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A few CNC build questions

Discussion in 'CNC Mills/Routers' started by Steven Hickerson, Jan 7, 2016.

  1. Steven Hickerson

    Steven Hickerson Well-Known
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    Hello everyone,
    First off, I apologize as I'm sure some of these questions have been asked before, but I'm not having much luck finding the specific answers I seek when searching. So I figured I'd post here and see if anyone had some feedback for me.

    To describe my goal, I'm currently in the research stage of planning to build my own diy cnc mill. My eventual plan is to have it with swapable heads so that I can use the same machine for routing, 3d printing, pen drawing, and possibly some vinyl / other cutting (not sure on the last yet but it's the least important to me). I'm looking to have my initial build have a printable area of at least 18" x 18" x ?? (haven't decided on a desired height yet, I'd say at least 8" though).

    So in doing research I came accross this website and it seems to have a lot of the parts that I'd like to make my build easier. The linear rails available and v wheels being a big part of the movement system I would like.

    However I have a question regarding the OpenRail. I really don't understand the point to using it in conjunction with a V Rail? All the video demonstrations of it show it being used like that, but it looks to me like you could simply slap that OpenRail on a piece of wood and have a working linear rail system for pretty cheap... I don't think I really plan to use wood for my linear rail I'm just illustrating how I don't see the need for both systems. Seems to me you can get by perfectly fine with either only a V Rail and solid V Wheels, or an OpenRail and Delrin V Wheels.

    Another question is concerning threaded rod vs timing belts. Specifically the Pros / Cons of each. One thing I read about timing belts is they are more prone to slippage with heavy loads. I do want my cnc to be able to mill aluminum so not sure if that might be an issue? However another thing I read said threaded rod was more prone to backlash and binding, which would be catastrophic as well. One appealing thing I like about using belts for movement is the easy of homing, I can simply switch off power to the steppers and move everything back into the zero position. I wouldn't be able to do that with threaded rods, would have to use the steppers to move it back.

    My final question for now (there will be plenty more later if communication is good on here I'm sure), in some videos I see a hose attached near the tip of the router. Some of them look like shop vac hoses, but others look like they are blowing air out (presumably to keep the work surface clear). Is one method substantially better than the other for this?

    Thanks for any answers!
     
  2. JustinTime

    JustinTime Master
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    OpenRail is being used because it is made out of steel and therefore harder than the V-Slot. If you run steel wheels on the aluminum the aluminum will ware out, the steel wouldn't. Yes, you can use it on wood, and it has been done a few times, but wood is prone to warping/bending/twisting whereas aluminum will stay, mostly, true through different situations like humidity and temperature.

    Moving the parts by hand is not a good thing unless the stepper controller has builtin protection against reverse voltage since moving the parts by hand turns the stepper from a motor to a generator and the faster you move the part the higher the voltage. Having said that I do use belts on my machines and I do move the parts by hand but I do it very slowly to prevent any possible damage. :D
     
  3. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    OpenRail is anodized aluminum and should not be used with steel V-wheels. As far as application to wood, wood also shrinks and swells with changes in humidity which could affect the C/C spacing of the rails and thus the wheel's grip on the rail. If you are trying to keep costs down, consider applying it to aluminum bar from the local home center and then mounting that to wood. This may or may not save you anything however over the costs of just using extrusions.

    As far as the difference screws and belt, look at the distance traveled with each turn of the stepper. With the OpenBuilds TR8*8 screws, that distance is 8mm. With GT3 belt, that distance is 60mm (assuming a 20 tooth gear). It takes a lot more effort on the part of the motor to move 60mm vs. 8mm so the amount of power each system will have is considerably different. There is also a difference in the resolution of the motion. If each turn represents 200 steps of the stepper motor and you have the steppers subdivided to 1/8th steps, the resolution would be (roughly) 0.0375mm for the belt driven system and 0.005mm for the screw driven system.
     
  4. Steven Hickerson

    Steven Hickerson Well-Known
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    Thanks for the replies. Yeah, after reading your replies and thinking about it more, I think I do like the lead screw driven system more. Even though I can't freely move it (and apparently shouldn't even if I could lol), I think I like the strength and higher resolution of the screws over the belt. I'll have to build a control panel to move the motors outside of software or something :)

    Right now I'm thinking of using a couple c-beams for my x,y axis with the lead screw in the middle of each (plan on doing dual motor drive for y axis). But haven't decided how I'm going to do z-axis yet. C-beam seems a little bulky for it.

    Another follow up question concerning the stepper motors. I'm looking at Nema 23s, I see some on the parts store here that are 175 oz/in torque. Is that going to be enough to handle things like aluminum? I found a set on amazon for not to much more that are 270 oz/in and one set that is a descent bit more that are 460 oz/in. Just not sure how much torque I actually need.
     
  5. Compton

    Compton New
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    @Rick 2.0 , what is the need then for both OpenRail and V-Groove? Why not just use V-Groove? I could understand using the OpenRail if you are mounting to traditional HIWIN rails.

    For my build I want to add a very lightweight free moving x-axis carriage to my router mount. Something that I can attach pens or markers to that as the CNC moves the carriage will allow the tool to move up and down smoothly with the contours of the material. My router mount has three vertical M4 holes 30mm apart that I want to use for attaching it. What would your suggestion be for this application?
     
  6. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    As for the reasoning behind both OpenRail and V-Slot.... not a clue.

    Attached is a concept for a pen holder. The rubber band at the back will hold gentle downward pressure on the pen. Just drop the gantry until the tip touches and then drop the gantry another 1/16" to maintain contact. You will probably want the z-gantry moving up and down with the terrain if there is a significant amount of vertical difference.

    The pen can be attached with rubber bands or hose clamps.

    Pen Holder.jpg Pen Holder Detail.jpg

    The detail to the right shows both the tension band and the stop bolt at the side.
     

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    GrayUK likes this.
  7. Compton

    Compton New
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    @Rick 2.0 Very cool design thanks for sharing, I think that is a great solution to use the rubber bands.
     

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