Robin S. wrote in message
news

"Dave Baker" wrote in message
...
Option 2. Stick a tight fitting 1/2" bar into the hole and fix a dial
gauge
to the milling head. Clock the bar up until you're concentric.

How do you count the number of sheep in a field? Count all the legs and
divide by four.

Hell no. Some of the sheep might only have three legs. First you count the
number of legs on each sheep, note down how many don't have 4, calculate the
average number of legs per sheep to several decimal places then count the
total number of legs again and divide by your average.


Perhaps he should just clock the hole?

Most hobby machinists have a plunger type DTI knocking about but not so many
have a lever type which is what you'd need to get inside a hole as small as
1/2" to clock it up. I was being charitable enough to assume that the OP
didn't either or he'd be using it already instead of the edge finder and we
wouldn't be answering the question in the first place so I was giving him
some methods that didn't rely on him having one.

If on the other hand he does have a lever type DTI, the primary use for
which is to clock up holes, and he knows how to use it and still hadn't
thought to apply it to this task I guess it would be rather like standing
next to your car in the driveway scratching your head and still being unable
to work out how you were going to get to the store 10 miles away without
walking.

The best method to use depends on how much accuracy you need, how much space
you have between the quill and the job to fit DTIs into, what operation is
going to be performed after the centralising and a bunch of other stuff all
of which require a brain to foresee any pitfalls with. A lever type DTI
might be best if you need to be centred to within a thou but other methods
are far quicker if you don't.

You can still cock the job up badly with a DTI if the mill head isn't
aligned to the direction of hole in the workpiece properly and you move the
knee down to make room to mount the DTI and then move it back again to mount
the tool.

I cut valve seats in cylinder heads on my Bridgeport. Every seat I cut means
getting the mill head aligned exactly with the direction of the hole in the
valve guide and then centred on the hole in the valve guide. I don't use a
DTI for any of this because it isn't the fastest or best way to do it. None
of the purpose built valve seat cutting systems use a DTI either for exactly
the same reasons.
--
Dave Baker - Puma Race Engines (www.pumaracing.co.uk)