Originally Posted by
wizard
First, thank you for the very in depth reply. It is exactly what I was looking for when I came here.
This is a very nice size for a fixed gantry machine.
Ultimately what you are willing to pay is up to you. I would lean towards making a rigid frame first and cutting corners on things you can easily upgrade latter. For example implement an old router for the spindle or implement acme lead screws over ball screws. A good machine tool frame often outlasts the stuff that is bolted to it and will almost certainly outlast the electronics.
This is something I have thought about, but I think I would rather save up longer to get everything I really want at once.
I would tend to go slightly larger that 2" square tubing and probably add additional support for the tubing carrying the X axis linear slides. Generally you want tubing thick enough to hold the screws that mount the linear slide rails or you buy tapped linear slide rails and use through bolts.
Currently my thought was 1/4" wall for the 2" square tubing and probably 3/8" for 4"x6" on the bridge.
You might want to make provisions for a thicker table.
What do you have in mind? 2"? I need to talk to a colleague of mine, because he may be able to find a scrap piece I could have in the range of 24"x36".
Square steel tubing is use in all sorts of machines and special machines for automation. There are several examples of very nice machines in this forum.
As for tolerances it is really up to you and what you want. If you want to do a good job on a welded steel frame you need to spend a bit of money. First the frame needs to be stressed relieved and then machined on a mill to flatten and square everything up. Another option is hand scraping everything in, you can also do things like epoxy leveling.
In the automation industry welded steel is the common approach to machine building. Here we almost always weld pads to the steel tubing to provide for linear rail mounting and assembly mounts. At some point in a machine design you are pretty much forced to use bolted together sub assemblies. In the case of your design you need to consider how you would machine the surfaces for the X Axis rails with the uprights welded in place. This largely depends upon what machinery you have access to locally. If you have the machinery to do it then welded in place uprights are the way to go. Those uprights should have in place mounting pads for the gantry beam which get machined at the same time. Doing it all in one setup keeps surfaces square and parallel as required. Having your frame machined simplifies a lot of things when it comes to final fit up of the machine. The gantry beam should have mounting feet that match up with the pads machined on the uprights.
You can of course use other techniques that take the expense of going to a machine shop out of the equation but this will result in a lot more work on your part. There are a couple of realities that you need to be aware of. For one square tubing isn't always square, at least not to the precision a machinist might expect. Think parallelogram. Second the surfaces of square tubbing are not always flat, they can be concave (most often) or sometimes convex. This will raise hell with profile rails if you try to bolt them directly to the tubing. Thus one of the goals with machining or leveling the mounting surfaces is to get the rails to set in the same plane.
By the way don't assume that a set of components all nicely finished from the machine shop will just bolt together with out any and fitting or tweaking. It really comes down to what you expect from the machine.
There are a lot of threads that touch upon this but be aware your goals or expectations are a factor. Also the types of drives you use impacts lead screw selection. In the end you need to trade off a lot of factors to get the results you want. I strongly recommend getting the engineering data available from the various ball screw makers so you have a handle on the various limits of operation.
You might also note that I talk about mounting pads and feet for the gantry beam. I bring this up because I doubt that you can get the beam square enough by welding it in place. Better to have the flexibility of a bolted connection there to allow adjustments to squareness of the Y & Z axis.
My basic plan for manufacturing the frame would be weld up the base, stress relieve if possible (Same contact for the table might be able to do it), and machine the top and bottom so that they are flat and parallel. I should be able to reach everything with the bridge being removable/level-able which is a fantastic idea. Today I have been looking at methods of making it possible to level and tram everything.
In a nut shell a good part of machine design is answering the question: how will I build this thing?