Using a Between-Centres Boring Bar

**franco** · 9th January 2011, 12:57 AM

In an e-mail to Anorak Bob discussing the building of the small lathe-mounted slotter shown here:

https://www.woodworkforums.com/attach...y-cutter-3.jpg

I mentioned that I had used a between-centres boring bar to ensure that the ram guide for the slotter was vertically centered on the lathe's axis. He suggested that I enlarge on the concept in a post on the forum.

Years ago when specialised machines for boring were less common, boring of items like engine cylinders was often done on a lathe by mounting the item on the lathe cross slide or saddle and a boring bar between centres, to carry out the operation. The lathe's power feed was used to move the item past a tool in the middle of the boring bar as the lathe rotated. Many lathes were supplied with T slots on the cross slide or saddle for this purpose. An example of how this was done can be seen in the third illustration on this page:

Page Title

Because the full length of the item being bored has to pass over the tool mounted in the middle of the boring bar, the boring bar must be at least twice as long as the item being bored. This can cause problems if the boring bar is too light for the job because the bar can flex more as it gets further from the support of the centres and produce an hourglass shaped bore.

The tool bit has to be advanced by a measurable amount for each cut, and I have seen illustrations of some very neat arrangements for doing this - cannot lay hands on one at the moment though. With smaller boring bars where room in the bore is limited a common arrangement was to have a bracket which was clamped onto the boring bar after each cut, and which had a dial indicator mounted on one end which contacted the nose of the tool, and a screw on the opposite side of the boring bar which contacted the back of the tool, and advanced the tool by the required amount.

The boring bar in the first photo below is the one I used to bore the ram guide on the slotter illustrated in the other thread, though I don't think the tool in it now was the one I used on that job. Don't know where that one came from. I have no idea what happened to the tool feeder bracket - after all it is about 18 years since I made it - I'm surprised the boring bar survived two house moves and 18 years without being used as stock for some other purpose.

The second photo shows the tool bit mounting in the centre of the bar.

I was half expecting that the bar would not be stiff enough to produce a bore with a consistent diameter throughout, but with light cuts and a sharp tool it turned out OK. If it had not, Plan B was to overbore the ends and fit bronze bushes of the required size, which would probably have been a better job anyway than running a steel ram in a steel guide - it's not exactly a high speed operation likely to cause galling though! However, it works well enough that I have no incentive to modify it at the moment.

franco

**Anorak Bob** · 9th January 2011, 01:42 AM

Thank you Frank.

Bob.

**nadroj** · 9th January 2011, 09:07 AM

An interesting point is that an advantage of boring between centres ensures a hole that isn't tapered. Any misalignment of the headstock results in a taper, if the job is rotating with the lathe's spindle.
But, if the centres aren't in alignment, it could end up elliptical!

Jordan

**electrosteam** · 9th January 2011, 09:36 AM

Jordan,
A comment on boring bar adjustment.
Recently, cannot remember where, I saw reference to a between centres boring bar with a boring head in the spindle holding the headstock end of the boring bar.

The boring head is set on centre initially, the job rough bored with normal tool adjustment on the boring bar, then the boring head adjusted for fine tool trimming.
With a good boring head, you can dial-up the required amount of cut.

The headstock end of the boring bar moves in an orbital motion, but still circular.

Anyone actually used this approach ?
John.

**franco** · 9th January 2011, 10:06 AM

Originally Posted by electrosteam

Jordan,

The boring head is set on centre initially, the job rough bored with normal tool adjustment on the boring bar, then the boring head adjusted for fine tool trimming.
With a good boring head, you can dial-up the required amount of cut.

The headstock end of the boring bar moves in an orbital motion, but still circular.

John.

An interesting idea - must keep in mind to try it next time I need to use a between centres bar. It would certainly simplify getting the adjustment for the finishing cuts correct without the need to make a dedicated fine adjustment device. Thanks for that.

franco

**Anorak Bob** · 9th January 2011, 11:07 AM

Below is an image of a boring bar micrometer from a 1955 Moore and Wright catalogue. It shouldn't be too hard to replicate.

George Thomas, in his Model Engineer's Workshop Manual, describes in depth, the construction of an adjustable bar. In his bar he used a round HSS cutter located in a 45 degree skewed cross hole. He states an advantage of this arrangement is the is the ease of measurement using a standard micrometer.

His boring bars are far more complicated than Frank's simple version.

If you had a set up similar to M&W's, and Frank's bar, I imagine setting the cutter would be as simple as dialing in the measurement required on the mic, positioning the mic on the boring bar and then moving the cutter to touch the micrometer spindle end. Then lock the cutter in place and commence boring.

**Anorak Bob** · 9th January 2011, 12:02 PM

John,

Please ignore the flimsiness of this pretend setup but is this what you are suggesting?

Even if you had a head that accommodated 3/4" boring bars, I can't imagine that you would approach the rigidity of a between centre bar, especially on deep boring jobs.

Obviously the one great advantage of the boring head is it's ability to bore blind holes.

Bob.

**electrosteam** · 9th January 2011, 12:22 PM

Bob,
No, your photo is not what I described.

The boring head in the spindle that I remember had a centre fitted and a standard between-centres bar installed between the boring head centre and the tailstock centre.

The rigidity will be reduced, but only to that imposed by the boring head which is designed to operate in this manner, just horizontal instead of vertical.

The boring bar will sweep out a cone, tip on the tailstock and base at the headstock.

There will be a tiny change in the rake angle between the cutting tool and the job, but if the technique is only used for the finishing cut(s), that should not be an issue.

A clever way to do the job.
John.

Dave J · 9th January 2011, 12:27 PM

I can't see that being any better than boring the usual way from the tool post, other than for awkward parts that cant be set on a face plate or in a chuck.
I took his post as still using a boring bar set up between centres (like in the first post)but having a centre in the boring head instead of a boring bar.

Dave

Dave J · 9th January 2011, 12:32 PM

I just noticed John replied while I was typing.

I am not sure how good that idea would work in actual set up, as it's best to have the larges bar possible so it is rigid. Also I think it would be a pain to try to get the amount that the tool moves over compared to the boring head movement. It would have to be worked out with how far the cutting tool is down the bar.

Dave

Dave J · 9th January 2011, 12:50 PM

Franco nice write up

For those researching this on the net look under align boring as well.

Here is a step by step photo process of it for those that need a visual.
Auxiliary Spindle for the Small Lathe

Dave

**Anorak Bob** · 9th January 2011, 12:54 PM

It's sunk in.

**Bryan** · 9th January 2011, 01:19 PM

Well and good, if you want to bore a taper.

Dave J · 9th January 2011, 01:34 PM

No taper, the job is bolted to the cross slide and travels with the carriage over the tool, which cut's the same diameter along it's length.

Dave

**Bryan** · 9th January 2011, 02:01 PM

A cunning stunt.

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a boring head in the spindle holding the headstock end of the boring bar.

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