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holmqer
10-24-2009, 11:11 PM
I was trying to mill a slot with a round bottom and started with an end mill of
the right diameter till I got the slot to the right depth minus the radius, then
went back with a ball end mill to get the bottom of the slot round.

The two flute flat bottom end mill did just fine, but when I swapped in the ball
end mill it chattered like mad unless I took very light cuts (0.005"). Is that
typical for this type of cutting tool, or is their something special one does
when using ball end mills?

I was cutting a 3/8" wide slot in 12L14

airsmith282
10-25-2009, 12:13 AM
when i want to make a radius cut i just use a 4 or 6 flute endmill and put my milling head on the angle i need and do it that way ..this way i get the exact depth and stuf i need for the task at hand

BobWarfield
10-25-2009, 12:27 AM
Keep in mind that with any ballnosed cutter, the effective diameter of the cutting tool varies with depth of cut. So let's say you take a 1/2" ballnosed endmill, and you have an 0.050" depth of cut. Further, let's assume this is your first pass, so the end of the cutter is down exactly 0.050" and only that part of the cutter is engaged.

What is the effective diameter?

Well, because of the ball, it is only 0.3" instead of the tool's 0.5" diameter. So you need the feeds and speeds for an 0.3" endmill rather than an 0.5".

The thing is, the point of the tip is hardly moving too.

Cheers,

BW

Carld
10-25-2009, 12:29 AM
What kind of mill do you have? I don't recall having much trouble with a BridgePort and a 1/2" ball end mill.

holmqer
10-25-2009, 06:30 AM
My mill is an X3

The task was to mill a 3/8" x 3/8" slot with a 3/16" radius bottom along the complete length of a 1/2" rod thus producing a crescent moon shaped cross section, 1/8" at its widest point..

oldtiffie
10-25-2009, 07:08 AM
I am not surprised that you had a problem milling a 3/8"W x 3/8"D slot followed by a 3/16" radius cut out of the bottom of the slot in a 1/2" rod. There was practically no effective clamping once the depth of cut passed the 3/8" deep mark and it sure didn't improve with the radius cut. You were effectively trying to grip a thin-walled "U" shape.

I suspect that the "chatter" may have been "flutter" in the job while being clamped. Were the slides that were not being used clamped while cutting ("Y" - across the table; "Z" - the milling head on the vertical dove-tail; and the quill - in the milling head) and was the cutter tight in the collet?

A (Sieg) "X3" mill in good condition would ordinarily do that job pretty easily.

Was the job clamped in the vise or to the table?

While 12L14 is a "free-machining" steel it is cold-rolled and may "move" under cutting at the best of times.

holmqer
10-25-2009, 09:34 AM
I am not surprised that you had a problem milling a 3/8"W x 3/8"D slot followed by a 3/16" radius cut out of the bottom of the slot in a 1/2" rod. There was practically no effective clamping once the depth of cut passed the 3/8" deep mark and it sure didn't improve with the radius cut. You were effectively trying to grip a thin-walled "U" shape.

I suspect that the "chatter" may have been "flutter" in the job while being clamped. Were the slides that were not being used clamped while cutting ("Y" - across the table; "Z" - the milling head on the vertical dove-tail; and the quill - in the milling head) and was the cutter tight in the collet?

A (Sieg) "X3" mill in good condition would ordinarily do that job pretty easily.

Was the job clamped in the vise or to the table?

While 12L14 is a "free-machining" steel it is cold-rolled and may "move" under cutting at the best of times.

The piece was held in a vise parallel to the X axis with the clamping force applied perpendicular to the direction of cut. There was perhapse 1/2" overhang to either side of the vise jaws.

I perhapse did a bad job explaining what I did. I cut a 3/8" wide, 3/16" deep slot with the standard end mill, then came back with the 3/8" ball end to add the 3/16" radius for a total depth of cut in the end of 3/8"

Initially I had tried the ball end at the start and it chattered, so I switched to the standard end mill to get the slot to the depth where I wanted to add the radius. I suspect that this initial chatter was related to what someone mentioned above, I as ignoring the fact that I effectively had a very small cutter and needed higher cutter speed.

I believe I had the ball end nice and tight in the collet, it was certainly as tight as the standard end mill which exhibited no chatter

Carld
10-25-2009, 11:05 AM
What probably happened is when you clamp the 1/2" rod in the vise and then start cutting the middle out of it the metal will collapse and require tightening the vise. I don't think there would be a reasonable way to do that job in a vise. I would have used V blocks and clamps and dowel pins in the slots to align it. You don't say but imply that the rod is about 6 or 7" long.

Once the slot is cut you can use ball bearings to hold it down with as clamping on the top of the slot is not good but you can do it.

holmqer
10-25-2009, 09:44 PM
I tried an experiment...

Used a progression of smaller end mills making deeper and deeper slots to approximate the final curved slot, then went through with the ball end mill to clean up the slot. If I cranked up the speed on the ball end, I was able to finish it chatter free.