Boring between centres

One of the first jobs I need to do when my ML7 is together (at long last), is to make and use a boring bar.

I have seen several photos of between centres boring and they always show the boring bar held in the 3 jaw chuck and a live centre at the tailstock end. I was a bit bothered about the associated run out of a 3 jaw. However I did satisfy myself that even if the bar is running out of true at the chuck end, any boring of items on the saddle would be unaffected by this.

Or have I missed something? Maybe a second order effect? I still have the urge to use the 4 jaw and get the bar running true at the chuck end. Is this a waste of time as I suspect it might be?

Andrew.

It makes no difference. Infact some body here suggested using a boring head to support one end of the bar as this allows for fine adjustment of the cutting diameter.

Certainly a second order effect. Makes the cut a tiny bit bigger than what would be expected from measuring the tool bit projection.

Relatively easy to calculate the magnitude of over cut for any given run out. Unless the chuck is very poor odds are that it will be un-important compared to the other frustrations involved in trying to bore exactly to size with such bars.

Using the chuck rather than centres means you cannot remove and replace the boring bar in exactly the same place without special precautions. Worst point when using between centres type bars is that its pretty much impossible to easily make decently accurate measurements of the bore without removing the bar.

General advice is to get the bore as close as maybe then turn the mating part to size.

Clive.

Hello Clive,

Many years ago I used a boring bar which had a "micrometer" adjustment for the tool. This made life a lot easier and I could bore without the usual sweat and finger crossing. I have never seen one since, but having looked in GHT's book on workshop practice. There is one described there, the type I used is much more like the ones shown in the Hemingway catalogue. It certainly beats the slacken off, pray move cutter , pray again and then tighten cutter!

Andrew.

Andrew,

There is no need to pray. Just measure across the bar and cutter tip using verniers or a micrometer after a cut then after the tip adjustment. The difference will give you the cut depth - no need for fancy micrometer adjustments etc.

Phil H

You can hold the boring bar in the four jaw chuck and deliberately run the bar off centre as adjusted by the chuck in order to make fine adjustment on the diameter of the hole bored. (note: fine adjustment, not big ones.)

Andrew,

Microbore by name and microbore by nature. I still use them on between centres bars driven by the usual "dog"

**LINK**

Not cheap, then nothing is these days.

Edited By KWIL on 01/05/2017 09:58:05

I'd be worried by the fact that the bar is going to be slightly bent if it is properly engaged with the running centre and the 3J is not dead true. This must put an unwanted side load on the centre bearings, and could even wobble the taper out of the socket slightly. I'd either run the bar between centers or true it up in a 4J.

Simples. Only stick the bar into the chuck a short distance and nip it up loosely, then put the tailstock centre in place, then finish tightening the chuck. No different from turning any shaft with a centre in the end.