Advantages of non-center-cutting endmill?

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WillAdams
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Advantages of non-center-cutting endmill?

Post by WillAdams » Tue Sep 17, 2013 4:25 pm

My next project involves through cuts in 1" rough-sawn hickory w/ a 1/8" bit.

I'm currently considering two different bits, one center-cutting, the other not:

http://www.amazon.com/dp/B0080CADJ8/ref ... 76MMW91ZWH
http://www.amazon.com/dp/B003BIEOWS/ref ... VZLD9XNFLC

I've drawn up the parts in InkScape and generated G-code using MakerCAM --- AFAICT, it's written so that it requires a center-cutting bit to allow for the plunge moves (AIUI, a spiraling-in-move would accommodate a non-center-cutting end mill).

Is the bit significantly more rigid? Less prone to run-out? What? Is there some advantage which would make changing my workflow worth it?
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twforeman
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Re: Advantages of non-center-cutting endmill?

Post by twforeman » Wed Sep 18, 2013 1:21 pm

There is no advantage, other than non-center cutting end mills are easier to make and should be (a little) cheaper.
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bobt
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Re: Advantages of non-center-cutting endmill?

Post by bobt » Wed Sep 18, 2013 5:24 pm

The disadvantage is that the end mill cannot be used to pierce the material and requires an existing hole or enters from the edge of the material. A non-center-cutting end mill is not a drill.

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cvoinescu
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Re: Advantages of non-center-cutting endmill?

Post by cvoinescu » Wed Sep 18, 2013 10:53 pm

The Guerrilla Guide to CNC Machining, Mold Making, and Resin Casting, a wonderful resource, has this to say about non-center-cutting endmills: avoid them if you can.

I am sure that in an industrial setting, it's sometimes worth giving up the ability to use certain toolpaths in order to save money by using non-center-cutting endmills, but they're really not worth bothering with in our case.
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jasonharper
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Re: Advantages of non-center-cutting endmill?

Post by jasonharper » Thu Sep 19, 2013 1:16 am

So how does a center-cutting endmill work, anyway? I mean - what's the mechanism for removing material that's exactly on the axis of the bit during a plunge, where there is theoretically no sideways motion to do cutting? My only guess is that they rely on there being at least a tiny bit of runout in the spindle, so that there isn't actually a point with no sideways motion.
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cvoinescu
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Re: Advantages of non-center-cutting endmill?

Post by cvoinescu » Thu Sep 19, 2013 6:38 pm

They rely on the fact that the infinitesimally small amount of material exactly dead center can be squished away, or gets ripped along with the nearby material. After all, how does a drill bit do it? By the same logic, the point should not be able to advance at all. If they relied on runout, there would still be a spot with no movement at all, just not in the center.
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