Originally Posted by
dmalicky
Good to hear you have the budget. You probably also know that Misumi offers pre-machining of the rail mounting surfaces -- that's highly recommended, unless you have access to equipment to do that yourself. As the gantry gets stiffer, the rails will have more tendency bind unless the mounting surfaces are very flat.
I modified my old GFS8 100200 model so it had 7.75" Z height from tool tip to gantry bottom, like your machine. For rigid endplates (i.e., just accounting for the contribution of the GFS), the deflection is 0.00315", for a stiffness of 32k lb/in. With very stiff Y and Z cars, spindle mount, uprights, and lower frame, I'm guessing that overall machine could hit 10k lb/in which is quite good (compared to most DIY routers). 20k lb/in is best for heavy cuts in alum; 10k should allow moderate cuts which is a lot better than the very light cuts of a typical router.
For more stiffness, the easiest way is to decrease the Z height, like Gerry suggested earlier, either by design or plywood spacers on the table. If we reduce the Z height by 2" (to 5.75"), Deflection = 0.00263", Stiffness 38k lb/in.
The next easiest alternative is to reinforce the 100200 extrusion, like you suggested. I tried some options in FEA:
- Mount/epoxy a 100x200x6mm standard alum tube to the back of the GFS8 100200: Deflection = 0.00185", Stiffness 54k lb/in.
- Same tube but wall thickness is 10mm: Deflection = 0.00151", Stiffness 66k lb/in.
So the GFS8 100200 can be improved a lot, if you find it's not stiff enough as is. You could add that tube before or after its built. It's not the most elegant or inexpensive solution, but it's reasonable and a pretty stiff way to stay with T-slot.
Going to a standard 8"x8" alum or steel tube gives much higher stiffness, but a whole different manufacturing process, much more work, and it doesn't sound like your cutting needs call for it.
That software is Creo Parametric/Simulate -- it is very capable but not easy to use. Solidworks is much easier and its FEA is also very good -- I am currently switching to it. Right, SW Premium is the one with good FEA. The entry level SW has a 'starter' FEA which has many limitations (but still good for some purposes). But the hard part about FEA is using it properly: it's very easy to make wildly unrealistic models, giving meaningless results. All FEA models are idealized to some degree, including the ones I've shown (e.g., perfect joints) but they should be reasonably close for good design decisions.