z axis drops

[petlaw63]

hello I am new to all this.
I am building a cnc router from scratch and it all working with limit switchs but my problem is that when my z axis goes to the top and triggers my limit switch the z axis drop to the bottom and don't know why i and useing a chain drive with the motor at the top i hope some can help me.

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[awerby]

It sounds like your limit switches are turning off the power when they are activated. Stepper motors hold their position when they're powered up, but when they lose power they will let the axis loose. If it's a Z axis with a heavy spindle, that will backdrive the screw and drop it to the table. If this is a stepper-powered machine, I'd suggest disconnecting the limit switches until you've got everything else working correctly. Unlike servos, steppers won't burn up from pushing against a stop. You don't really need limit switches at all on a machine like that, as long as it has a stop to keep the axis from overshooting and falling apart.

[petlaw63]

Thanks for you help . I turned off the limit switch for a and am just ueing it for homing now.
The z axis still drop when the power off but now i know why and might put a spring to take some of the weight.
Thanks again

[awerby]

Regular springs don't work that well for this. Even "constant force" springs tend to wear out quickly in this application. I've had best luck with gas springs, like the kind that hold up your hatch-back. Other people have used counterweights and pulleys.