Effect of Tensioning a Boring Bar

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

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

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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.

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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]

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

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

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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.

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?

I have modified smaller bars, 12mm shank with a piece of 5mm tungsten carbide inside a 5.2 mm hole and used a brass plug in the end. This has worked very well as a vibration dampener. This is the technique used by Summitomo in their vibration dampening bars. So you can make it work 3 ways , have the moving weight in a bar, use tension or compression, or do another method which is to lighten the cutting head. Mitsubishi have the Dimple bar technology that vibration dampens by changing the mass at the insert end of the bar, and it works also. For home I have the dimple bars and have added little weights to the inside as well to the none dimple bars. I have also made external holders with these weights in them for my work place. Had an issue with turning between centres and getting a harmonic. So I lenghthend the tool from the holder, and it worked. The tool is a 25mm shank and has a piece of 12mm carbide inside a 12.4mm hole. When the tuning tool is struck, it makes a dull thud instead of a ring. Again it works.

Neil

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I stand corrected, Duncan

... but I wonder why they [Kennametal] patented the viscous fluid device, if they don't use it.

MichaelG.

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?? ... did I mention anything about them not working ??

MichaelG.

Duncan.

I've just re-read you post, and I'm now not sure whether you intended to correct me, or not ... as it seems your comments are broadly consistent with the patent that I referenced.

... For my sanity, would you please clarify.

MichaelG.

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P.S. If some clever draughtsman could interpret item 54 in the patent drawings as a 'wedge' I would be grateful

Edited By Michael Gilligan on 20/08/2017 09:04:47

I'm getting confused myself now. The link provided by SOD to a kenametal patent is different to the one provided by Michael G. I always find reading patents hard going. Sometimes I think it is deliberate so that you won't be able to see how it works, but they will be able to stop you doing whatever it is and anything like it. Just in case there are any patent lawyers reading this, only joking!

​No I wasn't trying to correct you Michael G, someone had suggested that the radial screws were to adjust the preload in a simple all steel Dore type bar, and I don't think that is correct. I may have misread his post

SoD's post of 13:17 yesterday sums it all up well

There seem to be two Kennametal patents on two different techniques, one on bar structure and the other on damping. It may be that the two are related as the first mentions in claim 1 ""a tunable or tuned absorber inserted within the central cavity of the body of the bar,".

Reading patents isn't good for one's sanity, but a couple of points.

One is, start by reading the claims as they are the only thing that actually says what they are patenting. Look at the rest of the text and the drawings if you need to clarify what the claims are talking about. Often the text will describe all sorts of stuff that isn't in the claims, often because they started with much wider claims but were cut down by the examiner. This makes it easier to get the gist of what it's on about, though sometimes the claims are so obscure you either just have to read the rest of the verbiage or give up in disgust!

The other, companies usually patent a lot more than they use for two reasons. One, when developing a new product the cleverer ones try to surround what they actually do with a thicket of alternative techniques that their competitors might use instead, to protect their position. The other is, when they get involved in a patent battle they have things to bargain with and a way to get some return on development work they would otherwise not get a return on.

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Thanks for that, Duncan

It's an almost inevitable problem with posting on a sequential thread.

MichaelG.

Apologies if someone else has mentioned it already but I picked up a useful factoid reading about this stuff on the web. I found figures were given for the maximum depth one can expect to go with a cantilever boring bar before trouble starts with tapered cuts, poor finish or chatter etc. Approximately, the maximum depth is expressed as the ratio between shank diameter between and hole depth.

Roughly:

• HSS - About 1:2 (i.e a 1" diameter HSS boring bar OK up to 2" deep.)
• Carbide - About 1:4 or 1:6 depending on the exact carbide. Concern expressed about the danger of pure carbide boring bars shattering.
• Tuned Boring Bars - About 1:6 - 1:10 depending on type and material.
• Active compensation - up to 1:20

Of course tapered cuts, poor finish and chatter can all be managed by the operator, but where productivity matters it's more economic to use proper tooling. Fortunately Model Engineers who know the ropes can often replace expensive tools with time and skill, special boring bars being a good example.

An unsupported boring bar is more limited than I thought. Presumably boring a model engine cylinder on a lathe would be done best by mounting the work on the cross-slide and boring with a centre cutting boring bar supported at both ends by the chuck and tail-stock? Another new thing for me to try!

Dave

Edited By SillyOldDuffer on 20/08/2017 14:13:56

Edited By SillyOldDuffer on 20/08/2017 14:14:56

Here's an interesting read for those who haven't yet tired of the original topic:

US Patent 2648895 'Prestressing Structural Members' **LINK**

https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=US&NR=2648895A&KC=A&FT=D&ND=&date=19530818&DB=&locale=

Perhaps not directly relevant to the pre-tensioned boring bar, but getting close.

See column 5: "Referring now to Fig. 7 ..."

MichaelG.

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 Thinks ...

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

Edited By Michael Gilligan on 20/08/2017 19:55:30

Some boring bars are made out of special materials that have a higher than normal steel modulus at the end of a carbide bodied bar. But they cost real money. A lot of research is continually being done about torsional tuning of boring bars as well.

Then there are the supported bars, so are like an internal roller box for precision boring of long tubes etc.

Neil