[mpowley66]

Hi all, I'm new to CNC zone and joined to learn more about homemade CNC machines, I have bought the plans for the momus, and am planning on building it. I like the straightforward solid design, and I am thinking of changing the machine to increase its length (depth). I know that some others here have done this same thing, what problems did this cause and how did you overcome them? I know that by increasing the gantry length you increase the possibility for binding or that the gantry will deflect out of square with the frame, but to me increasing the depth should not mean a large sacrifice in rigidity.

At what length would you say that belt drive starts to become impractical? In my head making a machine that has a 36 or 48 " depth is simply a matter of making a longer frame, which is a simple change that require mostly more wood fabrication, in which I am fairly confident in my abilities.

Thanks, Mitch

[pikolo]

i wouldnt do it ....actually if you need to make a accurate cnc dont make the momus or make it with screws or even better with ballscrews...in the one i have built i am thinking of making the gantry fixed and put a moving table

[mpowley66]

Yes, I think you are right. After doing a little modelling in sketchup of the base, I realized that a wooden bed of that scale would likely not be very rigid without major modification. Also a router that is 48x16 would not be as useful for large work as a wider one, and widening would require changes to the design to prevent gantry deflection.
I think I will use the plans as is, this is probably already an ambitious enough project for me already.

[pikolo]

After alot of modeling and thinking about it ....i would recomend to make the gantry fixed!and put a moving table into play...this will add rigidity and you should only buy two linear cnc rails about 100 dolars or so...this is what i will do ...

[mpowley66]

Hi pikolo, the idea of making the table move and gantry rigid probably will increase rigidity as the original design was made to be accommodating of less than stellar installation of the rails on the top. However I believe that the idea of making the base or table move will have several consequences.
1 The complexity will increase, you know have to deal with essentially building two beds, and you can no longer work directly from the plans.
2 The momentum of the moving table will either reduce the x axis operating speed or require the use more powerful steppers.
3 This is a minor point but the design of the sides that hold up the gantry is now less than optimal as we no longer need long flat sides to run the x rails on.

[louieatienza]

Hi pikolo, the idea of making the table move and gantry rigid probably will increase rigidity as the original design was made to be accommodating of less than stellar installation of the rails on the top. However I believe that the idea of making the base or table move will have several consequences.
1 The complexity will increase, you know have to deal with essentially building two beds, and you can no longer work directly from the plans.
2 The momentum of the moving table will either reduce the x axis operating speed or require the use more powerful steppers.
3 This is a minor point but the design of the sides that hold up the gantry is now less than optimal as we no longer need long flat sides to run the x rails on.

1 - Moving the table is the less complex of the two, by far. And it can be center driven. The disadvantage of a moving table is that you wouldn't be able to do anything that may require a hole in the table, for example milling the edge of a board.
2 - I think the table would be fine as far as operating and top speed is concerned. The real concern would be acceleration, but these steppers are a lot stronger than you'd expect, and you do have the mechanical advantage of the screw. Only detriment is if you have to put a very heavy load on the table.
3 - You could still build torsion box sides with a fixed bridge. Likely this would be easier to level because you only have two anchor points, not two long rails to level.

The main disadvantage would be having less overall travel for the size of your rails, since you'd need more width between bearing blocks to support the table. But here again, if your linear rail system is strong enough you can mount the rails under the table, such that the blocks support the table at the machining area of the spindle. The other disadvantage is that there may be variable performance depending on the weight of what you have on the table, but I assure you that for what you're likely to put there, this should not be an issue.