Choosing between alternative X axis bearings designs

Hi all,
Apologies if this has already been asked but I'm weighing up my design choices and wanted some input about the different arangment of x axis bearings. I don't know what or if these two designs have an official name (so i didn't know what to google) so I've called them alternative A/B

(The pros and cons are based on my limited understanding, so please correct me if I'm wrong)

Alternative A
Attachment 27658

Pros: easy to implement and calibrate how parallel the guide rails are, more travel than alternative B
Cons: Tool ends up being further away from the axis than with alternative B

Alternative B
Attachment 27659


Pros: Tools can can be held closer to the axis, better for rigidity
Cons, much harder to check how parallel the rails are, less travel than alternaive A

Most of the DIY machines I've seen use A, but it would be great to get the merits of each one.

About my build
Work area: 300x400x150mm, fixed gantry
Frame 50x50x3mm Mild steel
Spindle: Chinese 2.2KW water cooled
Intended cutting Materials 6082 Aluminium

Thank you

Before choosing the bearing position you need to fix the gantry.

Separate tubes are weak.

You should have one large tube.

Your pros and cons are correct. There is no "best" for this design choice. They each have pros and cons. It partly depends on the other design compromises you make, for instance how much stick out the spindle will have due to other decisions.

If you choose good rigid gantry uprights, for example large tubes and not thin plates, you can get back the lost space under the gantry due to rails underneath.

For me, the hassle of trying to align rails in alternative B and the increased complexity and weight of the carriage make it not worthwhile. I prefer alternative A.

If you are going to weld up the gantry from steel box section then you can use the epoxy levelling method to get a flat surface to fit the rails to in option A, but you should make it more solid. My own gantry is made from two pieces of 100x50 box section welded together to give a 200mmm high face for the rails to fit on. As always with a DIY build, your best option depends very much on what tools or other workshop facilities you have access to. Option A makes the Z-axis assembly much simpler to build as well.

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Originally Posted by pippin88

Before choosing the bearing position you need to fix the gantry.

Separate tubes are weak.

You should have one large tube.

Your pros and cons are correct. There is no "best" for this design choice. They each have pros and cons. It partly depends on the other design compromises you make, for instance how much stick out the spindle will have due to other decisions.

If you choose good rigid gantry uprights, for example large tubes and not thin plates, you can get back the lost space under the gantry due to rails underneath.

For me, the hassle of trying to align rails in alternative B and the increased complexity and weight of the carriage make it not worthwhile. I prefer alternative A.

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Thanks pippin88, I think am leading to the same conclusion as you. My diagram wasn't the best, the rails will be on the tube going across only (along the x axis), the diagram shows that it also goes across the small section of box that is going in the Y direction but this won't be the case since this space will be used to house a motor and leadscrew bearing support, so the rails only sit on one section of tube.

Quote:

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Originally Posted by Kitwn

If you are going to weld up the gantry from steel box section then you can use the epoxy levelling method to get a flat surface to fit the rails to in option A, but you should make it more solid. My own gantry is made from two pieces of 100x50 box section welded together to give a 200mmm high face for the rails to fit on. As always with a DIY build, your best option depends very much on what tools or other workshop facilities you have access to. Option A makes the Z-axis assembly much simpler to build as well.

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By "more solid" do you mean by adding bracing? This is something I will be doing for sure. Yes they will be welded, though I am considering welding the highest x-axis guide box to the top of the frame and bolting the one closest to the ground to make fine adjustment in reference to the welded one, haven't thought this completely through yet. Do you have pictures, or a build log ? it would be great to see your machine Thanks for your input

ADJUSTMENT ADJUSTMENT ADJUSTMENT... I've said this many many times in posts but I'll say it again because cannot be stressed enough.

With this fixed gantry design, you are going to need plenty of adjustment. If you weld up the gantry completely and especially if welded to the base frame you won't be able to adjust the planes the rails sit on in relation to the other planes.
Any extra time spent thinking about how you can build in adjustment will pay back big time when setting up the machine and the accuracy you will get from it.

Regards the 50mm with 3mm box section then like I said in other thread, it's spaghetti-like properties mean you really will need to brace it good. Not so much for strength but for rigidity and resonance. The thin wall means it's resonant which causes vibrations at the tool, yes you can fill it with sand or concrete-like some have done, but this won't help a poorly designed and braced gantry.
This stage is critical to get right because it's the foundation everything else references from, Wobbley foundation = Shit machine simple as that.!

There's 3 key areas to building a great machine, get any one of them wrong and the machine will suffer.
#1 Base frame
#2 Strong Z-axis
#3 Good electronics.

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Originally Posted by eci22

By "more solid" do you mean by adding bracing? This is something I will be doing for sure. Yes they will be welded, though I am considering welding the highest x-axis guide box to the top of the frame and bolting the one closest to the ground to make fine adjustment in reference to the welded one, haven't thought this completely through yet. Do you have pictures, or a build log ? it would be great to see your machine Thanks for your input

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You do better following Jazzcnc's advice rather than mine on making the structure more solid. He's had years of experience and I've only built the one machine. However, you might consider restricting the 50x50x3 stuff you've already bought to just the base frame and start again with the gantry design.

I have been thinking about putting out some details on the forum as I'm in the middle of finishing the Z-axis at long last. Not so much a build-log on how to build a good machine but more about the compromises I've had to make to fit the available budget and footprint and being very remote from suppliers and professional workshops. It works, but I wouldn't advise anyone to copy it.

If I were starting from scratch I'd brew a nice big pot of tea, open a packet of Hobnobs (or the Australian equivalent) and read everything Jazzcnc and Boyan Silyavski (if you'd like to see some really heavy engineering) have written and a number of other build-logs before buying the first nut and bolt.

Kit

80x80x3 or better 100x100x3mm will greatly simplify the structure and the welding as no diagonals will be needed. 50x50mm is weak for such purpose.

Gantry could be welded from 2x 100x100x3 or thicker welded together or a distance for screw in between. gantry legs can be 2- 3 pieces of 100x100 welded together. Check my build 2. If you are not into welding, painting and so on, better make the gantry L style like Dean/ JAZZ CNC / makes his machines, from aluminum. it will greatly simplify it, though a bit more expensive.

Box section comes on 6m pieces here ion Spain

I know you would like a small machine but if you make it beefy it would make a lot of difference when machining aluminum, especially on the finish and precision