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  1. #1

    Thoughts/Ideas

    Hi,

    Just draughted up a design for a 4x4" router, (missing a few parts but the general design is there).
    Rails will be HGW25, HGR20 on the Z axis, 2510 and 2010 ballscrews respectively. Rough gantry wiehgt will be 80-85kg.
    Rails will be top/bottom mounted to minimise Z axis offset, with ballscrew at top.
    I will use some 8mm plate and bolt or weld to the box section, then skim flat to provide a decent surface for linear guides.
    Z axis will be supported by a couple of one way air cylinders coupled with a 5L reservoir.
    Driven by NEMA 34 with 4.5Nm holding force, closed loop system and dual drive Y. UCCNC controller AXBB-E.

    My main debate was whether to mount the gantry/X rails facing the same way like a lot of CNC routers (ballscrew in middle),
    or like my current design (top and bottom). The current design significantly increases the rail spacing and rigidity however moves
    the ballscrew to the top of the gantry. Being large linear bearings I was assuming that moments causing by the ballscrew acting on them from its
    current position wouldn't be significant. I like the idea of having the Z axis close to the gantry for rigidity, especially as I want to
    go through a fair bit of
    aluminium and hard wood.

    any thoughts/ideas/improvements welcomed.

    Cheers

  2. #2
    Join Date
    Feb 2017
    Posts
    59

    Re: Thoughts/Ideas

    Your design is very similar to the very popular PrintNC design for DIYers. The PrintNC is highly customizable, is made of bolt on steel tubing and seems to have large following and supportive community. I have not build one (yet) but I am reviewing the videos and freely available Fusion 360 model. Take a look at the video for all the details that have been thought through. It's amazing work.

    https://youtu.be/k_gGgE79noc

  3. #3
    Join Date
    Jul 2018
    Posts
    6254

    Re: Thoughts/Ideas

    Hi Geordie and Junior - Yes the PrintNC is clean and tidy. I disagree about putting two motors on one belt. In applications of displacement accuracy you have to use a stiff belt. This means the motors cannot share the load as you would think and only one motor will be dominant in each direction and you don't get twice the torque. If you need twice the torque then use a bigger motor or 5mm pitch screw. plus you then need two drivers which adds cost. if the motors are driven from the same driver they will perform poorly... Keep designing. Peter

    Re Geordies design about top bottom or front face. If you use SHS it will be unlikely the top and bottom faces will be parallel they will have to be faced off. The PrintNC gets away with various things as the connections are using printed plastic parts that can move. This compensates for the SHS crowning or cupping that is natural to those sections. If you use large SHS aluminium section you may have a better chance of the surfaces being flat and parallel. The common method of having the rails on the same face means the rail land can be machined in one setup which is quick and accurate. To machine the top and bottom in one set up means using a long lollypop type tool that can skim one side then move to the other... Keep at it Peter

  4. #4
    Join Date
    Jul 2018
    Posts
    6254

    Re: Thoughts/Ideas

    Hi Geordie - You have the gantry drive screw thru the gantry. This either traps the ballscrew or the gantry and Z assembly. Better to have the gantry land on the bearings without having to disassemble the ballscrews. The drives take a bit to set up and settle. Once done you don't want to go thru that too often. Design machines like pyramids each major bit sits on top of the other for easy assembly and disassembly. with no backtracking in the process. Plus don;t bury any screws heads or nuts so that when they come loose you can check them or retighten them without pulling the machine apart... in the saddle area this is difficult but doable...

    You also have the gantry wall rails in the middle of the SHS. This area is quite springy. Have them as close to the corner as possible to increase the local stiffness. Peter

  5. #5
    Quote Originally Posted by junior2016 View Post
    Your design is very similar to the very popular PrintNC design for DIYers. The PrintNC is highly customizable, is made of bolt on steel tubing and seems to have large following and supportive community. I have not build one (yet) but I am reviewing the videos and freely available Fusion 360 model. Take a look at the video for all the details that have been thought through. It's amazing work.

    https://youtu.be/k_gGgE79noc

    Hi, thanks for your reply,

    PRINTNC looks great, Its where I got a significant part of the design from, I'll have to look into some more infor and videos

    Kind regards
    John

  6. #6
    Quote Originally Posted by peteeng View Post
    Hi Geordie - You have the gantry drive screw thru the gantry. This either traps the ballscrew or the gantry and Z assembly. Better to have the gantry land on the bearings without having to disassemble the ballscrews. The drives take a bit to set up and settle. Once done you don't want to go thru that too often. Design machines like pyramids each major bit sits on top of the other for easy assembly and disassembly. with no backtracking in the process. Plus don;t bury any screws heads or nuts so that when they come loose you can check them or retighten them without pulling the machine apart... in the saddle area this is difficult but doable...

    You also have the gantry wall rails in the middle of the SHS. This area is quite springy. Have them as close to the corner as possible to increase the local stiffness. Peter

    Hi, thanks for your reply,
    Lot of good info here, makes sense. I'll probably move the gantry/X rails closer to the edge to gain more support. As for the ballscrew positioning that also makes better sense, I can mount the gantry to the rails without manoeuvering the ballscrew through, I can move the ballscrews to he side of the machine and use some mounting plates, may keep more swarf/chippings off them to.

    Cheers
    Kind regards
    John

  7. #7
    Quote Originally Posted by peteeng View Post
    Hi Geordie and Junior - Yes the PrintNC is clean and tidy. I disagree about putting two motors on one belt. In applications of displacement accuracy you have to use a stiff belt. This means the motors cannot share the load as you would think and only one motor will be dominant in each direction and you don't get twice the torque. If you need twice the torque then use a bigger motor or 5mm pitch screw. plus you then need two drivers which adds cost. if the motors are driven from the same driver they will perform poorly... Keep designing. Peter

    Re Geordies design about top bottom or front face. If you use SHS it will be unlikely the top and bottom faces will be parallel they will have to be faced off. The PrintNC gets away with various things as the connections are using printed plastic parts that can move. This compensates for the SHS crowning or cupping that is natural to those sections. If you use large SHS aluminium section you may have a better chance of the surfaces being flat and parallel. The common method of having the rails on the same face means the rail land can be machined in one setup which is quick and accurate. To machine the top and bottom in one set up means using a long lollypop type tool that can skim one side then move to the other... Keep at it Peter
    Hi, thanks for your reply, good info

    I won't be using any belts on this,all ballscrews will be direct drive to minimise parts needed, with a driver per motor, and the second Y motor slaved in the UCCNC software with it's own homing switch on separate input. The easier machining of the rail mounting plates definitely makes sense, I'll have a think about that.

    Cheers
    Kind regards
    John

  8. #8
    Join Date
    Jul 2018
    Posts
    6254

    Re: Thoughts/Ideas

    Hi John - When designing a machine its sometimes easier to design the structure first and not get bogged down by the drives. The structure is what's important and it needs to be simple and really rigid. Once you settle down the structure you then add the drives. The drive is secondary to the structure and if you try to combine them too early you will compromise the structure. It takes several design rounds to boil down a design. Maybe 20 rounds for a really good design.. Z axis are usually underdone. It took me 3 machines to get the Z stiff enough to my liking. If you start designing from the outside in you generally run out of geometry for the Z axis and try to put too much into a small space. So once you have a basic frame philosophy sort the Z first then the machine is built around the Z axis. Make the Z axis as big and stiff as you can, the printnc has a very thin Z axis plate this allows it to flex to take care of other inaccuracies. Plus the Z axis drive is plastic and sits in a space with no screws so can wiggle. If you make a rigid Z then the rail foundation surfaces need to be accurate or the cars jam (this has happened to me) Peter

  9. #9
    Quote Originally Posted by peteeng View Post
    Hi John - When designing a machine its sometimes easier to design the structure first and not get bogged down by the drives. The structure is what's important and it needs to be simple and really rigid. Once you settle down the structure you then add the drives. The drive is secondary to the structure and if you try to combine them too early you will compromise the structure. It takes several design rounds to boil down a design. Maybe 20 rounds for a really good design.. Z axis are usually underdone. It took me 3 machines to get the Z stiff enough to my liking. If you start designing from the outside in you generally run out of geometry for the Z axis and try to put too much into a small space. So once you have a basic frame philosophy sort the Z first then the machine is built around the Z axis. Make the Z axis as big and stiff as you can, the printnc has a very thin Z axis plate this allows it to flex to take care of other inaccuracies. Plus the Z axis drive is plastic and sits in a space with no screws so can wiggle. If you make a rigid Z then the rail foundation surfaces need to be accurate or the cars jam (this has happened to me) Peter

    Your probably right with that haha, I do have a tendency to overcomplicate things sometimes. I think going back to your post of putting the two rails on the same side will greatly simplify things, and allow me to focus on getting a solid frame and z plate mount set up before moving on. Thinking about it more putting the bearings on the same side will only move the head out another 30mm, and it would also provide support closer to the spindle. Having this weight slightly further out with such a large gantry member (200 x 200 x 6) probably wouldn't effect it too much. Glad I came on here as there's lots of good advice from people that have already been through designing and making.

    With regards to spacing would it be better attaching another member (such as a 100 x 100mm) on top of the existing gantry beam to increase the X rail spacing (increase from approx 175mm spacing to 275mm).

    Thanks john

  10. #10
    Quote Originally Posted by Geordie1996 View Post
    Your probably right with that haha, I do have a tendency to overcomplicate things sometimes. I think going back to your post of putting the two rails on the same side will greatly simplify things, and allow me to focus on getting a solid frame and z plate mount set up before moving on. Thinking about it more putting the bearings on the same side will only move the head out another 30mm, and it would also provide support closer to the spindle. Having this weight slightly further out with such a large gantry member (200 x 200 x 6) probably wouldn't effect it too much. Glad I came on here as there's lots of good advice from people that have already been through designing and making.

    With regards to spacing would it be better attaching another member (such as a 100 x 100mm) on top of the existing gantry beam to increase the X rail spacing (increase from approx 175mm spacing to 275mm).

    Thanks john
    Never mind I think I'm being a massive overkill with that, and would be adding unnecessary weight the gantry. I just measured the spacing of a 3x2m 5.5kW router at work and it's 200mm, so mine would be only 25mm narrower with 1/4 of the weight.

  11. #11
    Join Date
    Jul 2018
    Posts
    6254

    Re: Thoughts/Ideas

    Hi John - A 200x200x6mm is a very stiff gantry. Mine are typically 120x150x3mm and they are quite good over a 4ft sheet width (1300mm) I think your bearing spacing is fine for the moment. You need to dig into the Z axis soon. Peter

  12. #12

    Re: Thoughts/Ideas

    Cheers, getting stuck into the Z axis and changing the frame around, definitely looks simpler to produce like this.
    Have some bits of 30mm and 50mm plate already they will probably come in hnady.

  13. #13
    Join Date
    Jul 2018
    Posts
    6254

    Re: Thoughts/Ideas

    Hi Geordie- This machine when built will be "noisy" the large open tubes will have acoustic noise. A simple cap will fix that...This is not machine vibration... Looking good. Also check in the section manual that your hollow section corner radii are correct. They can be quite big... Peter

  14. #14

    Re: Thoughts/Ideas

    Thanks

    I can mill some nylon caps for the sections; I'll make sure I take the rads into account when i do a final draw up, the sections have about a 1/2" rad, so will adjust the rail seats/rails to suit.
    I think the only other thing I'll changes is to drop the whole Z assembly down 30mm, and increase z axis bearing spacing by 30mm. It'll reduce the 'overhang' and spread the bearings out a
    little further. Still gives me 160mm Z travel (approx 70mm cut capacity) so plenty of room for what i would be making.

  15. #15
    Join Date
    Jul 2018
    Posts
    6254

    Re: Thoughts/Ideas

    Hi Geordie- With the size of bits you have I'd go more Z. By the time you have a wasteboard a vice or fixture a long tool you will run out of Z easily. No less then 200mm travel 250mm is good. peter

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