Originally Posted by
wizard
This thread highlights how confusing it is to chose arbitrary assignments to X,Y & Z on these machines!!!!
In any event, lets look at your needs where you indicated about 20% usage in aluminum. You didn't get into specifics of what that means but having a long gantry axis works against you when it comes to getting good results. Think about it in this manner, you will want about ten times the stiffness to machine aluminum the way you machine wood. You are already starting out with questionable extrusions so that is a factor that makes using them in a long span even more questionable. While plastering a plate on the back of the extrusions will add some stiffness it isn't a huge win.
As I see it, in many cases the gantry ends up be the weakest link in the DIY machines out there. I suspect this is the case here also as the extrusions simply look thin in your rendering. That and you are almost always better off using box sections to hold the gantry its self up instead of plain plates. If you really think that the long gantry will work better for you consider an upgraded beam of larger extrusions or ideally a steel beam. Support the gantry properly. The rest of the machine looks pretty good though I would throw in at least a few gusset plates at each corner of the base frame; also make sure the "ladder rungs", at least the ones on the end, are the same dimensions as the legs of the ladder frame.
The 700mm is relatively short but you are indicating usage in aluminum. Now that means different things to different people but it also means that you need a stiffer machine than one dedicated to wood working. Also just because you see that somebody else has done something similar it doesn't mean it will be good enough to meet your expectations.
On the net it is hard to understand what your expectations are. However if you are using minimalist materials such as Openbuilds extrusions you likely are already limiting machine capability. As somebody has already mentioned we don't know what the cross sectional sizes are here. So we can only infer from the rendering what the actual parts are. If we better understand your expectations we might be able to better comment. Otherwise we are forced to comment in general terms and those comments reflect what we think you mean by 20% usage in aluminum.
There are a couple of problems here.
First the cross members, in the base, should be the same dimension as the axis members at least at the ends but ideally across all members. While you may see the cross members as a base for the table and waste board they also have a significant impact on the rigidity of the axis extrusions. With suitable fastening methods and plenty of gussets you can end up with a very rigid base.
The second issue here is that the only exposure I've had to the Openbuild hardware is what I've seen on line and frankly I'm not impressed. I'm not impressed with the idea of V rollers of any type running on aluminum extrusions for one. The second issue is that the extrusions appear to be very light weight. Beyond that you are implementing smaller sized extrusions for the cross members which has its own issue
Now about that light weight, a few months ago somebody put on line a picture of some square extrusions he apparently got salvage rights to. Now this was small square extrusions all of the same outside dimension but clearly different "inside". Now the small square tubing has limited machine building usages to begin with but in this case two of the extrusions where extremely light weight, one having hollow sections. I could easily imagine breaking that extrusion just like I will be breaking branches in my back yard i will be picking up in a hour or so. The point is not all extrusion are the same so you need to know the specifics of the items in question. Even the likes of Misumi sells extrusions with varying design elements including the wall thickness of the cross section.
I'm a bit long winded here. I can't say if the OpenBuilds hardware is suitable for your needs. However a bit of attention to design can make for far better performance from what ever materials you use which is the important point.