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  1. #1
    Join Date
    May 2007
    Posts
    86

    CNCRP Rack and Pinion vs. Lead Screw

    I am working on an 8020 design of a fixed gantry router. The X axis (across the gantry) will be about 34 inches wide. I am considering using the CNCRP Rack and Pinion system due to where the steppers mount, I can get about 4" more travel using this system vs. leadscrew based on the enclosure my router will need to fit in.

    So the question is. Is there any loss of precision with Rack and Pinion? Any loss of rigidity?

    Is R&P as good, better, worst than precision acme rod (1/2" one start) with delrin anti-backlash leadnuts?

    I haven't compared the costs yet...but I believe when you add up all of the mounts, bearings, needle bearings, bearing blocks, etc. It is probably a wash.

    This is the NEMA 23 system btw.

    Thoughts are appreciated.

    OK...did some more reading and thinking on this, so gonna edit this and see if my thoughts are correct. First off, the R&P system requires the use of the extended carriages. Any extra travel that I might gain, could be lost from having to switch to these. I'll have to evaluate that. Next, if I use a 10TPI, 1 start screw, that gives me 2000 steps per inch, put in Micro stepping and I can get up to 16000 (with the hobbycnc driver). With the R&P it looks like about 1527 steps per inch max.

    Curtis

    Ooops...wrong category. Don't see a way to move it though.

  2. #2
    Join Date
    Nov 2006
    Posts
    1036
    I think 34" is a little long for single start 10 pitch 1/2" lead screws. I have no experience but understand others use 5 start lead screws for machines of that size. On my 14x14 inch router, 75 ipm seems fast enough. That may be a little slow on the machine you're going to build.

  3. #3
    Join Date
    Mar 2003
    Posts
    35538
    Yes, 1/2-10 single start really isn't a realistic option for a router, as it's just too slow, usually limiting you to 75-100ipm or less. The R&P is capable of well over 1000ipm.

    Many people have built acme screw driven machines, and then upgraded to R&P, and I've never heard any of them have any complaints with the R&P.
    Gerry

    UCCNC 2017 Screenset
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    JointCAM - CNC Dovetails & Box Joints
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    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  4. #4
    Join Date
    Apr 2007
    Posts
    8082
    I don't have any problems with the r&p system versus ACME screws. They both have their place and the uses overlap for some size machines. From what I've seen accuracy is not really the issue. My 48" long 1/2-10 5 start ACME screws are happy jogging at 700 ipm and my 144" long r&p system jogs at 1,400 ipm. I don't actually use either of them at over 250 to 300 ipm. Mostly between 150 and 200 ipm.

    My r&p system has cut combination composite fiberglass/rhoacell foam/epoxy/solid carbon fiber plate parts that have been within a couple thousandths inch accuracy. I'll be checking the accuracy of the machine again next week for cutting a new set of parts from the same type of material. Paying work when I can get it.

    All of the calendars, signs, and Stargate stuff I've cut is just for entertainment and software use education. I really need to move on to Aspire and the more roundy looking stuff. Somehow, Marilyn Monroe just doesn't look quite right as a V carving.

    CarveOne
    CarveOne
    http://www.carveonecncwoodcraft.com

  5. #5
    Join Date
    May 2007
    Posts
    86
    I guess I should mention here that I would like to design towards aluminum cutting, but will probably mostly do a lot of softer stuff. I like the idea of R&P or using belts to drive the screw as it will keep the motors from sticking out the sides and maximize my space.

    Does anyone see any issues with using belts?

    Has anyone seen an R&P system used on the Y (lower) axis of a fixed gantry machine? I'm gonna play around in sketchup and work on a couple different designs.

    Thanks again for all the feedback.

    Curtis

  6. #6
    Join Date
    Apr 2007
    Posts
    8082
    A center mounted rack under the table would not be a problem, but getting the belt, gears, and motor under the table would be a little tricky. Ahren sells the gear set and belt as spare parts, so a custom plate can be designed to make it all work together.

    CarveOne
    CarveOne
    http://www.carveonecncwoodcraft.com

  7. #7
    Join Date
    May 2007
    Posts
    86
    Well here's a quick mockup of what a center mount R&P would look like for a fixed gantry setup. Still debating the advantage over leadscrew.

    In my case, I'm looking at this to increase Y travel...I think its a wash compared to what I can get with the leadscrew. I actually might be able to get more travel with the leadscrew for my space if I mount the stepper under the table and use a pully to drive the screw. I would then use the standard carriages on the sides instead of the extended and would gain 4 more inches as well as reduce the 3" of motor sticking out.

    Now for someone looking for a fixed gantry but more speed, this might have an advantage.

    Another thing to think about is the table...with such a narrow support under the table, I'm not sure if you'd have to worry as you cut towards the edges. I suspect you might need to, but the 8020 bolted to the top of the carriage might be stiff enough...not sure. I used the standard z axis plate with 4" CRS, you could probably get a better setup by using the High Z (or a custom plate) and 6" CRS or the dual 2.5" crs for 7" wide.

    Anyways...wanted to complete the thought here. Would be an interesting machine for sure.

    Curtis
    Attached Thumbnails Attached Thumbnails Screen shot 2011-12-10 at 11.03.57 PM.png   Screen shot 2011-12-10 at 11.03.40 PM.png  

  8. #8
    Join Date
    Oct 2008
    Posts
    1147
    R&P is great for a larger machine but for less than 4 feet of travel it's kind of over kill. Besides 1000 ipm will be hard to jog over a short distance. You'll wind up limiting it a lot lower. I think a 5 start screw would serve you much better. You'll get rapid speeds around 500 ipm if you want it and get greater resolution than a R&P setup would give you.

  9. #9
    Join Date
    Mar 2003
    Posts
    35538
    If your main goal is cutting aluminum, I'd probably get a 10mm pitch ballscrew instead of the acme, as the plastic acme nuts have a bit of flex in them, and the ballscrew will be more rigid. The ballscrew should be a little faster than the acme as well.

    1 anti backlash ballscrew RM2510-1200mm-C7 for CNC XYZ | eBay
    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

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