Effect of Tensioning a Boring Bar

Just hate to think what else he may put in his vice and give a twang to

I suppose the bar is a bit like a hack saw blade, if you just put them into the frame loosly they flop about from side to side as you try and cut, wind up the screw so they are taught and no more wandering.

Sandvik many years ago had tuned boring bars available for milling and turning. It just put the assembly into tension. For the turning one, there was an adjuster that could be accesses and adjusted while it was taking a cut. Much easier on a manual lathe than a cnc one. It works. So does the other way of vibration dampening and that is to place a loose heavy metal object in the boring bar a bit back from the cutting edge. Like on a 20mm bar, the weight is about 40mm back from the very front. They allow boring bars to turn up to about a 7 to 10 d limit.,Meaning a 20mm shank tool can be used down as far as 200mm . Just tungsten is often used for this weight and has about 1mm of for aft movement and about 0.3 total side way's movement.

Neil

The second moment of area of the flagpole is significantly increased (ie resistance to bending) by the guy ropes. The pretension in the guy ropes is only necessary to overcome / control the elasticity and gravitational droop of said ropes.

Incidentally, it would behave equally in tension and compression if the rope were instead a rod capable of withstanding compressive forces. Same improvement would be seen in a boring bar if you fitted it with appropriate (but impossible) guy ropes of course....

Murray

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.

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

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

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

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.

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

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

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.

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.

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...

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

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

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

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...

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

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

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