Spiral cutters vs. knife cutters

Which type of cutters is best for this special application? July 24, 2001

A buddy and I are trying to shape hardwood staves 1.5" inches wide x 1/4" thick, by cutting a 14" radius on the 1.5" side of the wood. We are seeking a smooth finish, with next to no cutter marks or chatter.

We have developed a tabletop cutting tool assembly consisting of a fixtured router mounted in a horizontal position and a linear rail actuator, with a vacuum chuck, to hold the thin slat of wood and run it past the cutting tool in the router to shape the wood. It works amazingly well, and can maintain .010 tolerance, but we have several issues.

The biggest one at this time is cutter type. We had a custom cutter made from a 1/2" diameter shank Onsrud cutter blank. (It's basically a paddle or knife cutter with a 1/2" shank to fit the router collet.) It leaves a great finish but makes a hell of a noise when cutting and causes the router to bog down when trying to cut more than .040 of material. (We'd like to take .050 to .080 with no problem.) If we go deeper, it chatters and leaves a poor finish. We are concerned with the life of the tool and the side load on the router.

We also tried a two-flute spiral endmill. The endmill mounted in the router cuts through the hardwood like butter, but left a very fine washboard finish or cutter marks.

In both cases, we're feeding at 2 to 3 inches per second, and the router is running at 20,000 rpm. We are looking to put a variable speed router in place of the other one to give us more options with feed and cutter speeds. What works better for cutting wood and leaving the better finish grade cut, cutter knives or spiral flutes? Or is it just a factor of feed and cutter speeds? I'm also concerned about the bog down on the router with the knife cutter. If the speed remained constant, I think it might be helpful. Should I maybe switch to a gearbox driven motor?

Forum Responses
From contributor T:
I read your query several times. At first I was tempted to answer your questions directly, i.e. feedspeed, spiral cutter, etc. Then it hit me.

You're trying to reinvent the wheel.

Woodworking is one of the oldest professions and its relevant machinery can make 100 toothpicks a second or 20 house doors a minute. Your mini staves are no exception.

You need a planer. And not even an expensive one. You'll replace the 12" wide knives with 2" wide concave ones, install a channel to capture the wood left/right as the geared feedworks transport the stave through the machine at 10-15' per minute with a finish equivalent to 80 grit and accuracy +- .003".

Better still, if you have a wide panel, you can design knives for the 12" wide cutterhead that will both mill the crown and part the panel into several pieces in the same path.

True, nothing's more tempting than sitting down with a millwright and sketching out your idea of a machine on a napkin. But when it begins to look like something you'll see 20 of at the next wood machinery trade show for a tenth of the cost of what you'll eventually have in the piece you're making if it even works, it's time to toss the napkin and reach for your wallet.

From the original questioner:
Your message is accurate in many ways. We are working on a budget, so everything we have used for prototyping has been available at next to no cost. The idea of buying an inexpensive planer and having the knives custom shaped to our required dimensions has been discussed. It's a good option for what we are trying to produce and I suspect with the proper knife configuration, would leave a finish as smooth as a baby's butt. All you have to do is look at moulding in your local lumber store to see that just about any shape can be put into a piece of wood. The equipment is out there and available.

On our project, I mentioned the o.d. cut only, I didn't elaborate to say that we have i.d. cuts and bevel cuts, which requires more tooling. Wood thickness is an issue. Currently, we take the wood down to .300 before shaping, so having our own planer would be advantageous and allow us to experiment with knives. The wood we had planed at a local lumber store had noticeable cutter marks. Is this what we can expect from a planer for cutter finish?

It all still comes back to my original question. Given our current router application, can we achieve a quality finish cut with a spiral two-flute cutter, or is a double-edged cutter going to do a better job? Or to take it a step further to relate to your comment, what do you consider an inexpensive planer, and if we bought one, can we achieve a smooth finish and dimension accurately to the +-.003?

One comment we received from a local woodworking store was "you can buy a few hundred dollar planer, modify the knives and feed assembly to meet your needs, and maybe you can achieve what you've already created." I'm not sure we're ready to invest just yet, but certainly an option.

From contributor T:
A cutting knife makes as many cuts per minute as the rpm. This is irrespective of the number of knives in the cutting tool--two, three, four, etc. The reason is too long and complicated for this post.

So to mill a good finish, you should shoot for 18 knife marks per inch in hardwood, +-3. Math that out to the feed speed in inches, then feet, needed on your machine and gear it so.

Spiral cutters were invented to reduce noise. That's all. A spiral cutter is designed such that when it's revolving there is enough length and spiral that somewhere on the wide board part of the knife is touching and therefore producing some hold-down influence as well as sound dampening (just as your finger will dampen/deaden your guitar string when lightly applied). A planer in a mill with straight knife blades bangs so hard on the 2x6x20' that the long board becomes a tuning fork of sorts and conducts the sound out well beyond the machine's ineffective sound enclosure.

A spiral cutter that falls short of wrapping 360 degrees around the shaft and, further, is not in contact with the work piece at some point at all times (read: needs to be a wide board) will not produce a better cut and may, indeed, produce a poorer one, since the difficulty in sharpening is increased tenfold.

We'll now hear from folks who swear by their spiral router bits. And they should swear. They're paying too much for little or no effect.

From contributor J:
You may want to investigate work done by Dr. John Stewart regarding helical (i.e. spiral) cutting tools, which reduce planer tear-out on many hardwood species.

From contributor T:
A board planed by HSS straight knives laid beside a board planed by a spiral cutter will be smoother, given the same factors such as feed speed, RPM, number of knives in the head, jointed or not.

The reason is simple, when you think about it. First, a spiral head is almost certain to be made from material that cannot be sharpened as keen as high-speed steel. Often, it also cannot be jointed to impart strength to the tip and show all knives cutting. Therefore, straight knives allow the ability to impart more cutter marks per inch, hence a smoother finish.

Also, re-sharpening a spiral arrangement is immensely more difficult than straight knives and can lead to distemper, weaknesses, skips and dings. Often, spiral planer heads are segmented and a segment, once dinged, must be replaced with a taller tooth and ground down to match the others, often leaving a line in the board.

If you can, lay the boards next to each other and pick the board that was cut with the spiral cutter. Do you think the cutter marks on the spiral cut board will show up at an angle? They won't show up at any discernable angle. Hence no visible advantage of spiral knives.

Just sound and chatter benefits. Straight HSS knives forever!

From contributor J:
The Helical TCT tool design patented by Dr Stewart (as sold by Newman Whitney) is easily ground and jointed. The slicing cutting action of a helical tool is proven to reduce tear-out in difficult species compared to conventional straight planer knives.

However, I agree with you that HSS tools will generally produce a visibly better surface because of better initial sharpness, but HSS will not stay sharp very long with boards that have surface dirt or grit. This is why most roughing planers have carbide knives (often helical to reduce tear-out) and finishing planers HSS.

From contributor T:
Yes, roughing planers do benefit from the helical design. Obviously finish here is not a factor, only size, enough hit and miss, and 6 knife marks per inch to allow for grading.

Re: "worried about the side load on the router": Aren't router bearings designed for side loads? I would be more worried if I only needed to drill holes, etc.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
I think you're selling spiral cutters short when you suggest they only reduce noise. Think of a cutterhead as performing "impacts per minute" rather than "cuts per inch." Each impact creates drag on the cutter head, trying to slow it down. A 180mm long cutterhead with 3 knives moving at 6000 rpm is 18000 180mm impacts a minute. The same 180mm head setup with 50mm knives staggered in 6 slots produces 32000 50mm impacts a minute. Spiral cutters reduce this further still, because only a small portion of the cutterhead is in contact with the wood at a given instant. So required power is much reduced, allowing the same machine to take deeper passes.

Comment from contributor B:
The advantage of the spiral cutterhead:

1. Reducing cutting resistance requiring less horsepower.

2. Smaller chips for easy dust collection.

3. Less down time.

4. Less noise.