Plunge vs. X/Y speeds?

[Phil Thien]

My tiny CNC router uses a Ryobi SS50 (like a RotoZip) as the "spindle." I use mostly 1/4" spiral bits.

I've been experimenting with various plunge and feed rates and wondered if you guys found a relationship between the two? Do you, for example, plunge at 1/2 the speed you feed? Or at the same rate?

I hope my question makes sense.

[Gary Campbell]

Phil...
Good question. Harder answer. There are 3 things that will limit the speed of any axis. The electromechanics of the drive, the ability of the bit to plunge into the material and the material itself. The process is complicated further by type of cutting. 2D, 3D, straight plunge, ramp in, etc.

In most cases OEM built CNC's have the same or higher "power" ratings on the Z as the X and Y, however...., the Z has to lift the weight of the cutting head and fight inertia on a fast plunge and retract, so some concessions must be made. With that in mind, screw or R&P ratios are usually geared to less speed (and more power) on the Z. Most OEM's add some sort of assist to try and gain "neutral buoyancy" of the Z. Coil springs, gas springs and regulated air cylinders are used most often. This means, if properly counterbalanced, most commercially produced CNC's will be able to match the Z federate to the X & Y for most normal cutting below250-300 ipm. This does not mean that any bit can plunge straight into any material at that speed, unless its a foam.

Using a slow Z feed on 3D work adds to the time spent tremendously. The control must calc in ramp and acceleration values to not exceed the Z feed, which usually means that the X or Y seldom operate at their set feed. In any case, you should test your machine and find how fast your z will travel and change direction without loosing steps. Then back off of that some and match it to an XY feed that is 125% of that number for 3D cutting. For aggressive 2D cutting your Z might stay at that number and the X & Y be 3-4 times that high. Sorry for the long answer, but in the end you will have to balance what is best for your machine, bit and material. And it may change for the next job!!!