Boring Smallish Hole

Tonights project was to bore a steel toothed pulley to end up a snug fit on a 11mm motor shaft, simple.

I put a hole through and gradually opened it up by drilling to 10mm, leaving the difference for boring to size with a small boring tool.

Could I get it to cut properly? no, and all the time it's getting closer to size!

It's obviously a problem with the way I ground the tool which is quite slender and inclined to be 'springy' which made matters worse.

Gave up in the end as it was heading rapidly for the scrap bin and will tackle it from fresh tomorrow.

It was just one of those evenings I guess but so frustrating!

Alan

You have to maximise the size of the boring tool, and minimise the amount of tooltip protuding from the bar to maximise stiffness.

Experiment with the tooltip grinding, once you get it right the material falls away and even a wee bendy unimat sl can bore perfect holes

edit

You may even have a perfect tooltip, but without stiffness it's useless and just scratches.

Edited By Ady1 on 19/06/2012 23:20:52

Use a sharp (8mm ish) endmill mounted in the toolpost at a slight angle to provide some clearance, and you'll be able do this job quite easily.

Martin.

Instead of boring, why not use an expanding reamer and gradually increase the size till you get the required fit or ream out the hole to 10mm and turn the shaft down to suit. It's easier to make the shaft fit the hole than making the hole fit the shaft.

Ah, too late was the cry "I've already made the hole larger than 10mm". Maybe next time ?.

I agree with blowlamp, but I would suggest using a slot drill as you can use a larger diameter without worrying about the 90 degree teeth touching the top or bottom of the bore.

Good idea about using a slot drill/endmill, however, I ground up a new 1/4 square HSS bit tonight making it only long enough to pass through the job (the bar I was using is at least twice as long as required and quite slender)

I found top rake was best applied with a Dremmel and small stone, then touched it up with a stone.

It cut like magic (well, like it should) easily got to size for a tight push fit, lovely.

I think my problem was identified by Ady1 in that I was getting a lot of spring in the tool which was different depending if it was cutting L to R or R to L.

Should have made a tool last night but thought I could get away with it - wrong!

Thanks to all

Alan

[on this forum] I am sure that someone will correct me if I am wrong ...

Surely the "right" tool for the job is a D-bit. This is the ultimate incarnation of a boring bar with "the biggest possible diameter, and the smallest possible tool-tip protrusion"

The end-mills and slot-drills, suggested earlier, work in a similar way; but with a reduced core diameter.

MichaelG.

Michael.

A D-bit is a simple kind of reamer, but we aren't using an endmil/slotmill in the same way at all in this situation. We're talking about using these milling cutters as one would use a conventional boring bar i.e. say an 8mm diameter cutter to bore a 10mm hole.

I do agree that a reamer of suitable size could also be used to do this job.

Martin.

Martin,

I should have been more specific; because there are two different tools both commonly known as a D-Bit. One, as you say, acts like a reamer ... The other has no rake or clearance on the side, and it cuts only on the front. [i.e. it is somewhere between a Drill and a Boring Tool]

Sometimes known as a Cannon Drill; once started, it is self-aligning and is particularly good for deep holes.

... There is a lengthy discussion about the differences, on this thread.

MichaelG.

Sometimes known as a Cannon Drill; once started, it is self-aligning and is particularly good for deep holes

Anyone got any advice on these self aligning d-bits?

Or is it just a case of shove it in and see what's best for you (like um...)

Very square shoulders? rounded shoulders?

medium cut angle on the front edges?

It's a bit unusual insofar that stiffness is not king if you can design a sensible system.

From my (pretty extensive) reading of deep boring, wood was often employed to help support the job in the basic manuals from the 1940s and 1950s

No book ever tried to explain a self aligning boring cutter

Edited By Ady1 on 21/06/2012 00:54:25

Emergency Edit:

I just had a preliminary read of the d-bit thread, will spend some time on it

Please ignore my query

hi Ady,

Try G H Thomas Book 'The Model Engineers Handbook' available from any good library. Tells you all you need to know.

Regards

Terry

Or you may have more luck looking for the book by Tubal Cain

Though I don't recall seeinganything about these type of cutters in it, maybe another book by Thomas?

J

Sounds like one and the same thing to me, but with the addition of a flat bottom face cutting edge. D-bit reamers don't have side (radial) clearance in the way that the cutters do in the other thread you linked to.

Martin.

Martin,

No ... the hole-boring version doesn't cut at all on the sides, only on the front [rather like a counterbore]

The tool is a "running fit" in the the finished hole, which is why it's self-aligning.

... The D is just over half the diameter of the tool; which puts the long edge above centre.

MichaelG.

...but unfortunately, for that reason, it only becomes self-aligning when the hole is deeper than the "D" section of the drill. So you need a pilot hole, or a guide block of some kind, that is at least that deep to get it started properly.

Regards,

Tony

So that tool can't cut a flat bottom face without leaving a central 'pip', whereas if made conventionally (half the diam), you would have the best of both worlds, in that it would both ream and face in one go, without losing any accuracy on diameter.

Martin.

Given that the original post was about opening-up the bore in a pulley ... from 10mm drilled, to 11mm bored ... that does seem irrelevant.

MichaelG.

OTOH it may just be that he's met that normal problem with boring bars, spring in the tool, and very slight rock of the saddle. Which means that the next cut always takes a touch more out than expected.

In which case, with each cut, cut on the inwards and outwards cuts. I always, when its critical do that twice, and then apply the next cut. Takes a touch longer, but with all the spring in the tooll taken out, increments proceed in an orderly manner, and it is not difficult to hit diameter with precision.

Edited By mgj on 21/06/2012 23:14:14