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RETIRED
14th September 2010, 09:34 AM
Following the post of Frank and Earnest http://www.woodworkforums.com/f8/tamed-skew-123523/ I have now read and studied the whole document.

It has taken a while to respond due to the fact I was waiting on a copy of the reference book that Frank had quoted from.

I have now read both and my comments are in red.

I am sorry the pics did not come out but if you reference F&E's original they are there.

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There seems to be a wide consensus that the skew chisel is the most versatile but the most difficult to control tool for wood turning. From various other threads on this forum you might be aware that I have been developing a tool which I believe provides most of the versatility of the skew but is much easier to control, and has many other advantages which far outnumber its limitations. This thread has the purpose of illustrating the theory behind its development and hopefully dispelling some unfounded prejudices about its effectiveness that have emerged from previous discussion.

To avoid reinventing wheels, I quote extensively from the book “Fundamentals of Woodturning” by Mike Darlow, Melaleuca Press, 1999. This does not necessarily mean that because it is already printed it must be true; it means only that the principles I am applying have already been researched and have been accepted by well respected professionals. Any extrapolation from these principles and any conclusion drawn from it is only my responsibility.

To avoid confusion, this is the meaning given to the main terms used:

A- rake angle: the angle between the top of the blade and the radius of the rotating piece
B- sharpening angle: the angle between the top and the bottom surfaces of the blade (bevel)
C- clearance angle: the angle between the bottom of the bevel and the tangent to the circumference of the rotating piece. Because the tangent to a circumference is perpendicular to the radius, the sum of these three angles is always 90 degrees (figure1).
D- side rake: the angle between the edge of the blade and a line parallel to the axis of rotation (figure 2).

In spindle turning, the only occasion where a “peeling” cut with a square straight blade applied perpendicularly to the axis of rotation is accepted practice is when parting with a parting tool. In all other circumstances some amount of side rake is considered desirable.

Given that the optimal presentation is at a side rake angle of about 45 degrees (Darlow p. 37), the presentation of a straight blade already skewed 25 degrees is only about 20 degrees off perpendicular, therefore the ease of movement on either side is improved in comparison with presenting a square blade (bedan or gouge ground square-across, for example).

As regards cutting, it does not matter whether the sharpening angle of the blade is made of two bevels as the skew chisel or of one bevel as the bedan. What changes is the ease of presentation. For the purposes of this discussion the bedan can be considered a crude version of the skew chisel where the advantages of the skewed blade and the reversibility allowed by the double bevel are traded off against the ease and speed of sharpening.
Not necessarily true. You can sharpen one side of a skew and obtain a sharp edge with minimal loss of bevel length.
A “planing” cut, that is a cut for truing the roughed surface of the turned cylinder, can be done with a roughing gouge “when the workpiece’s grain is not straight and axial”; otherwise, it should be done with a skew chisel (Darlow p.82). The flat blade gives a flatter finish, but the concave blade of the gouge is easier to manage because the corners are out of the way and can’t catch the wood. With the skew, instead, “If you plane with the short point the wood will tend to rive leaving an inferior surface. If you attempt to cut with the long point, the point will be pushed backwards, the blade will be banged down onto the toolrest and a spectacular catch will result” (Darlow p.83). True.

No reason is given for that behaviour, but it is fairly simple to deduce: the acute angle of the long point concentrates the force applied to it in a small area, and the side of the tool is blunt and can not go anywhere. Wrong.
One of the laws of woodturning is that the cutting edge must be supported by the tool rest.

In the case of the skew chisel this can't be done or it would not cut so by necessity the tool is supported on the rest at one corner of the blade and the cutting edge by the bevel behind the cut.

If you allow the cut to go above the mid point of the blade you are courting disaster because you no longer have bevel support and the chisel starts looking for it and naturally catches.

This is why the side rake angle must be maintained for the length of the cut by doing “The Lathe Tango”.

Which indicates the main limitation of the skew chisel: it works best only when the points are not touching the wood, True and it takes a lot of skill just to avoid that. False. It takes someone to show how to use it properly and then practise. No skill, just good hand, eye co-ordination.

The clearest examples come from those cuts that are best done with a skew because there are no better tools to do it, but even the skew does not work well because the point needs to be used. In rolling a bead, the starting V cut is done with the long point True and the rolling with the short point. Also true in some cases. In rolling big beads the chisel is used in the centre the same as in a planing cut. In both cases, the blunt side of the tool touches the wood before the sharp edge does. False. If you are doing the bead correctly the sharp edge at the point is slicing.

Looking at the side of the skew (figure 3) with a magnifying glass makes it evident that this is like hitting the wood with a hammer. The starting V cut “does not remove wood, it squashes it along each side of the vee” (Darlow p. 87) and any subsequent V cut should be stopped “short of the final bottom of a cusp to minimize crushing (Darlow p.85). You left out the most important paragraph “You can then cut the whole of the beads' surface with rolling cuts.”

Rolling the bead with the short point pushes the wood crushed with the blunt side There is no blunt side if done correctly, refer previous statement. in front of the cut and cleans it with the sharp edge that follows, but that’s difficult at the bottom of the vee.

For correctness refer to Darlow P 91 from top of page.

Figure 3

This deficiency is even more obvious in the “facing” cut, where there is nothing to hide the finish of the cut surface. The central point of crushed wood is there for everybody to see and the final cut is better done by rolling with the short point (Darlow pp.136-139). I think you have mis read that whole section. There is no torn or crushed grain at the finish. The “torn” bit you see is the shaving coming from the chisel on p139. Fig 7.19.

It stands to reason that the cut would be better and a catch less likely if the contact point were sharp instead of blunt. Sharpening the sides of a skew chisel, besides being difficult to do properly, is not done in practice because, if nothing else, it would mean that the whole cutting area would have to be reground often, shortening dramatically the life of the tool. A tool designed specifically for this purpose with one-sided bevel, let’s call it a “three sided bedan” (figure 4) would have basically the same drawbacks added to the limitations already mentioned before.
This is redundant because of preceding comments.
Figure 4

Here is where the use of carbide inserts becomes a viable option. The same geometry, albeit with a wider sharpening angle, can be achieved by mounting a square insert on a shaft that does not impede the presentation of the three bevels (figures 5 and 6). The following is a discussion of the advantages and disadvantages of this solution. You are basing this on misunderstood facts about the skew.

Figure 5
Figure 6

First of all, let’s get rid of a common misconception. It is frequently said that tungsten carbide (TC) can not be sharpened as well as high speed steel or carbon steel. The misconception hinges on the definition of “sharp”. The TC used by top brand manufacturers of woodworking machines can be, and is, sharpened to a very good edge. To establish an objective criterion, let’s define a very good edge as one that does not show any visible rounding (candle??) at 30x magnification (figure 7).

It might well be that at higher magnification a difference would become apparent, but for all practical purposes such an edge is better than any steel tool could hold for a reasonable amount of time. What is true is that, because the material is brittle, if the sharpening angle where as acute (25-30 degrees) as it can be with steel, the blade would snap too easily.
Figure 7

Commercially available inserts with a 55-60 degrees sharpening angle overcome this problem. What makes them blunt is not the fine abrasion that rounds steel edges: it is the accumulation of tiny shocks that chip off the edge (figure 8). Depending on the hardness of the timber and the roughness of the cut, this allows for hours, as compared to minutes, of satisfactory cutting. It is true that in a woodworking machine each TC “tooth” is used for only a fraction of the time (1/4 for a 4 spiral blades thicknesser, 1/60 for a 60 tooth circular saw etc.) and therefore can be expected to last longer than in woodturning. By rotating the square insert 180 degrees the three sided bevel presentation is obtained twice. Empirically, it has been found that an effective turning time of 6-10 hours with one insert can be achieved. Objective testing would be required to establish expected average durability in given circumstances.

Figure 8


The main factor that affects the effectiveness of TC as compared to steel in woodturning, therefore, is the wider sharpening angle of the insert blade, which reduces the width of the rake angle available. This does not affect scraping, that is presenting the blade at zero or negative rake angle, but affects cutting.

A good example of the difference to be expected can be found in the similarity with using a scraper instead of a bowl gouge to hollow a bowl. It is a well known fact that a bowl gouge produces a more undulated curve (rippled surface) while a scraper produces a smoother curve but more tear out, which generally requires more sanding. Both methods work well in the hands of an experienced turner but I think in the hands of a beginner a scraper is harder to use.

That the square insert is good for roughing is a given, that’s the way it was originally marketed for wood turning, there is no need to argue this point further. Agreed.

That it is good for hollowing and scraping the inside of hollow forms should also be reasonably self-evident. It is perfect for squaring the inside rim of the flat bottom of a vase, for example, and works well for wide radius bowls. The limitation here is that while a straight blade is the best one for rounding a convex curve, it is not as good for following a concave curve, the more so as the radius diminishes. At the worst, though, all that is required is to start sanding with a lower grit than would be necessary if a round blade had been used.

An important consideration in this respect is that the point of contact of the tool with the wood is the convergence of two 60 degree bevels??, which produces a 44˚ sharpening angle (figure 9). It is still a 45degree angle.

Figure 9

It is common knowledge that, while a skew sharpened at a 25-30˚ degree angle produces superior results, sharpening at an angle around 45˚ is recommended for less experienced turners because it is much easier to control. A better compromise angle is 40 degrees. Similarly, 45˚ is a very common grind for bowl gouges. Agreed.

The following is obviously an oversimplification of the matter but let’s accept it for the moment, to postpone a long dissertation on the reasons why this geometry compensates for the disadvantage of the wide sharpening angle of the insert blade: because the tool is used by thrusting the corner, the cut so obtained is comparable to the cut made by a detail gouge followed by a planing cut with a skew chisel. A properly used skew does it. See above. Because the thrust is almost parallel to the axis of rotation, even long and thin spindle work can be done without a steady. I find this hard to believe as there is always thrust towards the centreline of the timber. A scraping cut has little thrust towards the centre but the timber has a tendency to want to climb over the tip of the tool. Both methods require a steady whether it be by hand or other means.

Furthermore, the hexagonal shaft makes easy to present the blade at a 60˚ side rake for shear cutting and scraping when desirable.
The resulting quality of the surface is not substantially different to that obtainable with a more acute blade and becomes noticeably worse only for very soft timbers that are not really good for turning anyway. Tell that to the hundreds of turners who make their living from turning pine!

Here is the result of an empirical test: 30x magnification of the cross grain surface of a 140mm diameter cylinder of birch with patches softened by decay, after a facing cut done with a freshly sharpened (30˚) and honed HSS steel skew and a square insert with a blade already used for more than 1 hour. The photos are of the sound timber area, minor decay area and major decay area. It is evident that the difference between the cuts of the two tools is quite minor.

Facing cut with TC insert

Facing cut with HSS steel skew chisel

The only obvious limitation of this tool as a skew chisel is that a V cut is by necessity slightly larger than the sharpening angle of the tool that produces it. A 45˚ edge can not produce a V cut more acute than 46-47 degrees while a 25˚ edge can produce a V cut of 26-27 degrees.

This is not as big a limitation as it could appear, however. None of the V cuts in this photo (figure 10) is less than 46 degrees wide, having been produced with the insert tool. No comment.

Figure 10

In conclusion, my claim is that this tool can do virtually everything a skew chisel and a bowl gouge and a roughing gouge can do, much more easily and only slightly less proficiently than more sophisticated tools might. I don't think so. Others have tried with no success. These tools have their proponents but in most cases the users have not bothered to learn or practise using the correct tools for the job at hand.


You basically have a bedan that you are using upside down.

Frank&Earnest
14th September 2010, 01:46 PM
Thanks , I really appreciate the time you have spent in editing my paper. I take on board your comments that clarify and correct what I was trying to say and will edit the paper accordingly.

There are a couple of issues where one of us is either mathematically right or wrong, though. The most important one is whether the tangent point is with the top or the side of the skew. The other, which does not really affects the discussion either way, is whether the sharpening angle of the corner is 44 degrees (44.2 IIRC) or 45 degrees.

Given that, as you say, the first point is crucial to the credibility of the whole paper, I will review my calculations to see whether I blundered somewhere. Even visually, though, I think figures 6.45, 6.46 and 6.47 in Darlow's book are a dead giveaway.
Page 91, that you say supports your interpretation of the rolling cut, in my opinion supports mine. Your interpretation seems to me consistent with Darlow's definition of 'slide cut" given in that page.

And, yes, what I have is a bedan used upside down. With the advantage of the side edge that allows rolling it as a skew. Never said otherwise.:)