Straight Knives Versus Spiral Knives in a Planer

Pros and cons of helical and straight planer knives, and some tips on quality control. February 27, 2007

I've got a Shelix head in my 20" 4-post planer and like the surface quality. But I would like to speed up my surfacing and be able to feed multiple parts, so I'm looking at a 20" Italian machine with TERSA head.

I've not had experience with TERSA. Are they accurate enough to give a tear-out free finish in woods like hard maple and ribbon grain tropicals? How about noise? The shelix head is quiet, which is nice. How is the stock removal rate? Is it true that straight knives can handle a heavier cut? Last, does a segmented feed roller crush the edges of narrow parts if they are right on the edge of a roller?

Forum Responses
(Solid Wood Machining Forum)
From contributor J:
I'm not familiar with Shelix but our experience was the Tersa had a lot of chipout and the spiral was great. A lot of people swear by Tersa. It probably depends on the machine and application.

From contributor R:
Straight knives will not give as good a finish in wild grain. Straight knives when sharp and with a slow feed speed will do a good job on wild grain. I can still get tear-out on helical knives with a fast feed rate so the feed speed is half of the equation.

Straight knives require more power than a helical knife and will not take as a heavy a cut as a helical knife. Segmented feed rolls will not harm the edge of narrow stock. You already know which is quieter - helical!

From contributor C:

I've worked for years on Tersa head machines and they are great. I've heard a lot of bad things about helicals regarding track marks however, so you may want to be careful of that.

From contributor J:
Contributor C has a great point on the "track marks" We dealt with this problem with a different 4 head moulder. It was explained by Schult’s company that the teeth need to be sharpened on the head. If you replace them yourself with new ones, you will still get the lines and this held up to be true. It was with the Sidewinder helical head. If you’re just going to sand the stock anyway I wouldn't worry about those marks. Sending the head out can be costly.