Dampened boring bars for long overhangs

As only a hobby machinist but interested in everything mechanical I have Sandvik email me details of there tooling.

The latest shows a dampened boring bar with long overhang apart from coolant at high pressure being used how is the bar dampened.

It might be of use to us if it's not too complicated to make one.

Neil I have sent you a message.

Paul

I seem to remember some boring bars had carbide running down the centre to stop vibration.

Never used one myself and I expect they are professionally priced but a quick search reveals that most of them appear to have a mass / spring / damper assembly within the (hollow) body. The damper element being a viscous oil or mechanical friction mechanism.

Anyone here tried to make one of these?

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'fraid not ... But I have used the simple expedient of adding a good lump of Blu Tack

I can't comment on any of the alternative products, but the original stuff has remarkable damping properties. ... It may be the most thixotropic material known.

MichaelG.

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I have heard wrapping the bar in lead sheet can sometimes assist in removing harmonic vibrations.

Nick

Reminds me of that most unfortunate of vehicles, the 2CV. Even by French standards it was thrashed and violated mercilessly by the ugly stick, both on the drawing board and in the flesh. However, one notable feature was the front dampers which were mounted on the front swing arms without any form of coupling to the chassis. Unsprung mass was the least of their worries.

And also from a previous era, green Swarfega was a remarkable gel that seemed to be able to hold its form for months. Presumably that would make it "more" thixotropic? Is that how thixotropicity(?) works?

Murray

That 2CV damper was of course filled with oil, and worked very well, and never leaked, so never failed the MOT. I had a fail once 'cos the man had never seen one before and assumed some bits were missing. I am in the process of making a long boring bar, from pipe, and plan to fill it with lead. I will report back when done, about next xmas at the speed things are going just now.

I've tried soft solder wrapped around the end of a boring arm which seemed to improve things but I will try the Blu Tack suggested by Michael - that one hadn't occurred to me but sounds oddly appealing for some reason..

I have no idea what the mechanics involved in all of this are but it does seem to help by changing one of the many variables combining to cause the chatter in the first place. Obvious things like a speed change, a slight tool height adjustment and a squirt of cutting oil can all help to make a difference.

Beyond that, I don't do enough boring to get expert, so I'm always very pleased when it goes well. It's usually preceded by a bit of tweaking, general fiddling about, some bad language and a dollop of superstition (e.g. touching wood etc.) all of which also seem to help.

IanT

Sand in a tube might also be an option. I wonder if lead shot would work better than solid lead.

If you remember mixing custard from powder you were instructed to add a very little water to start with and mix. The paste at that point is a tixotropic substance.- sudden movement causes the particles to lock together, but slow movement is possible. Quicksand is another example. Blutack is just a paste (too much liquid phase) and swarfega is a gel so no better than a thick oil.

Once when doing some light boring the noise did not seem right I just gently rested my finger on the bar and it was better not to be recommended for H and S grounds.

Any spare fingers.

Paul

Gordon - apart from changing the mass of the bar which may have a marginal effect on the onset of squeal and chatter, filling the bar with lead doesn't seem likely to affect the outcome much. It's going to be a fine balance, as the very act of hollowing out the boring bar enough to affect anything will reduce its rigidity.

To make a real impact I suspect you will need to allow the internal mass to have some degree of movement against a spring plus a damping medium such as a viscous fluid to absorb enough energy to damp out any resonance.

Murray

Making the bar from tungsten ( if you can find any & expensive !) is one of the better ways of reducing vibration.

Solid carbide boring bars work very well as regards dampened vibration. Bit more expensive but worth it I have found.

Elastic bands stretched around the bar can often curb chatter or vibration. Just wind as many as can be fitted around the bar as tightly as possible without getting in the way of the material. It seems to change the frequency at which the bar will vibrate.

The best solution, if you can afford it is tungsten carbide, as previously advised.

The old adage,"increase the feed, decrease the speed" is also worth remembering.

Bob Strawson

I would imagine a hole down the centre, needs to be below the size were the strength reduces rapidly.
Then bond a hard rod in with a small gap using a rubber type solution.

The hardened rod will resist bending and the rubber solution will allow a small amount of flex for the boring bar and should help dampen the vibration.

Steve

I recently had to bore a hole in an aluminium casting to a depth of 150mm. My 1/2" dia. boring bar vibrated like mad so I made a new one with 3/4" square BMS that solved the problem without any complicated construction.

Edited By norman valentine on 23/07/2015 22:40:00

Obviously if you have room to fit a big f*** off boring bar, then you have a simple solution. The difficulty arises where the combination of small bore and big length results in enough spring to make it impossible to machine without encountering instability, even with fairly exotic materials for the bar.

Even if you eliminate the nonlinearities in the system (backlash etc), you still have a mass and spring resonant system that will be prone to resonance under certain conditions. The best solution is often to damp out the resonance rather than simply trying to move it out of the way. Given the wide range of operating conditions (load, speed, materials etc), it's going to be pretty difficult to move the resonances where they won't be encountered - and there isn't much of a choice of materials that would allow a significantly stiffer boring bar body. Carbide and expensive steels are possibly the best that are realistically available and we use those already.

I don't know if the systems used in high speed industrial tooling (mass, spring, damping fluid etc) would actually work in the conditions we encounter in our workshops. And to be honest, I doubt I would be able to do the calculations to start designing them myself. I suspect there will be quite a few PhDs in this area. Might be an area for experimentation!

Murray

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Assuming that you want an 'internal' solution [to keep the size down] ...

Some background reading on 'constrained-layer damping' would probably be useful.

MichaelG.

I forgot to mention that with a VFD you can change the rotational speed to potentially move away from the bar resonance that is causing the problem.

If a round boring bar is made tapered along its length it should reduce the likelihood of a problematic resonant frequency being generated. Colin