Choosing a boring head

Are you sitting comfortably?

If I were grinding this I would first do the grind that forms the near vertical forward face. Although this needs a slight angle it would be best done simply grinding to half diameter and then rotating the blank approx 5 degrees for the subsequent grinding. This is what it would then look like when viewed from the end, face 3 in your annotated pic is not critical nor is the fillet but that is likely to be left by your grinder.

Next will be the grind that forms the bottom edge of the tool, this is ground at 45degrees to the tools axis and also given 5 degrees of clearance by tilting the grinding rest downwards by that amount.

The final cut is the the other near vertical face, this is ground at 40 degrees to the tool axis so that it does not rub on the cut face and with a steeper clearance of 10 degreed minimum so the back edge does not rub

See angles marked on grinder table in first photo and then more tilt of th etable

Looking at the back of the tool you will have the 2nd & 3rd grind meeting at an 85degree angle and the two clearance angles will also show.

As I said face 3 is not critical and could just as easily be ground like this

None of the angles are set in stone so a bit either way won't hurt, the main thing to watch is that the back of the tool does not rub and you are clear where the red arrow points, as hole sizes get smaller you will need to alter the grind and make the 10deg steeper to maintain clearance or even grind a curved surface

Many thanks indeed!

You were right to ask if I was sitting comfortably. I needed to be.

I can't deny I also ran your sketches etc. past my better half, who instinctively understands geometrical stuff much better than I do and, unlike me, can draw and easily visualize how things will look from different angles.

I've started the grind on an old drill bit. I'm not too confident I'll hit quite the right angles freehanding it on the grinder, but I might cheat slightly on the finished item and use my crocker-type graver honing jig to set the side angles more precisely after roughing them on the grinder.

I'm beginning to wonder now whether I'm getting a boring head primarily in order to complete a particular job or in order to test and improve my ability to grind and hone single point cutting tools. The second objective now seems the more important of the two.

What are you intending to do about holding the Oilite bush for machining?

Like in the photo, or possibly with a vee block front and back and with the bushing raised up slightly on parallels to bring it level with the top of the vee block[s].

One concern I have is whether I can hold the bushing firmly enough without the vice jaws squeezing it out of round slightly. I intend to take very light cuts.

I'm assuming edge-finding for boring is typically done with a DTI, not a mechanical edge finder/wobbler.

You have two issues

Firstly as you mention will be distortion which is quite likely clamping as you have shown or even with a second Vee block, Better would be to bore a hole in a block of say aluminium then saw in half then hold the bush in what in effect will be nesting jaws.

Second issue is that Oilite bushed are made oversize in bot OD and ID with the idea being that when pressed into place they close down to the correct bore. You will need to work out how much oversize is needed so that you get a goof fit once installed. Alternative would be if you can fit to the part and then bore in position.

Jason

A most excellent description of how to grind the toolbit, hope you don't mind me stealing it for next time I get asked!

Further to potential distortion issues when enlarging a bush bore my procedure is to make an alloy holder with the correct bore to take the bush then drop it in boiling water to expand it so the bush slides in more easily.

Bore out.

Make an extraction mandrel a stiff sliding fit in the new bore and a touch undersize in the holder.

Drop it back in boiling water to expand and push the enlarged bush out.

Not tried it with oilite, which needs a specified nip on the OD, but its worked for a bush of about 30 thou wall thickness made to repair a direct in the casting bearing with almost no spare metal around it. (Third time was the charm!)

Clive

Many thanks for the further replies.

Jason, yes, I'd read about the likelihood of the bearing's inner diameter reducing after fitting. I think it may have been very wishful thinking on my part that I could hit the right ID with the bearing uninstalled and not have to rebore it after installation or scrap it entirely because I'd rashly overbored the whole 40mm depth of the bearing.

Given the additional need to encase the uninstalled bearing in a nest/sleeve of some sort in case it gets squeezed out of round in the vice, I'm not going to attempt to bore with the bearing uninstalled but bore each slice of bearing in situ using a setup like in the photo [held on parallels in the vice]. Obviously the bearing in the photo has not yet been installed, but it will be!

Hopefully the actual bearing won't need any clamping to stop it turning when it's being bored. I don't know the answer to that at present, unfortunately; all I can say is that it will be a good tight fit judging by the feel of things and by the OD of it and the ID of the big end.