Helical Head vs. Spiral Head

[jamil mehdi]

This is a public service announcement:

A lot of people use the terms "helical head" and "spiral head" interchangeably. I think it's important to know the differences between the two.

A helical head has cutting edges in line with the helix, so when the cutter hits the workpiece, it slices off a shaving.

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A spiral head has cutters at a 90 degree angle to the workpiece, and arranged in a helical pattern, so when a cutter hits the workpiece, it scoops, exactly like a 1/2" long jointer or planer knife.

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So what does that actually mean? Without going into it too deeply...

A helical head will cut more quietly because it's slicing wood fibers rather than gouging them. It will also retain it's edge longer than a spiral cutter will because there's less initial impact against the work surface. A helical head cutter will last longer than a spiral head cutter before it needs to be turned.

A spiral head is better than straight knives because if one gets dinged, you can turn it. It's also slightly quieter than straight knives because the cutting action is segmented across the whole diameter of the cutter head rather than at two or three specific points. But that's it. Beyond those two factors, there is no difference between a spiral head and straight knives.

If you want a helical head because you've heard about how quiet they are and how nice the finish is, and how long they last, you're going to be pretty let down if you get a spiral head because you think they're basically the same thing.

[james manutes]

Very well said . Thanks

[Tom Bender]

Interesting, In either case there are gaps between the cuts which will get cleaned up by the next set of cutters. That seems to favor straight knives in some ways.

A better cut might be made by one continuous spiral cutter. It would be challenging to make, align and sharpen. And it might make interesting chips.

[Andy D Jones]

Straight knives (or even continuous helical knives, if they exist) produce long shavings, albeit cross-grain ones.

Insert knives (no matter which pattern) produce smaller chips, which self-pack in less volume in the DC bin. Felder/Hammer has a video showing this.

Also, helical heads require inserts with a long radius on the edges (not corners), so that that the angled insert still cuts a straight-bottomed swipe in the face of the wood. Technically, it should (and may) be an elliptical edge, instead of a constant-radius (circular) curved edge, but a circular edge may be close enough.

-- Andy - Arlington TX

[Jared Sankovich]

The majority of the noise is not from the knives cuting the wood but from the knives compressing and chopping the air. There are a variety of quiet heads that have existed and none rely on shear angle, but rather a non continuous cutting edge.

[Erik Loza]

The majority of the noise is not from the knives cuting the wood but from the knives compressing and chopping the air. There are a variety of quiet heads that have existed and none rely on shear angle, but rather a non continuous cutting edge.

I agree but will add that Tersa heads are almost as quiet as spirals at idle. It's more to do with knife projection than straight vs. non-straight. Even though it's a straight knife, Tersa has very little projection. In other words, it's cross-sectional profile is a lot more like a spiral machine than like a conventional style jointer. Also, the Italian machines have lips with quieting fingers on each side of the cutterhead. So, there are other design considerations aside from just the style of knife.

Erik

[Robert Mayer]

I just bought a brand new Jet helical and a new DW735 (with straight knives). The DW735 actually leaves a tiny bit better surface (maybe because it has 2 speeds?). But I know that wont last long as the straight blades get worn. And of course on figured or maybe harder woods that wont be the case.