Straight Knife Planers with Grinders

Here's a good look at the hows and whys of grinding knives on the planer. October 19, 2013

(WOODWEB Member) :
I'm having trouble with my Byrd shelix head and thinking about a straight knife machine with a built in grinder. Does the sharpening you get with the grinder make a good quality edge? I've seen lots of big millwork companies who have giant planers and no grinders. They pull the knives and send them out. Why would anyone do that with built-in grinders available? I'm thinking of getting a wider planer anyhow and I don't think index heads come in 30"-36" wide planers. Anyone have experience running planers with built in grinders? What kind of time frame does it take to grind the knives and be back up and running? Seems like a good idea.

Forum Responses
(Solid Wood Machining Forum)
From contributor D:
I have run SCMI and EMA Casadei planers with grinders. Both were 24" industrial machines.

First, there are two ways to sharpen in the head.

Jointing is a technique where a fixed stone is held securely above the knives and then slowly lowered into the turning head to just touch the highest knives. This is called jointing and will sharpen and bring all cutting edges into the same cutting diameter. It will also create a land that makes for a louder noise and requires more power to do the same work.

Grinding is a powered grinding wheel traveling back and forth across the fixed knife. This sharpens the knives but has two drawbacks - the cutting diameter is slightly reduced, which will affect chipbreaker and perhaps even hold down settings. It also creates a metal and stone dust that gets all over the machine interior and will eventually nick those nice sharp knives. The jointing can also create some dust.

You may consider Tersa heads and knives. I could change the 3 knives on a 24" planer in about 5 minutes, and never need to fiddle with chip breakers or holdowns. The knives are also available in several types of steel and carbide.

From the original questioner:

Thanks for the info. What kind of price are you talking for the Tersa type system? I'm looking at buying a used industrial machine. Probably will only be able to find a 24" that's new enough to be accurate.

From contributor J:
I have a 20" SCM with grinder and wouldn't trade it for anything. I haven't tried the replaceable knives like Tersa, but I hear of a lot of guys using them. My concern would be cost over time, as you're replacing the knives with each change. I would consider them for my jointer, though, as they would make life a bit easier.

I had a carbide insert head on my last 20" planer, and they are nice, but I'll keep my straight knives over it.

With the grinder I can take a light pass to touch things up in less than 5 minutes. A full grind, which means re-setting the knife height, and waxing the tables while I'm at it, somewhere in the half hour range. When sharp it will leave a finish as clean or better than the carbide inserts. Of course that doesn't last a long time before you're down to a pretty darn good finish, which on most woods is fine.

I'd guess I can probably get anywhere between 4-6 light passes before having to do a full grind. As mentioned, each grind removes a minuscule amount of knife height. So after enough of them you have to re-set the knives and grind in order to maintain the correct cutting circle.

Although it does create a bit of fine dust, I haven't had any problems with it affecting the knives. Most of it just falls down onto the table where I wipe it off. Anything left likely gets sucked up by the dust collector as my machine has really, really good suction!

From contributor D:
The Tersa heads I used were part of the whole machine cost, so I can only guess. I believe that the last machine, in 1999, added about $2,000.00 to the cost.

The knives can be expensive, until you add in the cost of your time. Also they can easily be reset in the width of the machine. The biggest reason for a knife change was a nick or nicks, and shifting each knife laterally 1/8" made them go away. Now, if they are dull, that is different and replacement is called for. But realize there are two edges on each knife.

I also think your knot problem may be minimized/eliminated since the Tersa heads are solid, with a shallow gullet and miniscule knife projection. There is no way a knot can ride around, chewing things up.

Contributor J's experience is similar to mine - several grinds and then a reset. Faster that exchanging knives, but not as fast as Tersa. Now I am back to straight knives, and send them out for sharpening. I actually like the down time to get in the zone and set them just so, and wax everything, then admire my efforts on that first piece of pine that shimmers as it comes out.

From contributor C:
Tersa Knives at Global Tooling are pretty reasonable compared to sending solid knives. I pay $60(ish) for 4 -410mm HHS knives double sided. So $.46 an inch compared to $.75 ~$1.00 to send out and can change out in just a few minutes with great repeatability. in Canada has been adding a steep handling fee lately, so with the weak dollar I started using the Global guys. But they might be better if you're up that way.

From the original questioner:
Not sure I understand your Tersa head setup. From what I've seen they look similar to Byrd heads except the carbide is inset into the head and a groove is made in front of the cutting edge, effectively protecting the insert on 3 sides. Maybe Tersa makes many type heads? Yours sounds like a disposable knife cutter head setup? Are they carbide? Part of my problem is I don't know how long 410mm is. It seems like we might be talking about a couple different heads. I'd like to know all the options out there before I buy something new. There are a lot of used, nice big machines with 24" straight knife heads with build in grinders for $5k. Not many with helical insert heads, but I don't want to switch out to something that might have the same trouble I currently have. I am still working on the idea that the carbide I've been buying is more brittle and that spending more will give me better wear, but I have a feeling that when a loose knot bounces around in there it will break something the way the knives are set in the Byrd, and once the carbide breaks loose it has to break other carbide, as it's obviously very hard. If this is the way they run in general, I have to come up with an alternative as I get loose stuff in my planer and can't help it.

From contributor C:
The only Tersa I know is the disposable straight knife. But I don't know much. Many people like the helical heads because if you chip a tooth you can replace that individual one (or few) and move on. Big chip in a straight carbide knife is probably pretty painful. Lot of time grinding that out.

I use the HSS knives because they are cheap and I want high surface quality. If not abused they last quite a while.

The carbide Tersa can be resharpened for about half of new. They sell heads as well.

From contributor J:
You may want to check out the Tersa as I think you're confusing them with Byrd heads? I might be wrong and maybe they make an insert head too, but they're well known for their disposable straight knife heads, so I kinda doubt it?

By the way, 410mm is equivalent to a 16" head.

Just out of curiosity, what kind of wood are you planing? Loose knots should really be avoided if at all possible as they can damage more than just the knives! I don't get many, but I have a screwdriver within reach to quickly pop them out when I do find one.

From contributor G:
One of the reasons I love the Tersa head on my 20" SCMI is changing from HSS knives to carbide when I need to run teak - it takes about 4 minutes to change them. I have owned my carbide set for 4 years and still haven't used the second edges yet.

From contributor C:
On the lower speeds, how do you find the HSS works on tear-out prone curly wood, if you work that kind of material? I am really on the fence which head to use as an upgrade. My F1 is outfitted with Tersa and with lower feed rates and new knives I can pretty much make any wood look like glass. But I am feeding at around 8~12 FPM. I have only seen SAC that has the slower 13 FPM speed. I really don't want to go to carbide inserts on a helical because I am really liking the Tersa. I hand plane all my work (furniture) for the finished surface and don't want to be running the material through the abrasive planer as it is hard on the hand plane blades. That is, the aggregate left behind buried in the surface.

From contributor G:
I don't run many species very prone to tear out. I have an old Delta 13" that I retrofitted with a helical head but it killed the horsepower even after I installed a 7hp motor, but I kept it just for the curly stuff. The HSS at slow speeds with my SCMI 4500 rpm head does produce, generally speaking, some amazing results. The only drawback I can complain about is that the infeed rollers will deform/crush the edge of lumber if you are taking less than an 1/8", so you need to size the material to finished width after surfacing.

Thank you for keeping hand planes as a meaningful part of your work; you are keeping a discipline alive for the next generation. Some of us cannot afford to do that and you are one of the lucky ones!

From the original questioner:
Thanks for all the input. I run mostly domestic hardwoods, with some figured maple, though I think the helical head actually reduces tear out on our QS oak as well. On the straight knives I think there is a little blow out behind each ray, so the shear cut with the Byrd definitely helps with these figured woods. Also, we do some burly stuff when we can find it, which is mostly end grain. Probably will have to keep the Byrd head and find an big old junker planer with a straight knife and grinder to do my slabs, as they are prone to having knots and bark edges.