Feed speed clarification please...

BigJohnT wrote:

I wonder if you can use a VFD on a router to slow it down?

Chipload is simply defined as the thickness of a chip which is formed during the machining of a material. Chipload is critical because if the chip is the proper size, the chip will carry away the heat promoting long tool life. If the chip is too small, the heat is transferred to the cutting tool causing prematurely dulling. Too high of a chipload will cause an unsatisfactory edge finish, or part movement.

I think most people would be surprised to learn that you can actually cut at much lower speeds than are typically used. With the 7518 in my router table, I typically keep it at 16,000 rpm for work with 1/2” bits, and even slower with larger bits. With smaller bits, you may need to spin a bit faster.

I’ve been running big CNC routers for over 10 years, and rpm and feedrate are calculated using a recommended chipload, which is the amount of material (size of chip) each tooth or flute removes. Unfortunately, most carbide tipped bit manufacturers won’t give you a chipload, but instead a max rpm, which is more for safety than cut quality.

The biggest enemy with carbide router bits is heat. Heat is what causes carbide tools to dull. The cooler you keep them while cutting, the longer they will last. You keep tools cool by increasing the chip load; taking the biggest bite per tooth that you can. There are two ways to do this. Increase the rate the your feeding material into the bit (or pushing the router through material), or lowering the rpm.

There is a trade off, though. As chip load increases, cut quality can suffer. If that happens, then increase the rpm until you get an acceptable quality of cut. If you use any higher rpm’s than that, you’re just decreasing the tool life.

As for burn marks, that’s typically a sign of a dull bit, or to many rpm’s. When the bit is spinning too fast, it takes very small cuts, and does more “rubbing” the wood than cutting it. This creates tremendous friction, which quickly caused dulling. Which leads to a vicious cycle of more burning. If you’re getting burning, try lowering the rpm, or pushing the bit through faster. If it still burns, you may need a sharper bit. Some woods, like maple, and especially cherry, are very prone to burning. Be sure to use very sharp bits with these woods for best results.

Bottom line, if you have a variable speed router, try using lower rpm’s. You may be surprised that your cuts are the same quality, your bits may last longer, and you also get the added benefit of much less noise.