Unicorn Method and plane irons

I've done two tests with plane irons after noticing that a cheap plane iron "unicorned" on the bevel side felt like it had a slight loss in clearance, but didn't take any damage in a rosewood plane billet that looked to the eye to have little pebbles of silica in many of the pores. (I'll post the second test in another post below).

The first test was a traditionally sharpened iron (32 degree flat microbevel with the bevel side sharpened to 1 micron diamond) and then a unicorned profile on the same iron, planing the same piece of rosewood. The rounding seems to be protective but if the rosewood behavior with a flat iron was typical, then it could be shown to be far more protective than I guessed. primary grind on the flat iron was 25, and I thinned the primary bevel with the unicorned iron (to give as much room as possible for the round over to occur without eliminating clearance.

Back of the iron flattened with a washita in both cases, bevel sharpened with 1000 grit diamond before either 1 micron on the flat iron or unicorn on the other. The pictures you see are the back of the iron.

The straight sharpened iron after about 100-125 feet of planing (75 strokes of planing on the billet of rosewood). This is typical of the damage and each pock is about 1-2 thousandths deep. The worst of these was about double/triple the depth and twice as wide on the edge. The noticeable effect of this damage occurred in 10 strokes (shaving split), and then things went downhill relatively quickly and the iron was difficult to use by 75 strokes. I photo typical damage and not the worst because the worst only skews perception.

https://i.imgur.com/hVS0o7y.jpg

Little lines are left all over the work

This is the same iron unicorned. Same billet of wood, same 75 shavings.

https://i.imgur.com/jWNFBFW.jpg

I don't see any damage on it except for some pocking at the end of the deep scratches (this is the non-bevel side of the iron). it looks like I need to clean off the washita. The damage where those grooves terminate may have been created as part of buffing off the wire edge.

The unicorn iron still picked up clean smooth shavings at the same point of 75 shavings and I would guess it would've gone several times longer.

It did feel at the outset due to the rounded under profile like it didn't take the first shaving quite as easily at the end of the billet (the billet is rosewood and also irregular grain with runout at the close end, so it's a tough test). It was by no means "dull" or hard to start, but you could feel a difference. That difference was erased in 10 or 20 strokes, though.

The last or near last shaving from each sharpened profile is shown in the following picture - flat bevel on the left, unicorn iron on the right.

https://i.imgur.com/JZviysE.jpg?1

you can get an idea regarding which of the two scenarios will get work done in tough wood more efficiently.

wood like the rosewood billet is probably not widely used in the US. Most guitar makers finish parts with sandpaper and use power tool jigs to rough them in the first place. I plane them.

Cherry, soft maple, pine, mahogany are more commonly used in the US (and the latter only really by serious people). Interestingly, mahogany, khaya and limba also often come with silica - they plane easily, but the silica spoils them, so at least in some cases, the test above is applicable.

But for non-abusive woods, I wanted to see if the lack of clearance would cost some edge life. I could feel how crisply a flat sharpened iron takes the first shaving, and should wear evenly without contaminated or rough wood.

So, test two was to use a different iron (one that wouldn't last as long - the iron above was made with the same steel that LV uses for V11 - that steel is long wearing in clean wood but is not tougher when it comes to chipping at the very edge - subjectively, it's less tough than comparable O1 from what I've seen in use). The iron that I used for test two is an O1 iron of my own make, about 60 hardness I would guess. It's not soft, it's not overly hard, and it works well in normal use.

The test was to plane a hard maple board edge at 2 thousandths until it became a struggle to work. The board has a little bit of quartered grain at the far end that is harder to plane cleanly, so it gives a good signal of total failure coming soon.

The 32 degree flat bevel - 600 feet, 105 grams

the unicorn bevel - 520 feet, appx 80-85 grams (my scale goes in 5s and bobbles back and forth when a measurement is right in between the two).


This test favors the iron done first a little bit, but not as big as the difference in weight planed and feel. The feel of the unicorn iron is as expected. Before the iron is necessarily dull or worn much, clearance runs out and the iron slides across the more difficult to plane section. It's actually a different feel than one gets from a dulling iron.

I'd guessed prior to this test that the unicorned iron picked up a shaving in rosewood like an iron would after it had planed 10-20% of its life out. That turned out to be correct.

conclusions from both tests:

* unicorn fortification of the edge is very protective against minor nicking. In really nasty wood, it will probably last longer and the surface left on the wood is certainly better
* where the edge protection isn't needed, on average, the surface will probably remain better with the unicorned iron, but you will give up some edge life. The better you get at getting the right feel with the buffer, hopefully the less that will be, but I doubt it will ever be none unless you're a bad sharpener. The dulling of a plane iron includes rounding on the bevel side, and we're more or less imitating part of the profile of wear just at the very tip. This makes sense, but makes sense and test to confirm is better than "it only makes sense" (which is usually said by itself without proof in old wives tales).

For bevel up planes, the loss of clearance is no issue, it's on the opposite side, and the improvement in sharpness due to the protected edge is pretty lovely. If I actually used bevel up planes, I would sharpen all of them with the buffer on the bevel side. It's divine.

Hi David,
For BD planes, would it be reasonable to consider buffing the flat, instead of the bevel?

I'll drop this in here as I know it will interest DW, it comes from another thread posted today by Ross M. The video clips are astounding.

Why shaving dulls even the sharpest of razors | MIT News