Effect of Tensioning a Boring Bar

[SteveIParticipant @stevei]

Hi,

Is this what industry refer to as a "tunable" boring bar? A quick search seems to confirm that these tend to be much larger bars than the original inspiration for this thread discussed, bars with Ø40mm and above for bore rations of 10:1 (steel) and 15:1 (carbide). For smaller bars searching seems to confirm that solid carbide or solid carbide with a coolant through hole are the industrial choice.

Steve

[Martin KyteParticipant @martinkyte99762]

That would be my take on the subject Steve. I don't think you get any more rigidity with a tension rod just a different resonant frequency. It's tuning rather than damping.

regards Martin

[MuzzerParticipant @muzzer]

Posted by duncan webster on 18/08/2017 11:19:17:

The second moment of area of the flagpole itself isn't increased by the guy ropes, what you have done is convert the cantilever flagpole into a truss. The guy ropes are well outside the neutral axis of the pole and do have (very) significant effect. Very tall radio masts are often actually sat on a pivot and so the stiffness of the pole itself contributes nothing to the combined stffeness.

Depends where you are standing! We were talking about the radial stiffness of a point at the end of a beam (or flagpole or mast…), measured from the toolholder / ground or whatever. Admittedly the modulus of the rope and mast material are different but the contribution to the stiffness at the end of the beam/pole/mast is primarily due to the radial distance and section of the rope and the resulting net increase in moment of area etc. Increasing the radial position of the base of the guy ropes and/or their thickness will significantly affect the stiffness of the end of the beam etc.

Admittedly, unless the guy ropes are parallel, the equivalent moment of area varies along the length mast but there are ways to analyse that in the same way you would with a tapered bar.

Freestanding masts held with guy ropes surely takes the concept to a whole different place if you forgive the expression.

Murray

[Alan VosParticipant @alanvos39612]

A request was made to use Fusion 360 and finite element analysis. Here is my attempt at a Static Stress analysis.

This is a simplified model to determine whether putting a bar in tension affects its stiffness along an orthogonal axis. It is not intended to be real numbers.

The bar is 8mm diameter, 70mm along the Z-axis, made of 'steel'. The nominal tool is 1mm square. All one part, no joints. The end face of the bar that would be in the tool holder is fully constrained. 50N of force is applied normal to the top of the tool.

The result is a Y-axis deflection of the tool by -0.1325mm (total deflection -0.1325mm).

The analysis was run again with the addition of 20N, 50N, 100N, 200N, 500N and 1000N normal to, but away, from the tool end of the bar. The Y-axis deflection of the tool did not change, always -0.1325mm. The total deflection did creep up to -0.1326mm.

For completeness, I put a 4mm diameter hole down the middle and ran the analysis again. This time the Y-axis deflection was always -0.142mm.

Conclusion: Applying tension makes no difference to the deflection caused by forces acting on the tool.

A Modal Frequencies analysis did, as expected, show that these increase when in tension, but not at lot.

Note: F360 decided to put the 'Min:' marker where it is. I did not find any way to mark a point to take a measurement at.

[jimmy bParticipant @jimmyb]

All this "theory" is interesting, but simple fact is that anti vibration and tuneable boring bars exist.

[Martin KyteParticipant @martinkyte99762]

I think the analysis has established that tensioning does not affect the static deflection not that it has no effect on resonant frequency.

You are perfectly correct that tunable bars are extant.

I also pointed out that it can allow for carbide boring bars to be constructed with the carbide in tension which are more rigid than steel.

regards Martin

[Alan VosParticipant @alanvos39612]

As I said. A Modal Frequencies analysis did show that the modal/resonant frequencies are increased by adding tension. I didn't go into further detail as that is exactly what everybody expected.

[John HaineParticipant @johnhaine32865]

I did say in the original thread that one would not expect any change in the stiffness as the system must be linear for forces and deflections within the elastic limits of the material. The stiffness is a measure of the force required to deflect the end of the bar by a given amount. It depends on changes in the forces on small elements of the bar as it is loaded, and these do not depend on static load. Putting it another way, Young's Modulus is not a function of stress as long as the stress is below the elastic limit. This is what is shown by the finite element analysis.

Yes you can tune a boring bar, for example by putting weights on it and moving them around. But then it isn't the same bar from a vibrational point of view.

[SillyOldDufferModerator @sillyoldduffer]

Nice work Alan. What you've done with Fusion fits with my experimental data, i.e that increasing the tension does not have an effect on deflection.

I'm impressed by Martin's view that tension affects resonance rather than deflection. (Also Neil Lickfold & SteveI.) Perhaps like tightening a guitar string, adding tension with a push-rod moves the 'note' of the boring bar. If during a cut the frequency of a boring bar coincided with that of any chatter then the vibration would reinforce. Moving the resonant frequency of the bar would inhibit the action and therefore be a 'jolly good thing'. Of course KWIL said varying spindle speed should reduce chatter too, and that fits with my experience. Plus a good tip from Jimmy about loading boring bars with plasticene, thanks.

Despite taking on several new ideas I still wouldn't bet money on any of the answers though. I'm out of my depth. DrDaves' comment about the Laws of Nature rings true, but so does Neil's 'tensegrity structures'. Until I read Muzzer's comment about guy ropes that is! And Michael and Duncan have got me thinking too.

The issue has boosted my respect for the scientists, mathematians and engineers who worked out this stuff from scratch. We really are stood on the shoulders of giants.

Dave

PS. Me write a post breaking forum rules? Surely not! I did assume that no forum member would ever share serious literature with his wife or servants though. Perhaps that was unwise…

[duncan webster 1Participant @duncanwebster1]

Provided there is no external axial load, resonant frequency of a cantilevered bar is dependant on mass per unit length, second moment of area and length only.

**LINK**

The analogy with guitar strings is not good as they are subject to an external load. In this case the tube is stretched, but always along its neutral axis, similarly the rod is compressed along its neutral axis. If this has any effect, which I'm not sure about, the tube would tend to increase in frequency, the rod would decrease. However if it's a good fit both have to vibrate at the same frequency, so I'd expect it to cancel out. If it's not that good a fit there could be an effect, my money is on damping caused by relative movement and friction rather than changing the actual frequency

[Anonymous]

Chatter implies an unstable cutting regime; if it coincides with the resonant frequency of the tool (doesn't have to be a boring bar) then it becomes very noticable. Another way to eliminate the problem is to change cutting conditions; an increase in DOC and/or feed per rev, which loads the tool more, can help.

Andrew

[Clive HartlandParticipant @clivehartland94829]

The more I read this the more my eyes roll, at the rate we in model engineering bore and cut internally does it matter. I could believe it would if I were cutting something large but we still come back to optimum cut and feed rate to stop squealing/chatter. The geometry of the cutting tool has a lot to do with it and this is where you buy in tool bits and inserts made by experts. A one off job does not warrant this and it is far easier to grind up a tool for the job.

Clive

[RichardNParticipant @richardn]

Fascinating discussion, and while I follow the debate the principles of the calculations pass way over my head- apologies if my thoughts make no sense…

But is a boring tool a simple cantilevered beam with a vertical load? Under no cutting forces with a tool mounted in holder but not engaged I accept gravity tends to act vertically, but surely as one plunges into a bore, the cuttting load is initially vertically, but at the first hint of downward movement the shape of the bore will add an exponentially increasing side load… is this not therefore a torsional/rotational force rather than a simple vertical load? Or does the maths still work out the same…?

If you consider this a rotational force… and the inner compressive push rod is compressed by means of a helically inclined plane… will increased deflection created additional tension, and change the 'strength' or 'chatter resilience' or will the reality of amount of rotation be insufficient to be measurable…

[Neil WyattModerator @neilwyatt]

Posted by Martin Kyte on 18/08/2017 09:52:12:

Not convinced Niel. Ideally the two comparisons for the boring bar are 1. the unstressed bar and 2. a composite bar where the 'core' is in tension and the tubular 'shell' is in compression. As it's a thought experiment the second bar can be a single peice of steel as we don't have to actually make it. If the elastic properties of the steel are identical in compression and tension I cannot see a mechanism by which bar 2 can be stiffer to a static bending moment than bar 1. I can however understand that the prestressed bar could have different properties with respect to the dynamic transmission of shock waves in the same way that a musical string changes pitch under different stresses. (as has been mensioned earlier).

I have done some searching around on the web and have found patents for boring bars utilising carbide rings to create a more rigid outer tube and an inner core arranged to be in tension so that the outer rings are always in compression.

A converse scheme where the outer tube is in tension could be practical if a harder material could be suitably employed.

I suspect that the "stiffeness" of the bar we were originally discussing when this thread opened was much more related to the change in resonant frequency and does not affect the difflection under static load.

If you really want an analog you could do worse than to think of a central coil spring surrounded by a number of similar springs all of which are attached to a disc at the top end and similarly at the bottom with the exception of the central spring which is attached to a screw passing through a tapped hole in the bottom disc . This allows the system to be either unstressed or stressed by a varying ammount. I seems intuitive that the resonant frequency (off axis oscillatio) would increase when the screw is tightened but again I fail to see a mechanism which makes the system any stiffer to a side force at the top.

It's an intriguing puzzle and I am happy to be shown I am wrong, but as I said so far I am not convinced.

regards Martin

I think it's the core that's in compression, tension around the sides, which possibly changes the calculations?

[Neil WyattModerator @neilwyatt]

Alternatively, if you get chatter stick a lump of plasticine on the bar…

[MWParticipant @mw27036]

This tensioning idea does make sense if you're going to try and bore a large and long cylinder, say 250mm long, guarantees are you're not going to have the same size one end as you do the other, regardless of dialling in the cut.

Michael W

[Ian WelfordParticipant @ianwelford58739]

Can't comment on maths but practically , in wood turning circles, we use lead shot into the (hollow ) handle of deep boring tools as it dampens vibration. There it's the increase in mass as much as the deadening effect.

Ian

[SillyOldDufferModerator @sillyoldduffer]

Posted by RichardN on 18/08/2017 20:43:40:

…

 

But is a boring tool a simple cantilevered beam with a vertical load? Under no cutting forces with a tool mounted in holder but not engaged I accept gravity tends to act vertically, but surely as one plunges into a bore, the cuttting load is initially vertically, but at the first hint of downward movement the shape of the bore will add an exponentially increasing side load… is this not therefore a torsional/rotational force rather than a simple vertical load? Or does the maths still work out the same…?

If you consider this a rotational force… and the inner compressive push rod is compressed by means of a helically inclined plane… will increased deflection created additional tension, and change the 'strength' or 'chatter resilience' or will the reality of amount of rotation be insufficient to be measurable…

My response to those questions Richard is a resounding 'don't know'. I'm confident that a boring tool isn't a 'simple cantilevered beam with a vertical load' because – as you say – the cutter is forced sideways into the work, and then pushed downwards as it cuts. On top of that, any chatter will drive the boring bar with an oscillating force. I have no feel for the scale of forces involved.

After unsuccessfully searching the web for info on tensioned boring bars, I've decided that they 'don't work'. There's discussion about 'static deflection', like bending under gravity, and 'dynamic deflection', that is vibration. In a boring bar the first is minimised by making it as stubby as possible; by choosing a rigid cross-section, and by making it from a material with a high modulus of elasticity. Carbide is MUCH better than HSS in this application. The second is minimised by 'tuning' the bar to dampen vibration. This involves something like a pair of loose weights inside the boring bar near the cutting end. The weights are separated by a rubber pad and the space filled with oil. This type of bar is superior – even essential for getting a good finish – when boring long narrow holes. They work, for example, by mounting weights inside the boring bar near the cutting end. The way they work reminds me of the dampening mechanisms used to protect tall buildings in earthquake zones – a suitably mounted large weight at the top absorbs the energy. This US Patent is worth a read. At the really expensive end, there are boring bars that detect and compensate for vibration using electronics and servos in the tool-holder. Mega bucks apparently!

I think that a push-rod inside a boring bar is able to alter the frequency at which the bar vibrates. I'm yet to be convinced that the effect is useful. Using an internal push-rod to clamp the cutting in place is a good idea because it minimises the cross-section of the boring bar: it will fit into a smaller hole. That it also has an effect on tool vibration may have been exploited to boost sales.

In other worries, Duncan criticised the guitar string analogy. That's lead me to ask if there's any difference between a rod tensioned with a push-rod, and a rod tensioned by pulling against a frame:

The rod is represented as a blue glass pipe in this Fusion360 picture so you can see the innards. Not very clear but the push-rod's thread is only engaged for 20mm at the right hand end. The blue pipe is stretched when the screw push-rod bears on the fixed plug on the left.

In this one, unscrewing the rod pulls it into tension against the frame on the left.

Dave

 

 

Edited By SillyOldDuffer on 19/08/2017 13:19:25

[Michael GilliganParticipant @michaelgilligan61133]

Posted by RichardN on 18/08/2017 20:43:40:

But is a boring tool a simple cantilevered beam with a vertical load? Under no cutting forces with a tool mounted in holder but not engaged I accept gravity tends to act vertically, but surely as one plunges into a bore, the cuttting load is initially vertically, but at the first hint of downward movement the shape of the bore will add an exponentially increasing side load… is this not therefore a torsional/rotational force rather than a simple vertical load? Or does the maths still work out the same…?

.

Apologies, Richard … I owe you a response

I did very deliberately start my hypothesis with with the caveat "to a first approximation"

I have absolutely no doubt that the cutting action complicates things enormously … but I think it wise to try to walk before we run, and first consider the stiffness at the 'first instant' [which is before any significant deflection takes place].

Although the discussion has evolved to embrace wider aspects [particularly damping], I would like us to get a grip on Hemingway's claim that [quote] "Tension induced by the pushrod makes even the smallest tool surprisingly rigid" [/quote]

MichaelG.

.

Note: I am happy to make a working assumption that when they say "rigid" they mean "stiff"

[because in this context "rigid" is an absolute term; which would render both analysis and and experimentation superfluous]

[jimmy bParticipant @jimmyb]

here is a video on using Kennametal damped boring bars

**LINK**

theywork!!!!!!!!!!

[SillyOldDufferModerator @sillyoldduffer]

Posted by jimmy b on 19/08/2017 15:19:39:

here is a video on using Kennametal damped boring bars

**LINK**

theywork!!!!!!!!!!

Thanks Jimmy. Re-reading my own post, I'm ashamed to say it's not clear. I was trying to say:

• I don't believe in boring bars stiffened by push-rods (much), but
• I do believe in tuned / damped boring bars like those made by Kennametal. There's lots of evidence in favour of them.

Dave

[egaParticipant @ega]

Posted by jimmy b on 19/08/2017 15:19:39:

here is a video on using Kennametal damped boring bars

**LINK**

theywork!!!!!!!!!!

Interesting video but how many model engineers need a 1" or more diameter boring bar?

The adjustable tuning mechanism was not described. Do you know what principle is used?

[MWParticipant @mw27036]

Posted by ega on 19/08/2017 17:13:28:

Posted by jimmy b on 19/08/2017 15:19:39:

here is a video on using Kennametal damped boring bars

 

**LINK**

 

theywork!!!!!!!!!!

Interesting video but how many model engineers need a 1" or more diameter boring bar?

The adjustable tuning mechanism was not described. Do you know what principle is used?

I've got a 20mm boring bar, not far off it and I wouldn't consider myself a professional user.

Have I used it? Well yeah, once or twice but obviously big stuff doesn't come round often as much as smaller stuff.

(My lathe machine is far from a top-of-the-line or professional ones you occasionally see, but definitely bigger than a mini lathe.)

My guess would be that those screws on the surface are preloading the tension of the internal bar inside the tube. 

Michael W

Edited By Michael-w on 19/08/2017 22:33:12

[Michael GilliganParticipant @michaelgilligan61133]

Posted by ega on 19/08/2017 17:13:28:

The adjustable tuning mechanism was not described. Do you know what principle is used?

.

The Patent Office does … and thanks to 'espacenet' so do we:

**LINK**

https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=2017056977A1&KC=A1&FT=D&ND=3&date=20170302&DB=EPODOC&locale=en_EP

MichaelG.

[duncan webster 1Participant @duncanwebster1]

The Kenametal adjustable bar uses vibration absorber(s), the sideways screws would appear to be a means of adjusting these, This is nothing to do with axial preload. For a description of the principle see good old Wikipedia.
**LINK**
Same principle was used by Lanchester cars, so it's not exactly new

​A guitar string has no inherent sideways stiffness, the only reason it tries to stay straight is the applied end load, which always acts in a straight line between the two fixed ends. This is not the same as a tube preloaded by a close fitting push rod, where the force is always coaxial

​why has this stupid editor suddenly started going back to the start of the post when I press return?

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