Effect of Tensioning a Boring Bar

[Neil LickfoldParticipant @neillickfold44316]

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by duncan webster on 19/08/2017 23:40:29:

The Kenametal adjustable bar uses vibration absorber(s), the sideways screws would appear to be a means of adjusting these …

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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.

[jimmy bParticipant @jimmyb]

There are plenty of fluid filled boring bars. Thinking it don’t work, doesn’t make it so…..

[Michael GilliganParticipant @michaelgilligan61133]

Posted by jimmy b on 20/08/2017 07:34:08:
There are plenty of fluid filled boring bars. Thinking it don't work, doesn't make it so…..

.

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

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

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

[duncan webster 1Participant @duncanwebster1]

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

[John HaineParticipant @johnhaine32865]

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.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by duncan webster on 20/08/2017 11:29:01:

​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

.

Thanks for that, Duncan

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

MichaelG.

[SillyOldDufferModerator @sillyoldduffer]

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

[Michael GilliganParticipant @michaelgilligan61133]

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

[Neil LickfoldParticipant @neillickfold44316]

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

[FatgadgiParticipant @fatgadgi]

Hi John

Agree with your points – anything not claimed but put into the description prevents other people coming behind them and trying to patent other key features of their idea.

But many companies use the description in an offensive way as well as the claims.

As you say, you can only patent one invention per one patent, so adding more novels things to the description gives the opportunity to add things to the claims if the examiner complains, or even to pull off divisional patents further down the process, where the patent can be split into two (or more) if there are more than one potential invention.

I was trying to knock out a German competitor's granted patent in a hearing at the EPO in the Hague a few weeks ago. I won the battle when their key claim was deemed to be non-inventive, only to then loose the war when they managed to get agreement to move a point from the description into the claims.

I hate patents.

Cheers – Will

[SillyOldDufferModerator @sillyoldduffer]

Posted by Michael Gilligan on 20/08/2017 19:41:57:

…

Thinks …

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

Dunno about Dore/Hemmingway Bar, but this is what happened in my experimental set-up.

I modified the screw-tensioned pipe by turning a 4.5mm pin on the end of a 700mm long 10mm diameter steel rod. This was fitted to the pipe to provide a turning moment thus:

The other end of the turning bar was fitted with a pin so the deflection could be measured, with and without weights. I also applied the 'twang' test and measured how long it took the turning bar to stop vibrating.

The results:

The graph shows that, up to a point, increasing the pipe's tension increases deflection and also the time vibration persists!

But, in criticism of my experiment, the graph may be showing the effect of compression on the retaining pin rather than the effect of tension on the pipe. The pin bends. Back to the drawing board. Again…

Dave

[egaParticipant @ega]

Thanks to jimmy b and all the others for this fascinating glimpse into the world of patentese.

Bearing in mind that the original topic was *small* diameter boring bars my sense is that most of the methods of reducing chatter proposed would be impractical. The good old between centres boring bar may be the answer for the amateur.

[Michael GilliganParticipant @michaelgilligan61133]

After four pages of speculation … Wouldn't it be nice if Hemingway could tell us what they actually mean by "surprisingly rigid".

MichaelG.

[jimmy bParticipant @jimmyb]

Posted by Michael Gilligan on 21/08/2017 15:15:16:

After four pages of speculation … Wouldn't it be nice if Hemingway could tell us what they actually mean by "surprisingly rigid".

MichaelG.

[Martin KyteParticipant @martinkyte99762]

Maybe they mean surprisingly rigid in spite of having a hole up the middle.

regards Martin

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Martin Kyte on 21/08/2017 16:03:58:

Maybe they mean surprisingly rigid in spite of having a hole up the middle.

regards Martin

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Martin,

That is what I have always suspected …

i.e. we are wasting our time discussing "advertising puff"

… as I hinted on the 19th, if it was quantifiable they would presumably have known to use "stiff" not "rigid".

[by convention, a rigid body is one of "infinite stiffness"]

MichaelG.

[JasonBModerator @jasonb]

I suppose the only way we will ever know is if someone makes two boring bars, one with a grub screw in from the cutting end to retain the tool and another using the long hole and rod from the opposite end.

Make similar cuts using the same cutting tool and have a quantifiable way of measuring the results regarding deflection and chatter.

Anybody got some old Dore literature that may shed some light onto whether the claims about these bars are true or not?

[egaParticipant @ega]

JasonB:

"Anybody got some old Dore literature"

I don't think the original Dore bars had this feature (the idea of a long hole with push rod was probably already known when Arnold Throp came up with his castings).

See my 14 August post in the Dore small boring bar thread.

Edited By ega on 24/08/2017 10:27:18

[Graham MeekParticipant @grahammeek88282]

By way of the "Proof of the pudding is in the eating".

I recently had a series of bored holes in a bronze job that was causing me problems with chatter. My FB2 Mill being a geared head is not the best for boring holes especially combined with a single phase motor. However I have managed to get a good finish with my boring heads and boring bars in the past on Mild steel, Brass, Cast Iron and Aluminium.

The standard Emco design and my variations are shown above. All use an M4 grubscrew to lock the HSS tool bit.

While all the boring bars are made from Silver Steel only one is hardened, and one might think this last version would give a better result as regards less chatter. Using this bar gave little improvement, but there was an improvement.

After a lunch break I remembered I had some boring bars I made when I had my Unimat 3. These were made about the time George Thomas was writing a series of articles in ME on boring bars. His "design", (maybe cribbed from an earlier design), used a central push-rod to grip the tool bit. Again these bars are all made from Silver Steel, but left soft.

Thus a bar of the same length was retrieved from the stores and the boring bit currently being used, (continuity) inserted. The difference straight out of the box took me by surprise. The job was then completed without further problems.

I must conclude that in use the hollow boring bar under tension, does give a better result than a standard plain bar.

Regards

Gray,

[Martin ConnellyParticipant @martinconnelly55370]

Isn't the centre push rod under compression? Resonance is caused by a variety of factors, with a VFD it is easy to change rpm and go into and out of resonance. The stick out and stiffness of a boring bar, the rpm used, the depth of cut, the feed per rev, the geometry of the cutting edge and the material all have an effect. Change any one and you will get different results. I have read that putting a lump of plasticine on a boring bar can reduce chatter but have not resorted to it yet.

Martin C

[Bill Davies 2Participant @billdavies2]

Not tension, but… years ago, the company I worked for produced a small hobbing machine, which was being for producing gears for a brand of power drill. There was some chatter which the designers couldn't cure, so they brought in some engineers from a nearby university. They determined the resonant frequency and put a thin shaft up the bore of the hob spindle, with a mass on the end to vibrate in antiphase. It worked.

Bill

[duncan webster 1Participant @duncanwebster1]

Posted by Graham Meek on 06/02/2020 12:24:20:

By way of the "Proof of the pudding is in the eating".

……

While all the boring bars are made from Silver Steel only one is hardened, and one might think this last version would give a better result as regards less chatter. Using this bar gave little improvement, but there was an improvement.

……

Stiffness is governed by shape and Young's modulus, nothing to do with strength/hardness

[egaParticipant @ega]

In his ME manual GHT recounted how he had been challenged on the use of silver steel for his boring bars. Whilst, I think, conceding that they were no stiffer than if made in MS he preferred silver steel for its greater resistance to abrasion and the fact that it was more likely to hold up to nominal diameter (his bars were clamped in eccentric holders and a sloppy fit would have been unhelpful).

Edited By ega on 06/02/2020 14:05:05

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