Plane blade geometry, part 2
In the first of this two-part series, we saw that as a blade edge wears away in use you get a curved surface around the original sharp edge. Here we’ll look at how the worn curved surface between the blade and the wood affects plane performance.
The angular space between the blade and the wood being planed is called clearance angle. What is the optimum clearance angle? The correct answer is, positive clearance (any angle greater than zero) until the blade is blunt.
We saw in the bevel angle article that as the blade edge wears the bevel angle geometry is constantly changing. As the bevel angle changes so too does the clearance angle which has consequences on the performance of your plane if the clearance angle goes below zero.
Photo 1 shows a sharp blade profile using a 30° bevel with 12° clearance above the wood. This plane will work very well as it has positive clearance with a freshly sharpened blade.
Photo 2 shows the same blade but when it is blunt. You can see when the edge of the blade (represented by the black line) is in a position to cut wood you have a large curved area below the wood surface being planed, this is what is referred to as negative clearance.
Unless you can force this worn curved surface down into the wood to allow the edge to re-engage this plane will not take a shaving. In practice you may have the strength to make this blade take a shaving on a narrow piece of softwood but you would struggle to make it cut a full width shaving on a harder piece of wood. Most likely, before you get to this stage you would assess the blade as blunt and resharpen. But in essence you will have resharpened because you have lost clearance and not because your blade is actually blunt.
Photo 3 shows a sharp blade profile using a 30° bevel with 30° of clearance above the wood. This plane will work very well as it has positive clearance.
Photo 4 shows the same blade but when it is blunt. You can see a small negative clearance angle has developed and the planes performance will be starting to degrade because of this. But by starting with 30° of clearance the blade is basically blunt at about the same time the blade loses clearance so you have maximised the life of the edge before you need to re-sharpen.
Comparing the plane that started with 12° clearance to the plane with 30° clearance you can see for the same amount of planing the lower clearance angle plane will have its performance degraded by loss of clearance, about three times faster than the high clearance angle plane.
The important concept to take from these profiles is: the more clearance angle you start with the longer you can plane for before the constantly changing blade edge develops a negative clearance, which in turn will degrade the performance of your plane. If your blade edge becomes blunt at about the same time as it goes to a negative clearance angle, you can say that you started with the optimum clearance angle.
So what does this all mean to the woodworker? If you use planes to do small chamfers, edges and endgrain in softwoods you may have never come across this degrade in performance, simply because you can easily overpower the loss of clearance in these circumstances. If you use planes to smooth large flat surfaces (particularly in harder woods) you will have definitely come across this issue. I hope you didn’t self doubt and conclude you were doing something wrong.
Twenty five years ago, when I planed my first large surface which required taking many full width shavings, I was confused about the plane’s performance as the blade went from sharp to blunt. At first I was getting nice full width shavings, the plane was easy to push and I had no trouble maintaining a continuous shaving. Then I had to start to push down on the plane to make it take a continuous shaving, but it was still taking a nice shaving indicating the blade was still sharp. The longer I planed the more difficult it got to take a shaving, so I resharpened my blade.
This was a frustrating sequence of events particularly when the grain would tear as I pushed down harder. I didn’t know if it was me or the plane, but in any case I went out and bought a belt sander. This made it worse, but that is another story.
Five years later whilst on a military deployment to Malaysia I was introduced to wooden planes with a higher blade pitch which had 30° clearance angle compared to my original plane which had 15° of clearance. These high pitch planes didn’t have the issue of having to push down hard to make the blade take a full width shaving even though I sharpened my blades the same way. This was great and I didn’t give a second thought as to the why, I was just enjoying my woodwork so much more.
Fast forward to about 2010, I had a wealth of experience in making and using planes but I never really looked into why my first plane performed the way it did, until people started to ask me about the issues I had also as a beginner woodworker. During this time I came across the independent blade testing carried out by Steve Elliot and Brent Beach was able to fully understand why my first plane gave me so much grief and why the high pitch planes I was introduced to in Malaysia solved my problems. And more importantly I could explain this to woodworkers having the same issues.
Lastly, here is some practical advice if you plan on making furniture using hand tools. Irrespective of which type of plane you use, if you are getting the desired results for what you are making at the moment then just keep on doing it. Happy days!
On the other hand if you take on bolder projects and you start having issues with your plane’s performance, then understanding the contents of these two articles will help you move forward in your quest for making your ultimate woodwork project.
Personally, mastering how to hand plane any flat surface, big or small, without tear-out gave me great satisfaction but more importantly the confidence to take on any project I wanted and using any wood that I desired. Prior to that moment my woodwork was being stifled by the plane I was using. Empower yourself to do better woodwork by mastering this very important stepping stone in the woodwork journey.
Read part 1 of this series here.
Terry Gordon is a toolmaker in Alstonville, NSW. Learn more at www.hntgordon.com.au