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Z-Axis Travel

Discussion in 'CNC Mills/Routers' started by AlexHP, Aug 8, 2016.

  1. AlexHP

    AlexHP Well-Known
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    I've been doing a lot of research regarding the z-axis travel height/length (whichever...) and have been coming up confused. I've seen builds with z-axis travel from 2 inches to 12 inches. I have yet to find a good way of determining what my z-axis should be and how to determine it (well, I have found some calculations that require some information I don't have yet but...). Can anyone help me figure out a good starting point (as large as possible) in selecting what I can and cannot use for a z-axis length? I am making a 1000mmx1000mm work area machine with ballscrews for the linear motion and linear rails instead of wheels. I want to be able to mill metal (i.e. aluminum, maybe some others) as well as wood. I want to be able to have some good depth penetration of the spindle tool but also to be able to work on areas that might be thick (therefore I would need more z-axis height). This is the final part of my main design that I need to work out so I can post my design (still working on the electronics and such). Any help would be most appreciated. Thanks!!

    Alex...
     
  2. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    The real failure in Z-axis design is the approach of jacking the X-axis way up and then dangling the Z-axis back down to the work surface as a means of achieving Z-axis depth. This approach will not ever lead to satisfactory results unless you are doing little more than cutting foam. The problem is that when you are cutting the things that require the most rigidity (i.e. aluminum) the router is dangling down to its least rigid position. If this is the method you're wanting to use, stick with 2".

    If you're looking for alternate suggestions on how to get the best possible rigidity out of the Z-axis when you need it most, take a look at Richard Gouge's C-Beam Big Mouth. His approach of moving the entire X-axis gantry up and down increases the rigidity of the system as it gets closer to the base thus making it more usable for cutting thin stiff elements like aluminum but allows height where less rigidity is needed for doing something like carving initials on the top of medium sized wood boxes. With proper bracing on the back side of the uprights, there's no reason this couldn't easily provide a Z-axis depth of upwards of 12".
     
  3. AlexHP

    AlexHP Well-Known
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    Thanks, Rick. Food for thought...

    I am actually basing my design, roughly, on the Promoveo design that Ronald submitted. It should be interesting...

    Alex...
     
  4. EvanBruner

    EvanBruner Veteran
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    If you are using linear rails for all your axis, I wouldn't worry too much about rigidity as long as you make your plates thick and use big extrusions. I'm designing a sbr16 and 1605 ballscrew machine right now in fact. I plan on having a 250mm z axis that I can adjust it's height on the gantry to accommodate larger parts. With a z axis with 4 sbr16uu bearing blocks spaced 20mm apart I get a useable travel of roughly 150mm or 6 inches. That should be more than enough for my purposes. Don't forget to account for the bk12 and bf12 ballscrew supports.

    You can also do what rick 2.0 said about lifting the x axis rather than the router, but with ballscrews and linear rail it can get expensive quick. You'll also need another 4 motors (5 if you have two ballscrews for y).

    Let me know if you have any more questions, I'm happy to help :)
     
  5. AlexHP

    AlexHP Well-Known
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    Thanks, Evan. This is along the lines of what I was thinking, however, I was thinking about SBR12 rails about 300mm in length. Or possibly 400mm but I would have to redesign the plate I am using to attach it to in the z-axis direction. Do you think SBR12 is good enough or do you think I should go with the SBR16? Please advise. Thanks!

    Alex...
     
  6. EvanBruner

    EvanBruner Veteran
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    I've uploaded an image showing how I designed my z axis.
    To figure out how long you need the axis to be, first figure out how much clearance you want. That's going to be the distance between the bit and the spoilboard at its highest point. Start by thinking about the thickest material you want to cut, add an inch or two for clearance and you have your clearance height. I wanted roughly 150mm of travel, so A= 150mm. My bearings are each 45mm long and I have them spaced at 20mm apart. So B= 45 + 45 + 20= 110. So now I know that C = A+B= 260mm. I will need 260mm rails to get nearly 6 inches of travel between my z max and the spoilboard.

    Other considerations:
    -keep your z axis travel as low as you can. If you only ever plan on milling 1 or 2 inch thick material, you don't need 8 inches of travel. The longer the bit has to travel to the workpiece, the larger the moment exerted on your gantry and the more the material is going to fight you (resonance, chatter, gantry twist).
    -bearing spacing is important, the more the better. I need to fit my machine in a 28" enclosure so I kept the bearing spacing low to save space.
    -supporte linear rails work best with the load applied directly into the mounting surface, side loads to a lesser extent, and 'upside down' they aren't very effective. Keep this in mind when designing your x and y axis.


    If you haven't bought the rails yet I'd go with SBR16 because the price difference is so minimal. I don't think the difference in rigidity is all that much since the weak point on these builds is usually the extrusions they are mounted to. If you can find SBR12 for a significant savings over SBR16, go for it though I wasn't able to see much difference. I was able to get mine with ballscrews, bearings, and couplers for 270 shipped from China with good reviews. They won't be here for a few weeks so who knows how they'll turn out though. Z Axis Design.png
     
  7. AlexHP

    AlexHP Well-Known
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    Evan, thanks for the tips. I'm still sourcing parts but I may have a lead on SBR12 at a good price. I'll let you know. Also, I was wondering about switching the configuration around a bit. What if I mounted my bearings to the z-axis plate (on the y-axis gantry) and mounted my rails to the spindle cutting head instead? Then I could use longer rails and space the bearings further apart allowing for a longer z-axis travel yet still giving the necessary rigidity. What do you think?

    Alex...
     
  8. EvanBruner

    EvanBruner Veteran
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    That's a common configuration and plenty of people use that method. It's more weight for the stepper to move though, now that z stepper has to move 600-800mm of rails, the ballscrews, the router and its mount, and the plates used to join them all together. If you don't have a powerful enough motor you can start to see some z creep. Just something to think about.
     
  9. AlexHP

    AlexHP Well-Known
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    #9 AlexHP, Aug 9, 2016
    Last edited: Aug 9, 2016
  10. Barry Danks

    Barry Danks Veteran
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    You might want to stick with a stepper that is 1.8deg per step...Just my .02 ;)
     
  11. AlexHP

    AlexHP Well-Known
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    I'm curious about that... I've seen that many of the motors used are 1.8deg steps (200 steps per rev) instead of 0.9deg steps (400 steps per rev). Wouldn't the higher resolution motor give you better accuracy in your cuts and a smoother finish? Why is 1.8deg better than 0.9deg? Please explain. Thank you.

    Alex...
     
  12. Barry Danks

    Barry Danks Veteran
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    I've never seen the need for higher resolution steppers. For example an 4 start 2mm pitch rod which is the standard on open builds moves 8mm per rev divide that by steps per rev in this case 200 and you end up with .04mm per step and that is without any micro stepping
     
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