Help with lead screw choice

[EL34]

I am about to start building my router machine.
I actually need to have two machines but I need the router first.

Machine build one is a router/engraver/circuit board drilling machine.
The second machine would be more of a small benchtop milling machine for metals, mostly aluminums and plastics, but that is down the road a bit.

I ordered a bunch of the stuff today.
I don't have the exact design drawn out yet but it is based on a 3 axis router very similar to this.


Machine dimentions
Length = 30" - 22.5" of travel
width = 12" - about 6" of travel
Z axis = 8" - about 4" of travel

I am using precision ground steel shafts and bronze bearings just to start with.
I can upgrade the bearings after I get a working machine and figure out what needs changing.

Here's the shaft lengths and diameters
qty 2 x 30" x 3/4" diameter for the long axis.
qty 2 x 12" x 3/4" diameter for the side to side axis.
qty 2 x 8" x 1/2 diameter for the Z axis.

I ordered this 3 axis system from probotics
http://www.probotix.com/multi_axis_s...3axis_sidestep
These guys have awesome documention and diagrams on their site.

I will be using a free roto zip to start with, not a dremel tool.

I have a very cheap source for a couple types of ACME leadscrews but I am not sure if they would be the right thread pitch.
I can get
3/8" x 16 pitch
1/2" x 10 pitch

Would either of these threads be ok for this type of machine.
I will be drilling circuit board holes and engraving plastics mostly.

Any help is appreciated.

[rwwink]

The only thing I might suggest and the you figure the column loading and whipping for the screws. With the table at one end and the motor at the other as you try to accerate the table, the screw will whip..might take out bearings.
R. Wink

[EL34]

The only screws I think could whip woulb be the 30" ones.

The 12" screw is short enough not to worry about, especially since it is surrounded by 2 x 3/4" shafts

I was more interested in the 1/2 screws but they are 10 tpi.
Is 10 tpi common for a direct drive setup?

[IQChallenged]

I'm far from qualified to tell anyone what they should do. But in the interest of looking at every option you can take a look at my machine. I have no lead screws at all.

http://ProjectCNC.wordpress.com

[EL34]

That's pretty cool, hard to tell from the small images but it looks like you are running a rack and pinion gear setup?

I didn't have time to read all your site info.

[High Seas]

You might take a look at he JGRO Builds and see what folks are using there.

Fundamentally, you are at a decision point that is complicated by parts availability AND you probably knew the following already:
If you want to have rapid cut/traverse speeds -- you'll want a few threads on the screw per inch - so you get higher inches per minute. (or less teeth on a rack and pinion).

On the other side - if you're looking for higher resolution - you'll want many threads per inch, so you can move in small increments. (more teeth on Rack and pinion)
(You can improve resolution a bit buy $pending $ome more on high precision-cut ball screws and anti-backlash nuts, or precision cut racks.)

You can balance some this at bit with multi-stepping/microstepping - but then you get to watch out for motor resonance! Trade-offs, trade offs, trade offs.

Hey welcome aboard! Enjoy the journey! Jim

[Coogrrr]

a 1/2 10tpi leadscrew that has only 22.5 inches of travel would likely have to be spining faster than the stepper is likely going to go at 1/8 step resolution. and a 10tpi lead screw at 1/8 resolution typically 200 steps for one rotation means that you will turn that thing 200 steps * 10 to move one inch giving you .002 resolution. If you feel you need more resolutioln then yes you can buy a more "dense" like 16tpi lead screw and even in 1/2 diameter if you want. But honestly from what I have seen 24 inches of travel and doing a rapid the entire length would only whip when you get a good distance between the end bearing mount and the farthest push of your carrage.

What this all means is that anytime your carrage is in the middle of the table or in the middle half your removing the chance for whip anyway. So if you think you can spin that thing sooo fast AND have the carrage the farthest away before it hits the limit switch for long enough to make it whip I would high five you. Your likely never going to get the whip to happen on a real lead screw. DO NOT USE all thread rod beyond 12" as a rule I find.

Anyway I say 1/2 10tpi all the way unless you can and I mean be real serious about this say that you need more than .002 resolution. If you do then get the 16tpi rod and make sure you dont rapid that fast hell at a slow speed that would never whip moving the whole distance wouldnt take a minute and its rare you rapid that far for that long anyway. Larger tables well then all the bets are off but I would do a 48" 1/2 10tpi X or Y axis all day long.

I think your good to go.

Coog

[yanjue]

I have build a router and my screw have a pitch of 20 thread per inch (direct drive). With this setup, my resolution is 0.000025 inch per microstep (10 per step). This is too accurate and my feedrate in cutting is the same as rapid travel because if i increase the speed, the motor stop. I plan to upgrade my screw with 10tpi to increase the speed and the resolution will be 0.00005 inch per microstep.

So if you use screw with 10 of pitch and running fullstep, you will have a resolution of 0.0005 inch, i thing it's good for PCB and many other part. (you don't make part for plane).


Yannick

[IQChallenged]

Originally Posted by EL34 That's pretty cool, hard to tell from the small images but it looks like you are running a rack and pinion gear setup? I didn't have time to read all your site info.

Sorry. After I had made the post I looked my site over and realized there's not a good pic anywhere showing my propulsion. Yes. It's totally R&P. Does good too.

[MonoNeuron]

Personally, I like toothed belt designs because they don't wear as much and are relatively easy to set up. Lead screws and Rack and pinion all wear out unless you keep up the lube but the toothed belt runs dry and doesn't collect dust and detritus like a screw or gear and you can push these things far faster getting better rapids to the next cut point.
Cutting your stock will always be the same speed for a particular material like hard/soft wood BUT it is the rapids between each cut that saves a heck of a lot of time and belt drives allow this.
Look at your car engine. It (most probably) has a belt drive on the cam shaft that lasts 100,000 miles and longer at an average of 3-4 thousand revs.
I built my first router with a 24 tpi rolled thread and it was PAINFULLY slow. it took 9 hours to cut a relatively uncomplicated picture from DeskArt about 6" square. If you are on a budget, and I assume you are, and you really have to use the threaded rod the go the 10 tpi one and direct drive it. From your picture I would think a size 23 double stack stepper would be fine and the system you bought will be easy to set up and use.
I wish I could be there to see the look on your face the moment it jumps into action as it's a life-changing (and bank changing) experience. Then next on your list could be a laser table...................(maybe)
McMaster -Carr and Stock Drive Products have very comprehensive online catalogs with everything you need. McMaster stopped shipping overseas 2 years ago though so I now use SDP-SI instead and they have a great service which I would recommend to anyone.
If you need a teaser of a belt drive system the go here and have a look.
http://www.romaxxcnc.com/video.htm#

I hop this helps somewhat.
warm regards and WELCOME
Richard.

[edo]

I put 1/2" Acme 10TPI on my JGRO and I'm happy with it. I get over 80 IPM in 1/2 step mode and haven't had any problems. Let us know what you decide and how it works out for you.
Ed

[jeff2005]

I built a 36" x 40" router with 6" of z travel. I am using 1/2" - 10 acme with delrin nuts. Direct drive on the motors. The x & y leadscrews are overall 49" long. I get very little whip at 180 ipm rapids. Generaly I cut at under 60 ipm.
Hope this helps
Jeff