Understanding Chip Load

Explaining the term "chip load" --what it means, and why it matters. June 19, 2005

I was wondering about what chip load is. The value supplied by tool manufacturers for each tool has a particular chip load. For example a two-flute up/down 1/2" diameter compression bit has a recommended chip load of .024-.026 for (chip board) from Onsrud. Does this value represent the amount in inches that the tool removes for each revolution of the tool?

Forum Responses
(CNC Forum)
From contributor G:
The chip load refers to the size of the chip material after it has been machined.

From contributor M:
You can determine your chip load or figure your speeds and rpm to target the proper chip load with the following formulae.

Chip load = Feed rate / (RPM * # of Cutting Edges)
Feed rate = RPM * Cutting Edges * Chip load
RPM = Feed rate / (# of cutting edges * Chip load)

From contributor K:
As you can see from the formulae, chip load can help you to easily determine what your feed rate should be:

Chip load=.020in/Rev, which means a feedrate of 16,000*3*.020=960 i.p.m.

This is a good starting point, but there are also a couple of other considerations. First, chip loading is ultimately a measure of how much heat is taken away from the cut and consequently from the cutting tool. Too little will cause the tool to overheat and die prematurely. Of course too much will put too great a load on the tool and spindle and give a poor quality of cut.

Other factors contributing to heat buildup must also be considered. One is plunging versus ramping into a cut. Plunging creates a tremendous amount of heat and will quickly kill a tool that has the proper feed rate.

The other is deceleration at corners. If you are cutting a few large parts from a board, then the heat buildup as the spindle slows and/or stops when turning a corner will not be a big factor. But if you are cutting many smaller parts, this becomes a major factor. Also, with short movements at feed speed, the tool spends a lot of time at less than optimal feed rates, thus more heat buildup due to smaller chip loads.

The bottom line is that we never run our router at the maximum feed rate and RPM. We back off to about 60% (we are pretty conservative) so that the average chip load does not get too small.