Disadvantages of 2 start vs. 5 start?

[Biff]

In the designing stages of my machine, it'll probably end up looking similar to joe's 2006 with 2'x4' cutting area.

Since I'm in Canada I'm having a hard time finding 1/2"-10 5 start threaded rod. I've looked at getting up from the U.S. but the shipping charges really get up there. A local place that I like to deal with does have 2 start though for a very nice price. I need it for X,Y, and Z so (very approximate) 5', 3', and 1' pieces

I'm a newb but AFAIK 5 start is better then single start since it doesn't have to spin as fast - reducing whipping, allows the stepper to turn slower where they have more torque, and can be more efficient in power transfer. But I've read that although there's a 5:1 ratio in the pitch you don't get 5x the speed. Speed isn't really critical but I don't wan't to be getting frustrated with the 'slowness' either.

So where does 2 start fall inbetween the two? Is there a still a huge difference getting the 5 start over the 2? I'd hate to 'settle' on the 2 start and curse the machine everytime I use it for not going for the 5. But going with the (local) 2 start will probably save me over $100 and a lot of trouble.

[edit] a couple of additions,

#1. for got to mention that I plan on CNCRP's kit for the electronics
#2. it would appear the stuff from the local guy is only standard grad stuff, not the precision that I was looking at at mcmaster carr... but I don't know what ill effects that would have

[/edit]

[wildwestpat]

Hi Biff

The available motor power is possibly the first limit on the speed that an axis can reach. Yes the screw will tend to whip and this is undesirable. The size of your machine indicates that lead screws will need support at both ends. If you are going to use stepper motors there is a rapid loss of torque as the motor speed increases. Yes the very coarse pitch multi start screws require a slower motor speed to achieve the same rate of feed as a finner pitch screw, Thus there is a balancing act between the stepper motor torque fall off as the speed increases and the pitch of the lead screw. Obviously some form of gearing between the motor and the screw can help provided the motor has enough torque.

A good place to start is the paper by Irving2008

http://www.mycncuk.com/forums/showth......-A-tutorial

Irving has also produced a spread sheet which takes the tedium out of the necessary calculations and this can be found in the above tutorial.

Hope this helps get you started in the right direction - Regards Pat

[louieatienza]

Another thing to consider is the thread pitch will determine the mechanical resolution of your system. Let's say you're using steppers with the standard 1.8 degree turn per step, or 200 steps per revolution. Thus with 1/2"-5 start, your mechanical resolution is 1/400" or .0025". With 1/2"-2 start, you now have a mechanical resolution of 1/1000", or .001". So a 5-start will give you more speed, primarily with rapids, than a 2-start, given the same motor RPM but you will lose a little on resolution.

I'm pretty new to this as well, though Ive tried to learn as much as I can. Microstepping from what I gather may not necessarily give you greater precision, but will make the motors run way smoother at slower speeds, which helps the high lead screws.

I'm actually using 8-start (1 TPI effective) on my machine, but am using it mainly for roughing out parts that I finish by hand, so I can live with a 1/200" resolution. I probably won't be making PCB's or injection molds an time soon, though my next machine will have higher mechanical resolution.

You could of course gear down or up, and there's a good thread on just that in the Mechanical section.

Another thing to consider is the material you're cutting and your toolpath strategiy. If you have very few position changes, then it may not matter too much what your rapids are, because your cutting speed will depend more on your materials, the ridgidity of your machine, and the power of your spindle.

[george4657]

have you checked out SNB Inc. in Barrie
http://snbinc.ca/site/
they list 5 start 1/2-10 screws.
I have not dealt with then so can not off any opinion on their products.

George

[ger21]

I use 1/2-8 2 start. I have 45" of travel, and 60" screws. Whipping limits me to 150ipm.

You might want to call Roton. They''ll ship to Canada. www.roton.com
They don't have 5 start, but have 1/2-8 4 start, which is basically the same thing.

[Biff]

have you checked out SNB Inc. in Barrie
http://snbinc.ca/site/
they list 5 start 1/2-10 screws.
I have not dealt with then so can not off any opinion on their products.

George

They're close enough I can just go pick the stuff up! I think I just might owe you a pint for that! And if it looks as good as it does in their pics then I'll make it 2. You'll have to come to collect though .

[Biff]

Sorry for the double post but I wanted to follow up on louieatienza's post about precision. I've done the math too and came out with the 0.0025" fer full step, but the G540's use 1/10 microstepping so I thought that can out to a 'theoretical' precision of 0.00025". Yeah I know that's a best case scenario, and I don't intend to keep the machine at an exact 20°C and controlled humidity with machining only done during new moon phases in a clean room, BUT, Is it correct not to expect any more precion then a full step?

(sorry louieatienza's, I'm not intending to point a finger or center you out... just looking for clarification)

[louieatienza]

No worries biff...!

From what I understand, microstepping has more to do with giving steppers smoother motion at lower speeds...

Surely the microstep feature can give you more theoretical resolution, but if, for example, the backlash on your system or spindle runout or whatever, exceeds the precision of the components, thn you may not see the accuracy the math predicts...

[wildwestpat]

Hey Biff don't get too hung up on microstepping as a means of improving accuracy. Invoke micro stepping and the torque falls rapidly as does the maximum speed which limits the technique to the low orders of microsteps. There are issues in terms of linearity of both the basic as well as the micro step but that is a story and only of real interest to those making precision parts out of metal.

Regards Pat