I have come up with a way to sharpen end mills using the milling machine. In fact, the setup is tricky enough that I have to look at my own pictures to set it up right. The tool to be sharpened is held in a 5C collet in a fixture I got from ENCO for $38.99. (Perhaps I got it for a little less on sale.) This is on P 151 of the 95/96 full-line catalog, as #287-6840 'End Mill Grinding Fixture'. The little picture in the catalog on this tool is all the help you'll get from Enco in using this - there was no paper at all in the box. They show it on a standard surface grinder, I think. The grinder shaft is horizontal, but as long as the grinding is really done by horizontal traverse of the table, all surfaces will be flat, and so the plane of rotation shouldn't matter, I thought. So, if I put a cup wheel in the spindle of my mill, I should be able to grind the top of a surface, too. Of course, it gets a little more complicated when you do serious sharpening of mills that are badly worn or have a corner chipped off. Then you have to regrind the relief and gashing, too. If the mill is just slightly worn, just the primary relief can be done, in under 5 minutes.

Here's an overall view of the setup.

I suggest getting a book on end mill sharpening from a good library if you can. I would have never figured all this out without a good illustrated text. Try the library of a university with a mechanical engineering department, a tech school, or high school with a vocational department. These books will show how to do it with a cutter grinder, almost always with a cup-type diamond wheel. I have used both a 'flaring cup' type wheel of alumina (Aluminum Oxide) abrasive, as well as a diamond resin-bonded flaring cup wheel. The diamond wheel keeps a sharp edge longer, and leaves a much smoother finish on the cutting edges. This is actually similar to what is frequently used on a cutter grinder, and so makes translating the pictures in the book a little easier. The Cup-type wheel is essential for gashing. That side relief makes it possible to get UNDER one cutting edge to gash the other one. My alumina wheel is similar to Enco's # 391-1153. These are fine grain, hard abrasive, which seemed to be the right material. I take a swipe across the bottom of the wheel with a diamond dressing point before starting. Hold the diamond in a vise, or possibly in the collet of the fixture, and use the table feed to pass it under the wheel. Just make sure it is tipped away from the direction of the rotation of the wheel. Take off just a thousandth or so each pass, and the diamond will not show any visible wear.

Dressing the flaring cup wheel.

Anyway, the end mill grinding fixture holds the cutter in two possible positions. When set on its flat base, it holds the cutter at about 15 degrees in one plane, and at about 5 degrees in the other. This is the position you use for the primary relief, and it sharpens the cutting edge. it also applies about 5 degrees tilt, so the radial edge of the cutter is just a few thousandths farther out than the center. I never knew end mills were ground this way - the tilt is apparently much less on a factory-ground mill - but they work just fine this way. So, the trick here is to have the fixture mounted on the table with the 30 degree miter in the base facing the left, the wide part of the base clamped flat to the table (or in a vise). The end mill should be set so that the hub is in a detent, and the cutting facets are lying parallel to the Y axis. (If a 4 flute mill, then either pair lying parallel to the Y axis.) Position the table so the rear cutting edge can be fed into the front left quadrant of the wheel. Try adjusting the X position so that when the wheel edge approaches the center of the mill, it approximates the angle of the gashing that separates the two cutting edges at the center. After making a grinding pass, then rotate the hub 180 degrees, and make a pass on this facet to the same point on the Y position. Examine with jeweler's loupe (Super 8 mm projection lenses make FANTASTIC jeweler's loupes - although they won't stay in your eye socket) to see that the grinding goes all the way to the center. If not, advance the Y a little more and check again. When you've got it, then do all facets until the cutting edges are all sharp all the way across the edge.

Grinding the primary relief.

Step 2 is to grind the second relief, if needed. Now, tip the fixture onto the 30 degree facet, with the 30 degree facet to the left. Hold in a vise or clamp to the table. To make clamping easier, I cut a flat in the top of the fixture's bottom plate which is level when the fixture is on the 30 degree facet. I can place a clamp on that flat. But, I've found that the vise makes the work go several times faster, so I no longer use the flat I milled. You can see it in the photos, however. Now, with the end mill approaching the same quadrant of the wheel, bring the tool slowly into the wheel from the front, with the Y feed. Adjust X position until the tapered side of the cup wheel clears the cutting edges while grinding down the secondary reliefs. This is particularly tricky on 4-flute mills. This work is not as critical as the primary relief, which is actually part of the cutting edge. The secondary relief just provides room for chips to escape.

Grinding the secondary relief.

Gashing is used to make the center of a center-cutting end mill somewhat like the point of a drill bit. Gashing is necessary for major repair to an end mill, when .050" or so has to be ground off the length. This is done with the fixture set up as for the second relief, but the table is moved so that the end mill approaches the wheel from the FRONT side. Turn the hub a few notches counter-clockwise, one notch less than 90 degrees. (Should be 5 detents.) This has the 2 cutting edges almost parallel to the X axis, but the left edge is a little back, the right edge a little toward the front. Now, pass the mill back and forth with the X axis, while slowly stepping the table back with the Y axis. Observe the wheel cutting closer and closer to the right cutting edge. Stop before you cut into the right cutting edge, but get close enough to eliminate the large flat land at the center of the mill. Look at a new 2-flute mill to see what you are trying to achieve. What you want is this :
         xxxxxxxxxxxx gash here
            gash here xxxxxxxxxxxx

Gashing under the cutting edge.

This is the trickiest part by far! I have made several messes, had to repeat the first part after bad gashing attempts, etc. But I also have made a few end mills look just like they were absolutely new! I wish I had video taped those grinding attempts, so that I could see how I did it! But, I am getting better, and I can now routinely make end mills that will plunge properly. Start with really blitzed end mills, of which we all probably have a few. Keep the nearly good ones for last, so you can look at them and follow their contour.