Ball screws, Acme screws, threaded rod

[DJ Morrow]

I'm still finding my way around and one of the issues is the threaded rods of various sorts. Ball, Acme, hardware store stuff. So far, it seems to break down to cost, precision and availability.

Can someone go into a bit of detail for me when one type is accurate enough. And how about fine vs coarse thread ?

Any other enlightening details are always welcome.

[HuFlungDung]

Hi DJ,

There have been tons of threads discussing some part of this question, but most of the time it boils down to the end user accuracy requirements first, then what you feel like sinking into the project.

If you need high accuracy, you need precision ground ballscrews. This is the ultimate solution, and also the most costly. It provides you with a known pitch accuracy, over quite a decent distance, and has the potential for eliminating backlash, yet allowing high speed motion without undue friction, or rapid wear on the screw.

Then, comes the rolled thread products, starting with rolled thread ballscrews, which are formed entirely by squeezing the ball race profile into the screw, using hardened master rollers, perhaps an inch or two wide. This is a less controlled process, so the pitch of the threads may accumulate to considerable error over a long length of the stuff. Because of the tolerances, this grade of ballscrew must be assembled with greater clearance, so there is a slight amount of backlash built in. With crafty engineering, some of the guys eliminate most of the backlash with fancy nut designs, however, the accuracy is still not there, because the screw still floats in the spring-loaded nut.

Next, the acme rolled screw. This is likely one of the most popular screws for a machine with modest accuracy requirements. Again, fancy ideas for reducing the backlash have been laid out in many threads here. The bad thing about this might be in heavy usage, the wear of the screw and nut would require skilled maintenance to get the best performance out of your machine.

Finally, there is the rolled V thread. Much the same engineering required as using Acme threads. Some guys swear by their delrin nuts (do you solemnly swear, by your delrin nuts, so help you God ).

Fine thread, coarse thread, it may depend on what kind of motor speed and torque you have available. The pitch selection can help you get the motor speed into the range you have available. Generally, a finer pitch is going to improve the resolution of your stepper/encoder system. It will also reduce the top speed limit of your system.

Its really about a whole lot of compromises, boiled down and rendered by the energy released by burning your bank account

[abasir]

IMHO, the above explanation deserve a spot in the FAQ.

[DJ Morrow]

Wow. I'm going to have to read that a few more times but it is so very much appreciated Mr. Dung

[foamcutter]

HFD,
One question please. Can you give me an idea of a parameter in thousands of an inch for High Accuracy, and Modest Accuracy. I am not trying to be picky, it's just your idea of high accuracy may not be the same as mine and I'm trying to get a feel for your terms of accuracy. Thanks Ron

[HuFlungDung]

Hi Foamcutter,

.004" per foot for modest accuracy (a la Reid tool catalogue)

.0005" (or better) per foot for precision ground.

[foamcutter]

Thanks,
That gives me the idea of accuracy your talking about. Much appreciated. Ron

[WadeConner]

Hi all, I'm new to the CNC Router Bug thats going around.
I'd appreciate it if some of you who know more about lead screws to comment on this:

What do you think of these ball screws on ebay:

http://cgi.ebay.com/ws/eBayISAPI.dll...category=45045

I would couple them to this system:

http://209.41.165.153/stepper/FETcruiser.htm

(nema 34 150oz Steppers)

What kind of cutting speed and accuaracy will this give me (X&Y axis only)

Thanks,

Wade

[buscht]

Wade, that seems to be a great price on precision ball screws, especially since the milling is already done on each end and the ball nut has a mounting flange.

I bought rolled ball screws from McMaster Carr for about $50.00 per axis. but I still have to turn the ends down and make a mounting flange for the ball screw. Since, I can't do the lathe work, I am paying a machine shop. I am estimating another $35 per axis.

The Ebay screws are roughly 26" long, that will probably give you a working stroke of maybe 21". Subtracting the length of the nut, and bearing blocks on each end.

If you don't need the accuracy, look at www.mcmaster.com item
5966K16 nut and 5966k261 5/8 ball screw with a .203" lead.

T

[fred_m_48601]

hello

[sdantonio]

This thread will start you off. Acme, in their precission thread bosts 0.0003 per foot accuracy (see nookind.com)

http://www.cnczone.com/forums/showthread.php?t=34570

[Treischl]

There is one other thing you should be aware of with all this accuracy talk.

Most control software, Mach 3 being a typical example, has the ability to map the screw inaccuracies into a table which is used to adjust the machine travel. In other words, if you can do some precision measuring, you can make a pretty inaccurate screw quite accurate.

Extremely accurate machines are set up using this technique. A CNC jig grinder is an example, they use a very accurate screw to start with, but to get really accurate, they map the screw.

The type of screw also has an effect on your machine speed. Rolling friction is usually less than sliding friction. Therefore, ball screws allow for higher speeds.

On the machine I just built, I used acme screws from McMaster Carr, anti backlash nuts from DumpsterCNC. I am very satisfied with the performance. Using just my dial calipers (since I have no need of greater accuracy milling wood, cutting signs, etc) the machine looks to be good to about .003 in any foot. If I need this accuracy over a greater distance, I will start mapping the screws.

Good luck on your project!