"Tear out" discussion

Re: "Tear out" discussion

Ed Mulligan, Cape Cod

>For spindle turning, spindle blank grain runs parallel to the lathe axis so you are always removing wood with a cross-grain cut.

For bowl turning the grain is sometimes oriented perpendicular to the lathe axis. If so, the cut is largely on end grain.

Ed

Turning v. Planing

Steve Elliott

>I don't do turning but have a few thoughts on this anyway. One of the big differences between planing and turning is the direction the wood is cut. On a lathe the wood is cut at right angles to the grain direction, similar to planing across the grain. I plane across the grain in very difficult species like the African mahogany I'm using now. It's brittle and has interlocked grain so either direction parallel to the grain causes tearout.

The other difference in lathe work is the higher cutting angle of the tool.

Re: "Tear out" discussion

Paul in NJ

>Besides cutting perpendicular to the grain, you have the advantage of instanly adjusting the presentation of the cutting edge to the wood. It becomes instinctive to vary the pitch or the skew of the tool to match the wood being turned. You also burnish the wood for a smoother finish.

Paul Dzioba

Re: "Tear out" discussion

Denis Ch�nard, Orl�ans, Ont.

>While I can't disagree with the other responses, I'm with you regarding the speed being important. At 1500 RPM a 2" diameter spindle spins at 13 feet per second. At that speed I suspect that the fibers don't have the time to tear, and are severed on the spot. I don't know of anyone who can use a handplane at that speed...

It's kind of similar to why portable planers leave a better surface than stationary ones. Although the number of cuts per inch may be the same, the fact that most portable planers have only two knives require them to spin much faster (about double) to get the same number of CPI.

Anyway, that's my hypothesis.

DC

Re: "Tear out" discussion

wilbur

>My bet is that it is much more an issue of speed.

Nice tops in the picture, by the way. Suppose your tops are about 4" in diameter. If you did your finish turning on a Jet mini lathe at a medium speed (1450 rpm), then the circumference of the top is 4 times pi, or about 12 1/2". At 1450 rpm, the outside of the top would be zipping along at about 17 MPH. But of course you can go faster on the Jet mini lathe. The next higher speed, 2130 rpm takes that speed up to 25 MPH. The next speed at 2400 rpm is 28 MPH, and the top speed of 3950 rpm works out to be 47 MPH.

On the other hand, if I am planing a piece of wood about 12" long, and it takes me about 1 second to complete the stroke, my plane is chugging along at a mighty 0.68 MPH.

This means that even at medium speeds the effective speed of a turning tool is close to 25 times the speed of a hand plane.

One other way to look at this is that at 1450 rpm, working backwards from 0.68 MPH, you would have to get to within 0.08" of the center before the velocity of the turned object relative to a turning gouge would be as slow as a hand plane.

I think that with this difference in speed, it's not surprising that you can get a cleaner cut with a gouge on a lathe even if the gouge is not as sharp as a plane blade.

Re: "Tear out" discussion

Paul Kierstead

>Well, outside of tearout, the burnishing effect of riding the bevel certainly contributes to the look a fair bit as well I think. Perhaps if one planed across the grain and then burnished it, it might be similiar?

Re: "Tear out" discussion

George Makowski

>Gentlemen, I would question the basis of this discussion. There is tear out in wood turning, in both spindle and faceplate work. It can be minimized or sometimes eliminated, but it does happen with shear cuts and scraping cuts. It is more frequent in faceplate work.

In any case, it does seem to be closely related to the orientation of the grain, the hardness of the wood, turning speed, and cutting technique. If you turn something with interlocking grain, my chinese elm chisel mallet, for example, you will often get tear out with the best of efforts. The best case appears to be a straght-grained wood, hard, at the highest practicable speed, with shearing cuts.

Good turning,

George in AL

Cutting Downhill

Warren in Lancaster, PA

>Tearout can be avoided in either turning or planing by cutting downhill. In turning there is no problem with tearout as long as we are making tapered or rounded cuts where we start from a high point and work to a thinner area, but working the other direction from a small diameter ot large will give problems. If you want to see tearout problems in turning, try turning a straight cylinder on a piece where the grain doesn't line up with the axis, or is wavyor has a reversal.

Tearout can be avoided in planing if the direction is sufficiently downhill. If you cut the edge of a board, for example, just 5 or 10 degrees from the grain direction, so that you are definitely planing downhill you will have no problems with tearout.

yes but.....

bill tindall

>I considered this factor and at first thought it must be the important factor, for indeed much turning is down hill. But then in turning a cylinder I don't have any problem with what ever direction the grain is going and I turn right or left depending on my mood, not the grain. In this case it would be equivalent to planing across the face of a board as others have pointed out. With a sharp tool and a fine cut I can cut uphill and suffer vastly less tear out than I experience with a plane on maple. I make Christmas ornaments (hollow balls) and turn them from cedar with knots. I can turn right through a knot with the grain running every way, and it is glass smooth if the tool is fairly sharp. I will try one at very low speed and see if there is more tear.

Actually, as I consider it, turning a cylinder is not strictly like planing perpendicular to the grain because the lathe tool is skewed for best results. So, it is more like skew planing.

Re: yes but.....

Bill Lowrance in Geneva

>Naively:: I agree that zipping quickly through any cutting action seems to result in a cleaner cut. But in woodturning -- leaving cutting angle and burninshing and so on aside -- what factor is contributing to the resulting surface smoothness, some elastic bend/rebound time-constant of the fibers?

Re: yes but.....

Pam Niedermayer - Austin, TX

>I wonder if the heat from the friction does some of this work.

Pam

Re: yes but.....

wilbur

>Physics says that even turning at slow speeds will give you an effective blade velocity that will be much faster than a hand plane. In the example I gave above, a 4" top turning at 500 rpm (the slowest speed on a Jet mini lathe) will still be moving at a speed almost 9 times as fast as a hand plane.

Probably a better test is to see how smooth you can make your cedar knot with the piece of wood sitting on your bench with your turning tool, and then compare that to what you can do on the lathe.

Re: "Tear out" discussion

Dave OH

>My take, 2 inch+ wide cut versus 1/4 inch or less wide cut. Try to take of a 2 inch slice on the lathe and you will find the answer!

Oh, and all of the other stuff already mentioned.

Dave

Re: yes but.....

Warren in Lancaster, PA

>In turning a cylinder the problem is not one of moving the chisel to the right or to the left. The problem occurs when the direction of the fibers is some what different from the turning axis, as occurs when there is a crook in the grain. In other words there is runout. In this case the turning will be downhill on on side of the stock and uphill on the other. The one side of the stock will have a finer texture than the other when it is turned with a skew. Try turning a piece sawn 20 degrees from the grain direction and I think you will see what I mean.