What Can I Expect From A Long Hss End Mill--

[Bob V]

Hi,
Today I was milling some 6061 aluminum with a 3/8"diam, 4" long two flute HSS end mill. I was plunging into the aluminum to a depth of 1.250" The first plunge was a partial cut (basically off-center enlargement of a previously drilled hole). This went just fine-- the hole after plunging was about .380" in diameter.
Next, I plunged (not enlarging another hole- plunged right into a flat surface) into the same piece of aluminum with the same end mill to a depth of .750", and the hole diameter turned out to .450"?!?

I'm a beginner at all of this--
I expected a larger hole with the 4" long end mill-- but I thought it might be 3-5thou over the diameter of the end mill-- especially using HSS on aluminum. My mill has maybe .002 runout four inches down from the spindle- so I know the .075 increase in the size of the hole over the end mill diameter is not just runout from a long mill.
Question: is this the kind of thing I can expect from a long end mill? Very unpredictable?!?

Thanks in advance for the help.
Bob

 

[Tony Wells]

Even a center-cut end mill will not hold size all that close under those conditions. If you need a flat bottom hole on or near(er) to size, you should break it into 3 ops. Spot or center-drill, drill near depth (watch your point depth), then plunge the end mill. With aluminum, be sure and clear the chips often and use some lube like kero or WD unless you run flood. Mist is ok, but deep holes do better with oily lubes, IMO.

 

[JimDawson]

My mill has maybe .002 runout four inches down from the spindle- so I know the .075 increase in the size of the hole over the end mill diameter is not just runout from a long mill.
Question: is this the kind of thing I can expect from a long end mill? Very unpredictable?!?

That is about the result I would expect. An endmill is not a drill bit. They walk around when plunging with no pilot hole. The longer the endmill the worse the problem will be. When plunging into a previously drilled hole they work pretty good.

 

[cathead]

What can you expect? Expect to take it easy and proceed slowly if you are milling out a pocket or something. If it is basically a drilling operation, I would tend to use a drill.

 

[Bob V]

Thanks for all the help-I have one more hole to put in this piece of aluminum--so I will try spot drill, drill, and then use the reamer.
Bob

 

[mmcmdl]

What results were you looking for on the position , straightness and diameter of your hole ? Do you need a flat bottom or could you live with a drill point ? 1.250 is no where near a deep hole . For deep drilling aluminum , I most always pick up a Guhring drill and peck it to break or clear the chip . Use a coolant of some kind . Ream it to size using a flat bottom reamer if necessary .

 

[Bob V]

Hi,
I was really roughing with the end mill (should have used a drill as I've learned from this post). The finish milling of the pocket was to be done with a 7/16 end mill. The tolerances I was looking for for sides and depth of the pocket are +/- .004". The pocket width is .500" so I didn't ruin it with the .450 hole I got with the 3/8 end mill--I was just very surprised to get a hole .450 in aluminum using a .375 end mill-- but according to the posts I've received from the good folks on this forum-- I guess that's what I should have expected. The dimensions of the end mill I was using are: 3/8x3/8x2-1/2x4 HSS.
So If I had drilled out the 3/8 hole with a 3/8 drill--and I might have gotten about a .380 hole--could I use this long end mill to get within .004" tolerance for this pocket? BTW the pocket is about .750 inches long and .500" wide, and 1.250" deep. I thought once I got a hole almost to depth that I would be able to "creep up" on the sides and bottom of the pocket-- is this the right approach or am I all wet here! (I can't use a shorter end mill to finish the pocket because there's a protrusion on the piece of metal I'm working on that prevents getting the spindle any closer than about 1.5" from the surface that I'm working on.)
Thanks again,
Bob

 

[Paul in OKC]

I would have gone with a 7/16 drill, then a .5 endmill. One way to run a slot is to drill, then plunge cut with the .5 endmill about .1 at a time. Then run back and forth to clean up. With a rigid set up and as little slop as you say you have, can be done. If the 3/8 is all you have, then........

 

[Bob V]

Thanks, Paul,
--I had been wondering how to best cut with the side of an end mill--
Will the sides of a 3/8' end mill cut the full depth (1.25") of the pocket to "clean it up" maybe for the last .005"?--or do you need a larger diameter/stiffer end mill for this. In other words, if I went over and over the walls of this pocket with the 3/8 mill will I end up with a pocket with nearly parallel sides (+/-.002 top to bottom), or would it just end up much wider at the top due to flexing?
Bob

 

[Paul in OKC]

If you go over it a few times you should get a good clean up. I am just fond of the stiffest set up I can get! You may have a bit of a smaller slot in the bottom from 'push off', but should be in your tolerance range no problem.