View Full Version : Grinding Wheels for Sharpening Milling Cutters

Paul Alciatore
04-30-2003, 12:30 PM
I've sharpened drills and lathe tools on a bench grinder but haven't tried milling cutters yet. I have a shank type 45* cutter: HSS, 1.5", 3/4" shank, 12 flute. Also a 3/8" corner rounding cutter, HSS, 4 flute. Both are in good condition except that they are somewhat dull from use.

I plan to use my Unimat in a Mill/drill configuration with an indexing attaachment to hold the cutters and index to each flute. I know I need to set-up carefully and take light cuts, preferably on the faces of the flutes to preserve the diameters as best as possible.

My questions are about the kinds of grinding wheels to buy for this. What abrasive should I use? What grit? What do the hardness ratings (G, J, K, L...) mean? Any other considerations?

It looks like I need a flared cup or dish shaped wheel for these cutters because of the less than 90* interior angles. I know it's hard to keep a good 90* corner on even a simple disk shapped wheel, how do you do it on these even sharper angles?

It looks like any diameter wheel would work on the 45* cutter as the cut can be made in a straight pass. But the corner rounding cutter needs a small diameter wheel to match the original geometry. Are there any special considerations when selecting and using smaller wheels. The Unimat has speeds up to 10,000 RPM so I should be able to get a suitable linear velocity with small wheels.

Another question that has long been in the back of my mind is wear on the wheel during the sharpening operation on a single cutter. Will the 12th flute be the same as the first? Will the angled edge maintain the same small radius? What can be done to minimize these kinds of problems?

Any help would be appreciated.

04-30-2003, 02:38 PM
Here is Some general info on grinding.
for one M2 HHS is very hard on grinding wheels. M42 and cobalt are worse yet. depening on how bad the cutters are will determine how often you will need to redress the wheel.
Generaly in any manual surface or cutter grinding .002" is a heavy cut .001" is about the norm but this again depends on the wheel type and grit, Your experiance, and how much you need to remove.

I normally use a 46 grit wheel with a K or J rating but these break down fast and will not hold a corner long. and you mut remember that you will never get a trur sharp corner on your wheel no matter what you do. you can only get it as sharp as the diameter of the grit size at best. an this is why you need a 100 grit wheel for sharper then normal corners.

As for the angle of the wheel. Dress it flat and set the cutter at the correct angle This is the best way unless you feel like dressing in an angle.
Now you will need to get fancy here you need to do some figuring because if you set the 45* cutter up and grind each flute in there will be no relief on the flutes and will cut worse then if you would have used it dull.
you need to set the cutter up so you will grind the primary angle with about 2 to 7 degrees relief and the secondary angle at about 5 to 15 degrees relief.
Now your grinding a compound angle (45* cutter + relief angle) This is why cutter grinders have heads that move beacuse if you set up you cutter in say a Harig Grind All and set it at 45* either in a vise or by swinging it in relation to that table teh rotate the cutter in the fixure you will create an incorrect angle.

If you use a cutter grinder and set the head at your relief angle or dress the relief angle on the wheel of a fixed head machine and then rotate the cutter in the fixture you are using two independant axies to set your angles and they will now be correct to each other
Not so easy hu
Look in the machinery hand book on cutter grinding gives nice pictures of what's going on.

And as for you first flute to last flute is concern no they will not be the same the wheel will break down as you go around them. you can limit this to a very small differance by leaving .0005" to clean up. you can do this in one pass with a 60 or 100 grit wheel and next to nothing in wheel break down.
Now with the finer grits the wheel will load up sooner and bounce off the part. Once you've done some grinding you will know what to listen for and know when to redress the wheel, and the finer grit wheel will burn the parts faster so keep them cool and take lighter cuts like .0005".

For straight end mills
Unless you have a fixture that rotates as well as slides fore and aft you won't be able to do the side flutes only the faces.

The corner rounding cutter is a pain to re grind correctly but you can get away with just grinding in the flat surface of the cter instead of trying to gring the radius this will buy you some time.

Now this will produce a cutter that is just about perfect in geometry but don't think for a minute that all cutters are ground this good, Most are not and unless you have a tool grinder in your area that pay's very close attention to detail the end mills will not likly be better then +-.001 per flute and +- .002 on over all diameter granted this is good enough for every day use but if you grind your own you can produce better then you can buy.

Now if ytou are asking why would anyone need a cutter closer then that?
try running one at 8000 rpm and see what happens. If they don't chatter and scream they will at least leave a crapy finish especialy at higher feed rates.
if they'er real bad they'll throw the head out of balance and soon break in the part from an uneven cut.
I gave you all they above info because most people asume that endmills are perfect in diameter, symetry, and geo but most are not.
Good shops will order there endmilll from the grinder to there own specs.
Anyway there you go.

04-30-2003, 05:21 PM
I can sharpen a drill bit about 50 percent of the time on a grinder. I pick up all the thrown down drills on the construction jobs I am on. I have accumulated about 50 pounds like this.
I had a belt sander I had made with a grinder and one of the useless belt sanders hf sells. You could sharpen the tips of a drill on it so well they would cut you. I generally test a drill bit by gently rotating it on my finger. If it digs in it will cut somewhat. Now making it last or not run through metal so fast it breaks on exit is another skill. Generally acute angles I find only work on sheet metal.
So far, I have had NO luck sharpening end mills. I do have a dremel but not enough patience I am sure.
Thrud says I need a air chuck (whatever that is) I drawed up something using two stepper motors but it looks more complicated than I want to endure. Especially the programming.

04-30-2003, 06:44 PM
For sharpening drills 118* is the factory norm it will cut just about anything if given enough time.
generaly speaking the flatter the tip of the drill the harder the material you can use it on but the slower it will cut. the srarper the point the faster it will cut but the material needs to be softer like wood, and plastic.
It takes some practice to do it right and to get both side near the same.

Paul Alciatore
04-30-2003, 09:54 PM
Thanks to everybody. This is the kind of info I need.

Kerry, it sounds like you're recommending that I grind on the relief angle of the 45* cutter, not on the face. It's a Niagara mill that has only a single relief angle and it's flat is about .035" wide. From the looks of the flutes, I only need to take off 2 or 3 thousanths to get back to a razor edge. Would it be just as well to grind on the face of the flutes thereby leaving the relief angle alone? The flutes are straight so this should be do-able.

04-30-2003, 11:09 PM
You will change the rake angle by doing this but if you only need to take of a few thousands to clean them up it should be fine. just don't do it more then twice.

05-01-2003, 02:42 AM

CBN flairing cup is pretty handy, but truing CBN has to be done with a brake truing device (just as with Diamond wheels).

The CBN will sharpen any HSS alloy harder than 40C but not carbide. Using CBN on softer steel alloys (<40C or so) causes excessive wear of the CBN. Diamond should not be used with steel - only non-ferrous materials (ceramics, Carbides, aluminum) because of ionic transfer.

Paul Alciatore
05-01-2003, 10:16 AM

Yes, I really only need a few thousands. Without doing the math, I'm sure the rake angle will change a degree or so but isn't that better than a dull edge?


Not to worry about diamond. I've seen the prices and a new cutter is cheaper. Perhaps if I get into this and need to sharpen some carbide tools but for now, I'll learn with HSS and less expensive wheels. I'll look into CBN.

05-01-2003, 11:16 PM
Like i said it will be fine the first few times you sharpen it this way so long as you just clean up th edge a few thousanths. What will happen latter on is the the rake will be so negative that it will take more and more presure to do the same job. But this is in the extreme and with out seeing the cutter I can't say how long you can get away with it.
I would take the time and do it by the book you be happier with your work and proud of it too.


12-17-2004, 08:42 PM

[This message has been edited by Koloya (edited 12-18-2004).]