Do I need a multi start ACME leadscrew? What about trapezoidal?

[amanmazleigh]

Hello everyone, as per question. Do I need to use them? My machine will need leadscrews of dimensions 1500mm x 1000mm x 500mm. I would want to save money as much as I can, so if there's not a need for me to have multi start, then I wouldn't want to get it.

Also, I prefer metric measurements and realise that its hard to find metric ACME screws, so I turned to trapezoidal screws. Is this a good choice? Thanks guys!

[jcoats]

Are you out after speed, or is accuracy your big thing? Calculate how big a step will be with a 1 start screw and see if you can deal with 5 start or something else.

As far as 'do you have to have it now'? No. Unless your tolerances require it.

[amanmazleigh]

I guess I would rather have accuracy than speed. How can I calculate? Thanks for your reply.

[jcoats]

Most steppers do 200 steps per revolution.

Let's start with some numbers:

If you use it direct on a 1 start with a 20 pitch thread, you get 4000 steps per inch.

If you do the same thing but have a 4 start, that basically turns it into 200 steps x 20/4 = 1000 steps / inch

So with the 4 start, every step goes 4x as far.

This also assumes you do NOT do microstepping. Microstepping generates more steps and most controllers can do up to 8 if not 16 microsteps per step. Effectively increasing the theoretical resolution.

Also depending on the stepper you use. If you use the ones with 7.2:1 ratio, it takes 200 * 7.2 steps to make a revolution or 1500 steps/revolution

If you use microstepping it effectively reduces the power/torque available per step (by roughly the ratio of the size of the microsteps, 16 is 1/16th the torque of a 1 full step.

So like I said before... it depends. On what feed rates you NEED (versus want), size/power of your steppers, your choice for using microstepping (and how aggressively), whether you use a geared stepper or not, etc.

Personally I like to use spreadsheets when getting into this to keep my head from exploding with the minuchia of the mechanical design.

And you well might want a 5 start on X, 3 start on Y, and 1 start on Z, but in building YOU are responsible for what comes out. The good and the otherwise ... Lots of folks have good rules of thumb, but designing it to do what YOU want makes sense to me rather than using their guess about what is 'good enough' in their opinion.

And we haven't even gotten into the resolution/accuracy/repeatability discussion. Before asking about that, I suggest doing some reading of LOTS of forum posts.

Part of the joy of learning CNC is the learning, then the doing.

Enjoy the process.

[mike_Kilroy]

Jcoates hit it on the head. number of starts is TOTALLY IRRELEVANT AND NOT YOUR CONCERN.

find what PITCH screw you need for your app and buy that. whether it is built as 1 start, 2 startm, 4 start is of no concern to you; simply the final pitch. How the mfgr gets that is irrelevant.

Did I mention it does not matter? You got enough other worries than to worry about something totally irellevant like no of starts. We sell thomson and ballscrews.com screws. no of starts has no bearing on accuracy either. just ignore this. find the pitch you need and buy the accuracy per foot you need.

if you want help, download our free motioneering program and stick your machine parameters, weights, speeds, etc., into it and find what you need. avail free here:

http://www.kollmorgen.com/en-us/supp.../motioneering/

[jcoats]

Starts do matter, but not as much as the overall design performance. But if you start with single start, and you get some experience, THEN changing out to multi-start threads might help, but single start isn't bad, and it increases higher resolution (compared to multistart) easily, and keeps cost down.

If this is your first machine, go single start. Then you can have your own opinion

Later if you want to upgrade to multistart, you will need the new threaded rod and the associated nuts. It is a pretty easy upgrade to be done later if you find it is needed.

Enjoy, and keep asking questions (but read a bit more first, please)

.............
Sorry, I didn't read your ENTIRE post.

Yes, use ACME threaded rod for your first run. Not the easily found 'all-thread' that here in the states can be found anywhere easily.
Multi-start may be other shaped rather than ACME thread, just get anti-backlash nuts for whatever your thread is (typically from the same vendor, as your ground multistart rod comes from.

[amanmazleigh]

Hello everybody,

Thanks for the replies. I really appreciate the advice that was given. Initially I wanted to just use single start but I read somewhere around here about people wanting to change to multistart screws after they got the single start screws. So I was wondering if I should go straight to multistart, and hence this question. But after thinking about what you all had advised me with, I think single start is sufficient for my application because I do not need to have fast speeds. Just curious, if I were to get a 12mm diameter, 3mm pitch trapezoidal screw, how fast can the machine go? I can provide other information if needed. Thanks!

I had been looking for calculators for these speeds measurements but I forgot where I had found it.

[mike_Kilroy]

max speed is a function of length, diam, and type of bearing support on each end (single, none on 1 end, or thrust bearing one or both ends). check charts in back of just about every screw catalog.

I stand by my comment that number of starts has no bearing on picking the screw. Of course the screw ratings will changed based on number of starts, diam, etc, etc, but a single start screw can have very similar ratings to a multistart screw if same pitch. Again, it should not be a criteria for picking a screw - just your requirements such as speed, accuracy, load, etc should be used to pick a screw. Leave your options open and don't limit yourself by worrying about no of starts. Look at ALL screws that meet your requirements only.