Newbie questions about leadnuts and couplers

[dan2004]

December 7, 2006

Hello all,

I’m a first timer and I am doing research for the leadnuts and couplers that I am going to use building David Steele’s CNC Router table (Solsylva.com). I'm thinking about going with ½-10 Acme Rod and ½-10 Acme Couplers. I wanted to get a few opinions from you all regarding accuracy before I definitely decide to go that route.

If I use rodding that is 10 teeth per inch, then I assume that the motors will travel quicker up and down the axes then if I was to use more teeth per inch. Am I right about that?

But what I really need is precision while I am cutting, so I am thinking that I need more teeth per inch. Is 10 tpi pretty precise? What are you guys ideas about that?

Also, I was at McMaster Carr pricing Acme Rod and Acme Couplers.
The Acme rods come in:

Grade B7 Alloy Steel 18-8 Stainless Steel
Plain Steel
1018 Carbon Steel
Heat Treated 4140 Alloy Steel

The Acme Couplers come in:

Steel
Plain
Zinc plated
Stainless steel
Brass

Will it matter what type of material the rods and couplers are made of? The only advantage I could think of was brass is non magnetic and I was thinking that this would help in reducing noise that could interfer with signals.

Any input would be greatly appreciated. Thanks everyone!
Dan

[massajamesb]

IMO, more TPI will increase resolution. 10 TPI should be fine for a small cnc router such as you inquire about. The more teeth per inch you have, the slower rapids you will have, because the motor RPM's must increase to compensate for the number of turns per inch on the lead screw.
I.e., a 10 TPI thread must be spun a full revolution 10 times in order to move an inch. 5 TPI must be spun a full rev 5 times to move the machine 1 inch.
It is my opinion that the material used for thread and couplers is subject to your desire. I don't see normal steel or stainless giving you any trouble. I believe they are generally what most guys run.
In my router I am building, I am using regular steel Acme(not the hardware store kind,)from a cnc supplier, and steel lovejoy couplers.

[ger21]

Originally Posted by dan2004 But what I really need is precision while I am cutting, so I am thinking that I need more teeth per inch. Is 10 tpi pretty precise? What are you guys ideas about that?

You'll most likely be using stepper motors that have 200 steps per revolution. With 10 revolutions per inch, each step will move your machine .0005 inches. Is that precise enough? In reality, you'll have more backlash (slop) in the drive system than that. And you won't be able to build you machine nearly that precise either. So the 10 tpi is plenty precise.

Keep in mind that Acme comes in different grades, with different levels of precision. Without spending a LOT of money for ground screws (more than your entire machine will cost), you'll be getting rolled screws. These come in different grades. At the bottom end, regular acme threaded rod, you can get for about $10 or less for 6 ft. As you move up in precision, the price goes up. A decent quality acme leadscrew will cost about $30 for 6 ft. It has an accuracy rating of ±.009" per foot. http://www1.mscdirect.com/CGI/NNSRIT...MT4NO=15228276
The next step up is acme leadscrews with an accuracy of ±.003 per foot. This will cost you about $55 dollars for 6 ft. http://www1.mscdirect.com/CGI/NNSRIT...r=3906%50mode=

[dan2004]

massajamesb, gerry

Thanks for your replies.

Could you please tell me what an accuracy rating of +/-.009 or +/-.003 means? What does +/-.009 or +/-.003 measure?

Also, (sorry for the novice question) but what does it mean when you say "the slower rapids you will have"

Thanks again, really appreciate the work you guys do here on this site!
Dan

[massajamesb]

well, as far as rapids, if you run a stepper motor at 100 RPM on a 5 TPI thread, then you will move 20 inches/minute. The same application using a 10 TPI screw will only move 10 inches/minute. In order to get the speed of the machine back up to 20 inches/minute, you would have to increase voltage to the motor. You will eventually hit a wall where the motor loses steps, as it does not have sufficient torque to move the load. In this, your rapid speeds will be affected at some point, because you will only be able to spin your motors so fast.
So, in essence, the fewer teeth you have on the acme, the fewer turns of the motor it will take to move the machine a certain distance.
It is my opinion that 10 TPI will get you where you need to be.
I have found a good supplier on Ebay for ground acme thread with delrin nuts, turned down on both ends, one end for connection to a 1/4" shaft coupler, the other end for a 5/16 bearing. This is the supplier I will be using. I believe a 4' long, 1/2" thread, 10 TPI was about 30 bucks, with a delrin nut.

[ger21]

Originally Posted by dan2004 Could you please tell me what an accuracy rating of +/-.009 or +/-.003 means? What does +/-.009 or +/-.003 measure?

With +-.009 and 1/2-10 screw, if you spin the screw 120 times (to move one foot), you'll have moved somewhere between 11.991 inches (-.009) to 12.009 inches (+.009). That's the worst it can be. Most likely it will be better.