Feeds and Speeds
Speeds
The “Speeds” portion of the Feeds and Speeds combo refers to your spindle rpm. Determining the correct Speeds for a job is largely a question of determining how fast the tool can be spun without overheating it in the material you’re cutting. Once a tool overheats, it softens (well short of melting), and this causes the sharp edge to dull. It doesn’t have to get very dull before the tool is done. If you keep going with a dull edge, you’re likely to break the tool, but you’ll see a very deteriorated surface finish before that happens.
Your spindle speed is the biggest determiner of your tool’s life. Running too fast generates excess heat. Consider your spindle speed to be largely about maximizing tool life.
Feeds
"Feeds" refers to the feed rate (in mm per minute). Feed rate is all about the tradeoff between maximizing your material removal rate and being able to extract chips from the cut. 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. When that happens, the chips jam up in the flutes and pretty soon the cutter breaks.
I’m a Beginner, How About if I Just Run the Machine Super Slow? (Big Mistake!)
It’s a common misconception that you can “baby” the cut in order to be ultra conservative. Just run the spindle speed super slow and the feed rate slow too and you won’t break anything, right? Well, not exactly. Here’s some examples of what can happen if you run too slowly:
– If you reduce your spindle speed too much relative to the feed rate, 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 feed rate 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 feed rate creates a similar effect. It will suffice to say that running too slow is just as hard on your cutters as running them too fast, if not harder.
The 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.