How to centre a boring bar

Many thanks to you all and my apologies for not giving more details in my original message. I had managed to centre my tools by the simple vertical ruler method but as the boring bar runs parallel with the lathe bed I was unable to do so with the bar.

I liked Clive's optical indicator but still unsure of potential problems if the mirror should break! Despite that I have found some Perspex and may have a go at making one. I tried Normans height measure, the two suggestions of a centre tool in the tailstock and Ian SC suggestion for scribing and all these give a similar result. For ease of use I will stick with the headstock centre tool meantime.

I did like the idea from Marcus of a laser centre finder. As my eyesight is less than perfect it may be a useful addition to the workshop. I wonder if it was 'off the shelf' or home made?

Following all your help I tried boring a 32mm diameter and 35mm deep hole in a brass bar. I used a centre drill and then went on directly to a 18mm drill as a previous post suggested was best and to avoid pilot holes. Anyway I have managed to complete the job to the correct measurements - BUT: I cannot think of a way to flat face the bottom to remove the drill tip imprint. I have looked at several bulletin boards and the general view appears to be to live with the imprint rather than risk damaging the bored hole. I wonder if anyone has any thoughts on a 'simple' way of producing a flat bottom?

Thanks again to everyone for their help

I just run my boring bar across the bottom as you would when facing a bar, all done with a bar that takes the CCMT type tips and no tips were hurt

Edited By JasonB on 13/08/2014 18:06:48

For what my comments and methods are worth:

I made a Centre Height Gauge, similar to the one shown by NJH.

I set all my lathe tools to it, including replaceable tip boring tools, and am unaware of any real problems.

The bottom of a drilled hole can be faced flat by taking a series of small facing cuts, out to the cross slide setting of the finished bore. Obviously, starting near to the outer diameter of the bore, and moving the start point closer to the centreline as each cut is completed.

Eventually, the drill tip mark can be made to disappear.

If there is a small through hole, or one can be tolerated, then it is even easier, but it can be done with a blind bore.

Howard

If I need a flat-bottomed hole, I generally start off with one before boring by selecting a drill the same size or slightly less than an appropriate end-mill. Then run the end mill down after drilling to take out the drill depression.. Bore up to that surface as a limit then take a final surfacing cut.

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.

Niko,

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

MichaelG.

Michael,

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

Niko.

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.

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