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View Full Version : How to choose rack and pinion...?



kanankeban
12-02-2004, 09:06 PM
Hi...
I was trying to figure out what is the most appropiate rack and pinion setup for a 6ftx20ft table?
And also trying to figure out what will be a good servo-timing belt setup for this same table, I will be using a plasma torch instead of a router or spindle so not much torque is needed...maximum speed aprox...100 ipm
Could someone help me out in this decision, have to buy some rack , servos, and timing belt very soon... :banana:
Regards,
Hector

jcc3inc
12-03-2004, 10:24 AM
Dear Sir:

Sounds like the cutting machines we used to make!! (Retired now)

For a machine like you plan, I would consider 24 dp rack with 20 degree pressure angle. Of course the pinions are 20 degree also. I would use pinions with around 32 teeth so that several teeth are in contact at a time and there is no undercutting of the teeth. The 20 degree pa runs more smoothly that 14 1/2 degree gears. You can spring load the pinion into the rack to mimimize the backlash. We used gearboxes from Bison Gear; they made some low backlash units for the plasma cutter manufacturers. I had even bought some prototypes from W W Grainger for test!

The gear rack is available in 1/2 x 1/2 inch and 6 ft lengths. We purchased our rack from Standard Steel Specialties in Duncan, SC. I once checked the accuracy of a 6 foot piece and found it to have an accumulated error of around .020 overall.

If you mount your drive using a pivoting arrangement, you should place the pivot point over the pitch line of the gear rack; this results in no different gear loading if the force is in one direction or the other.

If you can place the rack with the teeth vertical, there is less tendency to have dross or dirt collect in the rack. Also, place some sort of shield on your machine so that the spatter from piercing cannot fall directly on your gear or the rack.

As to drives, we ran around 20:1 reduction overall including motor, belt, gearbox. Our motors were 2" diam with DC tachs. Our CNC was Burny. The max speed was 250 IPM, not too hard to come by. Main carriage weight was 50 to 150#.

What will you use for CNC? What plasma and what type material will you cut?
You will need an auto THC if you are in the business. Contact me if I can be of further help. jccinc@owc.net

Regards,
Jack C.

kanankeban
12-03-2004, 08:37 PM
Dear Sir:

Sounds like the cutting machines we used to make!! (Retired now)

For a machine like you plan, I would consider 24 dp rack with 20 degree pressure angle. Of course the pinions are 20 degree also. I would use pinions with around 32 teeth so that several teeth are in contact at a time and there is no undercutting of the teeth. The 20 degree pa runs more smoothly that 14 1/2 degree gears. You can spring load the pinion into the rack to mimimize the backlash. We used gearboxes from Bison Gear; they made some low backlash units for the plasma cutter manufacturers. I had even bought some prototypes from W W Grainger for test!

The gear rack is available in 1/2 x 1/2 inch and 6 ft lengths. We purchased our rack from Standard Steel Specialties in Duncan, SC. I once checked the accuracy of a 6 foot piece and found it to have an accumulated error of around .020 overall.

If you mount your drive using a pivoting arrangement, you should place the pivot point over the pitch line of the gear rack; this results in no different gear loading if the force is in one direction or the other.

If you can place the rack with the teeth vertical, there is less tendency to have dross or dirt collect in the rack. Also, place some sort of shield on your machine so that the spatter from piercing cannot fall directly on your gear or the rack.

As to drives, we ran around 20:1 reduction overall including motor, belt, gearbox. Our motors were 2" diam with DC tachs. Our CNC was Burny. The max speed was 250 IPM, not too hard to come by. Main carriage weight was 50 to 150#.

What will you use for CNC? What plasma and what type material will you cut?
You will need an auto THC if you are in the business. Contact me if I can be of further help. jccinc@owc.net

Regards,
Jack C.

I plan tu use maybe rutex or geckos with servos, for plasma a hypertherm 1650 o go for the high definition 4070 auch! for the THC the one from rutex or go for the one that hypertherm sells. I plan to cut 1" steel or maybe more...but the high production will be 1/2 and 5/8 carbon steel.
Your recomendations looks kind of heavy :) I see that kind of rack like for a 200lb gantry, but to be honest I dont have a clue, If you say so, and you have experience on that I will take your word an go in that direction. My gantry will be made of a 3x3" 80/20 extrusion on vee bearings or cam followers that kind og light, what advice do you give me in this kind of gantry...should I throw away this idea or build a heavier and more reinforced gantry?
Regards...
Hector

jcc3inc
12-08-2004, 10:19 AM
Dear Sir:

From the type steel you are cutting, have you considered oxy-fuel torches? You must have plasma to cut aluminum, stainless, and other non-ferrous materials, but oxy-fuel will do a good job on steel. You should definitely look into it!

As to your machine construction, the 3" aluminum is WAY TOO LIGHT for a 6 foot wide machine. The smaller machines we built used a 4 x 6 inch steel tube, I think it was 14 ga, for the main cross beam. We welded two cold drawn flats onto it, one on the top and one on the bottom edges. These were then ground flat to true the surface and another flat was bolted to the top edge to serve as the top machine ways.

Well, you know, there are lots of details which I could convey to you; maybe we ought to talk by phone, or maybe I could send you photos or possibly some drawings I still have. Why not E-mail me with as much info as you can think of. jccinc@owc.net

You might as well get the optimum system for the jobs that you are doing. Also please tell me where you are located.

Regards,
Jack C.