Thinking about feeds and speeds

I am starting to try and understand feeds and speeds and I would like to check my thought process.

OK, feed/speed. The main goal here is that your “chip load” - the amount of material removed by each flute - matches the amount prescribed by your cutter. There are also charts online which let you see reasonable chip loads for some common cutters.

Chip load is determined by a few factors, two of which are constant per-tool, and two that you can change:

  1. (constant) The number of flutes on the tool
  2. (constant) The radius of the tool
  3. (variable) The rotation speed of your spindle/router
  4. (variable) The linear speed you move through material

I have not sat down and worked out all the unit conversions here, but I think the number of flutes and rotation speed will work out to tell you how many times a flute comes into contact with your material per second. Since your machine will be moving through material at some rate, each contact will be into new material, and you will remove some amount which varies according to your tool size and rate that you move.

So how does this come together for feeds and speeds? I should pic a reasonable goal for chip load for the material that I’m working in, then set the speed on my router and the feeding speed through the material according to one of the feed+speed calculators which combines these.

My limits here are primarily how fast my machine can move and how slow my router can go. Since these are hobbyist machines, the routers actually tend to spin faster than the machine can really move through material accurately. So we tend to operate near the high end of the machine’s speed, and the low end of the router’s power.

The other thing I’m trying to figure out is depth of cut. I think this is a pretty separate calculation from chipload/feed+speed, but please tell me if I’m off here. As near as I can tell, the primary variables here are:

  1. Material hardness
  2. Machine stiffness

The ideal for my impatient self would be to cut to full depth in a single pass. But the deeper you cut, the more force you are asking from the machine. Each machine can vary (based on size, construction) on how much cutting force it can safely apply. The only way to know this for sure is to experiment per material / cutter to understand what your machine can handle.

That’s a wall of text, but I wanted to lay out what I understood (or thought I understood) before asking some questions.

  1. How do you check you have the right feed + speed? I know that spinning too fast / moving too slow can cause rubbing, smoking, and dust rather than chips. How do I know if I am spinning too slowly / moving too fast? Chatter? Something else?
  2. What am I looking/listening for in experimenting with cut depth? Skipping steps? Broken bits? Basically, as I cut deeper, how will I know that I am approaching or have gone “too deep”?

A lot of the topics on the forum say things like ‘you will hear when things are off’ but having run a few cuts I’m really not sure what I’m listening for. It sounds like a router.

Thanks in advance for reading and chiming in with answers, or corrections.

Here is a handy guide to the whole Chip Load concept…

You should be producing small/medium sized chips. If your CL is too small (too slow FR or too fast RPM) you will be producing very small chips/sawdust, then you are “rubbing” (also will cause excess heat, burn marks, dulling of bit, etc.). If your CL is too large (too fast FR or too slow RPM) , your chips may be larger, and you may hear the router “bog down”

This, and router bogging down. Recommended DOC for most specified CL is 1x bit diameter. I have seen charts where they recommend to reduce CL by 25% for 2x diameter DOC, and by 50% for 3x diameter DOC. Personally I would not go deeper than 2X diameter per pass. Trochoidal milling is an exception, as you can go full depth per pass using that method, but the time per pass is increased by a lot.

After you do a few cuts at various feeds and speeds, you will develop an ear for what sounds right and what doesn’t. It is normal for the router to slow down a bit in pitch when it goes from a free running no load state to a loaded state.

If the pitch drops too far (it may sound like it is struggling a bit), then you may need to increase the speed using the dial mid-cut (I needed to do this whenever my compressor kicked in during a cut, as the router was on the same circuit. Moving the router to a different circuit solved that issue)

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