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  1. #11
    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.
    Last edited by Neale; 27-06-2017 at 06:46 AM.

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  3. #12
    Neale, great explaining!

    If the machine cant move, there will be no dust

  4. #13
    Neale explained it there!

    It is worth pointing out that I am in the process of building a machine with side mounted rails so I've spent some time thinking about how to do it correctly, so I definitely can't say don't do it - just that you need to know HOW you are going to do it.

    You have a few significant issues with mounting them on the side. First, how do you make sure they are perfectly aligned so that neither rail goes up/down in height relative to the other along the axis travel... for me my solution to this is a 20mm thick precision ground eco-cast bed plate which will serve as my flatness reference, and the design allows me to insert a spacing shim between carriage and bed to get vertical alignment.

    The second issue is how do you ensure the rails are perfectly parallel (e.g. that they aren't further apart at the back than the front, and they don't have any bend that takes them closer/further apart along the run. This needs to be done to a very high accuracy - something in the region of a few microns. Your extrusions will not have sufficient flatness, straightness or dimensional tolerance to ensure this level of accuracy so how do you achieve it?

    For me, I switched to using more expensive extrusions from misumi which are milled on two sides give a proper precision milled surface on which to mount the rails. That only solves part of the problems however, as you still need to ensure that the rails are the same distance apart at any given point in the travel. My machine is quite a lot smaller so I managed to fit my cross bars (a job being served here by your extrusion bed) in the current CNC and measure them with a probe to 0.01mm resolution. I came out with a variation of around 0.06mm and have used a variety of 0.01-0.05mm shim material to hopefully take them to approximately the same length. My plan is to then make a bar that mounts across the span which can float if the rails diverge or converge along the travel, with a dial indicator mounted. I will then move along the axis and where the dial indicator diverges by more than a few microns I will insert some shim material.

    So, now that you've sorted all that out - next problem, how accurately do you think the gantry length is cut? If you are lucky it is probably within 0.3-0.5mm of what you have asked for. Problem is your rails are a fixed distance apart, and now you will either not be able to fit them between the gantry side plates (if the gantry was slightly shorter) or you will have a gap that then applies a load to the bearing carriages when tightened up. You will need to diligently shim either the gantry beam or the space between the side arms and carriages to make sure that there is no gap to cause problems.

    If you manage to figure out a solution to all of those issues, then there is little to worry about with side mounting - but I hope you can see that it is not as straight forward as you might think to avoid issues of misalignment.

    Personally, given your overall design there I would mount them on top (I would also personally go with some extrusions from misumi milled top and bottom for the side pieces there if the budget allows but it's not necessary) and figure out a little dust/swarf shield arrangement for them.

    Food for thought anyway.

    edit - to answer your question, lateral alignment is extremely easy on top mounted rails. You simply fasten one rail down as square as you can and then that serves as the reference for the second - you would just move the gantry along the travel fastening the second rail as you go. The issue with using extrusion is that vertically you are relying on the flatness and straightness of the extrusion which is not ideal but certainly not as problematic in that orientation (the gantry would likely twist to accommodate any small variation before causing damage to rails) but you will see plenty of people using self levelling epoxy pouring with a bridge to provide a flat mounting surface between sides. Easy option to get a respectable level of precision in that plane would be the top/bottom milled extrusions I mentioned but it depends upon your budget and precision desires.
    Last edited by Zeeflyboy; 27-06-2017 at 10:57 AM.

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  6. #14
    Quote Originally Posted by gamingan View Post
    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.

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