Re: Z-Axis problem - Not Enough Torque?

In your shoes I would try the following in this order.

1. Counterweight the head with a weight, steel cable and a couple of
pulleys. Make the counterweight greater in weight than the head.
The exact amount more than the weight of the head you can find by
experimentation, but at some point it will take up the backlash on the
leadscrew, killing two birds with one stone...

2. Toothed belt drive reduction on the 'Z'.. most of the time you
will hardly notice the reduction in speed..

I can't stress enough that the counterweight actually works... and it
is also probably the most effective, easiest and cheapest option.

Chris

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "WT" <pharmapack@...> wrote:
>
>
>
> Hi, all.
>
> I think I have a problem with my Z-axis not having enough torque and
will
> appreciate some suggestions and recommendations on what are the possible
> solutions.
>
> Following are the specs of my CNC retrofitted milling machine:
>
> 1) ZAY7032 Mill/Drill Machine, 2 HP spindle motor
>
> 2) X & Y-axis:
> converted to ballscrews: 20 mm diameter x 20 mm lead pitch (4 start
x 5 mm
> lead pitch),
> direct coupled to a stepper motor (Nema34, 4.2 N.m, 6.0A, 2.4Vdc),
> stepper driver capable up to 7.8A, 80VDC, microstepping (2, 4, 8,
16, 32,
> 64, 128, 5, 10, 25, 50, 125) and presently set at 5.7A, 25
microstepping, &
> 37VDC
> Mach3 - 250 steps/mm, 2000 mm/min
>
> 3) Z-axis:
> converted to ballscrew: 15 mm diameter x 40 mm lead pitch (3 start x
13.3333
> lead pitch),
> belt driven (same ratio/no reduction) by a stepper motor (Nema34,
4.2 N.m,
> 6.0A, 2.4Vdc),
> stepper driver (same as above) but set at 50 microstepping
> Mach3 - 250 steps/mm, 1000 mm/min
>
> 4) Power supply:
> Has multi-tap output capable of 37, 47, 52, 59, 65 VDC,
> currently using 37 Vdc,
> about 1 KVA
>
> Presently, the X & Y-axis seem to be working very well with no
backlash and
> lost step. However, the Z-axis seems incapable of moving up
correctly under
> load or when cutting or milling. Also, noticeable chatter or
vibration of
> the spindle head when milling MS plate at only 0.2 mm downfeed and 100
> mm/min feedrate.
>
> I know my ballscrews are very coarse pitch but I have to live with it as
> that is what is presently available to me. And hence, the reason I
set up
> the microsteppings to 25 (X & Y-axis) and 50 (Z-axis) to gain more
> resolution. Considering that the X and Y-axis are capable of moving the
> heavy X/Y table, is there any reason why Z-axis can not hold its
position
> under load?
>
> So, are there any solutions or suggestions to solve the problem
within the
> present hardware parameters? If not, what hardware changes should I
make?
>
> TIA for all your kind response.
>
> Best,
> WT
>