New thread in Motion Transmission

[chuckknigh]

Since this "Conference room" section is obviously well followed, I thought I'd use it to invite everyone over to the "Motion Transmission" section.

http://www.cnczone.com/forums/showth...&threadid=4178

I'd like to get this section of the project off to a good start. My intention is to establish a list of FAQs, and to answer them for the first time builder. Understanding the mechanics of how the table moves is SOOO important to understanding how CNC works.

I expect everyone to come on over, and at least submit a question...preferably an answer, too! I don't want to write this section, alone...

-- Chuck Knight

[Hobbiest]

would be nice to submit a question or reply, but system won't let me do so. Says I am not authorized access to post. Maybe the moderator can move this post there for me.
Chuck...I don't want to turn this thread into anything other than what it is, but I am confused as to why the positional accuracy of allthread is low, and it takes a bigger motor to move the same load as you state. The higher the thread count, the less distance that is covered by each interval of the motor, whether it be a stepper or a servo. Wouldn't this translate to higher accuracy? Also, the higher the number of threads per inch, the less force is needed to move, at least taken from simple physics (I am no physics wiz!!). The way it has always been explained to me, threads are simply an inclined plane wrapped around a shaft. More treads equals less incline, meaning more mechanical advantage. Great job getting this thread started, as it is something very needed IMO. As I find questions here and there on the site, I will answer them if I can here, or simply post the question here. Don't forget about trapezodal screws as well.

[ToyMaker]

All thread is the least efficient because of its surface; it's relatively rough. If you could polish the threads you could approach or meet the performance of acme. Also, it is mass produced and not especially accurate along its length.
Good acme screws are precision ground and polished.
Ball screws are best because rolling friction is *much* lower than sliding friction and they are precision ground and polished.

robotic regards,

Tom
= = = = =
Many a man owes his success to his first wife and his second wife to his success.
- - Jim Backus

[chuckknigh]

would be nice to submit a question or reply, but system won't let me do so.

I'm guessing that you never volunteered. The system requires, for these forums, that you belong to the "Volunteers" group for write access.

Chuck...I don't want to turn this thread into anything other than what it is, but I am confused as to why the positional accuracy of allthread is low, and it takes a bigger motor to move the same load as you state.

It's simple. It's not manufactured to very high tolerances, and the surface finish is lousy, so the positional accuracy is not very high. The Chinese ACME thread used in vises is not of very high quality, either.

Also, because of the shape of the thread, its power transfer efficiency is not as high as ACME. ACME was designed for power transmission, and just works better. Since it's more efficient, there are smaller losses. Smaller losses -> smaller motor for same results.

The higher the thread count, the less distance that is covered by each interval of the motor, whether it be a stepper or a servo. Wouldn't this translate to higher accuracy?

Assuming that the threads are made with equal precision, then yes. But the threads on allthread wander "all over the place," compared to something made with precision. Keep in mind, this wander is measured in .0001" units. For most uses it's perfectly fine, but for true precision work, a precision ground screw is a better choice.

You can get very high quality 60 degree threaded rod, but its cost approaches that of good quality ACME, and at the same price point, ACME is the better choice.

Also, the higher the number of threads per inch, the less force is needed to move, at least taken from simple physics (I am no physics wiz!!).

Essentially, it's just gearing it down. A higher thread count = higher force over less distance.

All else being equal, though, the screw thread with higher efficiency will deliver greater force with the same motor, since there are fewer losses.

The way it has always been explained to me, threads are simply an inclined plane wrapped around a shaft.

True, but remember also sliding friction. Moving a block up an inclined plane results in the same amount of work being done. But, if the box has wheels on it, sliding friction is eliminated. If the plane is smooth or lubricated, again to reduce friction, the losses are reduced.

-- Chuck Knight

P.S. Thanks for the question. It's definitely something that needs to be covered in more depth.