Nuances of Stropping

A while ago I received a question on social media regarding nuances of stropping technique and thought it might be helpful to others to post the exchange.

Question:  So here's an interesting topic, you are very clearly qualified to give advice on stropping your tools as an experienced and professional wood worker/craftsman and tool maker.  So when stropping I like how you say you slightly change the angle and I’m assuming that's for the efficiency of removing the burr, but for years I’ve always heard to make sure you strop flat to the bevel as to not round the edge, which has also been my experience of eventually having that cutting edge start to slightly convex over several stroppings if I am not being careful with staying flat and pressure.  Well now it seems to me a controversial topic because while the majority I have heard suggest to strop flat, I have only heard a couple Scandinavian tool makers suggest stropping at a slight angle to the edge. And well, now there’s you.  And I am very confident that you know what you’re doing and what works, and even why you do it.  But still a question remains on this method/technique, what about convexing the edge??  And I’m thinking of a sloyd knife where staying flat is crucial for performance. Could you please share your knowledge on this topic? Thank you!“

Answer:  That's a great question, the answer to which I'm sure will vary somewhat depending on which tool is under consideration and who you are asking.  I consider it good general advice to most woodworkers, for most tools and circumstances, to try to keep both backs and bevels of blades as flat as possible.  This is not because it is necessary that they be truly dead flat to perform reasonably well, but because I have seen the results of the sharpening efforts of many students and know that a lot of folks struggle to attain and maintain decent grind geometry.  So what I'm really saying is aim for dead flatness, but don't expect to necessarily actually achieve it.  But in the striving for theoretically 'perfect' geometry, you will likely achieve acceptable, good, or even excellent geometry.

When I talk about drawknife grind geometry, it should be understood that I'm referring only to bevel-down drawknives, where the bevel faces the workpiece.  Here, ideal grind geometry for best cutting performance and intuitive control is dead flat bevel and back.  There is some nuance, however, because other factors, such as edge retention and ease of sharpening are always at play.  As many will know, a microbevel vastly improves edge retention without affecting cut control very much, and that's all my drawknife stropping technique is really doing.  By stropping at a very slightly steeper angle than the overall grind angle, the strop subtly convexes the bevel just right near the edge.  The more pronounced this convexity, the more edge retention is improved, but also the more the intuitive steerage of the cut will be harmed.  So it is essentially a simple tradeoff.  Someone who loathes sharpening or whose drawknife work does not involve a lot of precision may elect to tolerate a lot more convexity in order spend less time maintaining their grind geometry close to the 'ideal', whereas someone who either enjoys sharpening or needs/enjoys the exquisite control of a near perfectly flat bevel may feel the extra maintenance time is more than justified. 

In the case of sloyd knives, I personally do not tolerate nearly as much convexity as I do on drawknives.  This owes to two main factors.  First, the sloyd is the main tool I use to make finishing cuts on spoons, where fine control is paramount, whereas the drawknife is more of a roughing tool for me.  Second, in carving a lot of axe handles, I use a drawknife with a degree of force and vigor that many would consider abusive to the tool, ripping aggressively through the toughest of dry hardwoods like ash and hickory, so I appreciate a bit of extra edge retention more than the ultimate in intuitive control.  That being said, my personal drawknife bevel is still obviously flatter than that of the more typical, casual user, of which I have seen many.  But back to sloyds, I am a lot more careful not only to keep the bevel flatter to the strop than with drawknives, but I'm also more particular about the leather itself.  I'll strop a drawknife with pretty much any old scrap of leather, but for sloyds I like to use a very hard leather, and only the smooth side.  My drawknife stropping technique is fast and loose with a slight tilt to the strop to create that tiny convex microbevel, whereas my sloyd stropping form is slow and tight with the bevel pressed firm and flat to the strop.