Z drive has no power

[hal]

I am new to this forum and have been using my machine for about six years without problems. Now it seems that my Z drive is "skipping" when plunging at 160 in/min. Parts that it has cut for years now will not cut to full depth due to the stepper motor missing steps. The router bit is right out of the box so that is not it

[richards]

Hal,

You're driving the Z-axis at 2.66 inches per second! I normally plunge at 0.5-ips. The only time that I set the Z-axis to a high speed is when I'm ramping into the cut.

[hal]

Thanks for the reply Mike. I would think that with a brand new half inch plunge bit that 3 inches per second would not be asking too much. I have installed the new Gecko drivers and increased the voltage slightly.

The thing that concerns me is that I used to cut this part with no problem at all, now it skips steps frequently. Indicating to me that the Z drive is loosing power. Do you know of any thing that could cause a loss of power in a stepper motor? The X and Y drives are stronger than ever with the new drivers.

Thanks again.

Hal

[richards]

Hal,
I would guess that the difference between the XY axes and the Z axis is the tool geometry. Most cutters handle side forces well, but very few cutters are designed for rapid plunging. Just yesterday I managed to fill the shop with smoke when I tried to plunge 3/8-inch cutters in MDF ultra-light. I've done that many times with regular MDF at the same speeds and with the same type of cutter, but the ultra-light was different - and the smoke got out.

When I drill, I like to use a drill bit. The V-shape is designed to plow through the material vertically with as little resistance as possible. And, with a spindle, where I can reduce the speed to emulate a drill press, everything works. On the other hand, a cutter, even one that is designed for plunging leaves no area to evacuate the chips. So heat builds up, chip compression occurs and problems escalate until the stepper motor is overwhelmed by the forces and steps are lost.

When I have to drill with a router bit, I peck drill. Each cycle goes no deeper than 1/4-inch and then the router bit is raised above the surface and then plunged again to the next depth. It is slower than using a drill bit, but it works for me.

Now to your actual question about the Z-drive loosing power. If you disconnect the Z-motor from the machine and then run it, if everything sounds good, then the motor/driver are working properly (most likely - a noisey motor/driver would mean that either something is mis-wired or that the stepper or the driver has a problem).

If you have a drill press handy, you might want to perform this experiment. Chuck up the same cutter that you use in the Shopbot and then plunge the cutter into the material using the same speed that you would use with the Shopbot. Note how much force it takes to make that plunge. Now measure the length of the 'handle' on the drill press. A stepper motor is rated in oz*in. My understanding of oz*in is how many ounces of force are required to 'lever' something when the length of the lever is one-inch. So, if your 'handle' is six-inches long and it takes five pounds of force to plunge the cutter into the material at the rated speed, you would be using 5 X 6 X 16 or 480 ounces of force. Add to that the mechanical forces of the Z-axis (spring, friction, etc.), and you'll probably find that the motor can't handle the force involved. In that example, just the plunging force is about 150% greater than the rating of a stepper motor at its slowest speed. The way around the problem is to go slower and to remove the debris by peck drilling if the holes are deep (and to use cutters designed for drilling instead of using cutters designed for routing).

In my case, since I use a spindle, I use Colombo's chart where they specify that a vertical plunge should be 1/10th the speed of a horizontal cut (to prevent damage to the bearings) and a vertical ramp at full speed of the horizontal cut if the ramp angle is no steeper than 20-degrees. Using that formula, I limit the plunge to about 0.50-inches per second, figuring that I will be using an X/Y speed of about 5 to 8 inches per second.

[hal]

Mike,

Thanks again for the information, much more than I expected. I disconnected the Z axis motor, moved it up and down manually and found that the bearing surfaces were very tight due to build up on the ground iron surfaces (where the bearing rides) This was causing a lot of extra resistance. I cleaned everything off and re-adjusted the bearings and now it moves up and down a lot better.

In reading your information it becomes clear to me that a slower plunge is a lot better if for no other reason than to preserve the bearings in the router.

I am using Rhino to design my parts and Mecsoft for the CAM. I think Rhino is wonderful and Mecsoft, in my opinion, leaves a lot to be desired.

I have reduced my plunge speed to .5in/sec and everything cuts beautifully now.

Thanks for sharing your experience and knowledge, I greatly appreciate your help.

Hal