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Is there a chart for drill hole dia vs. boring bar size?

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William Ayerst24/07/2021 09:16:39
259 forum posts

Dear all, good morning.

I've been casting my mind back to boring my steam engine cylinder and standard and I feel like I must be going mad.

While I was doing it I must have bought about four boring bars in HSS and brazed carbide in 8mm or 5/16" (i.e. to fit my toolpost and centre height) and the only time I've had success was when I ground away most of the support under the cutting face on a HSS tool, and along the length of the shaft to allow it into the hole without rubbing on either the bottom of the tool (or underside of the carbide tip). This meant that it was very bendy and required many spring passes - still ending up with a very small taper in the bore.

The standard and cylinder were both drilled 1/2" before boring, one ending at 5/8 and one at 3/4.

Is it as simple as a 5/16" boring bar needs to start with a 5/8" hole? Is there some kind of formula or reference chart I'm just missing?

Many thanks,

Les Jones 124/07/2021 09:26:34
2234 forum posts
153 photos

You seem to be assuming that the cutting tip is level with the top surface of the boring bar. If you make your boring bar with the cutting tip close to the centre line of the boring bar then the diameter of the main part of the boring bar can be closer to the starting hole size.


Andrew Johnston24/07/2021 09:42:53
6235 forum posts
676 photos

Posted by William Ayerst on 24/07/2021 09:16:39:

Is there some kind of formula or reference chart I'm just missing?

There is no magic formula. Insert boring bars and carbide bars from professional tool suppliers will have a minimum bore specified. Bars from other sources will be pot luck. Les is correct; although these are internal threading bars the principle is the same, the cutting area is on the centreline of the bar, not at the top:


Likewise with this home ground HSS bar:


On insert boring bars the insert area is cut away, and slopes down, for the same reason.


Pete Rimmer24/07/2021 10:11:55
1047 forum posts
58 photos

You need space for the chips that are produced to prevent packing of the hole so for short pieces you can use less clearance but longer bores you either add clearance, take smaller cuts, use some kind of flush or retract the tool more often.

Tony Pratt 124/07/2021 10:33:16
1648 forum posts
8 photos

No magic formula exists as far as I know, just experience.


not done it yet24/07/2021 11:15:27
6279 forum posts
20 photos

If you are not happy with your boring efforts, I would suggest drilling, boring nearly to size and finally reaming to size.

Reamers are more expensive than boring bars, but provide accurate and reproducible results.

Rod Renshaw24/07/2021 12:02:51
301 forum posts
2 photos


I agree with others- no table or chart.

Stiffness is important in single ended boring bars to avoid one end of the bore being bigger than the other. If I remember correctly the stiffness of a circular cantilever goes up with the 4th power of the diameter, so a small increase in the diameter increases the stiffness by a large amount. eg 8 mm is about 3.2 times as stiff as 6mm.

Increasing the effective length of a cantilever reduces it's stiffness by the power of 3 so, eg., if a bar sticks out of the holder by 50 mm it will be one nineth as stiff as if it stuck out 25mm. ( i think that's right! )

Hence the practice is to to use the biggest bar you have that will fit in the hole and cut without rubbing, with the minimum length possible projecting from the holder, and change to a bigger bar part way through the work if that is possible.

A few years ago I bought some solid carbide boring bars from China which were very cheap at the time, and which take replacable carbide tips. Solid carbide is about 4 times as stiff as steel and my boring has improved. Not sure if these are still cheaply available since the VAT changes.

If the set up is not very stiff due limited kit etc., then light cuts and spring cuts works but takes a lot longer to get a reasonable result.

We are amateurs, who cares if it takes a while!


Clive Foster24/07/2021 12:28:55
2817 forum posts
101 photos


I've had similar boring tool clearance issues. Its very easy when grinding HSS tooling for small bores to underestimate the angle needed under the tip to clear the bore you have just cut.

I ended up doing a quick bit of CAD work to produce a diagram clearly showing suitable grind angles to ensure clearance on various sizes of holes.

For my drawing I standardised on 1/16, 1.7 mm clearance between the bottom of the heel and the hole and reckoned to use mostly 1/4" , 6 mm blanks for the cutters except for 3/8" to 9 /16" , 10 to 15 mm, holes where I drew it for a 3/8" , 4 mm cutter blank.

The small one needs 45° rake to get clearance in a 3/8" , 10 mm Ø hole.

With 1/4", 6 mm cutters you need:-

50° angle in 9/16", 15 mm Ø holes;

60° in 3/4", 20 mm Ø holes;

65° does 1" to 1 1/2", 25 to 35 mm Ø holes;

Beyond 1 1/2", 35 mm Ø holes 70° does fine. I don't see much point in using a shallower angle for larger holes as the gain in strength / heat capability is tiny and unnecessary multiplication of tooling wasteful.

Above 2" , 50 mm Ø holes I tend to switch to 1/2", 12 mm tooling, again at 70° angle.

Above 4", 100 mm Ø holes 80° works fine and cuts down on grinding.

Obviously this doesn't take into account the space needed for the bar or shank carrying the actual cutter.

In the smaller sizes the tip has to be ground integral with the shank so you will be doing well to accommodate a basic shank diameter of 2/3 rds hole size. Which will need a fair bit of careful grinding to maximise support and retain some space for chips. As the actual cutting tip has to be ground at the centre line of the blank things rapidly get crowded.

PM me if you want a PDF of the picture.


Andrew Johnston24/07/2021 12:42:42
6235 forum posts
676 photos

Posted by Rod Renshaw on 24/07/2021 12:02:51:

Stiffness is important in single ended boring bars to avoid one end of the bore being bigger than the other.

This was discussed a while back and concluded that it is a myth. The bore may not be the diameter expected, hence the need for spring passes, but it won't be tapered. If the cutting conditions remain constant why should a boring bar cut differently at one end of the bore compared to the other?

The deflection for a given boring bar with constant load is proportional to the third power of the cantilever length and inversely proportional to the fourth power of the diameter. So doubling the length increases the deflection by a factor of 8. Doubling the diameter decreases the deflection by a factor of 16.


JasonB24/07/2021 13:23:01
21315 forum posts
2419 photos
1 articles

There is a lot going for the double ended type boring bars, on a lathe that only has 5/16" between bottom of tool holder and ctr height you can still use a 1/2" dia bar which will be stiffer than a 5/16" one as the cutting edge of the toolbit is only a couple of mm above the bars ctr. The small HSS bits are easy to grind and cheap to buy or just use old ctr drill shanks.

Your 3/4" bore could easily take a 1/2" dia one of these, maybe even a 5/8" one depending on your toolpost. and a 3/8" one would have started the trunk guide before swapping to a 1/2" one for the last few passes

Not hard to make if you don't want to buy them, I did a lot of my early boring with this type and still use large ones (upto 28mm dia) for things like engine cylinders upto 6" long.

Michael Gilligan24/07/2021 14:00:05
18734 forum posts
916 photos
Posted by Clive Foster on 24/07/2021 12:28:55:



PM me if you want a PDF of the picture.




I don’t wish to intrude, but If it’s of any help … I can very easily convert PDF to .jpg and post it for everyone to see.


ega24/07/2021 14:33:16
2242 forum posts
186 photos

I, for one, would like to see Clive's boring picture (and to know what MichaelG uses to do the conversion).

PS The OP would do well to read GHT on this topic generally.

Edited By ega on 24/07/2021 14:34:37

Clive Foster24/07/2021 14:50:34
2817 forum posts
101 photos


Thanks for the conversion offer but its something I can manage using Preview on the Mac.

Didn't really want to make it public 'cos its scrappy and something I'm a bit ashamed of. However if folks promise not to be too derisory :-

boring tool clearnace 150.jpg


Doesn't really come out well enough for my tastes and pride! Needs a serious re-work to make a nice jpg. Vectorworks produces a decent pdf of drawings but they don't convert cleanly.


Edited By Clive Foster on 24/07/2021 14:52:09

Rod Renshaw24/07/2021 15:25:25
301 forum posts
2 photos


I must have missed that thread.

I was told about tapered bores 40 years ago by an apparently authorititive source and believed him without thinking it through. Once you spelled it out there is clearly no reason for it to happen.

Whenever I have bored a hole and then checked it I congratulated myself on my technique!

I still think it worthwhile to try to have a stiff boring bar though as this allows a decent depth of cut and speeds up thr job, and also reduces or eliminates chatter and one gets a good/ better surface finish.

Thanks for correcting my error on the reduction in stiffness caused by length. I got my 2* 3 and my 3* 2 mixed up, for which I can only blame old age and the heat.


Michael Gilligan24/07/2021 15:25:32
18734 forum posts
916 photos

That’s entirely ‘fit for purpose’ Clive yes


Clive Foster24/07/2021 17:10:51
2817 forum posts
101 photos

To be fair about the tapered bore issue the operative words are, as Andrew states :-

"If the cutting conditions remain constant ..."

Which can be a little tricky for the home worker using a lighter machine to achieve. Especially when you factor any inexperience in both tool setting and grinding into the equation. At the small end of the scale its quite easy to create a rub prone tool shape that tends to ride away from the cut as boring proceeds due to requiring more stiffness than the tool possesses to hold it into cut. The tool may well appear to cut correct to size as it enters the work but, as the cut moves deeper, will jack itself out of cut until the tool shank stiffness is sufficient to generate enough force to stabilise things.

Generally such is an unfortunate combination of tip radius and clearance angles accentuated by a lack of sharpness. Hand honing a tool on the machine so "its really sharp for the last pass" can be counter productive. Its awfully easy, especially with a small tool, to get the honing angle just a touch off and turn a bluntish, but still cutting, tool into something whose properties are more akin to a scrubbing brush. Something I discovered to my mortification and Big Dens' great amusement more years ago than I care to admit. Big Den believed that the most effective teaching technique, once the basics had been shown, was to let folk get it wrong then show them how it should be done. He reckoned shame and embarrassment great motivators.

Then there was the time when an unfriendly piece of alloy produced a truly spectacular built up edge on the last cut which wasn't good for holding size.

Maybe there was some excuse with the piece of "found, just the right size and hard too" steel that had a soft core with thick hardened outer. Naturally muggins wanted a bore about 20 thou into the hard stuff which took great delight in blunting the tool as the final cut proceeded. Its dangerous to assume a hard outer on a soft core is a simple thin case hardened skin.

Personally I dislike spring passes. Its near impossible to hold real sharpness and exact shape for the last tenth or five with home ground tools so cutting conditions tend towards some indeterminate combination of cutting and rubbing. I suspect only the super-sharp carbide inserts wisely and frequently suggested for use with aluminium and its alloys are capable of proper, repeatable, cutting under such conditions on smaller machines. But with such tooling and a lathe with any reasonable pretensions to accuracy spring cuts won't be needed as the cut will be the size dialed in.

Needing spring cuts is invariably an indication of certain infelicities in the process. Whether machine, tooling or operator. I'm fortunate in that both my lathes are capable of both hefty cuts and holding tenths given half a chance.

Which rather cuts down on my margins of excuse when things go wrong!

Aiming to finish off with a proper cut rather than a half whisker shaving works best for me. If I have extra material I'm more likely to just bin a job thats half a thou(ish) out and make the necessary corrections on the finish cut dial position rather than futz around.

Most folk, especially those with lighter, often elderly, machines generally don't have that option but learning to work to size directly off the dials rather than cut, measure and skim is always worth while. If nothing else it saves a lot of time. Even if you do still have to spring pass occasionally.


old mart24/07/2021 20:32:05
3316 forum posts
203 photos

In my experiance using many different bars with carbide inserts, I would say no. Different shape inserts fitted to identical diameter bars project different ammounts. I try to mark each container with the minimum size hole that that bar requires, found by trial and error. The catalogues from the main industrial manufacturers usually state the minimum hole size for each of their products.

Andrew Johnston24/07/2021 22:02:42
6235 forum posts
676 photos
Posted by Rod Renshaw on 24/07/2021 15:25:25

I was told about tapered bores 40 years ago by an apparently authorititive source and believed him without thinking it through. Once you spelled it out there is clearly no reason for it to happen.

It's unfortunate that in many areas (not just model engineering) there are statements that may have been true, but the specific circumstances have been lost in the mists of time and they then become generic. I was aware of the tapered bore issue, but was always puzzled that I never seemed to produce one, even with significant overhang. Chatter, yes, but not tapered.

I'd agree that it makes sense to use the largest diameter and stiffest boring bar available in any given situation.


not done it yet25/07/2021 09:21:04
6279 forum posts
20 photos

I'd agree that it makes sense to use the largest diameter and stiffest boring bar available in any given situation.

The main problem arising is that most 25mm boring bars are not made to fit a lathe with an 8mm toolpost.🙂

I believe the most recent case of a ‘learner’ getting a tapered bore was due to using the compound slide for feeding the cutter?

JasonB25/07/2021 09:29:00
21315 forum posts
2419 photos
1 articles

You don't have to mount a big bar in your tool post, a simple length of square stock bored for the bar and split can be clamped directly to the top of the cross slide. Or in my case I did out the 4-way that came with the lathe and use one of the tool slots as a "vee block" and clamp the tool shank to that.

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