I’ve been trying to rack my brain over which of the two milling machine designs would fundamentally be more rigid.
The two I’m comparing are, using a gantry where its supports move at the same time as in the first picture. Verses an overhead gantry, where just the gantry moves, as in the last picture.
After doing some basic drawings my thoughts are that with design:
A = supported gantry. ( picture 1)
What ever leverage or force applied to the mill will be canceled ( in a perfect world ) by the opposing length of the support and hence to the bearing blocks.
B = overhead gantry. ( picture 2)
What ever leverage or force applied to the mill gets applied directly to the bearing blocks.
I was thinking of using the same principles of using a wrench, pictures of the bolts at the bottom.
Any thoughts or things I’ve missed? I know the construction of the mill will always make the final decision, but fundamentally would the supported gantry always be better?
If #2 is built as a cube, the whole assembly is far more rigid than #1 which is a plane; but that will be noticeable on big platforms, for heavy metals; but in desktops, for me, the extra weight and materials don’t justify #2. The gantry rigidity against the work is given by the distance between the two sliding point (at the side); you increase it, you gain rigidity, at the expense of travel
Figure the design from the bit back to the frame. You cutting speed, depth if cut, size of bit and material. You can only go so fast. 2x the forces in the rest of the design. Over kill just means bigger motors to move the higher mass with no added benifit.
My concern doesn’t even touch on rigidity. Driving two motors to move a single axis means you need to keep both motors synced. Racking(sp?), loss of squareness due to missed steps/motion, is an issue when you have to drive a pair of motors for a single axis.
At least that is the impression I have received from many people that have built/asked about such setups.
The first design is more common with respect to taking care of the racking issue. The two sides can be tied together underneath the table in ways that assist in maintaining squareness.
@Kyle_Kerr
It stands to reason that if you have two ends, and either can hang up, then putting your linear motion directly at those two ends can reduce racking. As opposed to trying to apply motive force at a mechanical disadvantage from the middle. Anyone that has pulled out a stuck dresser drawer intuitively grabs it at both ends, as opposed to right in the middle. Well, anyone that has two good arms I suppose.
Therein lies one of the pitfalls of the method as it applies to CNC though. Because no one starts out with two linear actuators. Each must be acquired, often at considerable cost. Two costs twice as much as one does. Installing two actuators is twice the effort as well.
But once you’ve gotten beyond all of that synchronization is not a problem because that is what stepper motors do. If both motors are the same angle (steps per revolution), torque, and your lead screws match, then it should not be an issue. It is best to use matched components.
Missing steps is always going to be a problem no matter how many drive elements your machine has. That needs to be avoided at all costs.
A third option would be to fix the overhead structure in place and move the work table. Takes up a lot more room, though. Oh! I just saw your post about the wooden one. Guess you already knew this!
