Newbie- one start or two start lead screw

[Pinhead]

Can someone explain to me the difference between a one start or two start lead screw.

With a two start do you gain twice the resolution and keep your cut speed up? I just don't know the difference and cannot find an explanation on my web searches.

Thanks for your help.

[acondit]

A 0.1" pitch 2 start screw is the same "resolution" as a 0.2" pitch 1 start screw, that is both are 5 turns per inch. For a single start screw turns per inch and threads per inch are the same, but for multi-start screws turns per inch is equal to threads per inch divided by the number of starts.

So if you had a 10 tpi single thread screw and wanted to increase the speed of your machine (assuming that you had enough power), you could replace that screw with either a 10 tpi 2 start or a 5 tpi single start to effectively increase the speed at which your machine could travel (again assuming that you have enough power). This is because the new screw moves something along the screw twice as far with each turn.

The trade off is resolution versus speed. For the same number of steps per rotation of the screw, going to a screw that moves something twice the distance with the same number of turns, means that it moves that thing (whatever it is) twice as far for each step (less resolution).

Alan

[MonoNeuron]

I just upgraded my router to a 12 start thread 2" pitch and it works great with a heap more speed at the router. I use Vexta 5 phase drives and with the 1/2 stepping mode, it gives you 1000 ppr on the motor and geared down 3:1 has plenty of power and resolution for what I need. I had a 2 start lead screw but it was way too slow and I didn't need the power it provided so I used the 12 start.
I would recommend any start lead screw from 2 starts and up for your router. More starts = more speed (but less power) 4 starts and 1" pitch (or greater) would be a good place to start. Just remember to gear down your motor to about 3:1 for the extra power/torque.
Steppers have plenty of power at low revs and lose power at higher speeds so it seemed logical to me to reduce the speed and increase the resolution (1000 ppr) of the motor. I get roughly 300 mm sec out of it that is usable but it is a bit scary so I slow it down just a tad.
Rich.
Are we long lost brothers. With a name like Pin Head you complement my Mononeuron. We would fit like 2 peas in a pod :-)

[rackmount]

hmmmm... not to contradict but...

As I understand it, pitch is expressed in terms of the travel of the nut per revolution... regardless of the pitch of each "start". So if you have a dual lead screw with a "pitch" of 5tpi (turns per inch) it's moves exactly the same distance per revolution (and hence has the same "pitch") as a single lead screw at 5tpi.

Again, as I understand it, multi lead screws are used because they can carry a higher load then single lead screws (because the load is distributed across the multiple leads).

I'm not a mechanical guy so if someone could correct any misconceptions I might have.. I would appreciate it.

[ger21]

Originally Posted by rackmount Again, as I understand it, multi lead screws are used because they can carry a higher load then single lead screws (because the load is distributed across the multiple leads).

Multiple start screws are more efficient than single start, and they allow higher leads (pitches?) than single start screws. Picture in your head a singles start screw with one turn per inch, or better yet, one turn per 2 inches (2" lead). I don't think a single thread every 2" would work very well, so you'd use something like this. Photo of a Nook Speedy metric screw.

[Metal1cutter]

Originally Posted by rackmount hmmmm... not to contradict but... As I understand it, pitch is expressed in terms of the travel of the nut per revolution... regardless of the pitch of each "start". So if you have a dual lead screw with a "pitch" of 5tpi (turns per inch) it's moves exactly the same distance per revolution (and hence has the same "pitch") as a single lead screw at 5tpi. Again, as I understand it, multi lead screws are used because they can carry a higher load then single lead screws (because the load is distributed across the multiple leads). I'm not a mechanical guy so if someone could correct any misconceptions I might have.. I would appreciate it.

The pitch is the distance from crest to crest on the thread. The lead on the other hand is the distance the nut will go for one revolution around the screw.

On a single lead screw the pitch and lead are the same.

Regards,

Stanley Dornfeld Quad-I.com