Vibration of a Boring Bar System

[Dennis in Southern Oregon]

I am investigating the design of boring bar systems for deep hollowing of wooden objects on a wood lathe. The bar rests upon two supports with one end overhanging. The cantilevered end has a scraping bit on the end that bores into a rotating wood work piece. The characteristics of the the tool bar (the cantilever) may or may not be the same as those of the support bar. The boring bar rests freely on the supports. I want to calculate the the resonance frequencies of the tool bar which will be excited by the scraping action. I am familiar with the various formulas for transverse vibration frequencies of a uniformly loaded cantilever. However, they do not apply directly here, because the effective stiffness of the cantilever will be reduced by the bending of the support bar, and (I believe) the mass of the support section will be reflected as a change of the effective mass of the cantilever. It seems to me that this problem ought to be a standard one, bit I have not found any references. Can anyone help?

[PinkertonD]

Hi Dennis, welcome to the forum.

I suspect there are too many unknowns when working with wood. The spacing between annual-rings can be so different as could the soft-spots and hard spots within the single piece of wood, that frequencies could range from nothing to a zillion, well almost. Feed rates would also have to come into it when considering vibration and the control thereof.

I would suggest working from the back forwards with trial and error. What's the largest diameter or area of cross-section of the bar that you can accommodate? I would then use that size as the max size for the boring-bar.

From there you could advance up the stiffness scale of various metals. That scale is usually pretty proportional with cost. The more rigid the material, the higher the cost.

Solid Silicon Carbide boring bars are commonly used in metalworking, but depending on diameter (or cross-sectional area) it can get expensive fast.

{edit}
If you are boring to differing depths from one project to the next, you could mount the bar in a holder that will allow more or less overhang/cantilever. Just sufficient sticking out to reach the desired depth.

[Kelly_Bramble]

So, how deep are we talking? Also, what diameter are you ging to bore?

[PinkertonD]

Making a peanut-bowl out of something like this?

http://vimeo.com/41892788

[Dennis in Southern Oregon]

Thanks for your response. I agree, wood is an imponderable. However, the dominant excitation frequency will be the rotational speed of the lathe. One can hope to minimize resonances by setting the rotational speed below or above the natural frequency of the boring bar. Of course, other considerations will limit what speeds can be used.

Dennis

[Dennis in Southern Oregon]

Kelly, I am providing engineering support for a fellow woodturner. So far he has achieved a boring depth of 48 inches using a boring bar consisting of a 48-inch tool bar made of 2.5-inch Schedule 80 steel pipe and a 7-ft support bar made of two parallel 3-inch Schedule 80 steel pipes. We hope to go much further. The objects being made are tall thin-walled wooden vases (art objects, not functional items).

Dennis

[Goingforsound]

Use round steel stock. A pipe is flexible and has less mass than bar stock.

It will be a good start. I cant help with the calculation.

[RWOLFEJR]

Hi Dennis,
You might want to look into a dampened or tuned bar?
check this or similar out...
http://www2.coromant.sandvik.com/cor.../C_2920_23.pdf

A quick cheat you might want to try would be to fill your hollow boring bar with urethane foam. You can buy spray cans of it at your harware store. Just need to be careful that the foam never gets hot enough to burn. The smoke from burning urethane foam is very bad for you... might even be deadly.

Maybe better yet might be to stuff it with foam rubber? Then if it didn't do you any good you could easily remove it to try something else. Maybe fill it with water? Maybe fill with heavy grease?

I can't help you with the whole vibration formula thing but there are piles of books and articles on the study of machining vibrations out there. I'd like to see the cutting part of this bar you're buddy is using. Does it have a little pecker on the front like a typical home owner wood bit to help support the cut? Slight tweaks in tool geometry can, as you know, make a huge difference. Maybe add a couple more cutting edges and reduce and spread out chip load?

Also... if he can stand going clear through the part and then "cork it" afterward... then you might be able to run a long smaller drill through it more easily then have means to support the boring bar from the other end?

And then if you want to get really big and deep you might want to look at doing this the way big wood columns are built. Pieced together and glued...

Good Luck,
Bob

[PinkertonD]

A quick cheat you might want to try would be to fill your hollow boring bar with urethane foam.

Or -- concrete.