[mikefalkner]

Is there a system/product/method to lock a z-axis in place when power is turned off?

I am building a special purpose 10-foot by 6-foot CNC table that uses rack & pinion on the z-axis (probably should have used a screw method for z-axis, but already built for now). Just connected the wires to my z-axis stepper motor, and the it works fine. It raises and lowers the heavy z-axis perfectly. But when I turn the power off (shutting down for the day, or if electric power is lost), the z-axis falls down hard (to be expected since it is heavy). I didn't think about this when designing the table (oh well ... that's learning the hard way). Now I need to research how to fabricate something to hold the z-axis in place with power is off. Has anyone experience this problem? How did you solve it? Or do you have any suggestions?

Thanks in advance for your help.
Mike Falkner

[Al_The_Man]

There are simple DC electric brakes by the likes of Electroid etc, Another way is a DC solenoid or air operated pin, that enters one of the rack teeth when power is off.
Al.

[Trotline]

Something equivalent to a hook & eye style latch would work, if you don't want to spend much. Or add a couple of bolts that protrude in a convenient place and twist loops on the end of a piece of wire shirt hanger. Or just make a padded prop that sits under your Home location. Just some low-rent ideas..

Luke

[DonFrambach]

The Z axis of my small mill falls when I power down. I don't do anything fancy. I just have a small board which I orient vertically and place below the spindle mount. I then lower the z axis until it is about 1/4" over the board. When I power down the z axis is supported by the board at the motor mount. KISS

[beluga282]

I had exactly the same problem with my design, also using rack and pinions.

It's very easy to explain my set-up in french ... But I'll try ...

The way to solve it is to decrease the "downward force" ... On my machine, I use two springs to retain the mechanism from falling.

With this set-up, the motor use approximately the same power to go downward or upward.

Hope that help !!!

Bye !!

[Dylwad]

I had exactly the same problem with my design, also using rack and pinions.

It's very easy to explain my set-up in french ... But I'll try ...

The way to solve it is to decrease the "downward force" ... On my machine, I use two springs to retain the mechanism from falling.

With this set-up, the motor use approximately the same power to go downward or upward.

Hope that help !!!

Bye !!

Counter weight would be the term not a bad idea, cheaper and easier than a brake.

Sent from tapatalk

[beluga282]

Counter weight would be the term

Thanks for the term !!!

[JerryBurks]

Gas spring? They have better constant force than coil springs and don't add even more mass, strings and pulleys like a counterweight.

Pretty cheap from McMaster with all possible forces and dimensions.

[tahustvedt]

I'm installing a gas cylinder with an expansion tank on my new Z. The tank is a large soda bottle, hidden inside the gantry, which keeps the force nearly constant since the gas won't compress much as the cylinder is compressed. It'll relieve the stepper when raising the Z as well, and the force can be adjuster by varying the pressure. You can find lots gas cylinders of various sizes on ebay if you can't source them locally. I have used this system before in a simulation rig.

This is my cylinder: http://www.ebay.com/itm/321027024266...84.m1439.l2649

Fittings: http://www.ebay.com/itm/140723150877...84.m1439.l2649

[mikefalkner]

When you use gas cylinders to hold the z-drive, does it truly hold in place the z-drive when power is turned off, or does the gas cylinders just slow down the eventual downward decline?

[tahustvedt]

With a pneumatic actuator, like I'm using, it depends on the pressure. You can set the pressure so that the force matches the weight of the Z exactly, for example. Pneumatic actuators are just cylinders with a piston, and no set pressure or dampening like a gas spring.

[Al_The_Man]

Usually when using Pneumatics a balance regulator is required to balance pressure on the compression stroke.
This is a common method of counter balance for the Knee mills.
.
Al.

[kevincnc]

Do you have a rear shaft on the stepper? If so,

http://www.cnczone.com/forums/cnc_wo...is_brakes.html

[tahustvedt]

Like I said, I'm using an air tank (1,5 l soda bottle) to increase the system volume so that the air compression as the slim pneumatic actuator collapses is negligible. Much simpler than a regulator.

[mikefalkner]

I never posted anything to CNCzone before, but this posting really provided lots of GREAT ideas to solution my original problem (How to lock z-axis when power is turned off). So here is my plans on implement some of these ideas

SIMPLE SOLUTIONS (cheapest cost)
I like the KISS one about just using a padded block under the z-axis when power is turned off. Just lower the z-axis to just about the block, and then turn the power off. Another simple idea was to just fabricate a simple hook to latch the z-axis in place before turning power off. I am using this method right now until I receive the items purchased on eBay today (see next paragraph).

FOOL-PROOF SOLUTION (for old people like me)
However, I wanted something more fool-proof since I am inclined at times in my older age to forget to do things (probably wouldn't forget if I had to do major repairs for failing to put the block under the z-drive, ha). So, I took "Al-the-Man" and "kevincnc" suggestions and researched electroid. I found a Fail-Safe electroid on eBay that locks the shaft in place when power is lost. I purchased an electroid that was listed on eBay (Electroid 30 BFSB 35 12 28V C Electronic Fail Safe Brake NEW). According to the manufacturer's catalog, it will hold up to 35 in-lbs static torque (or 560 oz-in), which is close to the rating of the stepper motor I am using on the z-drive. Since my stepper motor has dual axes (one in front and one in back), I will simply fabricate a mounting bracket to hold the electroid in place. Had to purchase a 1/2" to 3/4" sleeve shaft adapter on eBay (1/2" X 3/4" X 1-1/4" Motor Engine Shaft Adapter Sleeve Pulley Bore Reducer Bushing) since stepper motor shaft is only 1/2" and electroid clamp is 3/4". I already have a power supply to provide the 28vDC to the electroid, which will be connected to the same 120vAC supply as the power supply feeding my stepper motor drivers. So ... this is my solution – thanks to the wonderful suggestions many has made.

WONDERFUL "WEIGHT ASSISTANCE" IDEA
I also like the "tahustvedt" suggestion about pneumatic actuators to reduce the weight load on the stepper motor. He has a good idea, so I plan to play with that as well. Probably will not need it since my stepper motor is carrying the load, and I am implementing the failsafe electroid when power is turned off, but I just can't help thinking that a lighter z-axis load to the stepper motor wouldn't help.

Again, thanks for all the help and comments. It is nice to know others are out there to help you if you need it.

Mike Falkner

[Al_The_Man]

Don't forget to put a reverse EMF diode across the coil, they have quite the kick when they turn off.
Al.

[mikefalkner]

Al,
Thanks for the heads up regarding a reverse EMF diode across the coil to drain the voltage built up when power is turned off. I sure would had NOT enjoyed the kick!!! I will have to go the a local electronic supply house and get one for the electroid I purchased. Catalog said 28vDC, 0.29 amps, so should be no problem getting that small diode.
Mike Falkner

[Al_The_Man]

Any of the 1n4xxx series, should do I keep 1n4007 on hand as they cover most applications.
It is usually the rest of your electronics that does not like it.
Al.

[hamthis]

I think the counter weight idea is best. using pulley to pulley for the transfer. It would make the motor work easier too.

[kevincnc]

On a knee mill it makes sense. On a machine where it is on a z-axis that is whipping around in all directions it adds unnecessary mass to the system, and has to be precisely constrained in x and y. It aslo addes intertia to the z, and has a chance of bouncing during rapid accel/decels. If done properly it works, but IMO the brake is a better solution, especially on a higher performance machine.