Number of Knives and Planing Quality

More knives is not always better, and RPMs also play a role. Here are tips on obtaining good cut quality at various feed rates and RPM settings. May 28, 2006

As a general rule, it is well known that for a fixed speed, the more knives, the better the quality. Our company is facing a problem with this. We purchased a 12" planer running at 360fpm maximum. We have purchased 12" heads with 12 knives. This gives us a very good quality with 17 marks per inch. When we run widths going from 3 to 8", the machine could run to 280 to 360 fpm and the quality is okay. When we need to process pieces 10 and 12" wide, the speed should be slowed down to 150 to 180 fpm. But the quality we get is worst, showing some signs of chatter marks. The manufacturer of the machine says the problem is that there are too many knives for the speed. I don't agree with that. Is the manufacturer right?

Forum Responses
(Solid Wood Machining Forum)
From contributor C:
More knives is not always better. Your rpm's are a key component of determining the best feed rate. Feed rate directly affects the finish quality of the work. If the rate is too slow, knives dull more quickly and burning occurs. By running too many knives in a cutterhead at too slow a feed rate, you may also be experiencing hammer marks. These are caused by inadequate dust collection, or by the next knife in the cutterhead catching a chip cut by the previous knife and hammering it back into the work piece. Too many knives can also glaze (burnish) your work piece surface and create problems on the finishing end as well. Be mindful of maintaining proper chip loads to achieve the best possible material finish.

From contributor D:
360 fpm? That's about 15 mph. Surely that's a typo.

From contributor R:
Contributor C's response is right on the money. Here is another way to figure knife marks and feed rate.
Rpm of your machine X # 12 of knives =

Let's say 6000 rpm x 12 = 72,000
Feed speed Feet per minute X 12 (12" in a foot)
Yours maxes at 360 fpm
360 x 12= 4320 total inches
Divide 72,000 by 4320 = 16.6666 knife marks per inch.
When you slow it down to 180 feet per minute, take out 6 of the 12 of the knives.

From the original questioner:
Thanks. I will take out 6 knives and be sure to have perfect weight balance between them to avoid vibrations.

From contributor R:
When you take out half of the knives, be sure to use filler stock in the empty pockets. This will keep the bore of the cutterhead from becoming distorted. Weights of balance is critical within 1/10 of a gram, as you noted. Happy milling.