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Effect of Tensioning a Boring Bar

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Fatgadgi20/08/2017 20:16:18
178 forum posts
26 photos

Hi John

Agree with your points - anything not claimed but put into the description prevents other people coming behind them and trying to patent other key features of their idea.

But many companies use the description in an offensive way as well as the claims.

As you say, you can only patent one invention per one patent, so adding more novels things to the description gives the opportunity to add things to the claims if the examiner complains, or even to pull off divisional patents further down the process, where the patent can be split into two (or more) if there are more than one potential invention.

I was trying to knock out a German competitor's granted patent in a hearing at the EPO in the Hague a few weeks ago. I won the battle when their key claim was deemed to be non-inventive, only to then loose the war when they managed to get agreement to move a point from the description into the claims.

I hate patents.

Cheers - Will

SillyOldDuffer21/08/2017 14:19:04
8512 forum posts
1914 photos
Posted by Michael Gilligan on 20/08/2017 19:41:57:


dont know Thinks ...

Perhaps we should be considering the Dore/Hemingway boring bar's torsional stiffness, rather than its bending stiffness as a cantilever. ... Discuss please.

Dunno about Dore/Hemmingway Bar, but this is what happened in my experimental set-up.

I modified the screw-tensioned pipe by turning a 4.5mm pin on the end of a 700mm long 10mm diameter steel rod. This was fitted to the pipe to provide a turning moment thus:


The other end of the turning bar was fitted with a pin so the deflection could be measured, with and without weights. I also applied the 'twang' test and measured how long it took the turning bar to stop vibrating.


The results:


The graph shows that, up to a point, increasing the pipe's tension increases deflection and also the time vibration persists!

But, in criticism of my experiment, the graph may be showing the effect of compression on the retaining pin rather than the effect of tension on the pipe. The pin bends. Back to the drawing board. Again...


ega21/08/2017 14:37:19
2500 forum posts
200 photos

Thanks to jimmy b and all the others for this fascinating glimpse into the world of patentese.

Bearing in mind that the original topic was *small* diameter boring bars my sense is that most of the methods of reducing chatter proposed would be impractical. The good old between centres boring bar may be the answer for the amateur.

Michael Gilligan21/08/2017 15:15:16
20108 forum posts
1043 photos

After four pages of speculation ... Wouldn't it be nice if Hemingway could tell us what they actually mean by "surprisingly rigid".


jimmy b21/08/2017 15:51:08
780 forum posts
42 photos
Posted by Michael Gilligan on 21/08/2017 15:15:16:

After four pages of speculation ... Wouldn't it be nice if Hemingway could tell us what they actually mean by "surprisingly rigid".



Martin Kyte21/08/2017 16:03:58
2728 forum posts
48 photos

Maybe they mean surprisingly rigid in spite of having a hole up the middle.

regards Martin

Michael Gilligan21/08/2017 16:39:59
20108 forum posts
1043 photos
Posted by Martin Kyte on 21/08/2017 16:03:58:

Maybe they mean surprisingly rigid in spite of having a hole up the middle.

regards Martin



That is what I have always suspected ...

i.e. we are wasting our time discussing "advertising puff"

... as I hinted on the 19th, if it was quantifiable they would presumably have known to use "stiff" not "rigid".

[by convention, a rigid body is one of "infinite stiffness"]


JasonB21/08/2017 17:59:45
22603 forum posts
2643 photos
1 articles

I suppose the only way we will ever know is if someone makes two boring bars, one with a grub screw in from the cutting end to retain the tool and another using the long hole and rod from the opposite end.

Make similar cuts using the same cutting tool and have a quantifiable way of measuring the results regarding deflection and chatter.

Anybody got some old Dore literature that may shed some light onto whether the claims about these bars are true or not?

ega24/08/2017 10:24:08
2500 forum posts
200 photos


"Anybody got some old Dore literature"

I don't think the original Dore bars had this feature (the idea of a long hole with push rod was probably already known when Arnold Throp came up with his castings).

See my 14 August post in the Dore small boring bar thread.

Edited By ega on 24/08/2017 10:27:18

Graham Meek06/02/2020 12:24:20
465 forum posts
300 photos

By way of the "Proof of the pudding is in the eating".

I recently had a series of bored holes in a bronze job that was causing me problems with chatter. My FB2 Mill being a geared head is not the best for boring holes especially combined with a single phase motor. However I have managed to get a good finish with my boring heads and boring bars in the past on Mild steel, Brass, Cast Iron and Aluminium.

fig 14 a selection of boring tools.jpg

The standard Emco design and my variations are shown above. All use an M4 grubscrew to lock the HSS tool bit.

While all the boring bars are made from Silver Steel only one is hardened, and one might think this last version would give a better result as regards less chatter. Using this bar gave little improvement, but there was an improvement.

After a lunch break I remembered I had some boring bars I made when I had my Unimat 3. These were made about the time George Thomas was writing a series of articles in ME on boring bars. His "design", (maybe cribbed from an earlier design), used a central push-rod to grip the tool bit. Again these bars are all made from Silver Steel, but left soft.

Thus a bar of the same length was retrieved from the stores and the boring bit currently being used, (continuity) inserted. The difference straight out of the box took me by surprise. The job was then completed without further problems.

I must conclude that in use the hollow boring bar under tension, does give a better result than a standard plain bar.



Martin Connelly06/02/2020 13:01:00
2125 forum posts
222 photos

Isn't the centre push rod under compression? Resonance is caused by a variety of factors, with a VFD it is easy to change rpm and go into and out of resonance. The stick out and stiffness of a boring bar, the rpm used, the depth of cut, the feed per rev, the geometry of the cutting edge and the material all have an effect. Change any one and you will get different results. I have read that putting a lump of plasticine on a boring bar can reduce chatter but have not resorted to it yet.

Martin C

Bill Davies 206/02/2020 13:46:18
278 forum posts
11 photos

Not tension, but... years ago, the company I worked for produced a small hobbing machine, which was being for producing gears for a brand of power drill. There was some chatter which the designers couldn't cure, so they brought in some engineers from a nearby university. They determined the resonant frequency and put a thin shaft up the bore of the hob spindle, with a mass on the end to vibrate in antiphase. It worked.


duncan webster06/02/2020 13:48:55
3945 forum posts
63 photos
Posted by Graham Meek on 06/02/2020 12:24:20:

By way of the "Proof of the pudding is in the eating".


While all the boring bars are made from Silver Steel only one is hardened, and one might think this last version would give a better result as regards less chatter. Using this bar gave little improvement, but there was an improvement.


Stiffness is governed by shape and Young's modulus, nothing to do with strength/hardness

ega06/02/2020 14:04:39
2500 forum posts
200 photos

In his ME manual GHT recounted how he had been challenged on the use of silver steel for his boring bars. Whilst, I think, conceding that they were no stiffer than if made in MS he preferred silver steel for its greater resistance to abrasion and the fact that it was more likely to hold up to nominal diameter (his bars were clamped in eccentric holders and a sloppy fit would have been unhelpful).

Edited By ega on 06/02/2020 14:05:05

Michael Gilligan06/02/2020 15:14:42
20108 forum posts
1043 photos
Posted by Martin Connelly on 06/02/2020 13:01:00:

Isn't the centre push rod under compression? […]


Yes ... and therefore the outer ‘tube’ is in tension


Martin Connelly06/02/2020 15:50:11
2125 forum posts
222 photos

It is a system not just a tube. Some of the system is in tension and some in compression. This is to some extent the same as a piece of hot formed bar where the core and the surface may have part in compression and part in tension. It may change the resonant frequency of the boring bar but thinking it is due to tension alone seems debatable. Best bet is the biggest diameter you can get away with.

Martin C

Howard Lewis06/02/2020 16:26:36
6032 forum posts
14 photos

Being a simple fellow, this is my take on the situation.

I imagined a hollow boring bar to be a tube, with a threaded rod running through the centre, and fixed at the "inner" end with a centralising "top hat" sleeve, free, at the other end.

With all elements being circular, and symmetrical, the stiffness would be the same in all planes.

In the static state.

When the nut is tightened, the threaded rod is subjected to tension, and the outer tube to compression..

In the dynamic state, ie cutting, the tube is subjected to a, presumably, predominantly vertical downwards force

The tube then becomes a truss in which the lower half is under compression whilst the upper half is under tension, which is increased by the tensile load in the central rod. (Compare with the bracing wires on a bi plane ).

On the other hand; if the central rod is free at the "inner" end, and threaded through a fixed plug at the "outer" end, when the central rod is rotated, it tries to enter the tube, and by exerting force on the fixed, inner, plug, the rod comes under compression whilst the tube is subjected to tension.

The situation is then reversed, with the central rod being a strut, and the location of the compressive and tensile loads in the tube, when cutting are reversed. This would tend to decrease any tendency to bend downwards when subjected to cutting forces.

Tightening or rotating the central rod in the cases above, will slightly affect the stiffness of the assembly. This is evidenced by the small change in deflection as the load is increased, reported earlier in this post..

The change in stiffness will change the resonant frequency, but being only small, by only a small amount..

Adding weights, will increase the Moment of Inertia, and this decreases the resonant frequency.

Adding plasticine will only marginally increase the Moment of Inertia, slightly lowering the natural frequency, T he stiffness will increase marginally, but will add damping, which reduces the amplitude of vibration..

Filling a bar with fluid and sealing will mean that any deflection of the tube, is unlikely to produce any great change in volume, and therefor pressure within the tube. The fluid may well have a damping effect since the fluid may be expected to flow, aided by the effects of viscosity of the fluid.

A suitable comparison.MAY be that of the viscous torsional damper, where movement of the inner and outer elements, relative to each other, is resisted by the viscous fluid between the two parts. The energy absorbed in this process, in damping the movement will reappear in the form of heat and raised temperatures. The fluid is effectively acting as a friction brake between the two elements.

Have I understood things correctly?


Michael Gilligan06/02/2020 16:41:24
20108 forum posts
1043 photos
Posted by Martin Connelly on 06/02/2020 15:50:11:


It is a system not just a tube. Some of the system is in tension and some in compression.


Yes, I realise that, Martin

You posed a specific question, and I tried to respond helpfully.


Graham Meek06/02/2020 17:24:49
465 forum posts
300 photos

My initial post was done by way of a Practical Working example. All boring bars were the same length, diameter, cross section and material. As was the material being worked on and the same depth of cut and rate of feed used throughout.

The hardened bar was by way of an example, that there was no real gain. Even though there is a small change in Young's Modulus during heat treatment. A change that is usually so small that it is ignored, but none the less a change. I don't think I was advocating every boring bar should be hardened.

The only thing that changed was how the tool bit was held in the boring bar. In the final solution the push rod clamping the tool is in compression. What is resisting this compression load?

There has to be an "Equal and Opposite" load somewhere, or Newton got it wrong. The opposite to compression is tension, therefore the outer wall of the boring bar is under tension. If not then the tool bit would just drop out.

Whether this tension alters the resonant frequency, was not as far as I can see mentioned in my original post. However having worked with "Tuned" boring bars I suspect that this is what is really happening and was why I initially tried this type of boring bar.

Thus as I mentioned earlier the results of a bar under tension is better than one which is not, given the same Working parameters.



old mart06/02/2020 22:00:26
3724 forum posts
233 photos

Sandvik have long marketed tunable boring bars, we had them where I worked at least 20 years ago. I make my own solid carbide ones with steel heads silver soldered on and also tungsten Densimet bars in 12mm and 1/2". The Densimet can be machined and tapped. Both types are stiffer and denser than steel, that is their advantage.

Edited By old mart on 06/02/2020 22:07:02

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