On 2010-02-22, Christopher Tidy wrote:
Ed Huntress wrote:
Hi Ed,
The reason is thermal expansion of the caliper. If you're pushing for the
best accuracy, and particularly if the room is cold, holding it in your hand
for a few minutes can make a difference. If you warm a three-inch-long piece
of steel by 30 deg. F, from your 40 deg. shop to 70 deg., it will expand by
almost 0.001". If the piece you're measuring is the same temperature as the
caliper, and if they're both steel, the actual temperature matters little,
but a difference in their temperatures, if it's more than a few degrees, can
result in inaccurate measurement.
Right. I rarely wear gloves except for particularly dirty jobs. I prefer
barrier cream.
Gloves are a *very* bad idea when operating machine tools, but
when measuring with micrometers or calipers are a good way to reduce
thermal transfer resulting in errors. (Of course -- the thermal errors
can be in the workpiece as well -- if you have just turned a steel
workpiece in the lathe, it will measure significantly larger than after
it has cooled for a couple of hours -- which can lead to fit problems
when fitting a backplate to a lathe chuck for example. Finish the rough
turning, go eat dinner and read a book or watch a TV show, and come back
later to measure it and take the final passes to get the desired size.
[ ... ]
But it depends on whether you're measuring *relative* dimensions or
*absolute* dimensions. If it's the former, you don't need a well-calibrated
gage. If it's the latter, you'll never know for sure how accurately your
gages measure unless you check them from time to time against a gage block
(or a stack) that's somewhere in the middle of the gage's range, and another
one near its largest opening. For a 3" - 4" mike, I'd want at least a 3"
block (or a stack to make that dimension) and a 4" block or stack. I'd like
to have a 1/2" block, too, to wring with the 3" block to test the midrange.
What's a stack?
Multiple gauge blocks wrung together to make up a precise size.
A good set of blocks will have something like:
1.000"
2.000"
3.000"
4.000"
0.100"
0.200"
0.300"
0.400"
0.500"
0.600"
0.700"
0.800"
0.900"
0.110"
0.120"
0.130"
0.140"
0.150"
0.160"
0.170"
0.180"
0.190"
0.101"
0.102"
0.103"
0.104"
0.105"
0.106"
0.107"
0.108"
0.109"
0.1001"
0.1002"
0.1003"
0.1004"
0.1005"
0.1006"
0.1007"
0.1008"
0.1009"
And -- in a separate box, I also have:
0.10000"
0.10001"
0.10002"
0.10003"
0.10004"
0.10005"
0.10006"
0.10007"
0.10008"
0.10009"
"Wringing" two blocks together involves cleaning the surfaces
about to mate, crossing them at right angles, and rotating them until
the two blocks are parallel. A good block will cling to its neighbor,
and you can progress to a fairly long stack of blocks which can be
lifted by the top-most block and they will all stay together. (The
cheap Chinese sets, supposedly accurate to 0.000050", tend to not have a
good enough finish to bond together properly -- at least my set does
not. But I have a good B&S/"Jo"hansen block set which does this well,
and which comes with a certificate of calibration listing the actual
size of each block.
The combination of multiple blocks will allow you to build up
almost any dimension as long as it is shorter than the multiplicity of
ones which are just above '0.10000"' (they don't often make thinner ones
because they can warp more easily). As a result, some sine plates have
an extra section ground 0.20000" or 0.30000" below the main surface, so
you can build a stack of blocks which will achieve the small angle you
want to make.
If any of this is unclear, ask, and I or someone else will explain. FWIW,
most home-shop work doesn't require accurate measurement of absolute
dimensions. Usually we're trying to make two things fit together, and what
you need to know for that is their *relative* dimensions. If that's the
case, forget the gage blocks for now.
In this case, I think it's the absolute dimensions. The engine's piston
wobbles noticeably in the bore. It's an aluminium piston in a cast iron
bore, so I suspect there could be much more wear on the piston than the
bore (the bore looks good, without a ridge at the top).
Then again, softer materials tend to embed abrasive particles so
they wear away the harder material against which they are run. This is
the principle of a "lap".
At a guess, I would expect the measurement down into the bore at
right angles to the crankshaft axis to measure larger than the dimension
at that height parallel to the axis. (The forces from the crankshaft
and connecting rod would tend to apply more force along that cross axis.)
So I want to
know how well a brand new piston will fit in the existing bore, before I
buy one.
Understood -- and also to discover whether you need to bore
0.100" oversized to fit the next size piston up.
Note that the skirts of some pistons are designed to move with
temperature to fit well at operating temperature. The solid part near
the top remains more constant in dimensions than the bottom of the
skirts.
Good Luck,
DoN.
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