My friend needs some plastic (POM) bushings, OD is 13,3 mm length 6 mm and minor ID is nominally 9,8 mm.
No problem there.
BUT it needs internal recess 5 mm long and OD 10,0 mm into that 9,8 mm hole - in the middle = not a simple boring operation, because there has to be left "lip" on both sided.
Threfore there is only 0,2 mm to turn off from internal diameter and 0,5 mm wall (0,1 mm high) on both sides.
First try was to drill 9,8 hole and trying enlarge it to this 5 mm long d10mm recess with my trusty miniature boring bar that is made out of M6 HSS tap - only one tooth was left to do cutting. Has been working on steel and aluminium really well. Made pretty bad surface on this one and would not work on retun stroke (helix of the cutting edge - it was a tap after all!).
For second try I made a small boring bar, oval and 8 mm diameter. Drilled 3,2 mm hole to it with HSS-CO5 drill and used it's shank as a HSS tool bit. Sharped it like a trpetzoidal threading bit.
Works very well as a boring bar on forward direction, but does not produce good surface when in "groving mode". Pretty apparent when when consider material and direction of cut. I sharpened it to cut two directions: grove and bore, but it is hard to use, because it really does not cut well in return direction (which would be nice).
There are some fancy insert tooling to do internal copy turning, but size, cost and geometry limits greatly.
I am slightly baffled, this must not be very unique problem and I am pretty sure that it would be easy with different approach of tools.
The key is to finish bore/recess first and leave plenty of material outside to support the fancy ID features, then turn outside diameter, then finish one side an then part to length. Still not happy with ID recess quality.
This I'm having difficulties with:
* Small internal bore and the "plunge" is only 0,1 mm, this is pretty hard to do with dial (I put there dial indicator that is easy read but tough to "zero" into 9,8 mm diameter).
* Small bore diameter is hard to verify (measure) and I don't have the mating part to gauge it.
* I tried to grove the 5 length as a series of plunge cuts, it sort of works.
I managed to make one that seems to fit ok, First drilled it into 9,0 mm and then used the boring part to turn into 9,8 mm diameter where DTI was zeroed and then tried to cut the ID recess. Incertainties were what are the exact diamteters, measuring that is not trivial and boring bar does not seem to cut to correct diamter.
Zero degree rake (and this boring bar) cuts well OD, but not that well ID (with a very small DOC). There is enough rake, heel of the bit does not drag. 3,2 mm 8 mm long bit is hard to sharpen.
Watched this one:
Good, but I need different geometry. Much like on the top of this picture:
Pekka
