Knife Marks Per Inch on Wood Mouldings

by Russ Rego

Have you ever wondered why some of the elegant millwork you see in certain buildings looks superior to mouldings in different buildings? There could be many reasons for this phenomenon; this article will help you better understand just one of them.

Let's start with the term, moulding knife marks per inch (KMPI). Knife marks are present on every moulding manufactured, they are the result of using round cutter heads during the milling process. We estimate that ninety percent of all moulding manufactured today are milled with round cutter heads. Every piece of moulding manufactured, no matter what size or shape has a certain number of marks per inch, unsanded. Some knife marks are more noticeable than others, some mouldings have more marks per inch than others. It is a fact that the more moulding knife marks you have per inch, the shallower the scallop left on the wood, this will result in a smoother look and feel of the moulding.

We will explain how to control your moulding knife marks per inch, while making them as least noticeable as possible, as well as how to achieve top quality millwork production every time you manufacture mouldings

Listed below are some industry standard guidelines that most wood milling operation’s use as a guide to obtain their level of quality. Keep in mind, everybody's standards may be a little different. There is a reason why the price one pays for wood mouldings at a big box store is different from those purchased at a high end architectural millwork shop.

Moulding Knife Marks Per Inch Image
  • Low-grade millwork – Under 12 (KMPI)
  • Medium grade millwork 13-19 (KMPI)
  • Highest-grade millwork 20-25 (KMPI) AWI Standards

Here are three determining factors in determining knife marks per inch (KMPI).

  • Feed rate of moulder machinery.
  • Revolutions Per Minute of the cutter head (RPM).
  • Number of moulding knives cutting.

Most small millwork shop's own a single knife finish moulder (non- jointed). In this case only one of the above determining factors (the moulder's feed rate) will effectively change the knife marks per inch.

As a rule of thumb, twenty-five to thirty feet per minute feed rate on a non-jointed moulder will create enough wood chip flow to keep your tooling cool, providing you are milling at least .020“ in depth. If your feed rate is too slow, below 18 feet per minute, you will compromise tool life with excessive heat, causing you to sharpen the knives more often.

Most industrial moulder's have 6000 rpm spindles, with the latest technology we are now seeing industrial moulder's with 8000, 10,000, and even 12,000 rpm. All these machines have a single knife finish, meaning only one knife is doing the cutting, the opposite knife is balancing the other. We will use 6000 rpm in the example below, the math will change based off of the rpm you have on your machinery.

Below is the mathematical equation for obtaining Knife marks per inch:

RPM’s (X) Knives Finishing (÷ by) the Feed Rate in inches = Knife Marks Per Inch.

Six thousand rpm times one knife finishing ÷ thirty feet per minute equals three hundred sixty inches.

6000 rpm ÷ 360 inches = 16.666 knife marks per inch, this number of knife marks per inch would be considered medium grade, non-sanded millwork.

Six thousand (rpm) x one (knife finishing) ÷ twenty five feet per minute which equals three hundred inches, 6000 ÷ 300 = 20 kmpi, this would be considered high grade non-sanded millwork.

Look what happens when you slow down the feed rate to let us say twenty-five feet per minute:

Six thousand rpm times one knife finishing ÷ twenty-five feet per minute equals three hundred inches.

6000 rpm ÷ 300 inches = 20 knife marks per inch, this would be considered high grade non-sanded millwork.

Look what happens when we increase moulding machine revolutions to eight thousand with the same feed rate as in the first example:

Eight thousand rpm times one knife finishing ÷ thirty feet per minute equals three hundred and sixty inches.

8000 rpm ÷ 360 inches = 22.222 knife marks per inch, this would be considered high grade non-sanded millwork.

High speed moulder's (jointed) have more changeable options; the larger industrial mills can add more moulding knives in the cutter head while simultaneously changing to higher feed speeds to obtain the same amount of knife marks per inch as a non-jointed moulder.

We trust you have a better understanding on moulding knife marks per inch and how to control some aspects of your milling operation thru quality standards. If you have any questions, or if we can be of any help to your current moulding operation, please contact us for assistance, we would be happy to help you succeed.