Vertical Gantry Motion Management

[harryn]

Hi - I have started into the design of a Vertical Router / Mill (bores horizontally). The beginnings / plan are in the build log area.

From others on the forums, a common gantry weight seems to be in the 50 Kg / 100 lb range. Originally I thought this was a lot, but after adding it up, this actually seems pretty close.

Since my gantry for this unit rises vertically for the first motion axis, essentially the entire weight of the gantry is lifted by the stepper motor / belt drive on the way "up", but wants to pull the whole thing fast on the way "down".

Lifting 100 Lbs with a stepper motor is not that hard with the right setup, but the great change (gravity with / against the motion) going up /down seems like it could cause problems.

Is the usual method to counter weight large masses that need to go vertical ? Is this really needed, or just a good idea ?

I thought about springs, but since most spring forces are roughly Force = spring constant x distance stretched, this may or may not be a good path either. Maybe it is close enough to counter the majority of the gravity related forces ?

Thanks for any suggestions.

HarryN

[ger21]

Originally Posted by harryn Is the usual method to counter weight large masses that need to go vertical ? Is this really needed, or just a good idea ?

Depends on if the motor has enough power. You can get constant force springs at www.mcmaster.com

[Geof]

Here is an idea that is unconventional but might work if you are using ball screws.

The weight of the gantry translates into a back driving torque on the ball screw. This torque is opposing the stepper motion in one direction and assisting it in the other.

You want to counteract this torque in a manner that is independent of gantry position. As ger21 says you can get constant force springs or you could use an air cylinder with a large ballast tank so that displacing the cylinder does not appreciably change the air pressure, this is a form of constant force spring.

However, I am familiar with VMCs that simply support the weight of the head on the Z axis servo. I have several of different sizes and the stationary servo load varies from around 30% up to 65%. These motors like all servos have separate cooling fans so they do not overheat.

So I think it should be possible to mount a servo to the end of the ball screw and just apply sufficient current in the correct direction to it to counteract the back driving torque.

It is possible that the power supply for the servo would need a bit of sophistication because as soon as the stepper started moving the axis that would change the back emf being generated by the servo; the servo would act like a regenerative brake.

[harryn]

Thanks for the ideas. Probably all would work.

The solution that is closest to my current "plan" is the constant spring force method. I will add that to my ever growing parts list in the build thread. So far, the price to build is rising about $ 1 - 2,000 per week. Hope that stops pretty soon.