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weight?

Hi,

This is a rather general question, but here goes:

I hung around the woodworking newsgroups for a long while, and there were regular discussions of the advantages of weight. You want as much mass as possible to absorb vibration. Heavy machinery was quality machinery. Etc.

Now, is it just weight? That is, if I take my table saw at home, which is one of those hybrids that's like a cabinet saw but with an open base; it's fairly stable as-is. If I box in the legs with sheet metal and fill it up with concrete or lead or whatever, is it now the same as a one-ton oliver table saw in terms of running smoothly?

So if I've made designs to build a rose engine over the summer (I've decided to go through with it): it is normally a half-ton machine, much of that in the cast iron base, I would wager. All that is in steel and brass in the original will be replicated in steel and brass. But if I weld up some 1/4"-3/4" sheet metal to form the stand, can I just fill it up with concrete? Will that make it run smoother and give me better work?

There's the other question, too, that the machine vibrates itself, but someone running down the hall one floor above will vibrate the machine as well; so I want lots of mass on the one hand, but can't have it TOO rigidly bound to the floor. An engine-turner definitely told me that he had his machine in the concrete-floored basement of his house, and when somone walked too forcefully along the floor above, each step would be an apparent defect. So I might want ot think more about the complexities of dampening and insulation than merely about just blithely laying on mass?

thanks for the advice! -Bernard Arnest

Reply to
Bernard Arnest
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Reply to
RoyJ

Another thing I should have mentioned is that stiffness plays an important role in resisting constant cutting forces, as well as the cyclic forces which cause vibration. For example, if the table of your mill distorts due to cutting forces, you get an inaccurate component.

If vibration is a big issue for you, I'd suggest looking out for a proper 1 ton saw. Ultimately this is a much better solution to the problem.

Best wishes,

Chris

Reply to
Christopher Tidy

"Bernard Arnest" wrote in news: snipped-for-privacy@v46g2000cwv.googlegroups.com:

This might be a more elegant solution for a home built machine:

It will allow you to keep the weight way down and will be stiffer than cast iron and will absorb vibration as well as or better than a polymer concrete base.

You can use all steel welded construction which eliminates the problem of pouring a one off casting. It also makes machining the mounting surfaces a relative breeze.

Filling the base with concrete will add weight to be sure. But it won't always dampen vibration. If you look at a machine that uses a polymer concrete base, like a new Hardinge, you'll see that the entire base is made from polymer concrete. It's not just a steel or cast iron base filled with concrete. The polymer concrete is poured around the mounting structures, rather than the mounting structures being filled. This way vibraton damping is certain.

Stiffness is just as desireable as vibration damping in a precision machine tool. The problem is a welded steel base while its very stiff, rings like a bell. Machine tool design is all about trade offs. Heavy slides damp vibration very well but they are difficult to position accurately. Cast iron dampens vibration better than steel but it sags and flexes. Just ask anyone who has set up an engine lathe. The bed twists and sags easily. If the engine lathe bed was steel, it would stay straight. It would also vibrate terribly making the machine nearly useless.

Large diameter spindle bearings offer more support and can bear more load than a smaller diameter set. But at a given RPM the larger bearings will run hotter which isn't very desirable.

The key I guess is in finding the right balance for the application.

Reply to
D Murphy

I think weight is a by product as opposed to a design constraint. Vibration damping and stiffness is the design objective. Construction material is the key. Cast structures are more dead than formed structures and iron is more dead and stiffer than aluminum. Adding concrete to a formed construction changes its resonance frequency, but doesn't add much stiffness. Steve

Reply to
Steve Lusardi

"Bernard Arnest" wrote in message news: snipped-for-privacy@v46g2000cwv.googlegroups.com...

I was told by an old mechanical engineer that machine tools should be compared by the tonne. There is no substitute for copious amounts of cast iron!

Reply to
Tom Miller

does cast iron have different damping properties than, say, steel plates that have been welded or bolted together?

Reply to
Bernard Arnest

Re the defects OP (Bernard Arnest) mentioned, where he wrote

and you wrote

Although his damping example referred to a table saw, the machine he plans to build is a rose engine, as in (eg)

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or
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and in a pattern from such an engine, just as for a moire pattern, I think the tiniest variation can make an easily visible difference.

-jiw

Reply to
James Waldby

Yes. Cast iron has higher internal damping than mild steel.

Chris

Reply to
Christopher Tidy

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