Would you use a z axis with lead screw if the rest of your rig use ballscrews?

[HereinCS]

I've got x and y axis on nice ball screws. My z axis is really heavy (~25 lbs with N23 motor, not including the router) and has too much travel. I'm thinking of getting a smaller set up from k2. These use lead screws. Is this a bad idea? Will it compromise my system?

[RGeo]

Since you said router, I will assume you are cutting wood. The only problem using a lead screw is tool pressure. When the z-axis tool pressure spikes higher than the weight of your z-axis gravitational weight you will lose cutting depth. It might seem like 25# is a lot of weight, but depending on the speed of your cut, you can easily spike that amount of force. I would stick with ball-screws if I were you. This would also allow you to counterbalance the z-axis, which would be a good idea considering that your motor is fairly small. Remember, it is not only the weight you are fighting, it’s the inertia, i.e. the mass, gravity and velocity.

[Bubba]

Another thing to think about is an Acme thread is about 30% efficient as opposed to a ball screw at around 90%. That is to say that if the pitch were equal, it would take about 3x the power to run the acme screw.
On the other hand as this is the Z axis, the acme screw will hold its position when the power is off and the ball screw head will likely drop unless you have a brake on it.!

Yes, you can get acme in zero backlash as well as ball screws so that is a wash!

[RGeo]

Yes you can go with a so-called zero backlash acme, but then you are less efficient than a regular acme, and the torque is going to vary greatly depending on where you are on the length. This is true because when you load the nut, you have then taken away all of the designed clearance allows the pieces to operate together in a smooth manner. You actually begin to fight minute variations in pitch and geometrical deviation. In other words, if you adjust it to anything better than .005” backlash, you are going to experience variable torque. It will eventually smooth out, but not until it creates more backlash. What you end up with is some quickly worn out parts. I would say that .008” backlash is the best you will run at consistently, without too much advanced wear. If this is acceptable, then Bubba has you solution.
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Either way, Bubba is right about the brake. However, with a well counterbalanced system, and a permanent magnet motor, you can use a relay to short the motor windings when the motor power is off and provide some low torque breaking action. If you want to see if this will work for you, take your unconnected motor and turn the shaft, then short some of the motor leads together and try to turn it again, if it’s hard to turn, you have a permanent magnet motor and you can do what I said. You just have wire it in such a way that it disconnects the motor from the drive prior to it shorting the motor leads. A simple relay should do the trick.

[ger21]

Plastic nuts on multiple start acme can be up to 75% efficient.

I don't know what kind of nuts K2 uses, but plenty of people use nuts from www.dumpstercnc.com with excellent results, and probably much better than .008 backlash.

I'm sure with a heavy load that I have some backlash, but unloaded, I have less than .002 backlash on my Z axis with acme screw.

[RGeo]

I just checked out the site and saved it in my favorites. Heck, I'm willing to accept that my info is outdated.

[HereinCS]

Thanks for the feed back. I'll stick with my ballscrew z axis then. Good thought about the weight counter balancing the cut force. My set up does come with brakes on the bottom. Not sure how to work the break though. Is it just some low voltage on/off mechanism?

[RGeo]

It could be anything 24 48 or even 90VDC. It's motor specific. What is the make and model of your motor?

[HereinCS]

I have a Thompson set up. The brake is a Dynacorp 7099b-26005. It does say 24v so I'll give it a try. Is that round thing on top of the motor some sort of indexing device? Any use for it or should I just take it off?

[RGeo]

That round thing on the motor looks like an encoder to me. An encoder is a device that tells your control exactly where the motor is rotationally and how fast it’s turning; some of them even keep count of how many revolutions have been made. This leads me to believe that your motor is a servo motor rather than just a stepper motor. Stepper motors are often driven by a certain number of power pulses per revolution, however they are seldom set up to report back where they actually are like a full servo would (using an encoder).
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Do you already have the motor control, i.e. apm/driver? It's not that you cannot use an encoder with a stepper motor, it’s just that if someone bothered to put a stepper motor with an encoder on a drive assembly, they likely intended to drive it closed loop, like a full servo, rather than open loop like a stepper. It also sounds like you said the brake is separate from the motor? What motor are you using.

[johnmac]

If you are worried about how much weight the Z will have to lift, why not use a counter spring (or 2) to offset some of the weight? I have a ball screw on my Z and use a spring to reduce the amount of power needed to lift the router motor.

John

[HereinCS]

Originally Posted by johnmac If you are worried about how much weight the Z will have to lift, why not use a counter spring (or 2) to offset some of the weight? I have a ball screw on my Z and use a spring to reduce the amount of power needed to lift the router motor. John

I'm not so worried about the lift of the z axis. I was more worried that the heavy weight from the entire z axis ( maybe 30lbs with the router), will introduce a bit of flex on the y axis beam.

As far as the motor type on the picture, that's a nema23 stepper motor. I'm not sure if I've seen a stepper motor, encoder set up before. Does it look like it'll make a decent z axis?