Belt vrs. Ball Screw vrs. Lead Screw

[stevesplanes]

Hi

I have a small wood mill that I am trying to expand in one direction. The current ball screws have a pitch of 0.2 (5 tpi). They are driven by Nema 23's with 250 oz-inch holding torque. I have no success finding a 6 foot 5 tpi (0.2 pitch) ball screw and nut with at least 5 foot of travel. I am asuming that I need a 1 inch diameter ball screw for regidity.

Failing finding the ball screw, can I use a lead screw, or perhaps a belt? Mabee a gear and rack drive. What are the benifiets and drawbacks of each?

Steve (newbee)

[ViperTX]

Try Nook Industries....also McMaster Carr.

Ballscrews are more efficient then an ACME Leadscrew and more expensive for similar accuracies.

[wisp]

Forget using a belt. I had a kevlar zero-backlash belt on the X axis of my router. It was made by THK. The belt would still stretch about .02-.03 inches. Plus i had to use a planetary gearhead to get the torque needed to drive the belt with any force. I switched to a Kuroda ballscrew.

[NC Cams]

The key word to the requested comparison is RIGIDITY and its effect on preciseness of desired motion.

The least rigid alternative is the belt (low to mild friction depending on belt tightness needed to transmit desired power).

More rigid is an unpreloaded acme (a bit more friction depending on belt load/tightness)

Next Most rigid is a zero clearance acme (unfortunately , higher friction due to sliding contact)

Probably most rigid is a heavily preloaded ball screw (more efficient/less frictional torque than acme due to rolling action of balls)

Friction creates wear, sliding friction creates more wear potential than rolling friction.

Bottom line: if you do a load versus deflection/response curve, it becomes fairly obvious that a preloaded precision ball screw is the way to go for the highest accuracy/lowest friction/most rigid means of transmitting motion.

This is why most machining centers use them over any of the options (IE: belt, acme, etc). For a particular application however, belts, acmes or possibley even ropes, chains and and/or pulleys may be quite adequate.

[stevesplanes]

Thanks

Still looking for the ball screw. I have another reason for asking about belts and gear racks though. Although I am currently using a screw for my x-axis, it is not a full cantry. the screw goes through the center of an angle iron, which supports the y and z axis. The z-axis curently is below the x-y plane and plunges downword torard the work being carved. This arrangement gives me only 6 inches in the Z-axis and I need more. If I want more, I must break the x-y plane ad thus drive from both sides. I bought two racks and pinions from Tourch mate, but I do not know weather I should use them.

So If anyone has an opinion on how to expand my Z axis to about 24" and still be compatible with expanding the machine below, I'd love to hear it.

[phantomcow2]

HomeshopCNC should be able to sell you a 5/8 ballscrew at the size you need. I think they charge 1.20/ft.
5/8 might not be the ideal thing for a 5ft length though.
So check Mcmaster.
And make sure whatever you use is well supported on both ends. Use angular contact bearings on the end where your timing pulley or whatever is at, and radial bearings at the other end

[ViperTX]

stevesplanes.....do you need more z-axis for height reasons...like you need to stick thicker material.....if that's the only reason....I would drop your table and add the additional height there.

[stevesplanes]

ViperTX

I am sorry, I don't understand. I need to have 16 to 24 inches in the z-azis.
This might mean that I increase the hight of the machine by putting taller, 48 inches, aluminum extrusions and replace my 24 inch extrusions. Then I would be able to add 12 inches to my z-axis by buiding a table that can go 18 inches. This is not the only epansion I am planning. My first expansion is the x-axis. It is the one perpendicular to the blue angle iron. I have bought two 72 inc thonson 1 inch rods. My concern with this approch is the bulkness of the machine, but that is only a minor concern. The other is rigidity.

As for the screw driving from the center, if I go this way I must expand by increasing the hight of the frame. But if I go with racks on eithr side, then I can build a gantry which might be rigid.

Being an amature, newbe, I wanter to try to go with expanding the x-axis, meaning a longer drive screw, and then the y-axis and finally the z-azis. My fear is that 5/8 diameter drive screws might be too flexibel at 72" and the cost of a 72" 1" to 1.5 inch screw might be a little to high. Seign as I have two 5 foot racks, I might want to go to the side driving??