How to calculate proper lead screw?

[alwaysready]

Joe2000che brought up a valid ? in my build log concerning me using all threaded rod for the lead screws versus Acme.
http://www.cnczone.com/forums/showthread.php?t=34839
My initial thinking was that the lead screw
1. the all thread would wear out the Delrin before its self.
From what Joes said the acme will wear first... so my thinking was flawed.
2. i thought since the TPI on the all threaded rod would give more torque, since it has a higher thread count than the acme (1/2-13 versus 1/2-10)
Again my thinking was wrong, Due to the fact that the motors would be spinning faster which would put them out of their torque range.
3. i did not think at the time about whip on the X axis since i am running 4'.
and the acme is stonger versus the all threaded.
4. I was just using the standard plans and di not think about upgrading at the time..

SO, after geting all that straight i plan on using the zylotex motors with their kits. i planned on the 425oz motors which are 10.00 each more added to their kits.
What is the proper way to calculate the turns per inch
i first figured 1/2-10 acme but then it was recommended to go to 2 start which would be 5 turns per thread.
I assume that you would use the normal cutting speed of the router( for cutting wood or plastic what speed is that?) and then use the peak torque of the motor to get them to match correct?
Appreciate any help and Joe has been a great help also..Thakns Joe.
Also i do realized the cost aspect of using the all thread and the standard 1/2-10 acme then steping up to a 2 turn will cost 5 times more but i will evaluate this point and i see mcmaster has 5 different turn per inch 1/2 acme so im just trying to decide whats optimum and what it will cost me.
Thakns for any help Chris

[MrBean]

I'm not sure if this is of any use to you, but I wrote a quick and dirty calculator, for some drive calculations.

Ignore the beltdrive stuff. It's not accurate. Close, but not close enough.
Screwdrive type calculations should be ok though.

If you get an error when you try to run it, you are missing the .net framework. You can download that from M$ website.

Yeh I know it's a bit buggy, but it's ok for what I wanted.

Regards Terry.

[ger21]

Usually, you pick a target cutting speed and then design around that. Ideally, you need to know how much weight your moving, how fast you want to accelerate, how much friction is in the system... Also, you want to have a torque curve of the motors to determine what RPM is ideal for the speeds you want to go.

As to your other questions, unless you get some abrasive material in the Delrin nut, I don't see how the steel screw will wear faster than the Delrin?

More turns per inch will give more torque vs an acme screw at the same rpm, but the faster the stepper spins, the less torque it has, so that's not the case. And when you take into consideration that allthread is rougher, and not as efficient as acme, the acme will probably provide more torque, or at least the same.

Even a 2 ft screw will whip, if you spin it fast enough. 4 ft is a long screw, even for 1/2".

There really is no "normal cutting speed" Different materials require different feedrates, and you're router spindle will probably limit you from cutting at high speeds.

If you've seen David's thread, you'll see that changing to the 2 start screws has doubled his speed, and his machine runs better than with 1/2-10.
Everyone that I've seen switch to 2 start screws always gets much better performance, and is alway very happy with the results.

I did some calculations in another thread before, and using 1/2-8 2 start will provide a similar amount of force to 1/2-10 1 start due to the increased efficiency of 2 start screws, and the fact that the motor has much more torque when spinning 60% slower. So you basically get double the speed or better, with the same amount of force delivered, just from a different screw.

I'd say go with either 1/2-8 2 start or 1/2-10 2 start. Nuts from DumpsterCNC. You can get the 1/2-8 2 start cheaper than the 1/2-10 at www.mscdirect.com

[sdantonio]

A lot of people also pick the acme 1/2-10, single start mild steel because it's the cheepest. I know I did. The precision threaded acme is alloy steel (or stainless if you get the really good stuff) which should be harder and more wear resistant.

[alwaysready]

ok now someone show me where i can find 1/2-10 2 turn acme nuts with a hex. i even called mcmaster no luck. in the plans they call for lock nuts to hold them on the rod, but if i convert to acme i would use 2 nuts and jam them. but i cnat find a 2 start nut. the girl at mcmaster told me oh we have them i said where she said page... i said thats a 1 start she said its different then a 2 start,? its acme thread.... she found some but they are cylinder nuts so you can precision machine them yto your specs. i said i just want a plain nut Urgghh check enco ( no go on anything 2 start) and tried the big blue MSC book but couldnt find it, ill look again today when i get time...

[ger21]

Originally Posted by alwaysready ok now someone show me where i can find 1/2-10 2 turn acme nuts with a hex.

Even if you did find them, I'm not sure that they would lock together. Which is possibly why you won't find them.

[sdantonio]

Originally Posted by alwaysready ok now someone show me where i can find 1/2-10 2 turn acme nuts with a hex. i even called mcmaster no luck. in the plans they call for lock nuts to hold them on the rod, but if i convert to acme i would use 2 nuts and jam them. but i cnat find a 2 start nut. the girl at mcmaster told me oh we have them i said where she said page... i said thats a 1 start she said its different then a 2 start,? its acme thread.... she found some but they are cylinder nuts so you can precision machine them yto your specs. i said i just want a plain nut Urgghh check enco ( no go on anything 2 start) and tried the big blue MSC book but couldnt find it, ill look again today when i get time...

To the best of my knowlege they don't make them. They are available only for the single start. I would however love to be preven wrong and find a sopurce myself. For 2 start McMaster only has brass and plastic at $30 or so per nut.

Try dumpsterCNC. They may have something you can use. He makes collars specially for the multiple start screws. their delrin, but still usable for many applications

[alwaysready]

im going to leave it as is.
run it on threaded rod till i get it up and trunning and figure it all out when i can see the creature that im building then i will figure out a way to use the 2 start acme. at this point with out see it or even building one before or even seeing a cnc router in person. Ill still be happy when it moves and inch.

[sdantonio]

Originally Posted by alwaysready im going to leave it as is. run it on threaded rod till i get it up and trunning and figure it all out when i can see the creature that im building then i will figure out a way to use the 2 start acme. at this point with out see it or even building one before or even seeing a cnc router in person. Ill still be happy when it moves and inch.

Sounds like a plan to me.

the acme single start, readily available and sometimes less expensive than common threaded rod does have better, stronger threads and will hold better than the common hardware store stuff. And it has standard hex nuts available. And is reasonalbe inexpensive through someone like ENCO. That's what I'm using right now and it's working fine, though I plan to upgrade to ball screws soon.

[alwaysready]

alright new ? for you guys..
Are there advantages of a 1/2-10 2 start(5 turns per inch) versus a 1"-5 1 start(5 turn per inch) besides a little more rotating mass?
a 5/8-6 is only 14.00 at enco for 6'
a 1"-5 is only 20.00 for 6'

[ger21]

Actually , it's a lot more rotating mass. The 1/2-10 2 start weighs .53lbs per foot, the 1"-5 weighs 2.16 lbs per foot.

And the 2 start is more efficient. 57% efficiency vs 40% for the 1" screw. (with plastic nuts).

This info is available in the acme catalog at www. nookind.com

[alwaysready]

one of the reasons i brought up using the 5/8"-6 rod.
the 5/8-6 sounds the best then because it would give you the benefits of slowing the lead screw down(6 threads per inch versus 13 ) which would have less whip. larger diameter would allow a little more strength in the 4' span and a little more rotating mass but has the availability of hex nuts...
i know the 2 start is just more efficient but cost and parts availbility is hampering me from using it.... unless i buy a tap and make my own nuts...($$)

for now its going to be threaded rod.
then ill convert after its running probably to the 5/8-6 unless something else comes up thats a better idea