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I hesitate to describe this since the method may have been posted here before, or seem obvious, but it came up yesterday for me as I was milling four 3/8" by 2.5" long slots in some very tough (meaning poor quality) 1/2" plate steel.

I milled one slot by the method I used to use, which was basically milling the slot down about 20 thou deeper for each pass until I was through. Well, 20 thou into 0.5" is 25 passes to get through per slot. Each pass is slow on my old mill drill in this material. So it's quite tedious. It also puts a lot of wear on the sides of the flutes. Not only does the end have to cut, but the sides are continually rubbing with swarf as the cut deepens through each of the 25 passes.

Then as a comparison, for the other three slots I used a chain milling method with the same two-flute end mill. Basically plunging straight down 'til through, and then moving the table a little less than half the mill diameter further along the slot, and plunging again. Let's say I moved it over 0.15" per plunge on a .375 cutter. That's 17 plunges. Then I ran a cleanup pass down the length of the slot.

Each plunge takes maybe 1/2 the amount of time a slot pass takes. That's 17/25 x 0.5 = 0.34 or about 1/3 the amount of time slotting by the old method takes, plus a cleanup cut. This method also places most of the wear on the tip of the end mill instead of the sides, which is an advantage for me. In addition, swarf can always clear down the hole as opposed to building up in the slot with conventional slotting.

Since I have a very simple homemade end mill sharpener that is able to grind the flute ends, this slotting method greatly preserves the usefulness of my milling cutters by reducing flute edge wear when slotting. Well I think it also does reduce end wear as well, but I can't prove it.

The main disadvantage of slotting this way (with my machine's relative stiffness) is that the slot will have a slightly wavy edge with a single cleanup pass. I can take two cleanup passes, one for each edge so it looks perfect, but the slot will be slightly oversized. For the piece I was working on, the slot width wasn't critical so I did the two passes. I found a .003" cut per side did the trick.

Now this is not as great a disadvantage as it might seem, since I have a whole cardboard box full of donated machine shop end mills. Many of them have side sharpened flutes and are therefore slightly undersized. All I have t do with these is sharpen the ends, do the plunge slotting thing, and then two passes brings the slot to accurate size. And it takes just about 1/3 the amount of time of conventional slotting, while increasing the mileage before a re-sharpen of the end. For me that's a win-win. Or is it win-win-win....?

I hope this helps somebody else :cheers:

Lew Hartswick:
It's probably even faster if you use a drill for the "chain" drilling. A drill is the fastest way to remove material.  I have done that many times, of course it was on a Bridgeport.

Lew beat me to the punch but I'll offer this anyway...

Drills are the best bulk material removal tools plus they're cheap and easily resharpened.  Chain drill and then use the endmill to clean up the slot.

Use a 3/8 drill and move it one diameter plus a bit for each hole, say 1.1 * 0.375 = 0.413. Then you need 2.5 / 0.413 ~= 6 holes.

I'll try it Lew, but I'm not sure I could have milled easily between the much greater spacing required by drilling this 1/2" steel with my round column mill drill.

The advantage of using the end mill for my particular project yesterday (and possibly with other limited capacity hobby milling machines) was that the overlapping plunge cuts made the final slotting passes quick and easy, and the slots more accurate than simple chain drilling would have.

Aluminum, or a Bridgeport, different story.

Guys, I just gave it a try with .375" drill and chaining over .413 for three holes in the same 1/2" steel plate. The holes took a little over 2 minutes total to drill, then changeover to a fairly new US made carbide mill another minute. but after milling about a quarter the way between the first and second holes I had to stop because I didn't like the sound and feel. Definite warning signs of impending unhappiness.

I think for my particular machine, when doing difficult materials in sufficient thickness, I'm going to stick with the end mill plunge method. I appreciate your mentioning the chain drill alternative, and maybe it will work well for others.  :cheers:


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