Why is Z-speed set differently than X/Y-speed?

[jdervin]

I'm not sure why the Z-speed is always set slower than the X/Y-speed. I can imagine that it has something to do with stress on the bit when plunging vs. lateral cutting, but I can't think why it would matter when moving at the jog speed. All of my motors (X1, X2, Y, and Z) are identical, so I don't think that has anything to do with it. Do you know why Z-speeds are set differently than X/Y speeds?

I'd sure love to understand this better.

[richards]

Those of us who have spindles use a lower Z-speed when plunging to save the bearings on the spindle. My Colombo spindle has bearings designed for sideways pressure on the cutter. The chart on the PDS-Colombo website shows that you should use 10% of the lateral speed when doing a direct plunge (60 degrees to 90 degrees). When ramping into the cut, you can go progressively faster, until at 20-degrees or less, you can use the full lateral speed for plunging.

[9r2a5m70]

I'm not sure the entire reasoning behind it. But I do 3D (2 1/2D really) on the shopbot with modeled parts from Rhino. In Rhino you can adjust every little thing you want, I've tried to move the Z-speed to the same as the X, Y. It just ends up slowing the machine down in cut time. I've ran a few tests and it took about 20% longer if the Z speed is at the same as the X, Y. Also if you have the Z-speed to slow it does the same (although i'm sure this one is more obvious). Plus I would get a jumping Z which ultimately transferred to the finished part.

[knight_toolworks]

about the max on the z is 6ips. the z is what slows down 3d as it can't keep up.

[curtiss]

I believe the z pinon gear is different from x & y which has some effect on the plunge speed.

I like the "spiral" / ramping cut for most of my final profile cut outs. The z is always slowly going down with no sudden drops in the final profile.

[knight_toolworks]

my prtalpha has the same pinions as all the other axises.

[gc3]

http://www.shopbottools.com/BradysTricks.htm


this might help...

[thewoodcrafter]

Why would you need the Z to move fast (10 - 12 IPS) when it is only moving at most 2"?
Would you even know the difference from 6 - 12IPS?
When I cut 2D stuff my safe Z is only .25 above the material anyway.

[knight_toolworks]

it would only really benefit on 3d work. as the z is what slows it down for the most part.

[erik_f]

Like others have said...it has to do with ramping speeds...at least that is my understanding. If you are trying to move the z at 6 inches it may actually be slower due to a longer ramp up than a 3 inch speed move because of its shorter ramping needs. Also as you said in the first post...to protect the bit. I have a standard machine and more than a couple of times have lost Z position due to being overly aggressive...I think the only time I've lost X and Y position was due to poor communication between the computer and controller. High Z move speeds might work well at the start of a file, but at the bit dulls or gets any sort of build up on it you may have problems. I generally am very conservative with Z speeds.