Threaded rod vs. Gear Rack

[cpcapace]

I am in the process of doing research on building my own cnc, and was wondering my more people don't seem to use a gear rack instead of a threaded rod?

Is it just preferance? I have seen several professional CNC(multi cam and others), some use the rack and others use larger threaded rods?

[ger21]

Preference, cost, and ease of installation. Imo screws are easier to install, and are cheaper in smaller sizes. Most people here use 1/2" acme screws, which are cheap. As lengths get up around 5 ft, screw whip becomes an issue. At this point most seem to use rack and pinion, rather than go to larger screws, which cost more, and require more power to accelerate due to the greater inertia.

If you compare rack and pinion to ballscrews, rack and pinion is much cheaper. However, with rack and pinion, you typically need a gear reduction, which adds cost and complexity.

I'm currently working on a design with a 6 ft X axis. I originally planned on using 3/4" acme. Then started to look at rack and pinion as an alternative. But the complexity of the gear reduction turned me back to the acme, which in my case will be slightly cheaper than the rack and pinion would have been.

[Mike Nash]

Ebay is your friend (sometimes.) I picked up a bunch of Yaskawa servo drives and motors. They were a bunch of separate auctions and the motors and drives were on different auctions, but the motors have nice 10:1 planetary gearheads and the drives are step and direction. I wound up at about $150 per set (shipping included) on average for 11 sets. The blasted connectors added quite a bit to the total however. My (eventual) rack and pinion router will have a theoretical max speed of 1413 ipm but my ball screw mill will max at only 90 ipm. My mill has gotten bogged down on needy ways and my need of proper hand scraping spotting apparatus. My router is awaiting the mill completion.

(Dang that "missing argument" thing is still scary!)

[kylestrong87]

Im working on building a cnc machine right now... i had the same questions about ball versus rack and pinion... my machine is going to have 7'-8' x 3'x4' axis' someone told me i should get the rack and pinion because of motor size and speed... but gear ratios are pretty easy fellas. basically when you go from a bigger gear to a smaller grr the rpm is increased... if you go from a smaller to bigger the rpm is reduced. so if you go from say a 1 inch gear on the motor to a 5 inch gear on the then the rpm is reduced 5 times. so if the 1 inch is spinning at 125 rpm then the 5 inch gear is spinning at 25 rpm... so its not really rocket science... i might be wrong correct me if i am but that is basic gear knowlege... the part i dont know is im guess if its 200 steps at the 1inch then it would be 1000 steps at the 5 inch... the gear sizes were just example... im not sure if teeth actually change anything... if i were to guess it would be accuracy but dont qoute me i have no clue about teeth on gears