CubeXY Fabrication part 2, and X axis plate redesign

Made some more progress fabricating.

I made 4 brass soldering iron tips for melt-in inserts. The brass rod cost $6 (only used half of it), and it took about an hour to turn these.

I designed them for combination US and metric. They cover M2-M5 and 4-40 - 10-32.

I milled the bed plate (with a lot of help, thanks Anthony). While programming prototrak is like riding a bicycle for me, mastercam apparently isn't.

bed being milled

Top

Bottom

The 1/4" cast aluminum plate stock, purchased from midwest steel supply, arrived with a few pits in it. I guess despite being ground, basic material handling of these large sheets results in pits. One of the mill clamps also left a noticeable dent in an edge. Otherwise, it came out well. I'll probably end up lightly sanding the whole top anyways to improve bonding. 

Next was test milling linear bearing holes to find a good press-fit diameter. 

I had to turn a small part to push this out after I pressed it in.

Then actually milling the x-axis plate (again, thanks Anthony). 

This felt a little flimsy. Uh oh... I went back and checked math/FEA. First thing I noticed was that the big chamfers on each side interfere with the X-axis carriage at both travel limits. Oops. I calculated what the max possible acceleration of the Y-axis could be given it's estimated mass (~1kg), the motor torques and inertia, etc. Came out to about 64 m/s2, which is likely an over estimate because it doesn't account for friction or microstepping losses. I used that and 1g applied to a mock extruder assembly, the X-axis rail, and X-axis plate in an FEA model. I added radial restraints to the inside of the linear bearing holes and fixed constraints in the shoulder screw holes. The parts were "bonded"...bolt contacts are a pain to add in solidworks simulation, and they really slow it down. Considering the close screw spacing, I expect maybe 5% more deflection in reality than predicted by the bonded contacts. The results showed a max deflection at the nozzle location of about 0.19mm in Y and 0.07mm in Z. Some of the Z deflection came from 1g, which will be accounted for because that's constant. The rest came from torquing of the X-axis plate. That amount of Z is concerning, though...it'd probably cause poor bonding in one direction and extruder skipping in the other direction. The Y deflection is also concerning...that'd definitely be noticeable, and probably manifest as bad ringing or bulging layers. I tested both forward and reverse acceleration to see which was worse. I also did a mesh independence test. I then started modifying the design and running FEA. Took about 30 iterations, but finally settled on something better. 

I targeted half of both of the baseline deflections as my goal. I figured out I didn't have room in the Y-axis to make the plate thicker, which would have been the best way to improve bending stiffness due to Y-axis acceleration. So the plate can't be thicker than 1/4". Tests making the thin beam portion slightly wider helped Z deflection some, but didn't significantly help Y. I also tried making it thicker just around the linear bearings, where I did have some room to make it thicker, but that didn't help very much. The stress contours showed that almost all of the material around the linear bearings, except the portion near the thin rail mount beam, was essentially unloaded. Since I couldn't make the rail mount beam thicker, I had to change materials. Steel and stainless steels have a modulus of elasticity about 3x higher than aluminum. Unfortunately, they're also about 3x denser. I decided on SS304 because I can buy it for not much more than carbon steel, and it won't rust. One small advantage of using SS304 is that it's CTE will match that of the rail (some steel alloy) better than aluminum. I realized that extending the rail from 430mm long to 450mm would stiffen the transitions to the rail mount beam by effectively making that region thicker. Final results: Y deflection of 0.11mm and Z deflection of 0.038mm, 2x the mass of the X plate (about 10% more moving Y mass). Not terrible, I got most of the way to the 1/2 deflection goal. 

(Preliminary) Design of new X-axis plate

Final FEA run,100x exaggeration, showing contours of Y-axis deflection 

Contoured ends and pockets under the rail resulted in about 30% mass savings over solid. I also redesigned the X-axis belt tensioner part to accept the chamfers. The pulley blocks will need a chunk milled out for the longer rail and a M3 hole relocated, and the linear bearing blocks will need to be re-designed and re-printed. 

I'll probably go ahead and press the linear bearings into the old x-axis plate and use it to test the Y-axis shaft spacing and motion. 

To do:

1. Finalize re-design

2. Buy a longer MGN9 rail. oof

3. Buy a piece of SS304 bar stock

4. Re-machine pulley blocks

5. Re-print linear bearing blocks

6. Mill new X-axis plate

7. Get back on track.