[trentster]

Hi all - in the process of building very first CNC machine - been lots of fun and learning. Most of the parts have already come in. Tomorrow I plan on getting the 30/60 extrusions ordered and cut to size. I have run into a little bit of a hiccup and was wondering how you pros deal with this. The supported rails and ball screw That I purchased are both exactly 900mm tip to tip. I am not sure but I think I read its not a good idea to mount the supported rails directly up flush against each end plate due to possible thermal expansion warping the machine. I have left a 20mm gap on either side of the supported rail. This leaves me with a situation where the ballscrew end mount does not make it to the end plate and is kinda left hovering.

Also if you could have a look at my design and give opinions on whats done right and what may need work. I know I probably should have used thicker aluminium flat bar - but I am budget constrained and managed to get a lot of 10mm thick - so I am hoping that will suffice if I brace the joints etc. All opinions very welcome ;-)


Attached please find an annotated picture from the CAD model which shows the issue clearly.

File ?my-cnc-final - SketchUp Make 2015-04-23 03-43-46.png?

Any advice - much appreciated.

Thanks

Mark

[ger21]

I am not sure but I think I read its not a good idea to mount the supported rails directly up flush against each end plate due to possible thermal expansion warping the machine. I have left a 20mm gap on either side of the supported rail.

1mm gap will be more than enough.

If the rails are fully supported, they'll warp by just being bolted to the extrusion, as the shafts and extrusion will expand at different rates.

I think a bigger issue is the height and thickness of the sides, which can sway side to side under quick direction changes or high forces.

I'd extend the bottom plates out beyond the sides, and add triangular bracing on the outside.

You may also find the gantry "racking" with a center driven screw, if cutting near the ends with higher forces.

[trentster]

1mm gap will be more than enough.

If the rails are fully supported, they'll warp by just being bolted to the extrusion, as the shafts and extrusion will expand at different rates.

I think a bigger issue is the height and thickness of the sides, which can sway side to side under quick direction changes or high forces.

I'd extend the bottom plates out beyond the sides, and add triangular bracing on the outside.

You may also find the gantry "racking" with a center driven screw, if cutting near the ends with higher forces.

Thanks for the info, as for height of the sides should I be aiming for lowest height I can get away with for the Z axiz depth I need?

Also the main question was any idea how I get around the problem of the x axiz rails and ballscrew being the same length - how do I get the coupling to reach the nema stepper?

[ger21]

Put a spacer plate at the other end of the screw to move it closer to the stepper.

[trentster]

Put a spacer plate at the other end of the screw to move it closer to the stepper.

I think we may be misunderstanding each other, this is the issue I am talking about :

File ?my-cnc-final - SketchUp Make 2015-04-25 21-21-30.png?

Where would I put a spacer? like a big solid block of aluminium between front plate and the bearing block? Would I cut a large hole in the middle to allow screw to go through?

Thanks

Mark

[ger21]

Where would I put a spacer?

Between the frame and mounting block at the opposite end of the stepper. Which would move the screw towards the motor.