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Effect of Tensioning a Boring Bar

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duncan webster14/02/2020 22:48:31
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OK I've changed it and sent it off to the FE man. as I said, don't hold your breath

Graham Meek18/02/2020 12:24:03
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I managed to quantify the torque applied to the grubscrew on my boring bar yesterday. My 120 degree turn, produced by years of practice, produces a torque of 889 N mm. Unbrako recommend a torque of 1863 N mm on an M4 grubscrew, while HoloKrome recommend 2270 N mm. Unbrako even state this torque can be applied with a standard Allen Key.

While my torque is just under a 1000 N mm less than the lowest recommended value, this does still equate to a loading of 1111 N on the push rod, in old money 250 lb(f).

Regards

Gray,

 

Edited By Graham Meek on 18/02/2020 12:25:09

Kiwi Bloke22/02/2020 02:57:43
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...er, apologies for coming to the party a bit late. Interesting discussion, but is it blurred by terminological inexactitude? Have we decided whether doing something to the bar affects its stiffness (as in Young's modulus) when strained or only its resistance to initial deflection (as in pre-load)? I can't see how stiffness can be altered without material being changed to something stiffer.

Graham Meek22/02/2020 12:38:35
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In years gone by when I was learning my trade, mechanical engineering college courses included lab experiments.

These followed a set and logical pattern, Object, Method, Results and Conclusion.

The Object as I take it is the title of the thread, ie "Effect of Tensioning a Boring Bar"

My Method was to compare a Plain (solid) boring bar with a Pre-tensioned one. During actual machining and, using the same test equipment, the same overhang and the same diameter boring bars, two tests were devised to check each bar in Bending and Torsion.

The Results showed that during machining the Plain bar produced chatter, which was inaudible, while the pre-tensioned bar produced a bore with a very good finish.(Using the same spindle speed and Rate of Feed).

It also showed that there was a slight increase in the effort required to Bend the pre-tensioned bar through a fixed distance. While there was a much more pronounced increase in the Torsional effort required to displace the pre-tensioned bar through the same set distance.

The conclusions drawn from these tests are that the Pre-tensioned boring bar did benefit from being under tension, in that it produced better results. Both under machining conditions and under test. It was also concluded that the Push-rod was also experiencing a torsional loading which was of the opposite sense to the tool loading. (This was the subject of a second test, which proved this conclusion). It was also concluded that the Pre-tensioned boring bar was a complex system which needed greater knowledge to unpick the secret of why it produces better results, both dynamic and static.

I do not think the question is about "stiffness", I think the question should be, as a structure, "Is the pre-tensioned boring bar more Rigid".

There are two more test I have in mind, when time will allow, to show under working conditions what is happening.

Regards

Gray,

Michael Gilligan22/02/2020 14:34:29
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Posted by Graham Meek on 22/02/2020 12:38:35:

[…]

I do not think the question is about "stiffness", I think the question should be, as a structure, "Is the pre-tensioned boring bar more Rigid".

[…]

.

With the greatest respect, Gray ...

In ‘Mechanics’ Rigid is a term of convenience which means infinitely stiff, and there is no such concept as ‘more Rigid’

i.e. it is a convenient assumption, made when the calculations would otherwise be too difficult.

MichaelG.

Graham Meek22/02/2020 15:47:44
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Michael,

Point taken, and yes the calculations are beyond me. Despite my HNC in Mechanical Engineering (circa 1970).

I did however think if I used the word "stiffer" someone would only quote Young's Modulus, again.

The old wooden beams used to raise loads into upper storey's of buildings had one point of contact, the wall. The outer end thus has 4 degrees of freedom. Putting a brace or tie-rod between the free end of the beam and the wall directly above the beam, brings these degrees of freedom down to 2. Having two braces and splaying them apart where they attach to the wall above the beam reduces the degrees of freedom to 0. As Braces are added, the structure becomes "more Rigid", or "stiffer", plus the load carrying capacity goes up.

Regards

Gray,

Kiwi Bloke23/02/2020 09:51:48
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I didn't mean to offend Graham Meek by mentioning Young's modulus. Gray's experiments are far more useful in practice than pages of waffle and surmise and he deserves a vote of thanks from all who are interested in this subject.

I'm trying to understand what's going on. I of course accept the experimental evidence and can use this knowledge, to some extent, in future use of boring bars, but I'm the sort of awkward beggar who likes to know 'why', not just 'what'.

My belief is that the tensioned bar isn't much like the brace or guy-wire arrangement above (because it's a long thin structure), but that it is pre-loaded. This makes initial deflection (lateral and torsional) minimal, until the preload is overcome. My understanding is that preloading a sprung system moves the stress/strain curve sideways, but doesn't alter its slope. So, with appropriate preloading, stress, up to the preload, can be applied without producing strain. The stress/strain curve of the preloaded system is thus initially vertical i.e. infinite stiffness (where 'stiffness' means Young's modulus). Or perhaps not. Have I gone wrong somewhere?

Michael Gilligan23/02/2020 10:14:45
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Kiwi Bloke

I think your closing paragraph summarises the situation nicely.

As you will see from my earlier posts on this long thread; I have tried to draw comparison with pre-stressed concrete beams, pre-stressed steel beams, and vehicle springing ... but this appears to have fallen upon deaf ears.

I eagerly await the FE Analysis

MichaelG.

Graham Meek23/02/2020 12:43:04
167 forum posts
139 photos

Kiwi Bloke,

No offence was taken on my part by you mentioning Young's Modulus. I have you to thank for the Maximat Screwcutting Clutch design. You prompted me to take another look at the problem.

Young's Modulus is, as you rightly say, a Ratio of Stress over Strain. It's use in answers on this thread, sheds no more light on why the Plain solid boring is out performed by the pre-tensioned boring bar. If greater thought was applied to this problem we might be nearer an answer.

I too am struggling to understand what is going on, but I do like to see things through and by experimentation I have sorted in my own mind that the pre-tensioned bar is better. By my experimentation I have added values to a mostly theoretical thread.

Regards

Gray,

Michael,

I do not think your analogies were wrong, it sometimes pays to scale things up and look at the problem from a different perspective.

Like my example above with the wooden beam and the metal tie rods. The materials have not changed but the resultant structure is better. It is far removed from the boring bar problem, but the boring bar is just like the initial wooden beam attached at one end, that is to the tool post. Adding the push-rod changes the outcome, why it does escapes me.

As regards the FE analysis I hope you can draw conclusions from it, as it is all double dutch to me.

Regards

Gray,

Generally,

I received on Friday from a good friend an article by Martin Cleeve, Model Engineer 16th Sept 1966, pages 824-826. This, until Friday, was the first time I had read the article. I only started taking Model Engineer in Sept 1968, when I started my apprenticeship.

Martin's work is highly regarded by me, and many others. Thus it came as no surprise to me that his boring bar of choice was one using a push rod. His only drawback to using this type of bar was the need for the long drilled hole. He does however go on to describe a composite boring bar, part push-rod, part plain. That part I thought was a clever and an original bit of thinking. This latter bar does to a certain extent "put a stone in the ashes" on what I have been thinking. However he does say he could not see any difference between the bars in use.

The bars are threaded together, and in his "Practical Test" he states he had the joint "purposely left only finger tight", while plunging a 0.135" wide cutter into a 1" bore at 145 rpm.

Thus this is something that I would like to know the answer to "what is going on" with these bars. As this "structure" is even more complex.

Regards

Gray,

 

Edited By Graham Meek on 23/02/2020 12:45:57

DrDave23/02/2020 19:53:09
187 forum posts
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Posted by Graham Meek on 22/02/2020 12:38:35:

In years gone by when I was learning my trade, mechanical engineering college courses included lab experiments.

That rang a little bell in my memory. The lab experiments that we did on my Engineering course were intended, amongst other things, to be a practical demonstration of a theory. If the results did not support the theory, then either the theory was wrong, the measurements were in error or the assumptions were wrong.

A solid cantilever bar of the same geometry as those tested should require 71.6 N to deflect 0.05 mm. A pretensioned bar cannot be stiffer than a solid bar, for the same diameter. Graham’s tests showed 37.8 N (3.85 kg) to deflect this amount and the pretensioned bar 44.4N (i.e. stiffer than the solid bar).

In the bending boring bar tests that Graham did, I have no reason to believe that the measurements were wrong. Equally, we know from decades of experience that the theory of beam bending is correct. Therefore my assumptions must have been wrong. The assumption that is probably wrong is that the cantilever is (dare I say) rigidly built in at one end. As Michael said, this is a convenient assumption because it is very difficult to achieve in practice. Any real-world flexibility at the support will decrease the load required to reach the 0.05 mm deflection.

I think that we have been concentrating on the bar in isolation, rather than considering the whole system. Small changes in the support would have a noticeable effect on deflection and vibration and could be an answer to the “why do two boring bars have such a difference in chatter” question. Can I ask, Graham, was there any difference in the way that the two bars (solid and pretensioned) that you tested in bending were supported that could have affected the results?

Dave

DrDave23/02/2020 20:01:44
187 forum posts
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As an addition, to avoid cluttering my post, above, any more:

From a reference such as Roark, the maximum deflection of a cantilever loaded at the tip is delta = force x length^3/3EI.

Here, I = pi d^4/64 = 491 mm^4. E = 210 x 1000 MPa and length = 60 mm. So force to deflect 0.05 mm is 3EI delta/length^3 = 3 x 210 x 1000 x 491 x 0.05/60^3 = 71.6 N

Martin Kyte24/02/2020 09:47:41
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Posted by Kiwi Bloke on 23/02/2020 09:51:48:This makes initial deflection (lateral and torsional) minimal, until the preload is overcome. My understanding is that preloading a sprung system moves the stress/strain curve sideways, but doesn't alter its slope. So, with appropriate preloading, stress, up to the preload, can be applied without producing strain. The stress/strain curve of the preloaded system is thus initially vertical i.e. infinite stiffness (where 'stiffness' means Young's modulus). Or perhaps not. Have I gone wrong somewhere?

ummm, I think so. If you have a linear strees strain curve and preload puts you somewhere on the curve additional loads will still cause the same magnetude of extension or compression as without preload. Concrete beams are a bit of a red herring as the object is to ensure that all loads are compressive as far as possible.

I don't think the answer will be found in static deflection analysis but only by considering the dynamic system.

Boring bar chatter as far as I can make out, (and I do not set myself up as an expert in this field) appears to be be primarily modal rather than regenerative in nature. By this I mean that the bar bends into and out of the cut and also twists one way and t'other, with the tool tip oscillating in an elipse. I do not have the ability to do the maths but intuitively I can buy into the tensioning of the bar having an appreaciable effect on the energy transfer between the two modes of oscillation by causing the natural frequency of the two modes or oscillators to be non harmonic and thus making the dynamic stiffness or maybe resillience would be a better word? increase.

I do however note Graham's results on static tests which have not really been explained. Maybe the tesioned bar is twisting as well as bending as the load is applied. I don't know.

regards Martin

Michael Gilligan24/02/2020 10:00:49
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Posted by Martin Kyte on 24/02/2020 09:47:41:

[…]

Concrete beams are a bit of a red herring as the object is to ensure that all loads are compressive as far as possible.

[…]

.

Concrete beams are pre-stressed by a tensioned rod, not a compressed one, but:

As I commented to Duncan, earlier : the conceptual similarity remains ... it only requires a ‘change of sign’

Remember: in a bending beam ... the top is in extension; the bottom in compression, and there is a neutral axis.

MichaelG.

Martin Kyte24/02/2020 10:08:45
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Hi Michael

My point was that the prestressing of concrete was not an attempt to increase stiffness or stop vibration but to ensure that the beam remained in compressive load. As you correctly say the force vectors are the same albeit with the sign change.

regards Martin

Graham Meek24/02/2020 11:31:45
167 forum posts
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Posted by DrDave on 23/02/2020 19:53:09:

Can I ask, Graham, was there any difference in the way that the two bars (solid and pretensioned) that you tested in bending were supported that could have affected the results?

Dave

Dave,

Both types of the boring bars were held in exactly the same holder, using exactly the same sleeve, (the sleeve is dowel located to orient the slits in the sleeve and the clamp), with the same length of bar clamped in the sleeve.

The discrepancy in results in theory over practical can be best explained by a sentence on the Sandvik website. The sentence relates to the actual flexibility of the machine tool itself during boring operations. While my Emco Maximat Super 11 is fairly rigid it would not produce the same results as in the test lab. Under lab conditions the block holding the bar would be bolted to an anchorage which would be known not to move.

Unfortunately my workshop does not run to this level of lab equipment and so the lathe is pressed into service. The Maximat has clearances which are necessary in order for the slides to operate, (I did however lock those I could). As both tests were carried out under the same initial conditions I feel some continuity exists in my method.

The practical versus theory results were as I expected.

As I said initially the structure needs to be looked at as an overall package.

There is no doubt in my mind that the push rod is a pre-tensioned Torsion bar. The pre-tensioned boring bar is being presented to the work with a preset loading. Which is opposite to the load which is about to be applied by the cutting action. The stress in a tube in torsion is said to be concentrated in the surface layer, this is also pre-loaded by the reaction to the load on the push rod.

Neither of these additional loaded elements are present in the plain bar.

How this all relates to Martin Cleeve's version I have yet to fathom.

Regards

Gray,

Graham Meek27/02/2020 11:34:03
167 forum posts
139 photos

Following a little light reading a reference was made to Machinery's Handbook, which led me to the following.

In my 15th edition, at the bottom of page 356, is a paragraph entitled, "Shear Stresses Combined with Tension or Compression Stresses"

The paragraph opens saying the complicated calculations associated with the above can be avoided by using the accompanying table on page 357. The paragraph goes on to show an example where the Shear Stress (S) is divided by the Tension or Compression Stress (T). The example gives a product of 0.75 for S/T.

From the table on page 357, 0.75 gives factors of x = 1.401 and y= 1.2. Quoting from the book, "This means that in this case the maximum combined tension will be 1.401 times what it would have been if there had been no shear". The passage goes on to say the that the maximum Shear stress is 1.2 times what it would have been if there had been no tension or compression stresses.

If the product of the S/T ratio had been 0.05 then x = 1.002, but y would be a factor of 10.05. Similarly the product of S/T at 1.5 gives x = 2.08 and y = 1.05. (0.05 and 1.5 are the extent of the table)

For my part the above gives a clear indication that the addition of the tension loading in the boring bar is a win, win situation. Not only is the boring bar experiencing this combined effect but also the push-rod, albeit the push-rod is in compression.

Regards

Gray,

 

Edited By Graham Meek on 27/02/2020 11:36:02

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