G0704 'Z' Axis Stepper Brake-With Ballscrews!

[atomarc]

A common problem for many...when powered down, the Z axis ball screw auto rotates and lowers the spindle. I have found several variations of piggyback brakes for my NEMA 34 stepper that could solve the problem. My son showed me the method of shorting two poles together to lock the stepper rotor.

What do you folks use/do to solve this problem. I won't do bags of sand with pulleys and ropes for a counterbalance...too funky! The brake that goes on the driven shaft side of the stepper looks the nicest but it's $275. As luck would have it...my brand new Z axis stepper is only a single shaft unit. I could attempt to return it for a double shaft motor so I could use a back end magnetic brake if I had to.

What's the best way to go on this?

Thanks,

Stuart & Nick

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[Fastest1]

I havent had the issue myself. If it was a concern I would go the strut route. Are you running linear rails on your Z?
Having said that I like the sprocket, chain and weight systems too.

[atomarc]

Well heck...maybe the fact that I do have linear rails on the column exacerbates the back drive situation..didn't think of that! The strut thing isn't something I would consider. This has to be something braking the motor or screw proper. You can stop the auto rotation with very slight pressure so It certainly won't take much of a brake. I assume you're kidding about the sprocket-chain weight thingy!!

Stuart

[lcvette]

The linear's will certainly make things worse. That being said, struts are a really good idea to help take the load off the screw for normal movement as well. Should help give you high z speed as well to work in conjunction with the rail upgrade. Also will cut down on back feed through the stepper motor during direction changes and deceleration.

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[SCzEngrgGroup]

A gas spring would be a much better solution, as it will make the motors job easier, and will still work when power is off. And, they're dirt cheap - about $20-30. You can easily measure how much force is needed using a simple fish scale.

Regards,
Ray L.

[Jim Dawson]

Maybe an air spring? My machine is totally different than yours, but I use an air spring for Z backlash comp. I have seen routers with a brake on the Z leadscrew, nice and compact, but rather expensive.

[atomarc]

Although I failed to weigh all the factory stuff I removed from the head and compare it to the new stuff I've added, I don't think there is much weight difference. I'm using a NEMA 34 906 oz motor with a Roton (?) ballscrew and the whole thing moves so easily I can't imagine it's going to need any type of counterbalance, but I don't know. I don't mind the idea of a brake, but the idea of a gas strut tagged onto the column seems sort of hokey.

Has anyone every rigged up a relay circuit kind of a deal to electrically tie stepper motor leads together to help lock the rotor in 'no power' condition?

Stuart

[Jim Dawson]

Although I failed to weigh all the factory stuff I removed from the head and compare it to the new stuff I've added, I don't think there is much weight difference. I'm using a NEMA 34 906 oz motor with a Roton (?) ballscrew and the whole thing moves so easily I can't imagine it's going to need any type of counterbalance, but I don't know. I don't mind the idea of a brake, but the idea of a gas strut tagged onto the column seems sort of hokey.

Has anyone every rigged up a relay circuit kind of a deal to electrically tie stepper motor leads together to help lock the rotor in 'no power' condition?

Stuart

You could use a 4PDT relay and short both windings. BUT....you would need to energize the relay before powering up the stepper power. Not doing so would risk connecting the motor to the drive after the drive was powered up and that could fry the drive.

[lcvette]

Although I failed to weigh all the factory stuff I removed from the head and compare it to the new stuff I've added, I don't think there is much weight difference. I'm using a NEMA 34 906 oz motor with a Roton (?) ballscrew and the whole thing moves so easily I can't imagine it's going to need any type of counterbalance, but I don't know. I don't mind the idea of a brake, but the idea of a gas strut tagged onto the column seems sort of hokey.

Has anyone every rigged up a relay circuit kind of a deal to electrically tie stepper motor leads together to help lock the rotor in 'no power' condition?

Stuart

Think you're missing the point of the strut or counter balance. The easier everything moves, the less resistance there is to the head loading on the screw and little bearing supporting it. You are basically transmitting every bit of that load to the little bearing support at the top of the column. Because the head moves, that load is not static. In a peck drilling cycle, the head is rapidly reversing direction which puts a much higher inertial load on that bearing than the combined static measured weight. I'm not sure why you would consider a counter balance hokey as they are used on about every high end machining center. For smaller machines a gas strut is the most cost effective and last intrusive method and works excellent. It will also increase the life of the ballscrew and support bearing assembly. The benefits of a strut or pair of struts far outweighs a brake which is only there to handle the head from drifting during power down. It will also help to damper any backlash in the screw to help with surface finish. Obviously it's your machine but I personally would think it an excellent addition to an already great build.



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[atomarc]

Wow...lots of good information. I'll do some more homework on this for sure..thank you all.

Stuart

[DubMFG]

I was fairly sure that Hoss has a little blurb about just switching the polarity on his motors to prevent this from happening.

[lcvette]

I was fairly sure that Hoss has a little blurb about just switching the polarity on his motors to prevent this from happening.

I remember that to, think he added a switch on his control panel and it sparked some debate that got intense. I'm still a vote for the strut, lots of benefits and the cost is minor..lol

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[DubMFG]

I remember that to, think he added a switch on his control panel and it sparked some debate that got intense. I'm still a vote for the strut, lots of benefits and the cost is minor..lol

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Yep, I had a 66lb strut made for me by a local manufacturer, it works great. I'd suggest it as well.

[metalfixer]

<<<< I could attempt to return it for a double shaft motor so I could use a back end magnetic brake if I had to >>>>

I use the gas spring on my mill, they are replaceable, and are offered in different weights, but retain the same lengths.
Brakes are expensive, and brakes are available that mount directly to your motor between the coupling, no need for a
double shaft motor.
Larry

[atomarc]

Sheeesh! Certainly looks like the majority out there think a strut system is the best way to go. I'll start drawing cartoons on a napkin and see what develops.

Thanks,

Stuart & Nick

[atomarc]

There has been some great information shared in this thread so after chewing it over and playing with some struts I already had I decided to go this route. Thank you all for you excellent insight and input..you guys have the practical experience that I don't have.

Stuart