Why are my slots stupid?

T

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Hi all,
Little background. I'm new to machining, milling in particular. Every time I try to make a slot it always ends up oversized. I just made a set of brackets for a friend and the original size was supposed to be a 7/16" slot. I drilled a hole at the beginning and ending of the slot hoping that would make it better. Nope, the slot still ended up oversized. My 7/16" end mill was quite dull which I'm sure didn't help, but its not the sole reason. In the pics I actually went back in with a smaller sharper end mill to clean it up because it looked terrible so now they're even more oversized. My setup is a Bridgeport J head I was probably running 500rpm with a dull 4 flute HSS cutter. I also had a coolant mister spraying the end mill.
 

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Trying to cut a slot to width with an endmill of that width will always come out oversized. Drill your end holes, then finish the slot with something smaller - e.g. 3/8". Also, clamping can be tricky. Make sure you're not applying force that would want to push the slot closed as it's being formed. I suspect, in your case, it's the former.

GsT
 
How much oversize are they? Are you running the end mill in an end mill holder or a collet? Have you checked to see how much runout you have on the end mill?

A dull endmill will cause an oversize slot...

You should expect the slot to be oversized by two times the runout on the end mill, also.

To machine an accurate 7/16" slot, I would use a 3/8" end mill... make a cut right down the center, measure the width, then step over and cut both sides 1/2 the difference between what it measures and 7/16".

-Bear
 
Agreed, the only way to cut an accurate slot is in two passes with a smaller diameter end mill. However, if you don't need high accuracy, slots for bolts etc, I find that a straight cutting, 2 flute end mill or router bit will give the best one pass results I'm not sure why but I heard it long ago and it seems to work.
 
This is a common beginner issue- BTDT. All the above advice is valid, plus the 2 flute clears chips better, closer to size, the tradeoff being a rougher surface finish.
Never try to cut a slot cleanly to size in one pass, always cut undersize first if possible
Save your dull cutters for the really cruddy, non-critical work
 
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What you are experiencing is called "cutter deflection". It is the curse of apprentices, more experienced machinists know to expect it. Picture milling a slot from left to right. Looking down on the cutter, there is a thrust against the cutter at 90° to the direction of cut, caused by the resistance of the metal. There is no balancing force on the opposite side and the cutter is not perfectly rigid, so it bends (deflects) towards the top (away from you looking down). If the cut is from right to left the deflection will be towards you. This is most pronounced with small diameter cutters and even worse with dull cutters. One of the ways to avoid this is to rough out the slot with a smaller diameter cutter.
 
There are two similar types of milling cutter. The end mill which has 4 or more flutes & the slot drill which traditionally had 2 flutes, made & used for slotting. Through thin stock as your image shows a proper slot drill should be able to cut a decent size in one pass. They were made to different tolerances to end mills without being oversize. Having two flutes any deflection is allowed to return to the centre when both flutes are not cutting & when cutting, both cut equally.
 
An exception of a sort that I found when cutting metric keyways (and really any other), is that I reground inch endmills to a metric size (as a matter of convenience) if the keyway started from the end of the shaft, I could go full depth and complete the keyway in one pass and have it on size; the cutter could deflect, but in one pass it did not matter, I ended up with an accurate key width. Of course, when the feed was stopped, the cutter would un deflect, but no matter at the end of a keyway This was a great saving in time, not having to start with an undersize cutter and having to step over to achieve an accurate dimension. Having said that, the most accurate and time saving method of cutting keyways is with a plain mill cutter, as used on an arbor, or a woodruff style cutter, they will always on size.
 
Wow, this forum never disappoints! Thank you guys for the super informative responses.

So even in steel I should use a two flute cutter for slotting and to cut the slot undersize first...now I know.

Machining is wonderful. I love learning all this stuff. I wish i got into this as a profession.
 
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