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Tool rest height adjustment

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Panning for Montana gold, with Betsy, the mule!
Every time I pick up a different turning tool, I find myself adjusting the tool rest height, according to presentation of the cutting edge to the spindle centerline, while the tool itself is held level. This is a theoretical application, since there are variables that may exist for changing applications. Regardless, this is my generic tool rest height standard setting, prior to further consideration of variables encountered by the specific application.

Theoretically, if the cutting edge is presented to the turning, at exactly central to the spindle centerline, a better cut is possible, but not a given. The changing distances between the top of the tool rest, plus the thickness of the current turning tool will make this point of contact vary, and the adjustment of tool rest height may be necessary. To further the variance, the tilt of the tool on the top of the rest (fulcrum) will make a difference, according to individual turning styles. If the tool rest height, considering the thickness of the tool, is below the spindle centerline, the tool itself can be presented with the tip tilting up on it's fulcrum, and still hit the spindle centerline exactly.....and the opposite is true with a the tool rest higher, while the tool tip is tilted down on it's fulcrum.

The standard, or generic rule of thumb is to have the cut slightly below centerline for exterior surfaces, and slightly above centerline for interior of bowls.....and this is a pretty good catch-all rule that handles a variety of situations, but isn't calculating for perfection in presentation at the centerline, while taking into consideration tool rest height, tool thickness and the turner's style of presentation.

We all want "the perfect cut", and tool rest height is only one of the variables that is needed for a perfect presentation of the cutting edge.

-----odie-----
 
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hockenbery

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standard, or generic rule of thumb is to have the cut slightly below centerline for exterior surfaces, and slightly above centerline for interior of bowls.....
I use that rule for scrapers presented flat on the tool rest and level. It prevents bevel contact which is usually a catch.

Tool rest position is related to how high or low you hold the handle of the tool from level
Advanced turners develop a style and position the tool rest suited to their style.

For spindle work with skews and spindle gouges I usually have the tool rest a bit above center and cut well above center.
On small diameters I cut nearly on top of the work.


Every time I pick up a different turning tool, I find myself adjusting the tool rest height, according to presentation of the cutting edge to the spindle centerline, while the tool itself is held level.
When using a gouge on Hollow forms and bowls I set the tool rest to cut at center with the tool held level.
This puts the top of the rest slightly below center.
Then on most cuts I drop the handle to get a better slicing cut with tool contacting the wood above center.

A common mistake for beginners is setting the tool rest too far below center on bowls. This lets the wood drive down onto the cutting edge making a catch.

I almost always want the gouge contacting the wood at or above center.
 
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Not sure I want the "perfect cut", If I did, I would use a machinist style metal cutting lathe to hold a perfectly ground (angle and rake) cutter in the perfect position,
or use a CNC, thinking along the lines of "better is the enemy of good".
Sometimes I feel paralyzed by trying to find "The Best", but I still strive, still explore. I agree with the rule of thumb, cutting above inside, below outside, but feel it is more a help for learning initial setup. I have shifted the tool rest as little as a 16th to get a better cut and realized, hm, that little bit made a difference. Why, it was only a 16th, what was the feeling, how did I know? Five maybe ten years ago, I wouldn't have felt the need, or even realized what I had done. Can this be explained? I think that's why we have the ROT's. To me, the discovery of the nuances are part of the joys of the journey. Of course, in the shop, with no one around, no one sees the little smile when you make the discovery...
 
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I use that rule for scrapers presented flat on the tool rest and level. It prevents bevel contact which is usually a catch.

Tool rest position is related to how high or low you hold the handle of the tool from level
Advanced turners develop a style and position the tool rest suited to their style.

For spindle work with skews and spindle gouges I usually have the tool rest a bit above center and cut well above center.
On small diameters I cut nearly on top of the work.



When using a gouge on Hollow forms and bowls I set the tool rest to cut at center with the tool held level.
This puts the top of the rest slightly below center.
Then on most cuts I drop the handle to get a better slicing cut with tool contacting the wood above center.

A common mistake for beginners is setting the tool rest too far below center on bowls. This lets the wood drive down onto the cutting edge making a catch.

I almost always want the gouge contacting the wood at or above center.
Thanks for your reply. I'm still learning. What about pull cuts on the outside of bowls? With swept back side ground straight or arced?
 

hockenbery

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Thanks for your reply. I'm still learning. What about pull cuts on the outside of bowls? With swept back side ground straight or arced?
I use the Ellsworth grind most often for pull cuts.
I start with the tool rest with the gouge cutting at center when held level.
Then I drop the handle way down.
Depending on the shape I may lower or raise to tool rest.
I want the cut to be on the wing.
 
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Well, with scrapers, I always keep the handle raised and the tool pointing down a little bit. other than with NRSs. Much more so in bowls than in boxes. With gouges, most of the time, I am holding my tools just about level, so I want the rest at about center height. If it is a bit high, I don't adjust it, I just raise the handle a little bit. I drove poor Stuart crazy at a work shop I did with he and his dad. He always wanted it set at pretty much dead center.

Al, I don't get the part about a bevel rubbing with a scraper and it causing a catch. I know some have a very blunt angle on theirs, some thing like 80+ degrees, to the other extreme like Richard Raffen at about 45 degrees. If you try to rub the bevel, that would be a peeling cut, and if you try that one, just like trying to do peeling cuts with a SRG on bowls, as soon as you come off that bevel, you are essentially sticking your finger into the fan at the wrong angle, and massive catch. That is why I always have it pointing down a bit. Using scraping cuts and having the tool handle down, and the metal up, is suicidal.

I believe it is Lyle Jamieson who has his tool rests on threaded stock and a nut for adjusting the height. Never played with that idea, but it does have some merit, especially on some thing like the McNaughton coring system.

robo hippy
 

hockenbery

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Al, I don't get the part about a bevel rubbing with a scraper and it causing a catch. I know some have a very blunt angle on theirs, some thing like 80+ degrees, to the other extreme like Richard Raffen at about 45 degrees. If you try to rub the bevel, that would be a peeling cut,
It would be a peeling cut if you can control it, but the tool tends to just dig in deeper until something bad happens.
Bowl breaks, lathe breaks, Tool breaks.
Remember that video you showed me- you were coming across the bottom of a bowl and hit a center post - BAM!! The bevel made contact with the center post and dug in.

That said I did see a guy from Alaska turns bowls at an AAW symposium using bevel riding cuts with a scraper.
I can’t remember his name ( Buzz?.. ). I think he passed away.
 

hockenbery

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REVERSE TURNING.
When I’m reverse turning I raise the tool rest so that I’m cutting from the top when turning bas3 down to a pointy nub.
The big danger here with cutting at center is that the tool tip is easily pulled under the nub which breaks it and the bowl s loose.
Usually beats itself up on the tool rest and the center
 

Roger Wiegand

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I don't measure or try to figure out where the centerline is; I move the rest up or down until the tool cuts well and is easy to handle. Thinking about it all the time would make me crazy. I'd rather move the tool rest around to get an empirical answer. It doesn't feel as though it takes significant time or energy to do that.
 
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I have used bevel rubbing cuts with scrapers, handle dropped. It does provide a very high shear angle. Used to use a small 'inside' bowl scraper for that final finish cut across the bottom of a bowl, but then Doug Thompson came out with his fluteless gouge. Since mine are ground to 70 degree bevels, they could be used as finish cut tools, and BOB tools, but I have other tools I like better.

robo hippy
 
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Thanks - this kinda posts and most of the replies add value.
I use James Johnson tool rests on all exterior stuff - on the Oneway banjo, they are a teeny bit below center - any angle of the tool puts the edge at of above center.
Of course, the diameter of the work is a big part of the equation - 1/16" below is not a big deal on a 20" piece - on a smaller bowl or spindle could be catch-city.
Again, good stuff - being more than less a one-trick pony, I've come to realize it's the other "disciplines" that require the advance tool-handling skills
 
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That said I did see a guy from Alaska turns bowls at an AAW symposium using bevel riding cuts with a scraper.
I can’t remember his name ( Buzz?.. ). I think he passed away.
I would bet that he didn't hold the scraper flat on the tool rest, think about it would you use a skew flat on the tool rest? I remember seeing something about that technique several years ago.
 
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Once I do get the tool rest height where I want it, I use a small spring clamp to mark & hold that position. Since the clamp can be positioned in any direction it never needs to be in the way. If I rotate the rest I don't have to chase the height again. And if I change rests I can swap back later and be on or close to the mark. If I change tools and need to readjust the rest there's no screw or collar to deal with. I have several clamps, purchased for some other project, but as I recall the cheap house brand went for a dollar each.
 

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I would bet that he didn't hold the scraper flat on the tool rest, think about it would you use a skew flat on the tool rest? I remember seeing something about that technique several years ago.

A skew, flat on the tool rest is called a negative rake scraper....

robo hippy
American Woodturner August 2010 page 21 picture 22 shows to put the skew flat on the tool rest as a negative rake scrapper.
 
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