Left / right rotation bits

While browsing for new bits on Fraiser, I noticed that they offer some of their bits in left and right rotation, for example these upcut bits:

I’m very much a newbie on CNC so sorry if this is an obvious question to old-timers but what is the difference between these two bits? When should one I choose one over the other?


PS: these are 8mm bits, I’m in Europe so I use metric bits

Some woodworking machines can (or must) turn cutting bits in the opposite rotation, requiring left hand cut bits.

I have a horizontal mortiser like this. Multi-head industrial machines sometimes use both types of bits.

Your typical CNC router or spindle isn’t an application that can use LH cut bits.


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realizing this is not what you were asking but thought it might help with some explanation about directional bits.

It’s not upcut/downcut. Although I took the example of their upcut bits, they have the same right/left rotation option for some of their downcut bits.

The explanation from @YXYJack appears to be spot on. Thanks!

If I had to guess I would say you are looking at a generic picture of the bit, not the actual bit. That gives the impression that the flutes are spiraled in the same direction. I would suspect that anyone with a VFD controlled spindle could reverse direction if needed. I could see where it would be handy to prevent splintering of boards based on the grain of the wood. Don’t think of a hobby machine but a much larger one that can automatically change bits. The CNC could do one side of a board rotating in one direction, change bits, and then do the opposite side spinning the opposite direction.

VFD with spindle can switch and go opposite direction for those bits. Don’t try it without a spindle as you’ll burn them up and not really cut with the makita or regular router. I’d leave them alone until/if you get a spindle.