Made of aluminum, Made for aluminum: Would love design critique!

[Neale]

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Do you already have the 15mm rails for X and Y? If not, go to 20mm.

Consider 20mm rails for Z as well. Not needed for strength, but if you go with the two-plate Z structure as recommended, 20mm rails make it much easier to fit the ballscrew and nut between the plates.

[gamingan]

Thank you all for the incredibly useful suggestions; learned a lot by trying to think about why each of those suggestions made sense.

I've refined the design now (along with some significant changes in overall geometry) -- let's see if you guys like it.

Here is the 2nd version of the design...

Front view: http://i.imgur.com/Ik9s7uV.jpg


Back view: http://i.imgur.com/iKuChuA.jpg


Below are the changes I made:

1. Most important of all, altered the geometry so that the rails and ballscrews are protected. This also simplified the Gantry side plate design a lot!
2. Gantry side plates now are 20 mm thick (instead of 15 mm).
3. Strengthened the X-axis (i.e., the horizontal "gantry axis") by using 80 x 160 mm aluminum profile instead of 40 x 160.
4. Replaced ballscrews with 20 size (instead of 16), and linear bearings + rails with HGH20 size (instead of HGH15).
5. Added a fixed bearing block (with AC bearings) for Z-axis ballscrew (previously it was fully floating beneath the shaft coupler).
6. Added two more linear bearings (HGH15) for Z-axis.

Feedback?

I hope this one is OK to start assembling!

[Zeeflyboy]

Significantly better in most regards, nice job.

Consider going one step further and putting the rails on the top of the side extrusions, you can quite literally get rid of all unsupported gantry side arms and it is certainly easier for alignment purposes than side mount.

larger extrusion on X is a good decision, I would still go for heavy profile though if that's an option in that size.

Also consider flipping the Z-axis around so the router mounts to a plate with the rails and the carriages are fixed to the gantry side... 'tis what all the cool kids are doing these days. Gives better rigidity. I'd also say just upgrade those to 20mm rail too for simplicity's sake.

[routercnc]

... you can quite literally get rid of all unsupported gantry side arms and it is certainly easier for alignment purposes than side mount.

In case it is not obvious you have trapped the long axis rails between the gantry sides, which means you cannot easily control the pre-load on them when it is all assembled.
As drawn the side extrusions which sit on the bed - one of them needs to be adjustable laterally so that the pre-load on the rails can be set. Otherwise if you bolt this together rails may wear prematurely or bind.

[gamingan]

In case it is not obvious you have trapped the long axis rails between the gantry sides, which means you cannot easily control the pre-load on them when it is all assembled.
As drawn the side extrusions which sit on the bed - one of them needs to be adjustable laterally so that the pre-load on the rails can be set. Otherwise if you bolt this together rails may wear prematurely or bind.

Consider going one step further and putting the rails on the top of the side extrusions, you can quite literally get rid of all unsupported gantry side arms and it is certainly easier for alignment purposes than side mount.

Hmm... I will try to go this route but I really prefer the rails mounted on the sides because it also does a bit of dust shielding. Is there any reliable strategy you can think of (screw-based adjustment perhaps?) to adjust/align things if I *HAD* to go with rails on the side?

And to be honest, I don't see how top-mounting the rails allows adjustment either... routercnc mentioned the need to do lateral adjustment, which I agree with, but how exactly does this become possible in practice for the case of top-mounted rails?

Zeeflybloy: Regarding flipping things (rails <-> bearings) on the Z-axis: Will check to make sure it doesn't conflict with any dimensional constraints I have, and give that a try.

[Neale]

These Hiwin rails, for all practical purposes, have pretty well no alignment tolerance. They must be accurately aligned to, say, 0.01mm, in all dimensions. Can you control the width of the bed assembly, side extrusions, etc, gantry beam length, gantry spacer thickness, so that the total error in dimension is less than 0.01mm? The one big difference between home-built and commercial machines is that we don't have a giant machining centre that can give manufacturing accuracy. We have to build in adjustment at all key points. That's why the rails on top of the sides will let you build a good machine and your design will be a nightmare to set up. I understand your point about dust protection but keep asking yourself the question, "how can I adjust this on assembly?" For example, you could machine/shim the spacers to allow for gantry length errors, but how will you guarantee that the rails are parallel? Epoxy beds, bolts in oversize holes, etc, are all viable techniques in our workshops, however much they might be frowned on commercially!

You can spend as much time setting up as building, but that ability to tweak for accuracy is key. It has the potential to be a good machine if you can just sort out a few details like this.

[Lee Roberts]

Neale, great explaining!

If the machine cant move, there will be no dust

[A_Camera]

I really prefer the rails mounted on the sides because it also does a bit of dust shielding.

Dust is going to be an issue, regardless how you install the rails. A word of advice is that you already at this stage think about dust extraction. Not only the linear rails which needs to be protected from dust but also the room where you are planning the use of this machine. So personally, I'd mount those rails on the top, as said by others, but I'd also design a dust extractor, a simple dust shoe, as well as some side walls, which not only provides some protection for the rails, but also personal safety.