Determining Chip Load

In determining the feed rate, formulas ask for RPM and chip load. The RPMs is pretty easy to figure out, but what about chip load? When you’re cutting through MDF, you can’t exactly measure the size of your chips, can you?

I don’t want to post any equations if I am wrong without first clarifying that I am only making a guess here. So please, do not just run with these numbers if you happen to come across this post.

In my example, I am attempting to clean up and level out my spoil board/work surface. I do not need to take off much and looking to remove 1mm. I have a fairly large work area of 24x35 inches and this can take some time, so I would prefer to do this quickly yet preserve the life of my bits.
For a bit, I have a 2 flute half inch flat cutting bit (12.7).

I took this info from to get a benchmark of chip loads for different materials and bit sizes. Though I am unclear if depth makes a difference.
Chip Load

Going off of the lower value for MDF and a half inch bit, I’d get .025.
But if I look at I would get a large range of .005mm-.030mm.

.025 would seem reasonable for such a low depth, but calculators would then give me a feedrate of 1400. This is quite a bit of a jump from the 500 that seems to be a baseline. Now, I would love to increase the speed this much, but is it safe?

Feel free to jump in and add your two cents.

Unfortunately for me I have no way to measure RPM, so my chip load is calculated by ear.

Using the DeWalt 660, I start with 30,000RPMs and estimate after based on the speed controller setting. What’s really getting me is that this number is inches per minute, which is even more greater than the feed rate I’m using. Again, I’d love to go that fast, but this would match the speeds seen in accelerated videos.

I have not compared to the calc values. It is pretty clear how fast and deep I need to go after a few test cuts. I aim to cut at about 15mm/s and adjust the spindle and depth to match that. Most of those calcs are for extremely large cast iron machines so any numbers you get off that always assume the shallowest, slowest, lowest load. a half inch bit would be very large for this machine, I would stick with 1/4" or less I use 1/8" and 1/16", I do not even own any 1/4"…

I thought it was worth a try for the price I got it for to resurface my spoil board. Going over a 24x35 inch area with a 1/8 bit would drive me crazy.

I got one of these and ground off the bearing.

Haven’t tried it yet though. My current spoil board is plywood, and this wouldn’t work with that very well.

I used a cheap (in all senses of the word) harbor freight 1/2 inch straight router bit for surfacing the spoil board. It works just fine in the cnc. I think I got 3 for less than half what Barry paid for his nice rabbeting bit.

I keep forgetting about Harbor Freight. We didn’t have those in Alaska, so now that I’m back in the states it doesn’t occur to me.

I didn’t find one at Harbor Freight, but I picked one up at the TrueValue where I live.

It cuts nicely, but with everything being dust, I was wondering if I was going too slow and how much I could kick it up.