Suitable Threaded Bar

[gmc6711]

Hi there,

I'm new to cnc's and lookin at sourcing parts at the minute. Have managed to get some 20mm threaded bar. I would like to know if this would be suitable for screws. Also what sort of nots should I use along with these?

Heres some pics of the bar,

Cheers for your help.

Gmc

[jjdon]

I'm a newbie on this forum, but I'll take this one. Your threaded rod will make perfectly fine screws - for furniture, c clamps, repair work - wherever you need a screw. They are unsuitable for a screw drive for every reason there is - too many to list. Read the forums - there's a wealth of knowlege here. Look at Ball screws, for one-----

[Kipper]

In my opinion it depends on what level of accuracy you wish to attain! You'd certainly be able to machine things with them (one guy on my travels made some nice pieces in wood with just threaded rod) I believe the majority use "Delrin" for the nuts failing that make up some form of antibacklash nut. Trapezoidal (spelling?) thread is fairly cheap on fleabay...Either way it'll work....maybe not too accurate and you can always spend tons of cash later eeking the last ounce of accuracy by buying ballscrews etc There's always a better way of doing things if you have huge pockets lol Have fun :beer:

[OCNC]

Keep in mind that the finer the pitch of the thread the more revolutions it takes to get the drive nut to travel a certain distance. Therefore to get the same linear travel speed you have to turn a fine pitch screw faster than a coarse pitch screw. Stepper motors develop their greatest torque at low speeds. It's best to think in terms of what you want the machine to do and then choose the components accordingly.

Chris

Originally Posted by gmc6711 Hi there, I'm new to cnc's and lookin at sourcing parts at the minute. Have managed to get some 20mm threaded bar. I would like to know if this would be suitable for screws. Also what sort of nots should I use along with these? Heres some pics of the bar, Cheers for your help. Gmc

[jjdon]

My first response to this was vague, maybe. It's true that you CAN use plain old threaded rod. Why it's not a good idea: The best, most economical alternative would be Acme threaded rod, which is what is used on your run-of-the-mill lathes and milling machines. The Acme thread is a square thread, and is known as a "Power Thread". (I've read some posts here that dwarf my knowlege of threads, but this is basic stuff). It is efficient at transmitting power, and it also solves the other problem with threaded rod: It comes in 1/2-10. That means that 10 turns is 1 inch exactly. That has a great impact on your whole screw-turn/indexing/etc. system. While your threaded rod can do the job, and you could deal with the turns/inch or mm issue, in 6 months those threads are going to be gone, because they are not meant to take that kind of abuse in any way. You or I am going to invest however much money and 100+ hours in building a machine to do work and be precise and then in a short time it will be junk - and the junk part will be deep in the innards of the machine. Enco has 1/2-10 Acme rod for $5.74 for 3 feet, and nuts are $1.31 ea. Not a huge investment for the proper part.

[Rhodan]

Since you are building your first machine, I would say go with the regular threaded rod. There's a good chance your first machine just won't be very good at all, even if you bought brand new super-duper ballscrews, linear rails, and servo motors. So, if you're going to do a "practice", use the crappy parts and concentrate on the frame construction etc. Figure out how to make a nice stiff frame and how you want to mount everything by actually making and discarding bits and pieces. By the time the first machine is done, it should be good enuogh to make many of the parts for your second (keeper) machine. Thats when you can spend the big bucks for all the fancy schmansy ballscrews etc!

At least, thats my plan 8) I'm using threaded rods for screws, ready-rod coupling nuts for the travelling nut (suprisingly little backlash when new but don't expect that to last long though) and ball-bearing drawer slides. When its done I plan to use it to cut mounts etc for my second "real" machine, for which I already have a couple of leadscrews and linear rail/bearings sitting here and I'm always browsing ebay looking for more.

I do have machinist training from the Navy, made things like clamps and vices with a drill press, bluing plate, and a variety of hand-files. When we finally got to lathes and milling machines, the project was a sterling heat engine - 1/2 of which was made with the drill press and files, the other half (two cylinders, crankshaft, etc) were made on the lathe and milling machines.

I still expect my first machine would be crap no matter how good the component parts 8)

[gmc6711]

Cheers for the replies guys.

Like u say, my first one will just be an experiment. Then I can think about spending some proper cash on a real machine.

The Acme threaded or "Trapezoidal" rod sounds like good idea to me for the first mahine and I'll be lookin on ebay for some bargains.

Cheers again.

[Zumba]

I agree that your first machine will probably get scrapped. So the threaded rod will be just fine.

When you build the final one though, I wouldn't consider anything other than ballscrews. Note that with a good ballscrew, say 3/4" diameter x .200 pitch, placing the screw in a vertical position, the nut will spin from top to bottom with gravity alone. This lack is friction is exactly what you want.

Likewise, good linear rails that don't bind will make it such that the load on the motor is almost purely inertial, with very little frictional force.

You'd be surprised at how a tiny 270 oz-in NEMA 23 stepper motor powered with a 24v power supply and less than 2A of current is perfectly capable of moving a 100lb gantry at 150IPM, assuming ballscrews, quality linear rails, and 1.5 to 1 gear ratio.

Compare this to a mill table. A 9x42 weighs about 150-175 lbs, but would require at least a 640-oz in stepper with a 48v+ power source and 3 to 1 gearing to move it at 75-100 IPM.

Friction sucks.