Equation for calculating Feed Rate

I’ve shared this equation with a few CNCers and keep meaning to post it here in case anyone has trouble determining the right feed rate for different projects.

Spindle Speed X number of flutes on your bit X Chip Load = Feed Rate

For example: 18,000 spindle speed X 2 flute bit x 0.011 Chip load for hardwood being cut with a 1/4 diameter bit = 396 feed rate

This website will give you the preferred chip load for different materials being cut with different sized bits. https://www.cutter-shop.com/information/chip-load-chart.html

I recommend making a spreadsheet for yourself with the materials you often use and the bits you own for quick calculations.


@timafeich here is the feed rate calculation

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I’ve used this bit to flatten a slab before, and from that chip load chart that means its about .02" chip load. The packaging recommends a maximum of 18000 rpm, but i’ve used 15000 rpm before. From this info it means

15000 * 3 Flutes * .02 chip load = 900 in/min …

Does this seem really high, or is it just me? I’m not the most experienced CNC’er. When I’ve tried it before, it was going more like 80-100 inches/min. That might have been slow, but 900 seems high.

Does it just not translate well to that style of bit?

Yeah, for a huge bit like that 900 doesn’t seem crazy. Of course a lot of other factors play in, like cut depth. Going that speed at the bit’s full cut depth of 0.25 inch would be hard on the blade and hard on the material It would also create a lot of pressure on the wood, so you’d need to consider how the material is being held down and if it can handle that much push. But if you were taking off a millimeter at a time, 0.03 inches, and the material was very securly held down, yeah, the blade and material would be fine at that speed.

You can play around with the chip load too. If you’re just trying to remove material, you can go with the highest suggested chip load. If you wanted a smoother finish, you can try a smaller chip load.

We can reverse engineer what you did before too. Chip load = feed rate / (spindle speed X number of flutes). So, 100 feed rate / (15,000 x 3) = Chip load of 0.002 which is smaller even than the recommended size for a 1/8 inch bit.

I use a 2 flute 1.125 diameter bit on hardwood and run it at 684 with great results. Don’t be afraid to speed things up a bit :slight_smile:

This article has a great explanation of all of this too: https://inventables.zendesk.com/hc/en-us/articles/360012848433-Calculating-Your-Cut-Settings-Basic-Feeds-and-Speeds-Information

Interesting, thanks for the info! I tried again today with a little bit higher of a feed rate (more like 400, so still slower than recommended) and it worked fine. Seemed really fast though!

It was a resin river table thing, so I wasn’t sure how to deal with one setting for both the epoxy and the wood. I was getting a little bit of chipping in the resin when I was milling conventionally, but when I switched to climb milling it worked a lot better. Well lessons learned! Maybe next time I’ll speed it up a bit. In the past I’ve really just used the default settings in Vcarve and its worked ok, but i’ll be making sure they’re correct from now on.