Thoughts on a Skew Chisel:
A discourse about the ‘modern’ skew chisel and convexed grinding takes Rod Tallack back some 60 years, jogging a host of long-forgotten memories in the process
A discourse about the ‘modern’ skew chisel and convexed grinding takes Rod Tallack back some 60 years, jogging a host of long-forgotten memories in the process
As a subscriber to Woodturning, I browse through the articles each month not only looking for that little bit of inspiration that some of us need from time to time, but also because I have an interest in the technically orientated pieces. I came across one such article; its focus was the woodturners’ skew chisel. The image on the opening page showed a skew with a bevel grind that did not equate with my memory of the tool. The one I remember could be ground square across or with a skewed cutting edge, but the bevel would always have a convex bevel grind. My decision to explore this disparity further derives from having been a lecturer in machine woodworking for many years. In that role it was required that I pass on to the students some understanding of the basic skills required when operating a woodturning lathe and using the common tools.
I spent time looking on the internet for (in my view) a ‘properly’ ground skew chisel and I could not find anything approaching what I was looking for. There are all sorts, shapes and sizes of skew chisels available,
but no one sells – or seems to make – the original convex bevel-style tool to which I was introduced. Persisting, I eventually came across a YouTube video that showed a chisel being used in a manner to which
I could relate, but more of that later.
Made from the start
If we were to go back about 60 years, we would find my student self on a machine woodworking course. At one point, I required a handle for a burnisher. The lecturer said: “There’s a lathe over there, go and make one!” He gave me a few minutes’ instruction on using the skew chisel, together with a suitable piece of sessile oak (Quercus petraea) and a bit of copper tube for the ferrule. The handle and burnisher remain in my possession; although nothing special, it is functional and a memento of those forgotten times.
As my research failed to find an ‘original’ convex bevel design, I chose to obtain an unhandled 32mm wide HSS turning blade to grind with a convex bevel. I ground the end profile so the edge angle relative to the sides of the chisel was between 20° and 25°. I next ground for sharpness and it may be here that the problem arose, beginning the decline of the convex bevel-type skew chisels I experienced long ago. If the edge is ground convexed as a knife or axe – see the convex bevel grind illustration – the radius for the curve will be between 25x and 30x the thickness of the blade and that curve will produce a satisfactory sharpness angle. Utilising this curved face of the chisel will give very good control when used for turning on the lathe.
However, this grinding operation is challenging because the edge angle has to be maintained, while at the same time a reasonably correct curve has to be generated to produce the necessary sharpness angle.
An alternative is hollow or flat grinding. Grinding on the edge of a wheel fitted to a bench grinder will produce a concave surface, which is easy to hone when resting the heel and cutting edge on the sharpening stone. The sharpness angle shown on the geometry of a convex bevel grind illustration is about 20°, but in practice it will be greater as the cutting edge will wear quicker than the heel when honing.
A flat grind is achieved using a belt linisher or similar, but the comments remain the same as for the concave bevel when honing it, actually, hone the edge enough and it will become slightly convex.
The bought HHS tool blade I ground with a convex bevel to replicate the grind I know of old
Closeup of the convex bevel and edge
The geometry of a convex bevel grind
Geometry of a skew chisel with a concave bevel grind
A standard ground skew chisel. The bevel is a flat grind rather than hollow ground in this instance
A standard ground skew chisel. The bevel is a flat grind rather than hollow ground in this instance
Test case
Having created the convex bevel on the skew, and sharpened it, it needed to be tried and proved. It has, however, been a long time since I’ve done any basic woodturning – the work I now produce is very different and can be seen at www.sculptureturnery.co.uk. Nonetheless, borrowing a handle from an existing chisel and utilising it for the new, it was time to revive some of the abilities I had possessed 40 years ago, as a lecturer. I practised with the convex bevel skew on several pieces of softwood to create a cylinder with an acceptable finish straight from the chisel. I then located a length of sapele (Entandrophragma cylindricum) and a piece of copper tube in order to fashion a handle for the convex-bevel chisel. I started by partially shaping the handle, making heavy shaping cuts followed by refining cuts, to try and get as good a finish off the tool as possible. It performed as well as I remember from my past experience. All of this brings me back to the YouTube video.
In your browser type in ‘skew chisel’ and there will be numerous ‘how to…’ videos. In fact, it would appear that there are as many ways of ‘how to…’ as there are people prepared to submit videos, and not surprisingly they are all different. After watching a few of them, I found one that closely agreed with my understanding of the tool and how it can be used to best advantage. You can find it at: www.youtube.com/watch?v=XTAGmTEr_Mo. The author and I share some of the same frustrations: what was essentially a simple and functional tool has evolved in ways that can often limit its versatility. However, watch the video and make your own decisions.
Using the convex bevel skew to make some incisions, followed by planing cuts
Refinining the surface with planing cuts to create a smooth cylinder
Fashioning a tool handle using the skew chisel
A closeup of the cutting edge making a planing cut on the timber
The partially shaped handle with a clean finish straight off the tool
The finished handle fitted to the new convex ground skew blade ready for use
Sharpening and grinding
When I entered the field of machine woodworking in the mid-1940s the shaping, sharpening and grinding of moulding cutters was done free-hand. Toolrests and jigs did not exist at my place of work. Later as a lecturer, my students were instructed and encouraged into the techniques and skills for the free-hand grinding of cutters. Later still, I had a sharpening service trading with the woodworking and metal engineering industries, so I can claim a certain expertise on the subject of grinding and sharpening tools. There is, as yet, no acceptable definition for the sharpness of a cutting edge. Take two extremes; a razor blade is very sharp with a small sharpness angle, a scraper as used on a lathe is sharp with a very high angle, perhaps 90° sharpness. Abrasives are small grit-like particles irregular in shape with many sharp
corners; it is the sharp corners that do the cutting, whether you are sanding wood or grinding metal.
The finish from using an abrasive will be that of a ploughed field, the grit will produce furrows – even after honing. The magnified cutting edge images show the cutting edge is still relatively coarse.
All abrasives will create a ploughed field effect; finer grits will generate finer furrows and hence a smoother surface, but the cutting edge will be a series of points that feel sharp and cut satisfactorily for the required circumstances. Grinding using a larger/coarser grit will mean fewer but larger points that will fracture and break more easily and the edge will become blunt quicker. Conversely, a smaller/finer grit will form many smaller points that are less likely to fracture, resulting with the edge staying sharp longer.
A cutting edge is the line of intersection of two surfaces, the finer the finish on these surfaces, the better the edge, even so, the edge will end in a small radius. A smaller radius will produce a thinner shaving, which could be used as a measure of sharpness. It is possible to improve a cutting edge by smoothing out the furrows generated by the abrasive, using a leather strop in conjunction with a micro fine polishing compound.
The ploughed-field effect from sharpening
The ground cutting edge magnified 25 times, showing the furrowed surface
Honed cutting edge magnified 25 times, to show a cleaner less deeply furrowed cutting edge
A cutting edge is the line of intersection of two surfaces, which is in effect a micro radius
Magnified cutting edge on a convex ground bevel skew
Magnified cutting edge on a standard ground skew chisel
Conclusions
I am reliably informed that the convex grinding of cutting edges has been around for many years – possibly centuries due to sharpening of tools being undertaken by honing more than grinding. I also understand that and many turners use such a grind today.
The angle of deflection for the shavings of a convex ground chisel is small, compared to the larger deflection of the shavings from the proprietary ground chisel. I find that a larger deflection means a greater force will be applied to the chisel.
It is worth noting that there will be no detectable difference between the two sharpness angles when finishing cuts are being made, but where heavy shaping cuts are being completed, for instance turning
a table leg, then the chisel with the smaller sharpness angle will perform the best.
