For info a number of us have bought profile linear rails from 'fa-system' on ebay. They are used but the ones I bought still had plenty of life in them and have been running for a few years without problems. You'll have to cost it all up and make a choice.
Here's an example . . .
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There we go, third iteration. I've switched to square rails, 20mm. The frame is now a mix of 60x60mm and 80x80mm alu profiles. The alu plates are 20mm, except for the backpiece spanning the gantry, that one is 10mm.
The steppers should be more securely mounted, the gantry should be stiffer and hopefully I think it might be slightly easier to assemble.
What do you think? For some reason it just looks "wrong" to me, can't really put my finger on it.
Made some smaller adjustments, mostly cosmetic, but should have made it slightly more stable as well. Especially the table.
Any input is most welcome, I'm about to start ordering parts unless someone has any objections and/or improvements...
Looking pretty good now in my opinion. Again, if you are intent on lots of aluminium with a really good finish and short job times then the raised X-axis designs are really the way to go (i.e. not this layout).
However, with this design as it stands you should be able to cut wood, plastic, and the occasional aluminium without any problems. I'd say it fulfills your brief and will make a nice machine.
I'll just make this comment, which you can take or leave! The extrusion pieces between the gantry sides, rails, and lower plate - I can see why you've done it like that and it will help alot with the corner stiffness. But they look a little unfinished, if that sort of thing bothers you, then one of your first cnc jobs could be to cut out some 6-10mm aluminium plates to go over the ends and tidy it up a bit. The rear end cover plate could easily be larger and join all the gantry, side and lower plates together. In your link the rear view is shown in the 5th picture.
Good luck with the build and post back as you go.
I got the impression that I'd need either massive amounts of aluminium or preferably a welded steel frame for a raised x-axis to work. Neither would work particularly well for me (since I can't weld, nor work steel), so I think I'll go with this gantry design. I wont be doing massive amounts of milling, just the occasional part.
I'm one of those people that build mostly for the sake of building, not really with any clear goal in mind.
What kind of doc and feed speeds do you think I could expect from this? Something like 0.5mm and 1000mm/min, doing alu and still getting good finish and precision?
As for the aesthetics, I completely agree with you. As a matter of fact, I intentionally left that extrusion end open to have a fun first project to route. I would have liked making the rear end cover cover more (but not so much as to bump in to any high parts I'm cutting), but I'm working mostly with standard lengths and parts, meaning this was way cheaper and in my mind a small cosmetic tradeoff...
Thanks for the tips, I'll probably start ordering next week, need to make sure I'm completely decided first. The building will drag out a bit until the summer is over though.
This machine will be ok for medium duty aluminium cutting and will cut at those feeds and speeds but you will need some form of cooling with aluminium to get good results with finish. either Mist, flood or blown air.
One thing I would do is get rid of those openended profiles has the offer very little strength and they will actually make building harder.? How will you access the Bearing bolts.?
You will get the same or better strength from some simple 90deg triangle shaped plates bolted thru the gantry sides and into the bearing plates. Something like in the pics but made better looking this was just quick throw together.
This will also allow easy access to the bearing bolts and make simpler to build and adjust etc. You could drop that plate at rear or just use some Thin 2mm plate as a blank if you want to close gantry but I wouldn't and would leave open so chips fly thru.!
Also I wouldn't connect direct to the screws with steppers but use timing belts and pulleys. This will help with resonance and give other advantages by allowing ratio to be applied if needed. It's been discussed many times on forum so do a search if you need to know more.
Last edited by JAZZCNC; 03-07-2014 at 01:20 PM.
I've actually started building now. I won't really have time to maintain a buildlog here, but I am posting some status updates with pictures on my site/blog.
Currently got the main structure done, need to get the motors on there and a lot of fine tuning.
The rails aren't binding, but it's not super smooth. Got any tips on how to alleviate that?
But if they are new (which I think they are) they may need some grease.
More likely however is misalignment - how did you align everything? That wooden table does not look flat!
Which axis is not running smoothly X, Y, or Z? Most builders go to great lengths to get this alignment as good as possible including epoxy leveling prior to rail fitting (X axis, sometimes Y axis as well). Some builders use a DTI from a known flat reference surface, or a DTI from one rail to try and align the opposite rail.
Basically you have to remember that these types of rails are designed to be used by a professional machine tool design company and set up using machined datums etc. Have a search for the Hiwin fitment guide (either here, or on their website) to understand the lengths they expect the machine tool company to go to. You need to spend time on the alignment as best you can to get these running smoothly. As a start you can loosen one of the pairs of rails, run it back and forth, slowly tightening the rail bolts again. If this doesn't help then you may have a twist problem on the mounting surface. You may be able to help this with shims but I think you are better off getting the surface machined or epoxy levelled.
But do persevere with it because once set up accurately these units help give you a really high performance machine.
Well done on the actual build. I wouldn't have made all the design choices you went for (unconventional Z axis!), but I will say that you've made a good job of turning it into something real and that in the end you have ended up with a pretty capable machine.
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