I would like to use Ball Screws for my Z-Axis, mainly because of less backlash. However, unlike Acme Screws the Ball Screws will evidently let force on the nut turn the screw! (Backdriving).

Question:
So, if you mount a Ball Screw on the Z-Axis won't the weight of the router and other parts cause a continuous motion downwards because of gravity and try to turn the screw? Wouldn't your motor always be having to apply power upwards to counteract that?



From Nook Industries:
-----------------------------------------
BACKDRIVING
Normally, ball screws are used to
convert rotary motion into linear
motion. Backdriving is the result of
the load pushing axially on the
screw or nut to create rotary motion.
All ball screws, due to their high
efficiency, will backdrive. The
resulting torque is known as
“backdriving torque” and is the
torque required to hold a load
in position.
CAUTION - When using ball screws,
applications should be analyzed to
determine the necessity of a brake,
especially when the possibility of
injury may occur.

Ahhh - thats where the "holding force" on your stepper motor comes in! At least thats what I figure - thought I'd have that problem too and was going to add a pin to "set" the router in place - but haven't done that yet.
:cheers: Jim

The motor will keep the screw from backdriving. Yes there will be power at the motor at all times.

Eric

Your stepper needs to be powerfull enough to lift the Z axis, so it will certainly be powerfull enough to hold it under the force of backdrive. The way to use smaller steppers is to counterbalance or use a spring, not by adding static friction as in acme thread.

greetings,
John

Your stepper needs to be powerfull enough to lift the Z axis, so it will certainly be powerfull enough to hold it under the force of backdrive. The way to use smaller steppers is to counterbalance or use a spring, not by adding static friction (as in acme thread).

greetings,
John

Something went wrong, accidentely posted twice but it doesn't let me delete the second one.
Admin, help??

I am not shure, but may be he is asking when the power is off. That was my first impression, I am curious too.

The "ballscrew lover's" solution is to use a motor with a brake. That way when the power is off, the brake is engaged. If you'll pay attention to a lot of commercial designs, they will use a precision acme screw on the Z to keep it from free wheeling downward when not powered. If they use a Ball screw, the motor will have a brake.

Ah! Now I understand. Didn't think about power off. In my Zaxis theres a spring that is stronger than the weight of Z. Would be a funny sight having the Zaxis fall upwards.:D

John

My Z axis on my router is powered with a .2 pitch ball screw and a servo is running it. I have the ratio of 2:1 and with the 3-1/4 HP Porter Cable router I just have enough mechanical advantage to hold the weight when the power is off of the servo motor.

Thank God that I cannot afford a ballscrew, it would be an additional problem with design and I already have a bagful.

Hi,

One of the wonderful features of a vertical Z axis is that the weight of the spindle eliminates the backlash in the screw. If the detent torque of the motor isn't enough to prevent backdriving, I would use just enough spring to hold the axis in place, but I wouldn't balance it perfectly as that would introduce backlash back into the system.

Steve

Gas springs (like auto hatchback springs) come in a huge variety of sizes and forces and make a nice, neat countering force for a vertical axis. Even if the Z axis is not backdriving power-off, a countering force will make the Z quicker with less wear/tear on the motor and screw, and it will allow you to use a somewhat smaller Z motor as well.

But a brake system is excellent too, probably the best overall answer.:)

Anyone have some pictures of a brake for the Z-axis. Or does that have to be built into the motor? I haven't been able to find a picture, much less where to buy one. I probably don't need it but I might as well learn about them.

Thanks for any info...............Sam


Here is a picture of my new baby. Just got it on Ebay (.80" width, 9" travel, 5 TPI.)
Ain't she a beauty! If I had cigars I'd pass them out! :D
http://home.comcast.net/~kr4495/cnc/thk-kx.jpg

Samualt,

You are using servos, aren't you? I would not worry about it. The servo can easily hold it tight, without even getting warmed up.

Little do some users know, but when climb milling, the cutter is attempting to "backdrive" the ballscrew all the time. The servo acts more like a "metering device" because the cutter is actually driving the work ( in heavier cuts). In a carpenter's terms, climb milling is most definitely like "ripping from the wrong side" of your tablesaw.

Hi Samualt,

A little late but here is a screen shot of the design I'm going with. I am running the Z nut @ 1:1 with the stepper and so I won't get any help holding the assembly in place when the power is off. I plan to counter some of the weight but I don't want to take any chances. My boss' Multicam has a brake. It has slipped 2 times now and his Italian spindle did things he didn't like. Anyway, I'm going to use a cheap 110 volt solenoid and when the router comes on the solenoid will retract the spring loaded brake shoe. I found a solenoid in the C and H catalog that says continuous duty for around $6.00. Half amp draw shouldn't give me any trouble. Holler if you have any ideas or questions.

-Doug