Phase 1 Recap: Feeds and Speeds

A project log for Rocking the Horses

Learn how to operate your new CNC router by making a rocking horse, then learn a lot more by setting up for mass production.

JoshJosh 05/19/2016 at 16:130 Comments

(edited to bring more accurate feed/speed guidance)

It seems that there are quite a few different ways to calculate feeds and speeds, but I'll give you my take.

Feeds - How fast you cut (how fast tool travels when cutting), in units/minute.

Speeds - Rate at which a rotating tool spins

Chip Load - the size of the "bite" that the tool takes on every cut.

I tried to do some research to find out some other people's ideal settings for cutting wood, but I never found any and I just decided to give it a go.

My Experience

My tool starting out is a 1/4" diameter, 2-flute straight cutter. This means it has 2 cutting surfaces and no spiral. I didn't choose it for any reason other than I had it in a box that I kept when I sold the lathe and it was still packed new in wax. I started out just setting my spindle to the max speed (25,000 rpm) and cutting slowly (20 in/min). This worked just fine, but I didn't get wood chips, I got wood dust. As I cut parts, I slowly increased the speed.

What I found was that somewhere around 70 in/min the dust started to transition into fine chips of wood. Chips would be an over statement, as they were more like delicate shavings. This makes sense, because the calculations really aren't that hard. Chip size = Feed/(speed*flutes). So my chip size is 70/(25,000*2) = 0.0014". This is about 1/3 the thickness of a sheet of printer paper.

I eventually pushed the machine up to 100 inch/min and it had no problems cutting through at a depth of 1/4". Redoing the chip calculation, I should be getting 100/(25,000*2) = 0.002" thick chips. These were nice chips that the vacuum easily removed and there was little dust formed in the process. I decided that I will target a 0.002" chip thickness for this tool moving forward.

Method to Figure out Feeds/Speeds

So to answer the great question: where do I set my feeds and speeds? High spindle speed and low feed rate sounds like a logical place to start, but DON'T DO IT! You'll set yourself up to burn up your tool and your work. The problem is that even though our tools appear to be "sharp" when you look at them under a microscope, the "sharp" edge has a radius. <This article> does a great job of explaining, but basically the radius of that edge seems to be on the order of .0001" and if your chip load gets close to this number, your cutter doesn't cut, it "pushes" or rubs the material. I'd suggest shooting for 10X this and aiming for a .001" chip load.

Here is a good method for wood:

  1. Calculate a starting feed rate such that you will have a chip load of about .001" The formula is Feed = chip load * RPM * #flutes.
  2. If your machine can't travel that fast, then calculate your spindle speed RPM based on your max feed rate: Speed = Feed/(chip load * #flutes).
  3. If your tool is excessively heating your work piece, burning or melting it, Either SPEED UP your FEED rate, or REDUCE your spindle RPM. Chips carry most of the heat away from the work, so speeding up the feed rate makes more chips to cool the tool, while reducing the RPM produces less friction and bigger chips at the same feed. This also means you probably aren't hitting the calculated values for either feed or speed for the target chip load.
  4. If you are getting rough edges, or can see a significant line between each cut layer, you are probably getting excessive tool deflection. Reduce depth of cut, take more passes to cut your part. You can also reduce your chip load, but this has a side effect of causing more heat. I found many people said not to set depth of cut deeper than the cutter's diameter.
  5. If you are getting corners that are not well-defined, you may have a backlash or acceleration issue. Reduce your velocity slope (acceleration) so that your spindle head will slow down for changes in direction. Be wary that lower accelerations may cause your machine to fail to reach the specified feed rate in some areas with lots of direction changes. This means you should reduce your spindle speed to try to keep the chip load higher.
  6. Listen to your machine. The sounds it makes will give you some indication of what it's doing. If the sound changes drastically, it may be a warning to stop before something breaks!

I hope my experience will help someone else trying to start off! Feel free to ask me questions - I'm likely to have screwed up the same thing!