges which Slob referred to measure between two points, not two parallel lines, so the technique required is much easier to master.
Me, I use a pair of old-fashioned friction type, internal calipers, which also measure between two points .. but they too require a certain level of technique.
..... as will your new gauges for that matter, but nowhere near so much as with the vernier calipers.
I'm assuming that you will be after a slight interference fit for the bearing and will then sweat it in by heating the housing and pressing the bearing in place.
Therefore you would not be able to use the bearing as a gauge anyway.
The std method of measuring bores in a machine shop is to use "go" and "no go" plug gauges.
I suggest that you make yourself such a gauge out of a piece of scrap before machining the new piece.
I would select my desired interference on the 37mm bore (maybe something like 36.9 or 36.95mm) and make one gauge that size and another a few thou (in old money) smaller than that (maybe 36.8/36.85mm).
Then machine 'til the smaller plug goes in and, holding your breath, take a further light skim from there (or not, if the small plug goes in freely).
You can still use the material that you make the gauge from to make something else later.
This is the kind of rigmarole which makes machining one-off parts significantly more expensive than people realise ... and is why I am sometimes reluctant to take on such work.
ps. if you still don't have any paraffin to use as a cutting fluid, try WD40.
The problem with machining ally (without cutting fluid) is that the swarf easily gets friction-welded to the tool, making it cut both oversize and raggedly.
If it happens, I use a stanley blade to (carefully) scrape it off.
Here endeth the lesson.
