How to centre a boring bar

[speelwerkParticipant @speelwerk]

Also for what its worth; my experience with boring bars is to place on centre height or a little above, but that is for small to very (1.3 mm) small holes. I would love to have/use a boring bar for small diameter holes with inserts but sofar have not found one yet which goes below 5 mm. I now use very expensive KOMET ones but they are very difficult to sharpen out of hand. Niko.

[Michael GilliganParticipant @michaelgilligan61133]

Niko,

It's not what you are looking for, but I think you will appreciate this !!!

MichaelG.

[speelwerkParticipant @speelwerk]

Michael,

That is seriously impressive, no doubt it is out of my budget, but I can certainly appreciate it.

Niko.

[HopperParticipant @hopper]

Posted by Bogstandard2 on 13/08/2014 21:15:28:

Hopper,

'Ah, but most of them are not really negative rake, they just look that way. EG, they use a CCMT06 insert which is a positive rake insert. Trick is to set the tip of the insert well below centre, so the top surface of the insert is aligned with an imaginary line running from the centre of the work to the point where the tip contacts the diameter being bored.'

What a load of utter twaddle.

The actual reason the tips are set with negative rake to the mounting is to give more cutting clearance under the tip to allow holes very close to boring bar size to actually be bored giving much finer surface finishes because of less chatter, and also, if you set the tip below centre line rather than exactly on centre, you would end up with all sorts of cutting problems.

I do hope you don't work in industry using the method you just stated, if so, what scrap rate and tool breakage do you get?

John

Like I said, it works for me. (and I did say they were made that way to provide clearance, so am well aware of that.)

And like I said, I had tip breakage problems before trying this method. None since. (We are talking about use on my old Drummond M type here. Full size lathe in industry is a different matter — nice big rigid lathes that like negative rake.)

I know several others who have found the same result when trying it below centre.

Only thing is you cant drop it much below centre on small holes, or as you rightly say, you run into clearance problems on the heel of the insert.

Try it yourself sometime.

[Ian S CParticipant @iansc]

I feel that if the tool is a few thou below centre it will lift up a fraction to maximum radius, ie., the shallowest cut, if you must be of centre a few thou high seems ok, but with a stiff tool aim for centre height, I find this is the best for me, and there is no trouble with facing the bottom of holes, even when using a 1/8" wide 1" long boring bar turning out the inside of a little piston for a hot air engine from cast iron. Ian S C

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