Which is better 1/2"-10 OR 3/8"-12 acme rod

[mrbaseball]

I'm getting ready to order my acme precision screw rod and wanted to know which is better to use for the jgro style router?? 1/2"-10 OR 3/8"-12 the reason I'm asking is because these are the only availabe sizes for use with premade anti-backlash nuts.

Thanks

[jeffs555]

I would go with the 1/2-10. The 3/8-12 will start to whip at a lower speed than the 1/2-10. Depending on how long your screws are, and what kind of travel rates you want, it may not make any difference. Nook Industries has a lot of charts and calculators showing the strength and speed at which the different sizes of screws will whip. http://www.nookindustries.com/acme/AcmeCharts.cfm

If your steppers are marginal, the 3/8-12 will generate 2 lbs of force for each oz of stepper torque where the 1/2-10 will only give 1.6 lbs of force for each oz of stepper torque(assuming plastic nuts).

[mrbaseball]

Originally Posted by jeffs555 I would go with the 1/2-10. The 3/8-12 will start to whip at a lower speed than the 1/2-10. Depending on how long your screws are, and what kind of travel rates you want, it may not make any difference. Nook Industries has a lot of charts and calculators showing the strength and speed at which the different sizes of screws will whip. http://www.nookindustries.com/acme/AcmeCharts.cfm If your steppers are marginal, the 3/8-12 will generate 2 lbs of force for each oz of stepper torque where the 1/2-10 will only give 1.6 lbs of force for each oz of stepper torque(assuming plastic nuts).

OK, My steppers are 200oz/in so if my math thinking is correct using the 1/2-10 I would multiple the 1.6 x 200 = 320, so 320 would be my # of force?? So the 3/8-12 would be 400 # of force?? I'm only going to have a maximum table length of 36".

[widgitmaster]

At 36" screw length, the 3/8 dia. is way too flimsy, not to mention the 12 pitch gives you a lead of .08333333" per revolution!
The 1/2-10 would give you a lead of .1" /rev. That is easier to calculate feed rates with.

As mentioned in other posts in the zone, a nema23 with proper voltage and current will reach an RPM of 1000+; consequently, the 1/2 dia. screw length of 36" will probably have a maximum exposed thread length greater than 30", depending on the width of your slide carriage! So 30" at 1000+ rpm, that's pushing it! Your router would vibrate at rapid feed and any bow or deflection in the thread when stationary would be magnified while spinning!

My choice would be the .631 - 5 ball screws, it has a lead of .2"/rev. which is much nicer and faster!

Eric A.A.

[mrbaseball]

My choice would be the .631 - 5 ball screws, it has a lead of .2"/rev. which is much nicer and faster! Eric A.A.

But what do those ball screws cost?? I can get the acme rod from mcmaster.com for around $10 for 72 inch piece. This is my first CNC router so I"m trying to keep the cost down, until I can at least get some experience with this CNC stuff under my belt I'm going to have to pay someone to turn the ends down anyway so there is the added cost already

[ger21]

Force (ounces) = ((2 x pi x torque)/ lead) x screw efficiency.

You can get actual efficiences from Nook (link above), but I'll assume 50%

So, for 3/8-12 = 471 lbs

1/2-10 = 392 lbs

Keep in mind that this force is the holding force. When the steppers start spinning, the torque will drop fast.

[jeffs555]

Yes, that would be the maximum force it would generate, so you can see that either will give you way more force than you need. The 200oz/in is the stall rating of the steppers, and the torque drops off as the rpms increase. At 1000 rpm, the torque might be 1/4 or less of the rated torque.

The Nook charts show that for a 40" screw, if you had a double bearing on one end, and a single bearing on the other, the 1/2-10 rod could move a max of 70 ipm before the screw started whipping, and the 3/8-12 would start to whip at around 40ipm.

[mrbaseball]

Originally Posted by ger21 Keep in mind that this force is the holding force. When the steppers start spinning, the torque will drop fast.

Ahh, gotcha thanks for that little bit of information. It all is starting to make sense

[ger21]

The most cost effective screws I've seen are 1/2-8 2 start acme, which is 4 turns per inch. www.mscdirect.com $30.81 for 6 ft http://www1.mscdirect.com/CGI/NNSRIT...PMT4NO=2348251

You might want to look here for the ballscrews. They'll machine the ends for you too. http://www.homeshopcnc.com/page2.html

For you're first machine, you can't go wrong with the 1/2-10 for $10. Get everything working with that, but allow yourself room to upgrade in the future. Once you get the first machine working, you'll probably want to build a bigger, faster one anyway.