Imperfect slots

M

Maplehead

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Hi All

I am using the LMS micro mill with HHS end mills.
These slots are 3/16" wide and 5?8" long. I drill to 3/16" holes at the ends before the length-of-the-slot cut.
I dry run with the end mill just above the workpiece and it travels nicely along the proper line.
However, when I insert the end mill into the back side hole and then begin to slot towards the from end hole, I get the results you see pictured.
Why is the end mill going off track?
The third slot from the right is especially noticable.

Any and all advice is greatly appreciated. Go Bruins!

IMG_0398.JPG
 
Hi Maple. Are you locking the axis that you are not using to make the cut?

Oh and what is the thickness of the material and the material type.

David
 
Hi David

Steel sheet from Lowes, 16 gauge.
I am not locking the X axis. You think that's it?
 
Hi Maple, Well for sure I would suggest that you lock the X axis, assuming you are using the Y for cutting and see what happens on a scrap piece of material. If the material was a lot thicker than 16 awg, then I would would have suggested slotting with a narrower mill first, then offseting to each side to complete.

David
 
The two slots on the left and the far right slot were done with me making an 1/8" slot first, just to add some relief. They seem a bit better.
Will check on locking unused axis next time.
Thanks
 
When I was young and always in a hurry to get finished, my slots would turn out to be rough and "dog boned" on the ends, and I couldn't understand why. A 1/4" end mill should cut a 1/4" slot, right? I was discussing this with one of the old timers and he simply said, "Well, you have to cut slots with an undersized end mill and and then offset, or comp that end mill to cut to the finished width if you want to do accurate, good-looking work in a case like this. " Of all operations that will highlight flex in a milling machine, slotting seems to be the one that highlights it best. You may not think an 1/8" carbide end mill would generate enough pressure to flex a Bridgeport but brother, it will! Of course this method takes longer to do, but it consistently produces beautiful, on-size slots. I soon gave up on cutting corners and trying to save time with these operations, and have been doing it that way ever since. More than a few apprentices have asked why their slots are rough, off center, dog boned, and generally don't look like mine. I slide right into "Well, you have to cut slots with an undersized end mill..." :)

With the part shown, I think a 5/32" (.156") end mill would be about ideal. The initial cut will look rough and wander all over the place, but your offset of about .0155" per side should clean things up nicely.
 
I would certainly agree with using the 1/8 cutter, and then moving the work to clean up the two sides. This is the sort of situation where a DRO is really helpful!

ghostdncr types faster than I do!
 
Is the idea of the smaller end mill, after the initial slot cut, for avoiding climbing while the other side is cutting regular?
Is that what is creating the dog bone?
For small end mills, I have a 3/16" and 1/8". So I should create the slots using my 1/8" end mill, by three passes minimum?
 
Now that is an excellent job for a Haas mini mill or TM1, small inside work , holes and slots seem to be their strongpoint, excellent tools for such work.
 
You could try a 3/16 end cutting roughing end mill. Don't drill the end holes, use the the end mill to drill the hole and then move to the desired length. I've done this with good success.
 
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