End Mills

End mills are the primary cutting tools used in a vertical mill such as the mini-mills we are concerned with. Like drills, they come in a wide variety of styles and quality levels.

The most common ones you will need for getting started are 2-flute and 4-flute end mills. You can buy them individually as needed, or in sets.

Here’s a picture showing a variety of end mills:

The gold-colored mills have a very thin Titanium Nitrate (TiN, not tin) coating which helps to prevent chips from welding to the mill surface.

The ball end mill has a round tip, for milling grooves with a semi-circular cross-section such as you might use for a bearing race.

It is an example of some of the more specialized types of mill available.

Mills are also available in sets of 10 or 20 such as shown below. Import quality sets on sale range in price from around $20 for a set of 10 to around $50-$60 for a set of 20 TiN coated single-end mills.

Set of 10 4-flute single-end mills
Set of 20 2&4 flute single-end TiN coated end-mills

If you are just getting started, a set such as that on the left would be a good set to go with. You might want to also buy a few individual 2-flute mills just to compare the difference.

Shank Size#

The shank is the smooth portion of the mill shaft above (or between, in the case of a double-ended mill) the cutting edges. Mills of different diameters within a range have common shank sizes but shank sizes vary among larger and smaller mills.

For example, a 5/16" and a 3/8" mill both have a 3/8" diameter shank, while 1/2" and 7/16" mills have a 1/2" shank. This is important to know, since you will need the appropriate sized collet or end mill adaptor to match the shank size of the mill.

The mini-mill comes with 3/8" and 1/2" collets, but you will eventually need 5/8" and 3/4" collets and/or end mill adaptors to hold mills larger than 1/2".

2-Flute Versus 4-Flute#

2-Flute mills are generally used for cutting slots or grooves, while 4-flute mills are more often used for surface milling, using the end of the mill. In practice, I have found that 4-flute mills will do most of what a 2-flute mill can do, but not vice-versa.

But each type is best for certain operations.

Here are some interesting comments from the 7x group (1/21/04)

> What is the difference between 4 flute and 2 flute end mills? Is one better for Aluminum? > > A 2 flute cutter (or slot mill) is designed for “plunge” milling, ie you can plunge the mill directly into the work. > > If you look at the business end of the cutter, you will see that the two blades are of unequal length, one of the blades goes right across the middle. This allows the cutter to mill directly under it. > > It can be used to cut vertically or horizontally, but the horizontal cut is not likely to be as good as the 4 flute cutter. > > The 4 flute cutter (or end mill) is not designed to be “plunged”. Again, look at the cutters and you will find that they are all the same length, but there is a hole in the middle where nothing can be milled. This type of cutter can only be used to mill in a horizontal direction. Graham > > Um… That’s (sort of) the difference between center cutting and non-center cutting, but not really between 2 and 4 flute end mills. Some 4 flute end mills are center cutting (usually two flutes meet in the middle and two don’t). > > And, though I’ve never seen one, there’s no reason a 2 flute end mill couldn’t be non-center cutting. (Never saw an end mill with one “blade” longer than the other either… Not sure what that’s about.) The real difference has more to do with feeds, finish and such. > > Basically a cutter’s RPM is based on its composition and what you’re cutting with it. So how much is taken off per flute is, more or less, a “fixed” rate. > > How fast you can take off how much material is a function of how many flutes you have. The fewer flutes, the slower the feed must be. The more flutes, the faster the feeds can be. > > Also… I read a bit a while back that explained how 2 flute end mills were better for slotting and 4 flute end mills were better for finishing. The former had to do with the fact that the 2 flute cutter bites into both walls at the same time and, when cutting to the front, cannot cut the sides. > > Conversely, a 4 flute end mill will cut both the front and sides at the same time and the front cut will cause the cutter to flex, varying the depth of the side cuts making the finish rougher. > > Don’t claim to understand why a 4 flute cutter would give a better finish unless it’s simply because it’s stiffer…? > > As for aluminum… Though “regular” end mills will work with aluminum, there are end mills specially made for aluminum. > > “High helix”, I think they’re called. They claim to produce a better finish. Haven’t messed with them much myself. > > But I’m heading that way for some finishing work. I’ve only ever seen 2 flute versions of them. I’d love to find some 4 flute versions so I could increase my feeds…

Single-end Versus Double End#

Double end mills are somewhat less expensive than single end since you get two cutting ends on the same tool.

I prefer single end mills since one end of a double end mill sometimes becomes dull or damaged, but you lose track of that fact, since you are keeping it around because of the ‘good’ end.

Then you end up using the ‘bad’ end by mistake and messing up your project. I suppose I could grind off the ‘bad’ end to avoid this scenario.

Another reason I like single-end mills is that the mill sometimes becomes jammed in an end-mill adaptor and must be driven out using a soft metal rod.

This operation is likely to damage the top end of a double-end mill.