Two-wing versus three-wing cutters

[Phil Harding]

I've always assumed (yeah, I know:() that three-wing cutters would yield a cleaner cut than a similar two-wing cutter. So, when buying router bits, and now shaper cutters, I've always thought the three-wing was the better way to go. However, while reading the Shaper Handbook by Roger Cliffe and Michael Holtz I came across this statement:

"Two- or three-wing solid cutters are standard for most shaping operations. There is a certain amount of runout that occurs when a three-wing cutter is being used, and, as a consequence, only one of the three knives actually makes the cut. This can cause chatter and premature dulling. The two wing cutter has less heat buildup, more even wear on the cutter, and ultimately, a cleaner cut,"

I can accept the comment about heat buildup, but it would seem to me that a well made three-wing cutter should give a cleaner cut than a similarly made two-wing cutter. I would appreciate reading your comments.

Thanks,
--Phil

[Rod Sheridan]

Hi Phil, I disagree with the cutter runout statement regarding 3 wing cutters versus 2 wing, or 4 or 6 wing.

Feed rate however is an issue as the number of cutting edges increase.

A 2 wing cutter with a 0.01" chip load at 8,000 RPM needs a feed rate of 13 feet per minute.

A 3 wing cutter needs a feed rate of 20 feet per minute, a 6 wing 40 feet per minute.

People just aren't capable of feeding stock at those higher speeds, so we feed it slower, resulting in more heat, greater tool wear.

So for hand feeding I'd prefer less knives, power feed more knives.

P.S. As the feed rate is adjusted to take into count the number of knives, finish will be identical with different number of knives. the feed rate changes however.

Regards, Rod.

[David DeCristoforo]

I'm with Rod. The claim that three wing cutters are somehow inferior and prone to the kinds of problems you describe is nonsense unless you are talking about totally inferior tooling that is poorly made in which case those concerns might apply. A 3 wing cutter will provide 1/3 more cuts per minute at any given speed / feed rate ratio and that will only result in a cleaner cut.

[Ron Bontz]

Thanks Rod and Dave. Well spoken and informative.:)

[Phil Thien]

Some books assume industrial usage where power feeders are moving stock FAST. I read an article once where they provided a formula to determine how many thousandths your planer knives could be off before they would no longer cut.

It became apparent after studying the problem a little that they were talking about machines feeding 2x or 3x faster than hobby/small shop gear.

[Dick Strauss]

I believe a 3-knifed tool will also have a lower level of vibration since there are less additive harmonics.

[Matt Zettl]

I think it is very important to understand what Rod has said. To state it in another way, there is an ideal feed rate and cutterhead RPM for the material that is being worked. The "chip load" refers to the size of the chip being generated. Chip load = Feed rate / (RPM * # of Cutting Edges) Intuitively, one would think that slowing the feed rate would produce a better quality cut, and this is true to a point. However, if the feed rate is too slow, or the RPM too high, excessive heat begins to build up. Heat causes premature dulling of router and shaper bits.

A good way to see if the feed rate and RPM are appropriate is to look at the chips being produced. If you are making dust, then either the RPM is too high or the feed rate is too low. You should be making little shavings or chips. Also, if the bit is hot to the touch after you've been cutting for a couple of minutes, there is a problem. The bit should be warm at the most.

Again, Rod makes a very valid point that high feed rates are not easily possible when hand feeding. Increasing the number of flutes necessitates a proportionately higher feed rate. That is why router bits are traditionally only one or two flutes, because routers are generally hand fed. If the feed rate and RPM can be easily and safely controlled, there should be no difference in the cut quality between well manufactured 2, 3, or 4 flute cutters.

The best sources to learn about these issues are the CNC forums as well as the websites of manufactureres of CNC tooling, like Onsrud. Once I understood this principle, I had much better results with smoother cuts, less burning, and longer tool life.

[Brian Peters]

There is a lot of good information in this thread so I can only add to it saying that you shouldn't believe everything you read in books. I remember reading a shaper book years back and some of the ways he described working with the shaper were unacceptable and some of his opinions which were similar to the one you read were false. There is no imbalance to an odd number cutter head. Good cutters like Freeborn are perfectly designed to be balanced and give you a much better cut than a two wing cutter IMO with a feeder especially.

[Phil Harding]

Thanks, guys for all the feedback. What I've learned from your comments is that, all things being equal, a two-wing cutter should give the same results as a three-wing cutter. This assumes that the feed rate is properly adjusted to compensate for the number of cutters. I haven't really paid much attention to feed rate (except when I obviously get burning) because I always assumed (yeah, there's that word again) that slower = cleaner cut.

One of the interesting things Ive learned from comments in this thread and others is that woodworking is more than just shoving a board pass a cutter. A high level of understanding of what's really going on helps a lot.:D

-- Phil

[John Thompson]

Can't comment... yet! I just purchased a triple flute 45* chamfer that will be used in a few days. If I can remember.. I will comment at that time.

Sarge..

[Peter Quinn]

Other than insert knives, who makes two wing shaper cutters? Don't believe I have seen any yet. I understand that three wing is the de-facto standard for quality braised carbide and even old HSS shaper tooling. Are there two wing braised cutters? In any event, having never seen nor spun a two wing cutter, it is my considered opinion that three wing is better.:D And if there's chatter from a three wing, I haven't seen that either.