Blunt but Sharp Woodturning Tools

Discussion in 'Woodturning Discussion Forum' started by Dennis J Gooding, Oct 20, 2017.

  1. Dennis J Gooding

    Dennis J Gooding

    Joined:
    Apr 10, 2010
    Messages:
    226
    Location:
    Grants Pass Oregon
    There have been several posts recently on sharpening and the resultant sharpness of lathe cutting tools. While pondering one of these recently, I was struck by the realization that gouges and skew chisels are traditionally ground to very blunt angles compared to most other cutting tools such as razor blades, box cutters, wood carving tools, home cutlery, etc. The edge angles for skews and spindle gouges typically are 30 degrees or larger and for bowl gouges the angles range to at least 60 degrees.

    How then do we ever manage to turn wood with these tools even when they have a perfectly sharp edge? Apparently, my muscle memory learned the answer long before I even realized that there was a question. The simple answer is that we seldom present a gouge or skew chisel to the spinning wood so that the wood hits the cutting edge head on. Almost always, the tool is positioned so that the wood hits it at an oblique angle. That is, so the wood slides along the cutting edge as well as climbing over the edge. Exceptions to this include using a skew chisel or a spindle roughing gouge in a peeling cut to hog out material, and most cuts using a parting tool.

    It turns out that when the cutting edge is presented at a skewed angle to the moving wood, the edge appears less blunt to the oncoming wood. A good analogy is climbing a hill at an angle rather than climbing straight up. Applying a bit of Trigonometry 101 to the problem, I calculated the reduction in effective edge angle as the cutting edge is skewed more and more from a head-on presentation. The results are shown below for actual edge angles of 40 degrees and 60 degrees. It is worth noting that the same reductions in effective edge angle will be obtained even if the tool is dull, that is, the tool will appear less dull with increased skew angle. Also note that the edge angle of a gouge may not be the same as the nose angle as one moves away from the nose. Why the bluntness? I would guess that it evolved as a means of adding resistance to battering of the tool edge by the spinning wood.

    EffectiveAngle.JPG
     
    odie, charlie knighton and hockenbery like this.
  2. john lucas

    john lucas

    Joined:
    Apr 26, 2004
    Messages:
    5,832
    Location:
    Cookeville TN USA
    Dennis you are fight however there is another side to this also. Blunter (meaning less acute) edges are preferable for power cutter and also for a more robust edge. Take an Ax for example. You wouldn't sharpen an AX at 25 degrees. Same is true for wood chisels that are going to be hammered. I have 2 sets of wood chisels. One sharpened at 25 degrees for hand use only. The other is sharpened at 35 degrees because I may use my mallet to strike those. For woodturning tools most people grind their spindle roughing gouges pretty blunt so it will hole an edge for a long time. Actually most people probably don't think about it and just grind it to match whatever came from the factory. I've seen some ground about 70 degrees. I grind mine 45 degrees because I use it more than just for roughing.
     
    odie likes this.
  3. hockenbery

    hockenbery AAW Advisor Staff Member

    Joined:
    Apr 27, 2004
    Messages:
    4,317
    Location:
    Lakeland, Florida
    Home Page:
    While much bigger than the razor edge,
    Part of the beauty of an Ellsworth grind is the different edge angle you have available for offering cuts.
    The tip is about 60 degrees, the sweet spot just off the tip degrees.

    The handle down pull cut presents the 30 degree wing at a 45degree slicing angle which is makes about the best surface you can get with a gouge. The push cut with the nose is not nearly as clean.
    Rolling the tool to get a slicing cut with the sweet spot gets clean surface but not as clean as the pull cut.

    We have different tools with different edge we also have some tools with different edges on them.
     
    Last edited: Oct 20, 2017
    odie likes this.
  4. Bill Boehme

    Bill Boehme Administrator Staff Member

    Joined:
    Jan 27, 2005
    Messages:
    8,150
    Location:
    Dalworthington Gardens, TX
    Home Page:
    I'll agree with your statement that presenting the edge at an angle results in an apparent more acute cutting edge. However I believe that it is not quite accurate to say that the wood slides along the cutting edge, especially in the sense that we think of sliding as being equivalent to slicing as, for example, a butcher knife slicing through vegetables or a piece of meat where a significant length of the blade is used. A turning tool held at a shear angle is gaining the advantage of an apparently more acute cutting edge. However, only a very small portion of the edge of a turning tool is contacting the wood. If we could zoom in and see a single wood fiber being cut by a bowl gouge, I think that we would see is that it is no different than when the tool being held level except for the advantage of a more acute cutting edge. I've thought about this many times when hearing a demonstrator talking about the tool slicing through the wood when shear cutting. The trouble is that I haven't been able to rationalize that analogy in my head. Each fiber in the wood is being cut by just a tiny point on the cutting edge. Now, I'll agree that it looks like sliding or slicing, but there isn't any more sliding or slicing than when the cutting edge is horizontal (assuming that the lathe bed, tool rest, and spin axis are all horizontal and the wood is moving approximately straight down as it passes by the tool rest if the tool cutting is "on center"). I suppose that I don't really have a problem with referring to shear cutting as slicing since it resembles slicing, but at the microscopic level I don't think it is strictly accurate. Just my 2¢.
     
    odie likes this.
  5. Dennis J Gooding

    Dennis J Gooding

    Joined:
    Apr 10, 2010
    Messages:
    226
    Location:
    Grants Pass Oregon
    Bill, I used the terms sliding and climbing only to describe the direction that any given particle of wood is moving relative to the cutting edge.
     
    Bill Boehme likes this.
  6. Bill Boehme

    Bill Boehme Administrator Staff Member

    Joined:
    Jan 27, 2005
    Messages:
    8,150
    Location:
    Dalworthington Gardens, TX
    Home Page:
    Thanks for the clarification, Dennis.
     
  7. odie

    odie

    Joined:
    Dec 22, 2006
    Messages:
    4,432
    Location:
    Deep in the woods
    I had to read this post a couple of times, before I think I "get it".....then I'm not completely sure that I do......but I'm pretty sure there is some insight here that is grounded in truth. Bill is definitely a thinker, and how in the world am I going to apply this to my turning.....? As I ponder this, I can't help but think the application is not somewhere within the realm of critical thought, but more attuned to the connection between a possible subliminal connection between hand and mental awareness.

    -----odie-----
     
  8. Bill Boehme

    Bill Boehme Administrator Staff Member

    Joined:
    Jan 27, 2005
    Messages:
    8,150
    Location:
    Dalworthington Gardens, TX
    Home Page:
    Everybody develops their own turning style. I generally have the tool handle dropped really low ... so low that I need a special tool rest ... one that allows me to drop the tool handle to 60° or even lower. So, I present the tool to the wood at a high shear angle in a bevel gliding cut. I feel like this gives me a very smooth final surface.

    Of course there are many ways to skin a ... uh many different turning styles that deliver excellent results. :)

    BTW, here is the tool rest that I like: http://advancedlathetools.com/alt2017_011.htm
     
  9. john lucas

    john lucas

    Joined:
    Apr 26, 2004
    Messages:
    5,832
    Location:
    Cookeville TN USA
    I try to describe to people when I demo this slicing cut. If you take a skew and hold it so the wood comes across the blade at 90 degrees you get a rapid cut but a poor finish. If you "skew" the blade so the wood comes across at an angle you get a slower cut but a much more refined finish. Then I try to show them how to watch for this same cutting action with the bowl gouge or spindle gouge. I have a older no name gouge that has a very large U shape. by that I mean the U is not far from the width of the tool. When you grind this one with a Ellsworth grind you get an extremely thin wing like Al was talking about. Drop the handle a lot and do a pull cut and man does it remove wood and leaves a very clean finish at the same time.
    One thing that still confuses me. Often when turning really curly wood sometimes highly twisted figure I find that a small radius cutter will cut cleaner. I first discovered this when spindle turning my handles with a skew. I would have a spot or two that just wouldn't cut clean no matter the angle and how sharp the tool was. I could switch to my 3/8" detail gouge that has the same included sharpening angle and it will cut clean. both tools are rotated so the wood crosses the edge at the same angle. When I look at what is happening it appears that I am only taking 1/8" or less of a cut so you would think the difference in that extremely slight radius vs the flat edge wouldn't make a difference but it does. I've repeated this many times on bowls or mirror bodies. I use a smaller tool with a tighter radius and it cuts cleaner. anyone else see this.
     
  10. hockenbery

    hockenbery AAW Advisor Staff Member

    Joined:
    Apr 27, 2004
    Messages:
    4,317
    Location:
    Lakeland, Florida
    Home Page:
    I know you know the problem is caused by the twisty grain which results in some fibers being cut the wrong way.
    I think what happens is the straight edge of the skew gets under wrong way fibers lifting them a bit then either cutting them or breaking them the result is minor tear out.

    As you have observed the curved egde of the gouge doesn’t lift the fibers as much. It may also be due to the tool not quite as sharp so it is unable to get under the fiber.

    I get a similar problem on some NE crotch bowls I turn with the Ellsworth ground gouge. The bottom center tends to be almost flat and the crotch figure is a problem about 20% of the bowls and I get a tiny bit of tear out with the gouge.
    Sometimes repeated light cuts taking off whispers of wood fixes it. But I usually end up using a round nose scraper on these problem bowls. The scraper cleans it up almost to the quality of the gouge cut.
     
    Bill Boehme likes this.
  11. Dennis J Gooding

    Dennis J Gooding

    Joined:
    Apr 10, 2010
    Messages:
    226
    Location:
    Grants Pass Oregon
    Al, how do you make an accurate measurement of the edge angle of a tiny object like the wing of a skew? I have been wanting to study the effects of jig parameters on wing profile.
     
    Last edited: Oct 21, 2017
  12. hockenbery

    hockenbery AAW Advisor Staff Member

    Joined:
    Apr 27, 2004
    Messages:
    4,317
    Location:
    Lakeland, Florida
    Home Page:
    I don’t get an accurate measure unless I get an exact fit with a magic piece of plastic that came with my tormek. similar devices are available lots of places. Don’t use the tormek often. The little plastic I use once or twice a year. For me tool angles are not critical get within a few degrees and it’s good.

    I set up the Ellsworth jig pivot point so it is pretty accurate.
    Mearuring the wing of the Ellsworth you can put the 30 or 40 degree on the wing slide it toward the tip until it stops. Those would be the part of the wing with that angle.

    This has slots for 60, 40, 30, 25, 20 so if one fits exactly that is it.
    Lose fit tool angle is smaller, won’t slide on all the way tool angle is bigger.
    1974A2C4-EF71-4F9A-9CAC-C8B7A94B7B84.jpeg

    5726CE37-0884-4AFD-A033-9285C71A3C36.jpeg
     
    Bill Boehme likes this.
  13. Dennis J Gooding

    Dennis J Gooding

    Joined:
    Apr 10, 2010
    Messages:
    226
    Location:
    Grants Pass Oregon
    Thanks Al.
     
  14. john lucas

    john lucas

    Joined:
    Apr 26, 2004
    Messages:
    5,832
    Location:
    Cookeville TN USA
    Dennis I have a metal angle gauge that I purchased somewhere. It is round and has angles from about 20 to 45. It works for general edges. What I have done (and i"ll have to find it somewhere) is to cut 2 long triangles out of thin stiff plastic(think milk Jug but I think I used something else). Think 1/2" wide at the bottom and about 4" long. Anyway I put a pin through them at the large end. Then I can put them over the edge of the tool and squeeze them together until they match the sides. You may have to cut a little off the pointy end. When I have this set I simply take it over to a protractor and measure than angle. I will look in the morning and see if I still have that. If not I'll just make another one. I know I have a metal one I made but it has square ends and is too large to fit in my gouge. It was for measuring something else I had to duplicate on my metal lathe.
     
    Bill Boehme likes this.
  15. john lucas

    john lucas

    Joined:
    Apr 26, 2004
    Messages:
    5,832
    Location:
    Cookeville TN USA
    Al I've been playing with the Hunter tools lately for this type of tearout. Mike sells 3 different sizes of cutters and it seems like the smallest one gives the least tearout. The only problem is the small ones have really small bevels so it takes a pretty practiced hand to get a smooth clean bottom. I don't know what the name of the one tool I have is. I'll have to look. It's about 10" long and the last 5 inches taper down to fit that 6mm cutter. I have been comparing the cuts to my older #4 which has a 1/2" cutter.
     
    Bill Boehme likes this.
  16. Dennis J Gooding

    Dennis J Gooding

    Joined:
    Apr 10, 2010
    Messages:
    226
    Location:
    Grants Pass Oregon
    Thanks John with that hint, I believe I can put something together in my shop.
     
  17. hockenbery

    hockenbery AAW Advisor Staff Member

    Joined:
    Apr 27, 2004
    Messages:
    4,317
    Location:
    Lakeland, Florida
    Home Page:
    I may try the carbides. I have a #4 and the #2 Hunters.
    As you point out the small one needs a nice surface to ride on to keep a continous curve. Any ripples left by the #2 should sand out with 220. I’m sanding the traditional scraper surface with 220.
    A lot of folks are using the negative rake scrapers too.
    The gouge nails it about 80%. I think I have done this demo 9 times and only once have I used the scraper and that was on a Cuban Mahogany.
     
    Last edited: Oct 21, 2017
    Bill Boehme likes this.

Share This Page