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+-- Thread: Weak Links (/showthread.php?tid=7330268)
Weak Links - MichaelMouse - 05-17-2017
Getting around to reality, versus mechanical absurdity, where is the most likely place for runout to begin? Is it merely gravity acting on a long piece, or is it support for that long piece where it's mounted to the rigid spindle? Assuming the spindle has no runout, you begin at the chuck. We have seen multiple postings about inserts poorly manufactured by third parties, not snugging to the scroll itself, and similar. Certainly some potential there, but it is both consistent, whether the piece is long or wide, fairly easily located and repaired, barring a true manufacturing defect. Rule out.
Human error in mounting, however, is much more difficult to remedy because we seldom evaluate ourselves with the same scrutiny as we use in pursuit of things mechanical. The most common error leading to runout in mounting is failure to prevent racking by not bottoming the mortise or shouldering the tenon on the nose of the selected jaws. It's a newbie mistake, but we can all fall prey to misinterpreting a piece that will not behave as due to any other source but us. I learned long ago that a proper jaw face bottomed inside a mortise when reversing to hollow a piece will not run out. If it does when I hand spin, I release the hold, examine the mortise, and 9 times of ten discover a splinter or sawdust packed in it. Or I find a gap between chuck and shoulder on those odd times when I use an outside hold. Rule out.
Then there's the wood itself. The book calls it an anisotropic material, "the quality of exhibiting properties with different values when measured along axes in different directions" according to Brittanica. Means end grain, face grain and quarter grain exhibit different crushability, a fact for which we should compensate. Mount choices, like tenoning on a long grain piece are pretty obvious. The properties of face, quarter, and riff faces are similar enough that using a tenon is at all times appropriate. If we try to use a mortise, we risk splitting the piece along the grain. Two things we can do to help are to shoulder, and use a long hold. Two things we can do to hurt are hold short or chew a tenon. The second is a common error on cross-grain pieces, where there is a significant difference between face and end grain. Teeth compress and distort the hold, making a remount to true virtually impossible. Smooth, wedged (dovetail) holds bridge minor discrepancies, pull the face into bottoming in the mortise, and shouldering on the tenon. Excellent choice.
Of course, we should all remember that a single degree of deviation grows in absolute measurement from virtually nothing, to more than a mm at one meter. Regardless the shape of the spindle casting!
Talk of steadies encouraged. Type, location, and, of course purpose encouraged.
RE: Weak Links - Arlin Eastman - 05-17-2017
How about wood that is heavy on one side like a knot?
RE: Weak Links - MichaelMouse - 05-18-2017
(05-17-2017, 09:23 PM)Arlin Eastman Wrote: How about wood that is heavy on one side like a knot?
Not sure what the question is meant to resolve, but proper mounting is still key to obtaining circularity. Matters of balance, as discussed elsewhere, are best addressed by a firm understanding on the machine, appropriate hold and moderate centrifugal velocity.
In extreme cases, removing some soon-to-be-discarded mass from the heavy side with a drill might be required.
RE: Weak Links - Arlin Eastman - 05-18-2017
Not trying to resolve anything just thinking that if part of a knot was in a chuck that it would hold on the knot more and the soft part leave it unbalanced and if it was chucked again.
RE: Weak Links - MichaelMouse - 05-18-2017
I was confused by the word "heavy" in the sentence. Knot which is harder is more or less the same as a cross-grain hold with difference between end grain and face at 90 degrees. Answer's the same - try to size the jaws for maximum contact to spread the load over hard/soft differences if using toothed jaws. Smooth jaws should have no problem, unless it's a loose knot.