Effect of Tensioning a Boring Bar

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Martin Connelly on 06/02/2020 13:01:00:

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

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

MichaelG.

[Martin ConnellyParticipant @martinconnelly55370]

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 LewisParticipant @howardlewis46836]

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?

Howard

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Martin Connelly on 06/02/2020 15:50:11:

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

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Yes, I realise that, Martin

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

MichaelG.

[Graham MeekParticipant @grahammeek88282]

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.

Regards

Gray.

[old martParticipant @oldmart]

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.

 

https://www.google.co.uk/search?q=anti+vibration+boring+bar+design&tbm=isch&source=univ&sa=X&ved=2ahUKEwjT_8nU973nAhWCGuwKHV1zAzEQsAR6BAgJEAE&biw=1591&bih=839

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

[ken king, King DesignParticipant @kenkingkingdesign]

I remember that in late 50's/early 60's at the College of Aeronautics (as was), Cranfield, experiments were being carried out to examine and quantify the increase in strength of hollow struts which could be achieved by pressurising them. Being struts they were by definition loaded in compression, and the pressure would induce lonitudinal tension stress as well as hoop stress. If any of you can make a connection between this and boring bars I'd like to read it,

Cheers, Ken

[Michael GilliganParticipant @michaelgilligan61133]

Posted by ken king, King Design on 06/02/2020 23:03:01:

I remember that in late 50's/early 60's at the College of Aeronautics (as was), Cranfield, experiments were being carried out to examine and quantify the increase in strength of hollow struts which could be achieved by pressurising them. Being struts they were by definition loaded in compression, and the pressure would induce lonitudinal tension stress as well as hoop stress. If any of you can make a connection between this and boring bars I'd like to read it,

Cheers, Ken

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My tentative, but hopefully not too tenuous connection is that both examples are conceptually similar to ‘pre-stressed concrete’ beams … and that there may be ‘accessible’ descriptions available of of those.

I personally don’t have the math to do the calculations for a pre-stressed boring bar, but the concept seems reasonably intuitive .

MichaelG.

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P.S. We once had a forum member who was willing and able to produce Finite Element models; but he left when the chorus of ‘practical engineers’ decried his contributions.

[Neil LickfoldParticipant @neillickfold44316]

I can't do the math, but liken it to a plastic bag tube. Put pressure in it, and it is substantially more rigid than when the pressure drops, etc

Sandvik did boring bars with oil and a piston. As it is being used, (in a lathe) you can adjust the pressure and tune it to the situation it is in. I have not made one yet, am thinking about using it to fix a bar and see what happens.

When I get it done, will post the results here. The bar I am going to try has a 12mm diameter shank and a min bore diameter of 14mm. It would be great to be able to do the math t hing and figure out that if it has 120psi it becomes this rigid for example.

Neil

[Graham MeekParticipant @grahammeek88282]

Sandvik is a very good site for an insight into tips on the boring operation. Whilst biased more for industry there are a few gems of information hidden in the text.

There were several things that I picked up on. One the L/D 4 bar protrusion for a standard bar. Another which GHT seems to have got right form the start, is that Sandvik recommend mounting bars in sleeves rather than just clamping them in the toolholder.

Another thing I found interesting and one which I have found from experience. Was the size of the tip radius should not exceed the depth of cut. Lastly the diagram showing the tool forces during boring. These show, IF, the magnitude of the arrows is correct. That more of the cutting force is trying to push the bar out of the cut, ie along the diameter, than is trying to deflect the bar downwards. Obviously the resultant force will be combination of the two, (Vector Analysis).

Regards

Gray,

[Henry BrownParticipant @henrybrown95529]

Won't comment on any of the previous but when I worked all boring bars were made in house, they were upto three feet long and a couple of inches AF, sometimes larger. They were always made from nitriding steel and nitrided as it was thought it made them more resistant to chatter.

[egaParticipant @ega]

Posted by Graham Meek on 07/02/2020 11:09:16:

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There were several things that I picked up on. One the L/D 4 bar protrusion for a standard bar. Another which GHT seems to have got right form the start, is that Sandvik recommend mounting bars in sleeves rather than just clamping them in the toolholder.

I notice from old mart's link that Cutwel do a

Quick Change Anti-Vibration Tool Holder for Imperial Boring Bars

which looks very much like the GHT sleeve but costs over £118!

Thanks for those other useful pointers. I understand that the shape of the cutting edge also affects the direction in which the tool is pushed away from the bore – perhaps, therefore, another reason for using shop-made or modified cutting tools.

Edited By ega on 07/02/2020 11:28:18

Edited By ega on 07/02/2020 11:28:55

[MeunierParticipant @meunier]

Posted by Neil Lickfold on 07/02/2020 10:23:23:

./…..can't do the math, but liken it to a plastic bag tube. //The bar I am going to try has a 12mm diameter shank and a min bore diameter of 14mm. It would be great to be able to do the math t hing and figure out that if it has 120psi it becomes this rigid for example.

Neil

Neil, I like your plastic bag tube example and confess I am also unable to do the math.
Unless I have completely misunderstood your proposed bar dimensions, the min bore id of 14mm appears to be greater than the 12mm shank diameter.
DaveD.

Edited By Meunier on 07/02/2020 17:11:25

[Martin ConnellyParticipant @martinconnelly55370]

DaveD, diameter being bored not the internal boring bar diameter.

Martin C

[duncan webster 1Participant @duncanwebster1]

The pressurised bag and prestressed concrete analogies are not all that relevant. Concrete has a very low tensile strength, and so it is sometimes prestressed into compression so that when a bending moment is applied, say by end loading a cantilever, the concrete in the side which would have gone into tension actually just reduces its compression. (Experts please excuse my simplificaton!) Similarly the wall of a plastic bag cannot normally withstand compression, but inflating it puts the wall in tension so it can take a small bending load without the compression side buckling. Steel has the same young's modulus in tension and compression, it is governed by inter-atomic forces and as Graham says is little changed by hardening. It is mainly down to chemistry, and non stainless steels are mainly iron, even alloy steel like en24 is >95% iron

The downside of prestressed concrete comes when you try to demolish it, unless you are very careful, releasing the pre-stress can result in a sudden release of strain energy throwing bits of concrete all over the place. Think wire rope snapping.

I've no doubt Graham is right but the reason must lie elsewhere

[Michael GilliganParticipant @michaelgilligan61133]

Posted by duncan webster on 07/02/2020 18:29:37:

The pressurised bag and prestressed concrete analogies are not all that relevant. Concrete has a very low tensile strength, and so it is sometimes prestressed into compression so that when a bending moment is applied, say by end loading a cantilever, the concrete in the side which would have gone into tension actually just reduces its compression. […]

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I beg to differ, Duncan

… I only said that the cases were ‘conceptually similar’

Changing the sign of the ‘stressing’ from positive to negative seems fair to me for comparisons at this level.

MichaelG.

[duncan webster 1Participant @duncanwebster1]

Well we'll just have to agree to differ then

[Michael GilliganParticipant @michaelgilligan61133]

Interesting, and fairly recent article here: **LINK**

https://res.mdpi.com/d_attachment/applsci/applsci-09-03541/article_deploy/applsci-09-03541.pdf

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by duncan webster on 07/02/2020 18:29:37:

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[…] the concrete in the side which would have gone into tension actually just reduces its compression. […]

Steel has the same young's modulus in tension and compression,

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… and yet we happily use pre-loaded springs on vehicles, etc. … Mmm

MichaelG.

[duncan webster 1Participant @duncanwebster1]

Posted by Michael Gilligan on 08/02/2020 00:34:16:

Posted by duncan webster on 07/02/2020 18:29:37:

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[…] the concrete in the side which would have gone into tension actually just reduces its compression. […]

Steel has the same young's modulus in tension and compression,

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… and yet we happily use pre-loaded springs on vehicles, etc. … Mmm

MichaelG.

So your point is ????????

[Michael GilliganParticipant @michaelgilligan61133]

It’s O.K. Duncan … We have agreed to differ

I was just musing to myself.

MichaelG.

[MeunierParticipant @meunier]

Posted by Martin Connelly on 07/02/2020 17:41:01:

DaveD, diameter being bored not the internal boring bar diameter.

Martin C

Martin, thank you for pointing out what is now the obvious.
DaveD

[DrDaveParticipant @drdave]

My understanding of this is that the two parts of the boring bar, the tube and the rod up the centre, have different natural frequencies. Then, if one is excited near its resonant frequency and wants to “sing” the other is far from its resonant frequency and is “dead”. One part will thus act to reduce the resonant peak of its neighbour, effectively damping the system.

[Michael GilliganParticipant @michaelgilligan61133]

This is about large structural steel beams, prestressed by tensioned cables: **LINK**

https://www.researchgate.net/profile/Ahmer_Wadee/publication/265338326_Tensile_performance_of_prestressed_steel_elements/links/5a1323af0f7e9b1e572d74fb/Tensile-performance-of-prestressed-steel-elements.pdf?origin=publication_detail

So … it might take some mental juggling to see the relevance to boring bars prestressed by push rods … but it’s an impressive and convincing piece of work.

MichaelG.

[Graham MeekParticipant @grahammeek88282]

Having been reading extensively some of the documents posted, and others. I have come to the following conclusion.

Whether that conclusion is right or wrong, I cannot say, but it holds water for my way of thinking.

The boring bar is a cantilever beam with a load on the extreme free end. There is also a torsional element that is trying to twist the cutting tool about the centre-line. This twisting element will be greater the further the tool tip is away from the centre-line

Looking end on and using the points of the compass, with the tool bit in the East. As the tool starts to cut there will be a bending moment about the N-S axis pushing the tool away from the operator. There will also be a bending moment about E-W pushing the tool downwards. The resultant effect of these two loads will be to twist the cutting tip about the centreline.

In both cases the bending moments are resisted by tension in the North side and East side of the bar. When this tensile stress is within the strength of the bar everything is dandy.

However when the the tensile stresses exceed what he bar can handle then the twist in the bar becomes more prominent. The tool bit swings out of the cut, and as soon as there is no load on the bar immediately returns, or tries to return to the cut. The result of this is chatter or vibration.

If the induced, or static tension, (pre-tension), in the bar exceeds the last condition, then the bar continues to cut as it would in the case prior to chatter.

Whether there is a third element that alters the natural frequency of the bar or not, I have not been able to come to any conclusion on.

Regards

Gray,

[Author]

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