In article ,
Jon Elson wrote:
Other replies had much good info, but:
I set up the QC gearbox for 40 TPI, spindle for slowest speed (about 40
RPM), compound slide at 30 deg.
You might want to try threading at the highest speed you dare run at.
Since this is a very fine thread, you can probably do the threading
at several hundred RPM, at least.
Agreed -- as long as he has an adequate runout groove, so his
stopping point is not too critical. The only time that I thread without
a runout groove is on the CNC machine, where the tool automatically
retracts at the same point each pass. (I've never tried the trick of
threading to a hole, turning the spindle by hand the last turn or so.
That used to be seen on threads on old machine tools.)
[ ... ]
Tool deflection or workpiece deflection is very common in threading
operations. So, I'd expect it to take more infeed to get down to
final thread diameter. How do you measure it, with wires or triangles?
Without these special tools, measiring the true thread diameter is
almost impossible.
Agreed -- though if he has something portable enough to which he
is fitting it, that is quite possibly good enough. You can get thread
micrometers (either with permanent anvils for a single short range of
pitches, or with replaceable anvils, to cover a much wider range. The
same set of anvils works with both inch and metric (both 60 degree
threads), though you need special ones for Whitworth threads (55
degrees). As long as you stick with the same brand, you can use the
anvils from one micrometer with the other sizes. I've got 0-1" 1-2" and
2-3" Tesa (Brown & Sharp) thread micrometers sharing one set of anvils.
(All obtained in different auctions from eBay.) But they are expensive
for only a single thread.
The over-wires is the most accurate, as I can see the thread
mic's blade anvil deforming the thread if you don't use care. But
thread wires are awkward to handle -- though I have seen fixtures for
them to hold them to the micrometer anvil and spindle.
The only case when you can get the pitch diameter correct by
measuring the diameter over the crests is when adjusting a Geometric
style die head, as the thread form is complete in those.
[ ... ]
At the very end suggests deflection of the workpiece, though it
could be play in the gibs of the cross-slide or the compound. Or a
combination of them all.
Right! Does it make a crunching or snapping sound when threading at
that end? If so, you may need either a tailstock center or a follow
rest to support the workpiece and keep it from climbing up over the
threading tool. Long, thin workpieces will try to do this, and you can
actually see the part jumping up and down as the threading tool takes
bites out of it. Even without the tearing, you'll never get a proper
thread when the work is flexing like that.
Yep! He said "short" but he didn't (yet) say *how* short
relative to the diameter. The fact that he is still having problems
when going up from 3/8" to 1/2" suggests that it is either too long, or
that the chuck jaws need truing.
Enjoy,
DoN.
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