finishing cuts and end mills

[coldte]

Hi, What is the procedure when surfacing with an end mill ,do you surface nearly to size and finish with a higher speed and fine cut or not ?
the reason i ask is several times i have applied this method and found that the end mills do not seem to like it emitting a high pitch sound as though it is rubbing,must admit it is usually with the larger end mills.
coldte

 

[gbritnell]

The type of material you are cutting determines the spindle speed. Naturally if you are taking a very light cut you could speed the spindle up but it's not necessary. Generally you would take heavier cuts until you get close to your finished dimension and then you could take a light finishing cut. The depths of these rough and finish cuts are all related to how rigid your machine is. If you are using a Bridgeport or a Sherline it would make a difference. The high pitched sound you mention might be the chatter you're getting from the cutter because it's running too fast.
gbritnell

 

[Richard1]

I generally use a light finishing cut at the same speed or very slightly faster but with a slower feed rate. The noise you get could I suppose be chatter from a cutter running too fast.
Richard

 

[IanN]

Hi,

It is important that the milling cutter must cut rather than rub - the lightest cut you take should be at least 4-5 thou. and the feedrate should be steady and not be too slow.

There are two types of end mill - in the UK they are called End Mills and Slot Drills (in the USA the term End Mill is used for both types, but the slot drill is called a centre cutting end mill).

End mills (non-centre cutting) are designed for profiling (cutting on their side) not surfacing (cutting on the end) and you will get the best results if you use a slot drill for surfacing. If you have to use a non-centre cutting end mill for surfacing you will find the corners of the teeth are inclined to chip - this can be prevented by stoning a bevel on the sharp corner (it only has to be a very small amount, 10 - 20 thou or so, but make sure you maintain the clearance angle).

If you use a non-centre cutting end mill for surfacing, you will get better results if you take several narrow cuts rather than one wide cut - a good rule is to make the maximum width of cut 1/4 of the cutter diameter.

As for speed - I don't change the speed for finishing cuts, but the usual rules apply: Cut at a low speed and the job takes longer but the cutter lasts longer. Cut at a higher speed and the life of the cutter is reduced, but the job takes less time.

Ian.

P.S. It is, of course, possible to break all of the above rules and get good results. I.

 

[RobWilson]

*bang*

 

[rklopp]

I often use the same speed and feed for finishing as roughing, but take a much lighter depth of cut. I don't usually back off on feed, unless surface finish is critical. Some times, I even have to increase the feed during light finishing cuts to avoid chatter. For me, the most important reason to take a finishing cut is to get rid of the effects of cutter deflection, which improves accuracy. It's always surprising to me how much an endmill and workpiece deflect during a heavy cut. If you try to end-mill a slot in one pass, you will find that the slot is out of position laterally due to cutter deflection.

 

[BobWarfield]

Lots of good advice here. I feed slower for finishing but keep the spindle rpm at the recommended speed. I don't feed a lot slower in most cases as that can lead to rubbing. Less depth of cut finishing, as has been said.

Another really important thing for quality of finish is getting rid of the chips. Chip recutting is just not a happy thing for surface finishes. I like a strong air blast perhaps with a little bit of coolant mist.

Lastly, for best finish appearance, the fewer passes over the surface the better. For that reason, a nice fly cutter often leaves a very nice finish. That reminds me, a good tramming is also pretty essential to the best finish.

Best,

BW