In Milwaukee, it's Lake Michigan, specifically from an area
called "Jones Island". It's much less desirable as a parking
location now that the sewage plant has been enlarged, though.
That and the bridge above it seems to have structural problems:
Someone makes inside diameter "micrometers" that have three hardened, convex
surfaces and can measure down to the levels you need. However, most, if not
all I've ever owned or seen are metric and quite expensive. You can check
eBay for used ones, but you get what you pay for sometimes...
A cheaper fix might be to have someone make you a "go / no go" guage. It's
basically a hardened steel rod ground to .001 unsersize at one end and your
necessary size at the other with a taper of .001 or less over a 3" length,
etc. Honing companies love it when I send these to them as we have a lot of
3", 5" and 10" tubes that are hard chrome coated honed out by them and need
them to within .0005" I think we had ours made for about the price of the
used device I describe above... And at least it won't break if I drop it
and could be somehwat repaired manualy if it got a burr / gouge and if it
wasn't too hard.
Joe Agro, Jr.
My eBay: http://tinyurl.com/3n8gj
Know a good travel agent? I need one. Really.
I once used a cheap attachment for a bestest indicator... it was bought
to check an internal ring groove
It mounts to a bestest type indicator and you adjust it to the bore size.
I'm sorry I don't recall the name or brand
Using T and dial gauges has been covered in another post. There's a cracking
good cheap way to measure bores if you have a bit of spare time. Get a chunk
of scrap metal and turn up a top hat bung on the lathe. The larger (top)
part should be bigger than the bore by half an inch or more so it can sit on
the bore. The smaller lower diameter should be a couple of thou smaller than
the bore you are trying to gauge. Sit the bung in/on the bore and mount a
DTI perpendicularly against it. Push it back and forth across the bore and
read the movement on the DTI. With an accurate measurement of the plug part
of the bung plus the movement on the DTI you can gauge the bore very
closely. Of course you can only measure the top of the bore this way and you
need a good undercut between the small and large part of the bung so it
doesn't foul the bore on the chamfer between the two.
There's another method which no one ever thinks of but engine builders come
across all the time without necessarily realising what it tells them. If you
insert a piston ring into a bore the gap will be a function of bore size. If
you can measure the gap with feeler gauges to within a thou then you are
measuring the bore diameter to within a thou/pi (about a third of a thou).
You need to ensure the ring is square in the bore and a close fitting plug
to push it level with is handy. It's no substitute for a dial bore
comparator but it works.
Go/no go gauges are probably the most accurate but it takes time to make
them and they still won't tell you about ovality.
Dave Baker - Puma Race Engines (www.pumaracing.co.uk)
If you're talking about cylindrical plug type gages, I'm afraid I'd have to
disagree where it comes to accuracy. Unless you have them in tenth
increments, such as the Deltronic pins that are readily available here in
the States, they are a fast gage, but often tell you nothing about the size
of a bore, only that you are too small, on size with the given range, or
beyond. It's often desirable to know the exact size, and that's a function
the typical go/no-go gages don't provide. They are more a gage of
production than anything.
There's a variation of this technique in Guy Lautard's Machinist's
Bedside Reader #1, using a gage rod inside the bore that's just a few
thou shorter than the bore. The side-to-side movement of the rod is
measured with a ruler, in sixteenths (!) right at the bore's edge--to
calculate the difference between the actual bore, and the gage rod's
length in thousandths.
That sixteenths measurement can be done to the nearest 64th, just
expressed as decimal--i.e., 15/64ths = 3.75 sixteenths.
Instead of a rod, I'd use a flat plate cut to that same top hat shape,
whose edges are milled parallel and beveled to almost a point.
Place it in the bore snug to one side and scribe a thin line with a
knife on the outside face, then tilt the plate over to the other side of
the bore and scribe another line. Get the measurement in sixteenths, and
plug it into this formula:
C= A squared, divided by 2B
where A = number of 16ths of an inch of the plate's side movement at one
end (to the nearest 64th, but expressed as a decimal sixteenth)
B = width of the plate, in inches, measured as accurately as possible
C = the diff in thousandths of an inch between the actual bore and the
You're supposed to get to a half thou accuracy with this, assuming the
plate's edges are parallel to that tolerance as is your measurement of
take da "ma" offa dot com fer eemayl
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There are cheap dial bore gages available, and sometimes by changing
the cheap indicator in them, you can get reasonably accurate results
with them. However:
Brown and Sharpe sells the inside micrometers, they measure on three
points. No good, they will NOT show whether the bore is round like an
O or shaped like a goose egg. Federal made some dial bores with a
quite wide range, but three points, also not what you want and
fantasitically expensive. The inside micrometers, the "adjustable end
measureing rods" made by Starrett, B&S and others can do it, but
require practice and a good feel with them. Standard probably makes
the best dial bores available, but unless you have need for them,
damned expensive, in that size range $500 plus. The Sunnen honing
gage is probably the best available, but let's get down to earth
again, more than $3000 for the one I had.
Plugs won't show out of round condition, three point won't show out of
round, you need two point, and there are some out of round conditions
that even two point won't show. (Your chances of seeing that are
somewhere between slim and none.) For a quick and dirty, "one off",
if you can find an old used good quality dial bore, Standard, older
Federal, even Boise, it will probably do the job nicely if the
indicator is good. A cheaper dial bore can be made to work, but they
are all dependent on what you use to set them. IF you can find a
setting ring that's within a couple thou of the size, that would be
the best, just adjust your zero to compensate. Dial bores are
comparators, they compare what you make to a known standard.
Buy a cheapie, see if you can get any repeatability against known
settings, it'll probably work well enough. CHeck it against your
standard before and after each measurement and you can be reasonably
sure that the result will be good.
It is actually pretty common to turn thin walled parts in a three jaw
that will show round when the part is actually triangular. A plug
gauge and a two point gauge will show this. If it gauges larger with
the two point gauge than it does with a plug gauge then there's a good
chance it's a three sided bore. These shapes are called REULEAUX
traingles. You can draw one by first drawing an equilateral triangle.
Then draw three intersecting arcs using as the radius the length of
one side. This will give you a a rounded triangle with a constant
And this condition, at least, would show up with one of the
three-legged tri-mikes or similar. If it measures differently when you
rotate the mic from pointing to where the chuck jaws contacted to
half-way between them, you can be pretty sure that this is what you
have. (Mark where the chuck jaws contact before removing it, so you know
what orientations to try. And this would *not* be caught by the
telescoping gauge or the inside micrometer with extension tubes.
But if you are working something this thin, that is an argument
for a 6-jaw chuck to minimize the springing. Or even better, turn a ring
with about a 1" radial thickness and a slit to put between the jaws and
the OD of the workpiece, so the force of the jaws is better spread.
Make the ring at least as thick as the length of jaw engagement, and
ideally somewhat longer.
I did not consider ellipticality from wear in my suggestions
before, simply because he was apparently making this *new*. The spring
from thin walls, however, is another matter.
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When running a centerless grinder the same thing can occur when running on
center. It's important to be above, or below, at all times, which
eliminates the condition. A part so ground will mic perfectly round, yet
be three sided.
You may be able to get a good reading with your telescoping gauges. I
hate 'em but sometimes use 'em for speed. It's longer to set up a bore
gauge than it takes to measure 1 bore with a telescoping gauge. Here's
how I do it: Like everyone says, put it into the bore at an angle and
then tighten the thing. Not too tight. Just enough to keep it wherever
it ends up after rocking it through the bore. After rocking through
the bore by holding the very end of the handle grasp it in the middle
of the telescoping part and rock it back through the bore. You will
have to rock the gauge side to side to get it to go back through the
bore. Don't force it. It should go through one spot with just a tiny
bit of drag. If it goes through easy then it wasn't centered the first
time. You will have to try a few times to get the feel of it. Then,
mike the gauge and rock the gauge through the mike with the same
amount of drag you got in the bore. Finally, after measuring the gauge
by rocking it through the mike measure it by screwing the mike down
onto it several times until you find the largest reading. Compare this
reading to the one you got rocking it through the mike. It should be
just a couple tenths larger. As a test, turn two slugs to the
different diameters. The one measured by rocking the gauge should just
fit. The one that is turned to the diameter measured by just bringing
the mike to touch the gauge should not. Most important is to use
always a light touch. Nothing should be forced.
The Enco gages are too sticky for repeatable precision. Get a
Starrett in the size you need. Then practice with it.
I once knew an old machinist who could do .0002 repeatedly with a
simple spring-leg inside caliper and a mike. I asked him how he did
that. He said, "feel", thus exhausting his conversational quota for
Guy Lautard describes a "rocking" technique in one of his Bedside
Readers that works pretty well without relying so much on feel.
As simple as his reply was, it spoke volumes. It's something one
learns------and for those that have the capability to apply it, it's no big
deal. From reading these comments, I'm beginning to wonder if it's
something that many may never master.
Dunno, Don. The rocking technique, to me, is a part of the "feel" that
makes it work. What Eric had to say about applying telescoping gages is
pretty much right on. It's a matter of doing it enough times until you
understand what you are "feeling".
It was mentioned earlier that it is rather difficult to use an inside
mic with the extension handle in deep bores for one can't reach the
I've always set the inside mic to an outside mic, maybe a thou
undersize, then see if it slides in. If it's loose, I'll open it a half
thou, and try again. If it's too tight, dial down half the original
adjustment. Repeat the process until you get the desired feel.
I always thought of using this process as turning the mic into a go/no
go gauge, and for a half thou tolerance, this is certainly accurate
enough. With practice, a couple of tenths is possible, for those
Right on! That's exactly how to use them. If you measure over the inside
mics, you can do work within .0002" and know you're there. Because the
inside mics have a tendency to not repeat exactly (because of removable
members) it's a good idea to not trust the direct readings, although mine
seem to be pretty reliable.
Another reason to measure over the ID mikes is as a sanity check
on the reading. I've always found the starrett ones read right
on, but because the thimbles read the other way around, it's
easy to get a number that is 25 thou off. Putting them inside
an OD micrometer does a nice job of catching that mistake.
Most folks can read the OD mike much more reliably than the
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