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How to centre a boring bar

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David Hatfield 112/08/2014 21:43:07
5 forum posts

I'm sure this is one of those stupid questions but here goes anyay:

I have very limited experience of lathe work but have managed to accurately centre several cutting tools. Howver I'm not sure how to centre a boring bar. I have an A2Z tool post and a Sumitoma boring bar. The only way a can think is to drill a center hole in some scrap bar and try and use this to centre the boring bar. Can anyone suggest how best to achieve a true centre on the bar?

Thanks and apologies if this has already been covered (I did check back posts but couldn't find anything relevant.



John Haine12/08/2014 22:07:46
4189 forum posts
242 photos

What do you mean by "centre" the bar? Do you mean align it's long axis on the spindle centre line? Or the insert cutting edge at centre height? Or the long axis parallel to but not necessarily at the same height as the spindle axis?

The Sandvik boring bars I have have a flat on their top and bottom surfaces which ensures that when clamped in the holder the insert is correctly oriented for the right rake angles. I set the tool height in the same way as for a normal turning tool using a height gauge. All that's left is setting the tool axis relative to the spindle axis, which you can do by setting the tool post square to the topslide, or even just by eye.

does this help, have I understood your question?

Clive Foster12/08/2014 22:31:57
2838 forum posts
103 photos

I find a simple optical tool centre height indicator versatile, effective and easy to use. Easy to make too and inherently right with no tricky adjustments. Mine is a "lightly modified to use components to hand" version of a design featured many years ago in Model Engineers Workshop. Probably second or third year of publication.

Concept is to set a suitable size blade of thick perspex, or other transparent plastic, vertically upright on a base heavy enough to stay put but small enough to slide into the right places on the lathe. Within reason the thicker the perspex is the better.  Mine is only 4 mm, 10 or 12 would have been much better. Using a sharp centre in either spindle or tailstock scribe a line on each side of the perspex blade. Depending on the machine you may need to put a bridge piece across the ways to support the gauge or you may find it conveinient to work off the cross slide. I put two sets of lines on mine, one at each height. Arrange a small mirror at 45° so you can look down and see both lines in the mirror.

To set centre height put the gauge in a position where you can see both lines and the tool tip in the mirror. Adjust the tool height until the images of the tip and both lines co-incide so effectively you see a single line running across the cutting edge. Tool is now on centre height. Big advatage of this device is that tool tip and gauge don't have to be right up against each other, anywhere within a couple or three inches will do so long as you have a clear line of sight. It works fine if you simply brigh the tip up to lightly touch the appropriate line but obviously you don't want to scratch things.

Three (poor) pictures of mine may help illustrate the idea. Two more in my photo album.



in use.jpg


gauge on lathe.jpg

Extra lines and associated mirror bracket fixing holes are for other uses.  One is in wrong place.  Ooops!


Edited By Clive Foster on 12/08/2014 22:36:08

NJH13/08/2014 00:12:12
2314 forum posts
139 photos

My quick and simple solution was to make this:-

Tool Height

Works for turning or boring tools.

Blade was set to centre height and locked.



Edited By NJH on 13/08/2014 00:13:52

_Paul_13/08/2014 01:31:27
543 forum posts
31 photos

Clive I do like your optical centre height indicator I can think of a situations when that will be handy, I do have a question though when cutting the mirror to size is that classed as breaking said mirror and subject to the superstitious 7 years bad luck?


mechman4813/08/2014 06:12:26
2933 forum posts
460 photos

Get a small mirrored mosaic wall tile ... small inspection mirror... ? thinking.


JasonB13/08/2014 07:31:47
21467 forum posts
2455 photos
1 articles

Use one of your already centred tools to scribe a light line across a faced bit of bar in the chuck and set your boring bar tip to that.

If the tip is angled as it is on a lot of indexable bars then the cutting point on the tip wants to be on ctr


Clive Foster13/08/2014 08:42:48
2838 forum posts
103 photos

Given that my shop gremlin colony is large, hyper-active and on a 50 speed diet I didn't notice any bad luck effects from breaking the original mirror and cutting a small piece out of the remains. Probably past 7 years by now anyway.

If you use insert tooling it might be just as easy to make a simple block toolpost dedicated to that tool, or retrieve the factory 4-way if your lathe came with such, and just swop the toolpost over when boring. Leaves the expensive QC holders for other duties where the ready adjustability is important. Inserts of any quality are accurately sized so no need to move the shank once set.

I find the general run of small insert boring tools over-long making them prone to chatter and resticting cut. Three or four inches overhang isn't needed when boring short, inch or less long, holes. If you do go the dedicated route might be worth experimenting with a u section sleeve to wrap round the overhang portion and clamping on that as well as on the square shank. Might be a bit tricky to get it just so but should reduce considerably cut chatter tendencies. Such a dedicated post arrangment would be ideal for rear mounting especially useful if it is made so as to turn an accurate 180° and also carries a parting off blade. Its been reported that an inverted boring tool produces better results in smaller holes than the conventional type due to chips falling clear of the tool rather than settling on top.


Robbo13/08/2014 09:54:19
1504 forum posts
142 photos

For those interested, Clive's Perspex height gauge was in an article by Ted Wale in MEW 102, Nov 2004.

Do I remember correctly that Ted is from Nova Scotia?


Jon Gibbs13/08/2014 10:28:38
739 forum posts

I really like the perspex sighting device and can see it being useful but am I the only one who thinks the blade type is a bit superfluous?

Isn't it just as easy and quick to stick a sharp centre in the tailstock and reference off that?

What am I missing?


Chris Trice13/08/2014 11:13:42
1362 forum posts
9 photos
If you don't have any gadgets to set the tool, stick a centre in the headstock spindle or face the end of a piece of bar leaving just the smallest pip in the centre.
Doubletop13/08/2014 11:31:00
437 forum posts
4 photos
Posted by Chris Trice on 13/08/2014 11:13:42:
If you don't have any gadgets to set the tool, stick a centre in the headstock spindle or face the end of a piece of bar leaving just the smallest pip in the centre.

Unfortunately with indexed tips you are just as likely to break the tip off the tool before you get it right.

As most of us should have a 6” rule John B's method of testing against a bar should be achievable. I don't know why I don't do that on the lathe, I do it for quick centering of bar in the mill.



Edited By Doubletop on 13/08/2014 11:31:47

Clive Foster13/08/2014 12:20:59
2838 forum posts
103 photos


Thanks for crediting Ted Wale with the optical centre gauge design. Saved me digging out the back numbers this evening. Students of how to miss the b.... obvious may be amused to know that I used devices of similar principle in the optics lab for many years without realising that the principle easily translated to a lathe tool setting gauge.


Referencing off a tailstock centre is fine if you have room to swing the tool post round. Many folk scribe a line on the tailstock barrel and use that, some machines were factory fitted with such a line.

To my mind the biggest advantage of the optical gauge is its ability to work at a distance from the tool tip. The angle of view through the mirror is good with negative rakes too. All other systems require you to either get close and personal to the tip or set-up the lathe to help. If needing to gauge a tool mid job this frequently isn't practical. In an ideal world one would be able to set all the tools needed for a job before starting. Far as I can see the rule for that is X + 3 holders where X is the number of holders you have!

The trapping a ruler method is sensitive to stock diameter, 1/2" is about largest before you start getting noticable errors.


Ian S C13/08/2014 12:52:41
7468 forum posts
230 photos

Not long after getting my lathe I fitted a centre in the tail stock, and scribed a line across the blade of my 6" square, Some time after that I added another line at the centre height of my 6" Vertex rotary table(3 mm higher than the lathe). Ian S C

John McNamara13/08/2014 13:07:44
1314 forum posts
113 photos


The right height for a boring bar should it be on centre or a little above or below?
Sometime if the bar is long and slender I find a tiny bit above works better to resist the tendency of the tool to dig in and chatter.

One rule that should not be broken is to keep boring bars sharp if ground or with a nice unworn replicable insert. A small tip radios rather than a large contact area at the cutting tip will usually chatter less and give a better finish particularly when working with small holes and thin boring bars that flex.

There is also the problem of making sure the body of the boring bar clears the hole and allows the chips to clear. Ideally this should not be a problem if you have the correct size bar for the job. However needs must....
Sometimes you have to tweak the rules using the tool you have to hand to get the job done. Just start slowly and see what you can get away with!


Nigel Bennett13/08/2014 13:18:34
416 forum posts
11 photos

I have often machined holes with a boring tool in my lathe, setting the tip height well above centre height. Yes, the tool geometry is probably wrong, but it means that it is possible to have a little more meat on the tool to support it without it rubbing on the bore. Meatier tools are always an advantage in deep bores.

If the boring tool height is way out, remember that the amount of metal you take off will be a bit less than the cross-slide dial (or DRO) would have you believe.

The tip was given to me by my apprentice school turning instructor, rather more years ago than I care to remember.

Hopper13/08/2014 13:21:09
5505 forum posts
137 photos
Posted by Bogstandard2 on 13/08/2014 10:53:56:

Unfortunately, over the last few years, and especially with boring bars using replaceable tips, some of the methods are now no use for setting, as the tips have negative top rake.

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.

They are made like this to provide more clearance for high speed CNC work etc. It took me a few broken tips to work this out. But now I find if the boring bar is not behaving itself, lower it some more below centre.

Works for me.

JasonB13/08/2014 13:28:49
21467 forum posts
2455 photos
1 articles

I have always set the tip of mine on ctr height and not had any problems. Also a lot of these bars are said to be capable of facing the bottom of a blind bored hole, can't see how they can do that with the tip below centre?

Also that imaginary line will constantly vary depending on the dia of the hole as its being cut and wil only be right on teh finished cut??


Edited By JasonB on 13/08/2014 13:31:26

Jon Gibbs13/08/2014 13:58:42
739 forum posts
Posted by Clive Foster on 13/08/2014 12:20:59:



To my mind the biggest advantage of the optical gauge is its ability to work at a distance from the tool tip. The angle of view through the mirror is good with negative rakes too. All other systems require you to either get close and personal to the tip or set-up the lathe to help.



Many thanks Clive - I can appreciate both the "at a distance" advantage and the elegance of the of the optical method.


Marcus Bowman13/08/2014 14:30:54
174 forum posts

I sometimes use a Laser Centre Finder in the tailstock chuck. It shines its beam right along the lathe axis, so you want to adjust the cutting edge to split the beam, so to speak. I use a Polarising filter to get a really small beam.


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