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« on: August 30, 2009, 09:48:24 PM »
BALL SCREWS
I already had ball screws. Both the ball screws are easily adjustable for backlash. The intent is to minimize the backlash ( you never have zero but you can try ). You can do all kinds of calculations for the ball screws, but practically speaking, experience on sizing and mounting from past experience is all that is needed for this conversion.
The Z axis will use a 16mm diameter x 5MM ball screw. The X axis will use a 12mm diameter x 4mmm screw. You are somewhat limited on a conversion as to what will fit and yet the basic mechanical integrity for supporting them is very important.
One important criteria will be resolution ( just how small of an axis move can be made).
Steppers will be used since the existing controller is for steppers, is portable, and is used for other CNC machines. Here are a steps per unit calcs and a look at resolution:
Z Axis
5mm x 0.0394 mm/inch = .197” / revolution ( 1/0.197 = 5.0761 PITCH ) along with a 2:1 pulley reduction
STEPPER x MICRO STEPS x SCREW PITCH x PULLEY REDUCTION = STEPS / UNIT
200 Steps / rev X 10 X 5.0761 = 20304 STEPS / UNIT
RESOLUTION: 1/ 20304 = .00005”
Note that the mounting plate will be such that the pulley ratio can be changed as I may want
to increase the Z axis velocity later after testing.
X axis
4mm x 0.0394 mm/inch = .1576” / revolution ( 1/0.1576 = 6.3452 PITCH )
STEPPER x MICRO STEPS x SCREW PITCH x PULLEY REDUCTION = STEPS / UNIT
200 Steps / rev X 10 X 6.3452 = 12690 STEPS / UNIT
RESOLUTION: 1/ 12690 = .00007”
There are tradeoffs to made. I choose to do a 2:1 pulley reduction on the Z because I wanted more torque delivered while cutting threads. The X resolution is fine but doing a reduction makes the mounting somewhat more complex. Another consideration will be how much holding torque will be available. The Sherline never skipped when using 300 oz-in steppers and the steppers were direct mounted. I'll need to take a look at the speeds I will be running when doing heavier work along with threading considerations.
RICH