Ball screw or Acme lead screws

[bgolash]

Hi

I`m wondering how many of you have used acme leadscrews instead
of the usual ballscrew system.

[sdantonio]

Can't afford ball screws. Wish I could

[iXô]

Hello, can someone tell me what is the main difference between ball and acme screw (pro and cons ?).



Thanks.

[sdantonio]

About Precision Acme Threaded Rods

For a simple and effective way to convert rotary motion into smooth, precise linear motion, choose precision Acme threaded rods. Although not as mechanically efficient as ball screws (a precision Acme threaded rod system operates at about 30% efficiency, while a ball screw system operates at about 90% or better), it is more economical and ideal for uses involving lower speeds and fewer cycles.

Precision Acme threads are broader, stronger, and more square than standard V-shaped threads, so they're better suited for load carrying. They have a 2C (self-centering) thread fit and a smooth finish that increases rod life by reducing friction. In addition, the 2C thread fit is tighter and more precise than our general purpose Acme threaded rods (see page 3097), which have a thread fit of 2G (general). Thread fits 2C and 2G are compatible. The lead accuracy (the variation of distance a nut travels in a complete turn) over the length of the rod is 0.009" or better per foot.

Note: To prevent galling (roughness on the surface of metal caused by friction) in most linear motion applications, we recommend using rods and nuts made of different materials. Both bronze and plastic nuts work well with all of the rod materials we offer.

Acme Size— The outer diameter of the rod (measured from the crest of one thread to the crest on the other side of the rod) and the number of threads per inch. For example, a rod with an Acme size of 1/2"-8 has a diameter of 1/2" with 8 threads per inch.

Turns per Inch— For single-start rods, turns per inch equals the number of threads per inch. For multiple-start rods, turns per inch equals the number of threads per inch divided by the number of starts on the rod. To determine threads per inch, place a rule on your rod and count the number of threads in one inch.

Starts— The number of independent threads on a rod. Single- start rods are most common; multiple-start rods allow the nut to travel faster by moving farther in one turn.

To determine the number of starts, look at the end of your threaded rod (see diagram). The end of a single-start rod is an offset circle. The end of a two-start rod is shaped like a football. For more than two starts, count the number of corners on the end of the rod.

About Ball Screws and Ball Nuts

Ball nuts contain ball bearings that circulate in the groove of the screw, reducing friction between the nut and the screw. The result is precise, stepless positioning control over the full length of the ball screw and the conversion of rotary motion to linear motion more efficiently than Acme precision threaded rods. Load capacities are based on a ball screw and ball nut used together.

Screw Lead— The distance a load is moved with one revolution of the ball screw. For proper fit, choose ball nuts with the same screw lead and diameter as the ball screw.

[sdantonio]

p.s. lets give proper credit where credit it due. Previous post stollen in it's entirity from the mcmaster-carr catalog.

[InspirationTool]

I do light production on my machines. Hundreds of parts.

I won't buy a machine with acme screws again.

They are too hard to remove the backlash from, and worse, the backlash changes over time. You also get higher speed with the ballscrews.

-Jeff

[Bubba]

In my original coversion, I tried to use the acme screws on my mill/drill.
Wrong move, they quickly wore out the bronze nuts and backlash became unmangeable. Converted to Ballscrews and the backlash went away and had smoother operation.

Bubba

[totally_screwed]

One problem with ballscrews is that due to the very low friction, when using the mill / lathe in manual mode, you have to make sure that the slides are locked. This is because of back-feeding whereby the cutting forces move the slides because the nuts to turn the ballscrews. Because normal i.e. Acme lead screws have much more friction, this is rather less of a problem.

Almost needless to say, in precision work and especially with heavy cuts, the slides ought probably to be locked.

Chris

[Bubba]

totaly,
That is the EXACT reason you don't need hand wheels on a cnc'd machine.
I read many threads about the backfeed problem trying to use ballscrews in a manual mode.
I have found the use of either MDI or continous jog to be the answer and haven't looked back!
I am in the process of making my lathe cnc and it will NOT have handwheels. There is no need and if I want some quick feed, there is jog in my control software and if I want to be accurate, there is MDI!

Just my .02

Bubba

[jimluu]

Actually, I find that ball screws are not that expensive unless you go for the super high grade ones. I scores a couple of 84" ones with the nuts for 200 shipped from ebay. Had to grind off some rust, but they're otherwise new. One of them do have a detectable backlash though (if some one have ideas one how to get rid of it, please let me know). In fact, even brand new they are not that expensive. Go to roton.com. I have to get used ones b/c here in colorado springs, no one would mill the ends for me. What does get expensive are the accessories, such as the flange, the block mounts, etc.

[sdantonio]

One thing that was pointed out to me by Gerry, for my application, it would be a real pain in the a to keep the wood dust from fouling the nut. With no coolent and fine dust that gets everywhere (even past the dust collector at times) using a ball screw may be a self defeating thing. A ball screw cover would go a long way in this, but wouldn't be a complete solution.

[whoafat]

Hi, this newbie is looking for examples of CNC conversions using a ballscrwew on the X-axis and the stepper mounted on the left side of the table. I anticipate one obvious negative from a left-handed motor design. Each time you want to remove the table, you may have to remove the motor. I'm not sure that will be a big nuisance. Just in case, I have not ruled out the what looks like the ubiqutous right-hand mounted stepper approach. I appreciate all the help I can get,
Wilfred