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Thin Wall Squealing (Read 318 times)
 
Don Stephan
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Thin Wall Squealing
Jul 4th, 2017 at 7:11pm
 
Not sure if squealing is the technical term, but sometimes when I'm trying to turn a bowl with a thin wall as the gouge starts down the inside from the rim I hear a squealing sound.  Immediately I'll place a couple fingers on the outside of the bowl to try to dampen the vibration of the wood, and try to reduce pressure of the gouge on the wood.  If it's been several minutes, I'll also touch up the bevel so the cutting edge is sharp.

Yesterday turning a martini glass shape in some green holly, none of these helped, and I noticed my fingers were encountering more friction on the outside of the bowl than typical with some other woods.  So I looked at the tip of the bowl gouge and noticed that the primary bevel, that extending back from the cutting tip, was at least 3/16" wide.  I widened the secondary bevel on my coarse grinding wheel so that the primary bevel was less than 1/8" wide.  That stopped the squealing.
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Tom Coghill
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Re: Thin Wall Squealing
Reply #1 - Jul 5th, 2017 at 11:49am
 
Don,

What you have done here (I believe) is to shorten the bevel surface which gets the accrual cutting tip of the tool more on the wood.  I have found that;

The closer to that fine line of just barely riding the bevel the better.

Other ways I have found to do this are:

Raise the handle in relation to the tool rest

Elevate the tool rest - to get the tip of the tool closer to the absolute centerline of the turning.

All of these MUST BE DONE WITH CAUTION (sneak up on it) as these are the first steps toward getting a bad reaction between you and the wood (a catch).

When I get to finishing the inside of a bowl (or in you case a goblet) I try to shear cut as much as possible.  This can be tricky and I have found turning in reverse to be an advantage when doing this.  This shear cut, much like shear scraping the outside of the bowl, leaves me with a WONDERFUL surface, eliminating a lot of sanding  Thumbs Up.

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Ed Weber
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Re: Thin Wall Squealing
Reply #2 - Jul 5th, 2017 at 1:52pm
 
Good tip Don
I can't say I really had the need to measure but having a secondary bevel (sometimes a tertiary) helps many little turning annoyances. As always, you results will probably be different depending on the wood and/or curvature of the piece.
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robert baccus
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Re: Thin Wall Squealing
Reply #3 - Jul 5th, 2017 at 11:11pm
 
Years ago I discovered(for me) the bottom feeder grind which is simply allowing the bevel to rub whatever the degree of approach the tool is to the wood.  In later years I applied the the technique to large roughing out gouges and found them to be wonderful on exterior vases and bowls when ground to a 70-75 deg. bevel.  Almost impossible to catch, removes wood like a beaver and does a really fine finishing cut.  No catchy name yet but someone will supply that.  Like the bottom feeder it seems to take years to catch on but it works.
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Don Stephan
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Re: Thin Wall Squealing
Reply #4 - Jul 6th, 2017 at 7:55am
 
While this roughing gouge angle might work for you, the brief description might cause problems for others.

Every gouge should be used in a manner that allows the bevel to parallel the surface of the wood, sometimes referred to as riding or gliding the bevel. 

Most images of a roughing gouge in use, that I can recall right now, show the gouge essentially perpendicular to the lathe bed, or perpendicular to the wood, and angled up from the handle to the cutting tip, what might be called "spindle fashion" as if roughing out a table leg (or the outside of an urn). 

Scrapers used flat on the tool rest on the outside of bowls sometimes are essentially parallel to the lathe bed, or perpendicular to the wood.  Scrapers used flat on the tool rest on the inside bottom of bowls are  again essentially perpendicular to the wood.  Every recommendation I can recall for scraper use flat on the tool rest perpendicular to the wood has called for the scraper to angle down slightly from the handle to the tip, because of the likelihood of a catch if held flat or angled up from the handle to the tip.

I use a "bottom feeder" gouge with a bevel angle of 65 degrees across the inside bottom of a bowl because a gouge with a lesser angle cannot glide the bevel across the bottom without the shaft of the gouge hitting the rim of the bowl, not because a higher angle automatically follows the wood better.  The gouge is rotated so that the flute is facing approx 2 o'clock and the tool is angled to the right as well - my bottom feeder gouge is not perpendicular to the wood like a scraper would be.  When I use my bottom feeder gouge the tool is angled up from the handle to the tip.  This is opposite the tool angle of a scraper flat on the tool rest on the inside bottom of a bowl.

If a roughing gouge is used (in spindle fashion) at a 45 deg horizontal angle to the wood, the cutting edge meets the wood at an angle and makes a slicing cut.  If that roughing gouge is used perpendicular to the wood, the cut is more like a scrape and will not be as smooth.  I would expect major catches if I used a roughing gouge (in spindle fashion) perpendicular to the wood with a steep bevel angle like that of a scraper.
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