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Small boring bar for POM bushing internal grooving


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:


Why not try this.... with a piece of scrap, set up your boring/grooving tool, and bore out to the depth of the groove. Easy to measure using internal mic or bore gauges. Once you're happy you're at dead nuts; set DRO to zero if you have one, or handwheels if not.

Now cut a "proper one", sneak up on the 9.8mm depth. Once that's done, work off the handwheels to get your 10.0mm x 5mm internal bore.

The surface finish, I can't help with; but how critical is it? Presumably it's for a feature of whatever the bushing is mounting on, or is it an oil space? Either way, I can't imagine it needs to be mirror finished.

Not bad idea at all! Thank you.

Sort of tried that before: Made extra long 9,8 mm hole on POM and then bored 10,00 short recess end of to "zero" the boring bar.

Made a new try with aluminium yesterday and found the problem.  :bang: It pretty much stems out of feature creep: to make the boring bar holder more universal it became very "tight fit" to the hole that has to be bored. It was fine until I sharpened the HSS bit something like third time and tried out different cutting geometries. Shank almost rubs the 9,8 mm hole with sharpened cutting bit - centre height and location of the cutting edge in relation of the boring bar has changed from initial design during sharpening.

I made perfect 10 mm bore in POM (now that part is sorted). Needeed just more rake on HSS bit and a little "high" setting of the cutting edge. And some space for swarf and boring part front side at the bottom of the bore.

I am planning to improve the boring bar (mainly make it more oval and mill out some unnecessary material to give more clearance) and make new more narrow grooving bit.



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