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  1. #1

    Rack and pinion vs ballscrew

    Designing cnc router approx 32 inch y axis and 72 inch x axis. So I know most will say go rack and pinion cause of ballscrew whip I understand this so guess my real question in a rack and pinion system the pinion gear is not really meant to be just cramped against the rack by spring pressure . Racks and pinion gears are actually designed to have a clearance maintained between them how are you all accomplishing this or are you not. Yes you will have some backlash but if gap is correct should be minimal. Ideal the thing is with ballscrew less backlash and easy replacement if does wear. I would like to see rapid speed maybe 500ipm and cutting speeds of 350ipm. Appreciate any thought

  2. #2
    Community Moderator ger21's Avatar
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    Re: Rack and pinion vs ballscrew

    99% of the DIY rack and pinion machines spring load the pinion into the rack.
    I've purchased some helical rack for a new machine, where the pinion will be fixed (on a gearbox), but it'll be a while before I get it built.
    Gerry

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  3. #3

    Re: Rack and pinion vs ballscrew

    Yeah I've seen a lot are using helical racks fig that would be a lot better. What if a guy used say 25mm lead ballscrew then only need like 500rpm to get to say the 500ipm rapid moves. But assume you would affect the accuracy and not sure how the steeper lead angle would play with the steppers

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    Re: Rack and pinion vs ballscrew

    Hi Dfish - The issue with achieving the correct gap all along the rack is that home builders don't have access to mills big enough to machine the rack land to the correct tolerance. Then the machine also has to be correct for the entire length to achieve the correct gap. So a spring loaded system whether straight or helix is preferable. The ballscrew suppliers have formulas for checking length and whip calculations. You can also go travelling nut. 1800mm is not long for a ballscrew... Balancing speeds and forces is always a juggle. Best plan is to decide your required rapids and cutting feed speeds then back calculate to your motor requirements. Servos give you the best speed/torque range but cost more then steppers. If your only cutting timber consider large belts they have lots of advantages at that length... Peter

  5. #5

    Re: Rack and pinion vs ballscrew

    Really like the ballscrew idea better. The other thing is most of these steppers don't talk about how many rpm's they turn or show you a torque curve related to the rpm's. Would rather use a 10mm lead and that's puts me turning like 1270rpm to reach 500ipm rpm and 890 rpm for 350ipm feed rates max of course think motors I did see rpms on some were 1500 and some were 3000. So even the ones with 1500 at 880rpm for federated that is still only 3/5 of the rpm max for the motor would think i would still end up with decent torque. Any input appreciated. Now need to check into the whip specs

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    Re: Rack and pinion vs ballscrew

    Hi DFish - most stepper suppliers have the speed/torque curve chart available just ask. Look at stepperonline most of their motors have torque curves attached...But they generally are a linear decline from stall torque (zero rpm) to no torque at 1000rpm. Hybrids do a little better but the $$$ to performance isn't there I feel at the moment. Better to put the $$$ elsewhere. I usually don't count on much useable torque past 500rpm... especially with a ball screw, its inertia chews up a lot of torque, even 50% of it. Also use as high a voltage possible 50-60V your motors won't go fast at 24V. Need low inductance and high voltage. Search forum heaps of data on that topic here and electronics not my strong suite... many cheap steppers have high inductance and won't spin much... lots of comments in here on that.

    be careful with torque estimators that don't include the ballscrew inertia..

    Steppers won't run or provide drive at 1270 or 890rpm so you will need a gearbox or pulley say 3:1 so then 1270/3= 423 so good but need lots of torque depends on your acceleration or tooling loads which is next next question...

    Now servos provide full torque over entire speed range, asian servos maybe your answer 0-3000rpm easy. By the time you muck about with gears etc you will be at similiar cost...Peter

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  7. #7

    Re: Rack and pinion vs ballscrew

    Servo in that price range sounds possible not sure what size best fits a router for cutting hardwoods, plywood, aluminum and never steel. I know almost nothing about calculating inertia and sizing the motors to match and I'm sure that weight of the gantry will come into play

  8. #8
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    Re: Rack and pinion vs ballscrew

    Hi DFish - Firstly you finalise structure so you know the weight of everything. Then you can use suppliers calculators to calculate torque required to drive machine at required accelerations. The power and torque required to move something slowly and cut is quite small. Its the linear and rotational acceleration requirement that drives the torque up. Peter

  9. #9
    Community Moderator ger21's Avatar
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    Re: Rack and pinion vs ballscrew

    Yeah I've seen a lot are using helical racks fig that would be a lot better. What if a guy used say 25mm lead ballscrew then only need like 500rpm to get to say the 500ipm rapid moves. But assume you would affect the accuracy and not sure how the steeper lead angle would play with the steppers
    Because steppers have a limited resolution, and lose torque as speed increases, there's always a tradeoff between speed and resolution. What you want to have is somewhere between 1/2"-1" of travel per stepper revolution.

    Really like the ballscrew idea better. The other thing is most of these steppers don't talk about how many rpm's they turn or show you a torque curve related to the rpm's. Would rather use a 10mm lead and that's puts me turning like 1270rpm to reach 500ipm rpm and 890 rpm for 350ipm feed rates max of course think motors I did see rpms on some were 1500 and some were 3000. So even the ones with 1500 at 880rpm for federated that is still only 3/5 of the rpm max for the motor would think i would still end up with decent torque
    Smaller motors will typically have more torque at higher rpm's then larger motors. Max rpm ratings for a stepper are useless. If you have good drives, a big power supply, and low inductance motors, you should be able to get a max useable rpm somewhere between 700-1200rpm.

    A big issue that comes into play is acceleration. Since almost all hobby level controls use linear acceleration, you need the same amount of torque to accelerate at 100rpm or 700 rpm. So, acceleration is what really limits your top speed.

    And one other thing to consider with ballscrews, is that they have a lot of inertia, and require much more torque to accelerate. That's why most faster DIY machines use rack and pinion.
    Gerry

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  10. #10
    Community Moderator ger21's Avatar
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    Re: Rack and pinion vs ballscrew

    Hi Dfish - The issue with achieving the correct gap all along the rack is that home builders don't have access to mills big enough to machine the rack land to the correct tolerance.
    Yes, this is critical. I have access to a big machine that I used to machine the mounting surfaces for my linear rails and gear rack.
    Gerry

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  11. #11
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    Re: Rack and pinion vs ballscrew

    Hi Dfish - your going to need a large screw with the numbers your talking, maybe a driven nut for you it is. and in another thread a fellow set up a jig and tested the Hiwin formula and it was correct....But people do put in supports half way along to stop the screw drooping and this works. The support has to be able to move away as the nut goes by. Its usually on a spring. Peter

  12. #12

    Re: Rack and pinion vs ballscrew

    So was in my mind thinking 25 to 30 mm dia ballscrew and probably 25mm linear rails now going with the helix rack and pinion very well maybe and option hoping I can weld frame have it stress relief and machined at a decent price where I used to work that may work will probably do two designs one from steel and other from aluminum extrusion we shall see don't mind the Chinese servos if they hold up ok

  13. #13

    Re: Rack and pinion vs ballscrew

    What I really wish is the someone in US would distribute Chinese routers as bare bones machines with quality linear rails and helical racks with a lubrication system in place and you just add the rest yourself motors, controller, spindle
    , etc. They could offer sizes better suited for home shops. I mean I would ideally like something where I could machine half a sheet of Baltic birch so say 32 inch x 62 inch travel and setup in a way where on the end of the machine I could stand boards vertically for dovetailing so say maybe and extra 4 to 6 inch in y axis that more or less goes into space and you could hopefully stand say a 36 inch long board vertically wouldn't this be awesome and so much quicker and affordable but still pretty much let you customize the machine. Maybe one of you all have them good China contacts. Heck maybe some of you would even want and extended gantry for a 4th axis off to the side

  14. #14

    Re: Rack and pinion vs ballscrew

    And here's the other thing say I go with Jianken 2.5 kw spindle what can I expect for cutting depth and speed in say hardwoods, plywood, MDF. I mean if the spindle can't cut at 350ipm and a decent depth of cut then no use having a machine with feedrates that fast

  15. #15
    Community Moderator ger21's Avatar
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    Re: Rack and pinion vs ballscrew

    Lots of factors come into play with regards to cutting speeds. It's complicated.

    First, 350ipm is actually not very fast. The spindle shouldn't really have any trouble with that.
    However, these cheap spindles have most of their torque at 24,000 (or max) RPM. Depending on the tool size, you might find that the ideal RPM at 350IPM is only 13,000-15,000rpm, where the spindle has less torque.


    Then you'll find that machine rigidity may come into play. If the machine flexes or vibrates, it can effect cut quality, and require slower feedrates, or shallower cuts.
    Same thing with work holding. You may find that you can't hold the part down well enough to cut at higher speeds.

    I mean if the spindle can't cut at 350ipm and a decent depth of cut then no use having a machine with feedrates that fast
    There certainly is. After using the machine for a while, you'll wish it was faster. Everyone does.

    Bottom line, is that very few machines are limited by spindle power with a 2.2-2.5Kw spindle. You'll typically see other issues restricting speed before you run out of spindle power.
    Gerry

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  16. #16

    Re: Rack and pinion vs ballscrew

    So maybe ideally I would want to try and match spindle with federate a little better and shoot for maybe 500 or 600ipm federate to take advantage of the spindle rpm and torque better when needed assuming the machine would be rigid enough to handle it and Zito can hold the work down

    - - - Updated - - -

    Sorry not Zito supposed to be I can hold the work down

  17. #17
    Community Moderator ger21's Avatar
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    Re: Rack and pinion vs ballscrew

    Every machine is different, and the cutting speeds you use will be what works best for your machine, the tooling you are using, and the material you are cutting.
    Note that if you double the cutting speed, you are also doubling the cutting forces, and the torque needed to make the cut. Depending on depth of cut and tool size, 600ipm may not be attainable.
    Gerry

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  18. #18

    Re: Rack and pinion vs ballscrew

    I understand that would rather see the limits closer to you can't push that dia cutter that fast in that material weakness be closer to the strength of the cutter not the unobtainable federate or machine rigidity

  19. #19
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    Re: Rack and pinion vs ballscrew

    ...another way I've seen somewhere on cnczone is Timing Belt/Gear axis drive. The cost maybe a lot cheaper then ball screws if, you do not need the rigidity
    https://www.mcmaster.com/timing-belt...iming-belts-8/

    DJ

  20. #20
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    Re: Rack and pinion vs ballscrew

    Hi Dfish - What your talking about is definitely servo territory. So now you have to start on the structure. I'd find a thread here were someone has already done it or find a commercial machine to use as a benchmark. If you have access to stress relief then steel is your go to. AL extrusions will limit your possibilities. I suggest a high rail design to start... I also suggest you fabricate the gantry vs using std tube in this way you can control thicknesses and size better and have metal behind the rails. So much to get through... Peter

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