One ball screw or two?

[colinkent]

Hi

I am looking to build my first machine and have an idea for a gantery type cnc running on 20mm round shafts. My question if i use a ball screw on one side of the X axis above the round shaft is this going to be ok or do i have to use a ball screws on both sides? I have seen some machines and planes that only have 1 screw on the outside of the X axis, some have 2 screws one either side driven by belts and then there are others that have two screws and two steppers

Which is the best way to go?

Colin

[Zumba]

It depends on your machine design.

If your gantry is a 3-piece design (two verticals + bridge), you'll definitely want two screws, one on each side, to counteract flex. Use one motor for each screw. The single motor/double screw belt drive method IMO is a total waste of effort.

If your gantry is a 4-piece design (two vertcials, upper bridge, and lower bridge beneath the table), then you only need a single screw and motor. A well designed gantry with quality linear rails (THK, Hiwin) should be rigid enough that you don't need two screws to keep it tracking straight.

[CarbideBob]

I would use one but design the machine to add another later. There are all kinds of problems using a dual screw drive. How do you synchronize the 2 screws? No two screws will have exactly the same motion, no 2 pulleys are exactly the same, and no 2 stepper motor have the same step sizes (typical coil placement is +/-10%). When using 2 screws you have to design in some compliance (or slop) to prevent binding. Keep the bearings on the drive side spread as far as possible. This helps prevent twisting (this is known as abbe error).

[ger21]

Originally Posted by CarbideBob There are all kinds of problems using a dual screw drive.

Yet plenty of people are doing it succesfully with no problems.

When using 2 screws you have to design in some compliance (or slop) to prevent binding.

Most people go with 2 screws to get rid of "compliance".

No two screws will have exactly the same motion, no 2 pulleys are exactly the same, and no 2 stepper motor have the same step sizes (typical coil placement is +/-10%).

If we're talking about a homebuilt machine, more than likely the differences you mentioned won't be noticeable.

As for step size error, if you're using 5tpi screws, you'll have 1000 steps per inch. With 20% error, you might have an error of .0002 between the screws.

For a very high precision machine, you have a valid point. But for the typical homebuilt machine, the errors you bring up would be negligible. IMO of course.

[CarbideBob]

I guess my point is it's easier (and cheaper) to get the system up and running with one screw and then you can play around with the second screw later. Dual screw designs can do a great job of removing twist from an axis are almost a necessity on very wide spans. Doing some thinking about bearing spacing and drive nut placement to minimize twisting loads can often eliminate the need for 2 screws.