Fixed Gantry Design Question

[ynnek]

I have not seen a fixed gantry with the Y axis in the center of the work space. My thinking is this would increase the usable work area & also possibly make for an even more ridgid machine. Thoughts?

[CarveOne]

It looks like a good concept to me, but the router and cutter needs to be centered, not the gantry. I like the diagonal structure that makes the gantry more rigid.

That way the frame and table can be configured so the table can be moved forward and backward from the rear edge to the front edge of the table.

Likewise, the frame and gantry needs to be wide enough that the cutter can reach each of the side edges of the table.

[ynnek]

you are correct. The spindle would need to be centered, not the gantry.

[Phife]

I really like the long axis to be the bridge on a fixed gantry machine, it helps reduce the foot print of the machine and maintain many of the benefits of both moving gantry and fixed bridge gantry.

Ive seen several CR Onsrud machine with this configuration
CNC Routers, CNC Mills, Inverted Routers, and Tooling by C.R. Onsrud Inc.

Im doing something similar with my machine, but Its going to be a much different design.

http://www.cnczone.com/forums/diy-cn..._ridgid_z.html

I like your design, seems like it would be quite sturdy and solid.

[Mr Overkill]

That design looks great, I am definitely adding it to my ideas book! Where were you planning to mount the rails on the gantry - on the side or on the top & bottom faces? IMHO, a top & bottom mount would give less deflection and would save horizontal space.

Something else you could try to reduce cost and improve the frame stiffness is to add a flat MDF base, which makes sure the frame stays square and generally helps dampen vibration. That would optionally allow the table to be widened and supported from the outer X axis extrusions and eliminate the two inner extrusions. Even if the cutter can not reach the edges of the table, its always handy to have extra space for fixturing.

[DonFrambach]

I think your design is off to a very good start. I recently made a small fixed gantry router where the moving table axis is slightly longer than the fixed gantry axis. If/when I make another router, I'll strongly consider making the fixed gantry axis the longer of the two (maybe by a lot!).

[ynnek]

That design looks great, I am definitely adding it to my ideas book! Where were you planning to mount the rails on the gantry - on the side or on the top & bottom faces? IMHO, a top & bottom mount would give less deflection and would save horizontal space.

Something else you could try to reduce cost and improve the frame stiffness is to add a flat MDF base, which makes sure the frame stays square and generally helps dampen vibration. That would optionally allow the table to be widened and supported from the outer X axis extrusions and eliminate the two inner extrusions. Even if the cutter can not reach the edges of the table, its always handy to have extra space for fixturing.

I have not gotten that far in the design yeat. I will be working on this over the weekend.

[ynnek]

My thoughts on my gantry rails are to put 1 set on each the top & bottom & 2 sets on the front ti minimise deflection of the z axis. Also I am thinking of spreading 4 sets of rails across the table. with this setup I think I should be able to machine aluminum accurately, that is if I wanted to.

[Mr Overkill]

Its quite unusual to use more than two rails peraxis. There are several reasons: It's much harder to get the rails lined up: the rails need the be perfectly straight and if you are using supported rails, the surface they are mounted on has to be machined flat. Next, the cost is doubled. Lastly, you will probably get more rigidity from 2 thicker rails than 4 thinner ones. I forget the formula, but doubling the rail diameter vastly increases stiffness, while costing less than double (based on a quick look at the McMaster catalog).

You can further reduce deflection and increase load capacity by using either extra long bearings or 2 bearings per rail. The lower rail that is nearest to the cutter has the most effect on deflection, so I would look at making that rail the stiffest and saving some money on the upper rail.

[ynnek]

here is my latest render. Still a work in progress. Thanks for all of the input. This machine will have a cutting area of about 28" square. Should I go with nema 23 or nema 34 motors. I plan on using ball scres on all axii

[Mr Overkill]

Your design is looking even better! I see you are using linear carraiges, which are usually more compact and stiffer than round rails - just be careful about keeping dust out of them.

I'm no expert on motor sizes, however a guy whom i trust has put a good case for using smaller diameter motors if you want faster rapids and don't need huge torque to push larger cutters thru hard material.

[ynnek]

Speed is not what I am shooting for. I am looking more for rigidity & accuracy cutting mainly plastics (Delrin, uhmw, nylatron, & hdpe) & hopefully the ability to machine aluminum. Thanks for the comments.