SB CG1/Feed Rate

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Introduction

What are Feeds and Speeds and Why do they Matter?

There are some basic concepts and terms machinists use to discuss feeds and speeds that everyone should be familiar with.

Speeds

"Speeds" refers to your spindle speed in rpm (revolutions per minute). In a series of experiments performed early on in machining, it was determined that your spindle speed is the biggest determiner of your tool's life. Running too fast generates excess heat (there are others ways to generate heat too), which softens the tool and ultimately allows the edge to dull.


Feed rate

"Feeds" refers to the feedrate, in some linear unit per minute (mm/min or inch/min). Feedrate is all about the tradeoff between maximizing your material removal rate. Material removal rate is how fast in cubic units your mill is making chips, the faster the better for most machinists, right up until it creates problems.

The most common problem is tool breakage or chipping when you feed too quickly.


How About if I Just Run the Machine Super Slow?

It's a common mis-conception that you can "baby" the cut in order to be ultra conservative. Just run the spindle speed way slow and the feedrate way slow and you won't break anything, right? Well, not exactly.

  • Metal is a very unforgiving material.
  • Plastics, wood, and other softer materials can also have problems from improper feeds and speeds, but metal is the most sensitive.

Here's some examples of what can happen if you run too slowly:

- If you reduce your spindle speed too much relative to the feedrate, you're forcing the flutes of your cutter to take of too much material. The endmill is being pushed too fast into the cut and the chips get too big. You can easily break a cutter this way.

- If you reduce your feedrate too much relative to spindle speed, you will soon cause your cutter flutes to start "rubbing" or "burnishing" the workpiece instead of shearing or cutting chips. Many machinists will tell you the fastest way to dull a cutter is just to run it with the spindle reversed and make a pass, but having too slow a feedrate creates a similar effect.

There's a Sweet Spot for Feeds and Speeds

Yes! That's exactly right, there is a Sweet Spot for every cutting operation. It's not a point that has to be hit exactly, but at the same time, it is not very large either, and there are penalties if you miss it completely.

The more difficult the material you're cutting, the smaller the sweet spot and the greater the penalties.

Once you know where the Sweet Spot is, you can tweak your cutting parameters within that envelope to maximize Material Removal Rates, Surface Finish, or Tool Life. In fact, you can often maximize any two of the three, just not all three at once.