The Truth is Plane to See?

Many woodworkers have become familiar with insert tooling, where small carbide cutters are installed in place for brazed-in carbide:

The advantages to the inserted tooling are numerous: replacing the blades (or simply rotating them in some cases) always will re-establish the original tool dimension and its cutting geometry. Re-sharpening a fixed blade version of the above tool will of course will reduce the dimension of the cutter. The insert, since no brazing is involved, can/should be of even harder carbide then otherwise, and this should result in a longer cutter life.

The possibility to insert knives of varying profiles into the same head also means that the cutter head is more versatile. Changing blades in an insert tooling system can be measured in a few seconds (as with the Tersa system) or longer, and since the knives are held in an indexed position there is not futzing about for ages with a dial indicator to set the knives in a multi-knife cutter all to the same height. Dull blades are not re-sharpenable (at least if you want to maintain original dimensions), and this definitely annoys some people. That said, HSS steel is highly recyclable, and carbide does fetch a pretty decent price as scrap. I’ve read that a coffee can full of worn carbide inserts weighs about 40~60 lbs and is worth about $200 as scrap (though you may have to ship it off if a recycler for that material is not local to your area).

The downsides to insert tooling relate mostly to the inherent (philosophical/ethical) negatives of disposability, and the potential or temptation this presents for some manufacturers to specify cheaper and cheaper insert tooling steel. The good news with that of course is that if the factory blades are junk, there may be better quality aftermarket products available.

One way to reduce cutting effort with a blade is to reduce the cutting angle by shearing the blade to the side. In machines, this takes the form of cutter heads with helical blades. Here’s the old skool look, in this case a milling machine cutter:

And a newer application of a helical blade on a jointer:

The above set up is another potential application for insert tooling – small carbide/HSS cutters distributed around the cutterhead can take the place of single larger blades spanning the head. Here’s a ‘typical’ planer/jointer cutterhead with rotatable 4-sided cutters:

Now, from what I have gathered, these insert-tool helical cutterheads seem to have their pluses and minuses – while the helical orientation promises (and delivers) a cleaner shearing cut and quieter operation, a lot of these sort of heads tend to leave fine lines on the wood surface, the more so the smaller the diameter of the cutter head. The lines result from geometry: orienting the 4-sided knives to the side slightly so they shear the cut results in a cutting profile that is very slightly convex. That is what produces the slight lines – it’s a weird compound angle effect. Most people seem to accept this shortcoming in exchange for the benefits, and since most woodworkers do not obtain the material’s final finish surface out of a planer, but rather by sanding, it may be of little consequence.

I’m not here today however to talk about insert tooling in the sense of woodworking machines, and I’m sure there are many places online where folks debate these matters for hours on end. More power to them. No, today I wanted to show you a different approach to insert tooling I came across the other day. I kinda get a kick out of it and have no idea how well it works, but it’s cool and gives me pause to smile:

Comments? ;^)

4 Replies to “The Truth is Plane to See?”

  1. It does make you wonder how well it works vs a single blade. Also the adjusting and tuning would be an interesting display to watch as well as the pulling. It does stir up the imagination and questions. It may be the same location you found this photo, if not here is a link you may like with more photos:

  2. “orienting the 4-sided knives to the side slightly so they shear the cut results in a cutting profile that is very slightly convex”

    Not so. Do the math (or model/draw it out if you prefer). Canting a square insert would produce a concave cut, but no such insert tooling exists. The Byrd Shelix planer head pictured uses slight convex sided “square” inserts to slightly overcompensate for this effect. Most other spiral cutter heads (the ones with truly square inserts) arrange the insert centres on a spiral, but the cutting edge is square to direction of the cut.

  3. That's a good point Tim, and I appreciate that you saw fit to comment. I had understood the cutters were perfectly square in section on all these types of heads (not being especially familiar with the brand you mentioned) and canted, which would produce a slightly non-flat cut, however if the inserts are slightly convex then that effect would be corrected. In fact I can now see looking closely at the picture that the knives are slightly convex, which I hadn't noticed before.

    Thanks for that!

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