[kanankeban]

Ok...im confused
Designing a 6x20' capacity plasma/router table...
1. which rack to choose?
14.5 pressure angle or 20?
2. i know i will need gear reduction suposing ill be using servos in the range of 3000RPM at aprox 240ipm what reduction do i need? (20:1, 10:1, 5:1, 3:1, 2:1?) and will this afect which pinion to use with the rack? so what reduction and pinion should i use?
14.5 angle, 24 teeth?
20 angle, 30 teeth?
Ill rather understand the issues involved making this decision and make the decision myself but if is really hard to explain you can go to www.mcmaster.com and choose something for me...it has been hard finding someone that sells 20 pressure angle rack in 6 ft. online if you know of some other place please let me know.
After this ill go to the servo and gear reduction decision...ill know ill use timing belts for the reduction...but nothing for sure now.
Kindest Regards,
Hector

[energyforce]

240in/min / 3000rev/min = 0.08 in/rev
circumference = Pixd = 0.08in --> d = 0.025in gear

You need some serious reduction if you are planning to mount a gear on a 3000rpm output shaft if you want to run it at 240ipms.

If your motor is a torgue monster, either a stepper or servo that would be happy running at low speeds, i.e. 500 to 1000rpms you will still need quite a bit of reduction to achieve speeds this slow.

I would suggest a gearhead. Suppose you get one with a 20:1 reduction with your conditions requested
3000rpms becomes 150rpm, you then get 1.6in/rev (required gear)
d = 0.5 inch gear which is better. Your torgue numbers will be out of the roof!!! So you should be able to run a LOT smaller of a motor compared to the one you were planning on running with no reduction.

Suggestion buy a gearmotor servo?

As for your gears, if you can go to your public library and grab a machine design book you can pick the correct pitch. This highly depends on your pinion size! The machien design book will have an equation and you plug in numbers till you get within a safe range. If you do it this way your pinion and rack will last a LOT longer than just saying ohh ill go with a 20 degree pressure angle.

Do you know your loads and acceleration yet this is a big factor in selection as well. How fast do you want to achieve 240ipms, i.e. 0 to 240ipm in 1 second? How heavy is your s#it, also add 30percent to it to account for resistance and efficiencies.

Erik

[RICHARD ZASTROW]

kanankeban, 20deg. pressure angle rack is difficult to find. Usually, rack stock is in short lengths, like 36". I would follow the advice from energyforce and use gearhead(s) to get the reduction you need. Use the largest pinion practical. There is a thing called rolling efficiency that calls for larger dia.'s. The more teeth in the pinion, the closer to perfect tooth form; also wear is distributed among more teeth. There are way systems available with rack & pinion built into them including the leading "v" roller system.
Good Luck

[JerryFlyGuy]

Hector, I've just gone through this w/ my system. From your size's I'm assuming your using it for Plazma? Have you got your Gantry designed? Do you have any idea's how much it weigh's? If I was to design a plasma table, I'd go w/ as big a Nema 23 servo as I could. Then I'd go onto the website of the plasma unit I'm wanting to use, and find out how fast it will cut the plate thickness I want to cut. Then I'd go on ebay, and find the gear heads I need [ search for Bayside Stealth or Bayside motion] the Nema 23 gear heads are usually dirt cheap. I've seem them recently for $30. Once I've found the gearheads I can buy or have bought. Then I'd do the math to figure out what size pinon I need [ DIA of pinion is all you need to know at this point] to get a rapid speed of about 25% more than the rated cutting speed [rated cutting speed X 1.25].

Next I'd calculate out what type for force this motor/gearhead/pinion DIA is going to generate, from this I'd then spec out the gear Diametrical Pitch [DP]

Once this is all done, check your resolution per step from your motor [ should be really small if your using a decent quad. encoder] and you know your spec's. I'd recommend the 20deg gearing over the 14.5 as it has better gear interface, it will be a bit noiser but be better in the backlash dept.
A great place for spec's is Boston Gear go to their open gearing section sign up so you can get to the cataloges [ signing up is easy and they've never bugged me since] . Pay special attention to the torque loads allowed for your spec'd gears as you can usually easily exceed the spur gear ratings pretty easily [ I'd shoot for 50% or less of the max rating].

I just purchased 60' of 12DP rack in 6' lengths from my local Bearing & Transmision store, and it wasn't to expensive.. I'm doing a milling maching that is 20' x 10' x 4'.

Jerry.. [ forget the vision's of sugar plumb's dancing in my head.. I've got gears and cutters and tools.. ]