As funny (or punny) as the title sounds, I'm being serious. I always wondered why machines that involved keeping something straight with two tracks always had one track with play/tolerance. It makes a lot more sense to me now.
After spending a (well looking back, not so much) fruitless day of machining on Saturday to try and fix up the angle frame to get it straight so my x stage had no play, I was discouraged and quite frustrated with the process (you try successfully redrilling a countersunk hole without making the entire bolt assembly loose). I felt I wouldnt be able to get the frame properly straight, and all my wonderful visions of having a working 3d printer seemed so much farther away (have I mentioned we haven't even gotten the cartesian system done yet)? It was a little depressing for me, but then again I often find that I get discouraged too easily.
In either case we were left with an x stage that had too much play that would affect the positioning. I spent the rest of the day and most of today thinking about ways to deal with it (including cursing myself for drilling the holes on a drill press, dealing with countersinks, and the like). We considered covering up the countersunk holes with a bolt and washer assembly to remove their self-aligning nature but a quick test found that the three bolts coming together on each corner of the frame meant the bolt head had to be thinner than any bolt head known to man. What's worse was that looking at the top of the bolted frame I found there was no gaps between the horizontal and vertical members of the aluminum frame. If anything the frame members weren't poorly drilled, they weren't exactly (and I mean exactly) the right lengths.
Wait a minute. right lengths. Right angles. Aluminum angles.
Then it struck me. Why should I rely on horizontal alignment using something as finicky as two parallel tracks (or similary two rods with linear bearings on them)? Even the slightest mismatch in the tracks means they'll wobble, or worse bind. Why don't I just use one of the aluminum angles to keep myself horizontally straight? Extruded Aluminum Angle is way more accurate then my fumblings with a mill, so it should be perfect for ensuring something travels straight.
So here's the new plan: I remove the side friction wheels on one side of the x stage and place them on the other side, sandwiching an aluminum angle between two sets of bearings. This means the horizontal alignment of the x stage depends upon the straightness of the aluminum angle (which I think is fairly straight). It also means the frame doesn't have to be precisely square and can be off by a few thousands/hundredths of an inch without wobbling/binding issues. The same approach will be taken with the z stage so that the building platform travels straight up and down and doesn't wobble left and right.
Furthermore the "clamping bearings" will be mounted in slotted holes rather than simple drill holes. This arrrangment (similar to the adjustable tightness idler pulleys in the makerbot) allows me to deal with any uncertainties in the hole location by simple adjusting the position of the clamping bearings. In hindsight I should have applied this principle of "slots, not holes" more vigourously, but hey better late than never. If you're confused about what I'm talking about, don't worry. I'll make it all clear once I construct it and take a photo.
Things are looking brighter than they were yesterday, and I think there were two major design lessons that I learned.
1. stage sliding should not rely on two unparallel tracks to stay straight. Trust me even with a mill/drill press with caliper marked and center-punched holes, the tracks will be unparallel. Well that's probably more a testament to our machining skills, but hey it's hard.
2. Unless your machinery is capable of machining to within 0.00001 inches (okay thats a slight exaggeration, but only slight) or you're willing to pay a professional machinist to do a job 10x better, don't anticipate anything straight enough (you'll get bends/non-90 degree corners). Use slots/slightly larger holes so you can deal with the lack of straightness with the bolting.
Hopefully it works. We'll see next week.
-Eric
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