Last week I wrote a brief introduction to end mills. Big shout-out to the one person who read it! Today I’d like to further test everyone’s patience and go into it a bit further. We’ll start by talking about straight end mills, which are by far the most common.

Straight Cutters For Straight Shapes

Most of the shapes I make with my milling machine boil down to a bunch of straight lines and square corners. And for those, a straight end mill is just the ticket.

Varieties of straight end mill
Four straight end mills. From left: a 2-flute, a 4-flute, a 4-flute with a coating of titanium nitride, and a 4-flute roughing end mill.

Straight end mills cut a shape that’s straight, and parallel to the machine spindle. They cut on their sides and on their bottoms. Most straight end mills have smooth sides, and they produce smooth-sided cuts. But roughing mills, like the one on the right in the above picture, do not. As you might imagine, those big teeth leave a rough surface, hence the name. On the other hand, those big teeth mean that a roughing mill can remove an astonishing amount of metal. If I don’t need a beautiful finish, it saves a lot of time!

Some end mills are capable of cutting straight down like a drill bit. These are called center cutting. Other end mills can’t do this because they don’t have cutting flutes all the way across on the bottom. Consequently, they have to feed into the work from the side.

Center cutting end mills, like the three on the left, have cutting flutes that meet in the middle. Non-center cutting end mills do not.
The three leftmost are center cutting. The big one on the right is not.

Not A Question About Music

The photo above shows straight end mills with two, three, four and six flutes, or cutting edges. The choice comes down mostly to the material I’m cutting. The chips coming off aluminum tend to weld themselves to each other (and to the work), so it’s important to have a lot of room between flutes for the chips to clear out of the way. I usually use two-flute cutters for aluminum, or occasionally a three-flute. I would only use a 4-flute if I had nothing else that could work.

Oil-hardening tool steel, on the other hand, has no such issues. But it’s tough, and a cutter can flex if it’s fed in too aggressively. A four-flute end mill is stiffer than a two-flute, so I would reach for the four-flute first.

Picking The Right One

You can buy an astonishing array of sizes of end mills. In general I choose the biggest one the work allows, provided my machine has enough power and rigidity to handle it. After that I think about what will produce the best surface finish, given the material I’m cutting. Since I’m just doing this for myself, and at least partly for my own enjoyment, I don’t have to worry about how quickly my cutter operates. But I do care how long the cutter will last. So that’s a consideration too.

For commercial shops cutter selection is a very deep rabbit hole indeed. A quick look at the catalogs of big end mill manufacturers like Niagara and M. A. Ford have huge ranges of products, and tools to help machinists figure out what will best suit their purpose.

For a commercial shop, it might make sense to buy a $200 end mill if it will cut five minutes off the time to make a $100 part, as long as they’re making a lot of parts. But in my basement shop, almost everything I make is the only one of them I will ever make. So I tend to concentrate on finding something that will work, and that I already own.

I hope that gives you a little insight into straight end mills. Thanks for reading!