Cut faster grooves

Large flange undercut

I remember when I was machining; I used to get in an argument with a fellow machinist about metal removal rates on a lathe verses a mill. He would argue that his milling cutter had 10 inserts. I would come back saying that I had 100 horsepower and my insert was always in the material.

If you think about it… there are advantages to running more than a single insert. Applying this concept to a lathe in turning mode can increase productivity especially on grooves.

Lets think about the difficult situation where the underside of a flange has to be cut from solid material. You have only got two choices: You can plunge out the material with the widest grooving tool your machine can handle or use two opposing tools, one for the topside of the groove and then changing to a reverse tool to do the bottom side of the groove. The first option is slow and very noisy because of chatter. Every one in your shop will run for earplugs. The 2^nd option is inferior because it has to take tiny cuts due to how far the tools are sticking out from the tool post  

I’ve tried and tested every possibility known to man to solve this problem and then I read about chip thinning theory and had a engineering brainstorm. Button cutters exhibit chip thinning around .03 depth of cut.  What does this have to do with anything? For the same reason on a mill you can extend a high feed cutter out 7 times it’s diameter and still run 200 inches per minute. The cutting force is transferred axially, through the spindle instead of side-to-side reducing vibration and deflection during the cut.

In reality, we have to stick out our button cutter or prune tool as it is called on a lathe much farther than 7 times it’s cutting diameter We need further stiffening. You can only weld supporting steel to the cutter as wide as the cutter itself otherwise it will obstruct the cut. Since our prune tool holds a 3/8 button this does not allow for much backing or support.

What if we support our tool with another tool? Now we have to tools welded together with a ½” gusset in between supporting each other. A 1-½ inch wide double button cutter composed of two 3/8” button prune tool on each side with a supporting ½” spacer in the middle. Be sure to leave at least a one inch gap before the button inserts, not extending the support in the middle past this point.

There is more than extra support that is gained by welding these tools together. Now every pass that is taken in the Z-axis to remove material gets to travel 1 ½” less than the groove width. When I explain the path this tool travels to cut you will see that this is a big savings in time when converted to inches per minute.  

We all know that high feed mills travel at incredible feed rates. Since this tool follows the same principle it to will travel very fast up and down. So fast in it will scare the most hardy of operators and pile up chips like no other groover.  

Begin feeding this tool in at an angle. Always keeping .03 depth of cut. On a lathe this equates to .06 in diameter. This is accomplished by zigzagging up and down. Always moving in. A problem arises when you get to more than a ¼ depth of the button inserts. The tool no longer is chip thinning and begins to cut a very wise chip. At this point you will have to slow down the Z-axis feed .05 before it reaches the corner and then kick it back up when you change directions.  

G01 Z-11.0 X49.0 F.11

G01 Z-11.05 F.020

G01 Z-5.05 X48.94 F.11

G01 Z-5.0 F.02

…  

Yes, that is inches per revolution. Rock and roll baby. Let the chips fly. 

Large machined groove