Y-Axis Designs

Hi,

I've started compiling different plans and ideas for my very own diy cnc build. There are a lot of things to consider and me being a perfectionist I don't like building something just to find out that I screwed up or could've done better. So, without further ado I decided to come here and ask some more experienced people for help.

My first concern is the y-sled:

I see two reasonable ways of positioning the ballscrew nut without adding any angled pieces to the y-sled. I want to avoid angled pieces because they would contribute slack and could thus (hypothetically) reduce accuracy.

These are the two different options I see including some pros and cons:

Option A: Ballscrew in between the linear rails
- requires spacers on linear guides to clear the nut
- less space for placing the z-axis rails

Option B: Ballscrew above the linear rails
+ thin sled
+ better clearance for the ballscrew (less dust)
- taller sled
- power transmission offset from the sled's center

Here a little image as visualization aid: LINK

You may notice I don't see any point in choosing option A over option B, most CNC mills I've seen so far however use option B. Thus I decided to start this thread and hopefully initiate a discussion about different designs. I'm a computer scientist, not a mechanical engineer, but if you happen to have any mathematical or more technical input, please go ahead.

Regards,
Cyclonit

Option A is best - the problem with the other one is that the rails (obviously) offer no support parallel to the direction of travel, so when a force is applied parallel to the Y-axis, the lower bearings can move slightly along the rails without the ballnut moving. It wont move very much, but that still means you have a lower stiffness. It's therefore not as strong as having the ballnut directly between the rails.

You should find this document useful:
http://www.thk.com/sites/default/fil...l/a/ee_A15.pdf

Also, depending on the design of the rest of the machine, you may be better off putting the Z-axis rails on the Z-axis plate (which holds the spindle) and the bearings on the Y-axis carraige. It tends to reduce the overhang. I'm not keen on mounting the rails directly to the aluminium extrusion either...

Jonathan is correct that "A" is technicly the better option but I've built machines with ballscrews in all locations, on front, up high and at rear and I've yet to see any significan't differance that makes me say don't use any of them. The only time I'd shy away from any of the others is if machine is going to cut hard and aggresively in hard materials.

I also don't agree that mounting profiled linear rails directly to extrusion is a bad thing provided the rail is wide enough and obviously profile is straight good HD quality. If done correctly then it can be very accurate and easy to use. I do this all the time and assure you it's accurate enough for any router type machine. If your wanting milling machine levels of strength and accuracy then your using the wrong material any way.!!