No worries. I post build logs of custom PC cases that I'm building, which often don't get too many responses/replies as well:
No worries. I post build logs of custom PC cases that I'm building, which often don't get too many responses/replies as well:
I'm watching, find what you are doing very interesting, please continue.
I need to go shoot some pictures with recent progress, but let me try to give a quick update. I don't have any "action" shots since between the lithium grease on the rails and the sweat that poured out of me from the heat, getting the beam + rail assemblies mounted was a slippery, ugly process that didn't lend itself to also having a camera nearby. It took much, much longer to get done what I had wanted, and in the process, I ended up ruining the spacing between rails and the table. It's not horrendously off, but this is an area where I'd almost rather be a centimeter off than be a few millimeters off. I've got an idea for a basic alignment "jig" that will help, but it means a further delay before completing the mechanical build.
Some thoughts and lessons-learned from this process:
1) Butt joints are better. When I put the design together, I had really wanted to use butt joints to mount the beams that support the linear guides. However, because I got the beams milled, the machining necessary to use the "double blind" joints was not available. I had Misumi custom quote such machining for me, and the answer was 4x the cost. Thanks, but no thanks. That being said, I had also thought about just getting the ends tapped, which leads to lesson #2.
2) Don't get wedded to immaterial specifications. Part of the point of highlighting the Misumi services in my build is that Misumi puts a lot of power into a buyer's hands with its Web Ordering System (WOS, in their lingo). Unlike 80/20, which requires you to fill out an offline order form (maybe the ACAD plugin is more automatic?), Misumi has a pretty impressive online configuration engine for common machining tasks. That being said, there are tolerances and limitations for what they are willing to offer customers. This created two issues which had a collectively easy answer, but ones I did not see at the time. Maybe someone will find this helpful when faced with a similar dilemma.
For the GFS (super heavy) extrusions I am using for corner posts, Misumi did not offer the M12 counterboring I wanted to use that would allow the butt end of the rail beams to be screwed into the sides of the corner posts. Since I wanted all of the table support "joists" to have a common dimension (1060mm), the corner posts are joined to the beams on the 45mm wide side (see the model in post #6). Because I had not "done the math" and was still suspicious that I was getting an equivalent or better product at a much cheaper price, I was mentally wedded to the GFS extrusion (heavier is better, especially for a critical structural member, right?). If I had switched to the HFS extrusion (which is only marginally less strong than the Bosch "heavy" extrusion I was planning to use), then I could have had the M12 holes and counterboring done. That was the first problem, and one I thought a lot about. The next problem was that I rushed the configuration process a bit more than I should have (anxious to get materials and get building), so I could not find a good way around the next issue, which is what prevented me from doing the right thing, as I see it now.
Misumi will machine holes only in even millimeter spacing from the "left" end (pick one end to be the "left" and keep it consistent for the rest of the machining). This seems simple enough, except the hole centers for the 4590 extrusions are 22.5mm from the nearest sides of the extrusion and 45mm center-to-center. What this means is that if your extrusion is an integer multiple of a millimeter (e.g., 900mm), and you tried to have machining that would create a flush end-joint, then you would be off by half a millimeter. Your first hole would be at either 22mm (with the joined member too "high") or at 23mm (with the joined member too low), creating a 0.5mm gap, which is enough to feel and certainly worrying when you are aiming for sub-millimeter accuracy.
What I did not see at the time was that Misumi offers extrusion cuts at 0.5mm intervals. This was an important oversight for me. While I am still convinced that it is an oversight for Misumi not to offer hole machining at the half-millimeter level (and not to offer M12 machining with the GFS extrusions), the practical answer for me should have been to a) switch to the HFS extrusion with the M12 machining, and b) to go with either a post length of 899.5mm or 900.5mm so I could get the machining done with the correct spacing between the table and the rails. From a work height standpoint, that half millimeter was irrelevant, but it was material to the machining ordering process, and I blatantly missed it. Correcting this would be too expensive, so now I go with the alternative approach I had devised, which is a combination joint using both joining plates and gussets. More on that in a bit.
The upside, I guess: the joints that hold the machine together are going to fail a long, long time before the GFS end-posts get crushed.
Last edited by Bear5k; 07-04-2011 at 11:45 AM.
Also, please add Misumi 's hole/boring nomenclature to some of the above. For example, what's their code for counterboring ?
It's a little odd and, in general, better done using the product configuration engine rather than ordering outright. Here is a part from a recent order:Also, please add Misumi 's hole/boring nomenclature to some of the above. For example, what's their code for counterboring ?
HFS8-4590-384-Z12-YA169-YB214 ($25.20/ea.; $15.28 without machining)
Translating the above:
- HFS8- this is the "standard" extrusion (HFS) with a t-slot geared for M8 screws, equivalent to Bosch's Heavy ("H") extrusion. I have not compared for critical dimensional tolerances of new vs. new between Misumi and Bosch, so my opinions on comparability are based upon limited data. The "8-45" t-slots are the only ones where the "8" refers to the target screw size. All of the others are the actual slot width. Based upon my experience with manufacturing systems, I'd guess this nomenclature came about due to a system limitation somewhere (e.g., the HFS6 extrusions have a 6mm wide t-slot opening).
- 4590 - this is the profile, 45mm * 90mm.
- 384 - this is the length in millimeters.
- Z12 - Machined holes for 12mm nominal (nominal plus clearance, or in this case, it is 13mm by spec).
- YA169 - First hole ("A") is a counterbore in the horizontal direction ("Y"), 169mm from the left end. Horizontal counterbores go in both t-slots for the rectangular extrusions.
- YB214 - Second hole ("B") is a counterbore in the horizontal direction ("Y"), 214mm from the left end.
If I had wanted vertical counterbores, the first hole would have been designated XAnnn, the second hole: XBnnn, and so forth. You can have both horizontal and vertical counterbores without requiring a custom quote, but you can't change the nominal hole size (e.g., M8 vertical and M12 horizontal). You also can't get counterbores and wrench holes via the online configuration engine; that requires a custom quote, as well.
Here's the relevant catalog page for counterbores:
Misumi PDF Viewer
...and wrench holes:
Misumi PDF Viewer
This will give you an outline for the various types of machining for cuts and joints:
Misumi PDF Viewer
More importantly, here is the web page to configure everything for the HFS and GFS 4545 and 4590 profiles:
Aluminum Extrusions - 8 Series, Base 45, Four-Side Slots | Mechanical Components for Assembly Automation - Misumi eCatalog
As mentioned above, I did manage to get the rail that had the worst baked-on "gunk" soaked in citric acid for a while. However, the garbage can I used had a hole in it, and the rail pierced the thin garbage bag while I was scrubbing it under water. So, the rail isn't as clean as it probably should be, but it's a lot better. If it proves to be a problem. I'll detach the rails and re-soak them later with a dedicated apparatus (probably a sonotube concrete form with some end caps fabricated). Let's get to the pics, though.
First up, attaching the rail support beams (pictures 1 and 2). In true belt-and-suspenders fashion (redundancy for strength), I am using both a gusset and a mounting plate for this critical join. I finger-tightened the nuts, but in trying to wrestle the 7' long beams, I managed to knock them out of alignment. Fixable, but an irritant nonetheless. You can see the gap between the gusset and the bottom of the beam on the left side. I just didn't have the energy to fix that until I get an alignment tool.
Note: these are GFS8-4590 beams with the 45mm sides milled "flat" (+/- 0.1mm max deviation). I'm sure one can get better tolerances, but not for anywhere close to the same price in my experience -- if you can find someone even willing to do the job. The bigger "cost" of using the milled surface, as mentioned previously, is that Misumi won't do certain types of machining on the milled surfaces. It's a trade-off, but I'm not regretting getting the milled surfaces, merely that I didn't get the ends tapped and the corner posts counterbored.
The third picture shows the center legs installed, plus some extra structure I added to stiffen up the middle of the rails and ensure the width didn't vary too much over the length of the machine. These legs are pretty critical since they are needed to help control deflection. If I spanned the entire 2130mm, my maximum deflection with a 75kg load would be over 0.3mm. When I cut that span in half, the deflection drops to 0.04mm. I looked at several of the "commercial" (DIY+) offerings in this size/price class, and found instances where there were long spans with single extrusions, and I worried -- a lot -- about whether the deflection was going to eat away any of the supposed accuracy in the machines. Once you play around with the deflection calculators a bit, having long, unsupported spans will not look like a good idea. As it is now, I will watch what my results are to see whether I need to add more legs/structure in the future.
Pictures three and four show how I used a some spare gussets to act as clamping surfaces to draw the beam in tight with the corner posts. Butt joints (e.g., screws, dedicated joint hardware) are good because they draw the pieces together. Gussets and mounting plates don't do that, so you have to add in the appropriate force to make a tight connection. This was about the only thing that went smoothly with this work step.
Next to last, you can see the alternate use for the 4590 gussets. My eBay special SR30 rails and carriages don't have a compatible set of holes for any of the Bosch/Misumi extrusion series I am using. However, the Bosch 4590 gussets look like they will act as a good interface between the carriages and the gantry. The offset load shouldn't be a problem (well within spec, even with a pessimistic load for the gantry), but we will see how that bears out over time.
Finally, the results of the acid bath. There was a lot of surface rust and some tar-like substance (baked-on grease?). The rail is working a lot better, but does have some surface pitting.
More in a few days.
You have a great thread going here, one of the best I have read in sometime.... I am looking to either buy or build a machine in the same size as yours so this is all very good info. Are you going to have a BOM available at some point?
With temperatures a frigid 94 degrees outside (higher in the garage, er, um, shop), I was able to get some critical work done today on the parts that tie the machine base and X-axis into the gantry.
Having done some more detailed design work based upon my current progress, I decided to have the bottom pieces of the gantry re-done to include holes where the drive bar should be located to minimize my chance of racking. previously, I had left this relatively open, but after getting hit by reality with where the BK and BF brackets need to be mounted, it became important that I get this positioning correct from the outset, rather than pick a spot, and then adjust later as I find out how much racking occurred at each new position. I had also originally thought about investing in a decent mill with DRO, but learning precision machining on what should have been production parts originally seems like an even more expensive idea. I'll continue to pay someone else to do that for the time being until I get this machine built.
Machining Base Extrusions
As previewed, above, the gantry will ride on base extrusions that are 384mm long, and that have 12mm nominal machining that is done "horizontally". In the first picture, you can see what "horizontally" means: there are two rows of holes, one in each t-slot. These are "clearance" or "through" holes, so they are sized up a bit from what the nominal dimension is. In this case, a 12mm screw is given a 13mm hole.
In order to expedite assembly, I decided to take a risk and use locating pins instead of buying a Heli-Coil kit to tap the ends of the drive bar. Having never tapped a hole in my life, and having never used a Heli-Coil kit before, I'd rather go with something that depended a little less on my manual skills and more on my design skills. If need be, I reasoned that I could always try the Heli-Coil approach as "Plan B", but the locating pins were going to be cheaper, anyway, so they stayed "Plan A".
If I had to do it over again, the right answer was to get the ends of the drive bar tapped from the factory (the HFS 4590 get a Heli-Coil insert at the factory since the hole opening is 12.5mm for an M12 hole, while the GFS extrusions get a "proper" traditional M12 tap with its 10.5mm hole).
The locating pins were an interesting find in Misumi's catalog. There are pages and pages of options, but ultimately, I found one that looked like it would work with a 13mm base (to fit in the 13mm hole with no clearance), and that could be machined for a 12.5mm "nose". It also had the option for a tapped base (M8) that would work well to hold it snugly in place.
While I needed to give the nose a few love taps with the rubber hammer to get it set correctly (that zero clearance thing), the nose fit snugly and securely into the opening in the end of the drive bar with a snug fit. It was then a formality to secure them with a screw, and then attach the drive bar to the gantry base with "double blind joint" hardware.
For those who are reading averse, pictures 1 - 11 take you through the build step-by-step.
Attaching to the Carriages
Once I had the drive bar attached, it was time to attach the gantry base extrusions to the SR30 linear guide rails. This was done using Bosch 4590 gussets with the locating tabs pinched off. Since the 20mm screws I had on-hand weren't quite long enough to attach the gusset to the extrusion, I had to go up to a 30mm M8 screw and use a bunch of washers as spacers. It wasn't ideal, but it's pretty solid, even without the second set of screws in-place yet. A "post assembly" t-nut sits in the t-slot to round-out the hardware.
This was a pretty nice solution (low cost, seemingly effective for the moment) to how to attach the gantry and the 4590 extrusion to the carriages. If I were using 4080 extrusion, then the native hole spacing in the extrusion would have matched the hole spacing on the carriages (mostly), but with the 4590 extrusion I had to improvise. I'm hoping this solution turns out to be a winner over the long-term.
You can see this come together in pictures 12 and 13.
Attaching the Ball Nut
A drive bar wouldn't be much of a drive bar if it didn't drive, so the final step in today's build was attaching the drive bar to the "ball nut bracket" so that the ball screw could move the gantry along the X-axis. The drive bar has two M8 nominal counterbores spaced 50mm apart in the center of the bar (it is 1200mm long). In reality, since I was going with 110mm long screws that I had on-hand from a minimum quantity purchase, I probably should have just gone with wrench holes, saving a shekel or two. However, a couple of washers gain helped with keeping everything in-line.
Note: unless you are looking to pay big bucks from a specialty dealer, longer screws do not come fully-threaded in my experience. Instead, there will be some amount at the end that is threaded, and the rest of the screw will have an unthreaded "shoulder". In the case of the 110mm screws, the threading was only about 25mm, leaving an 85mm long shoulder. There wasn't quite enough threading to snug the screws tight inside the counterbored holes, so the washers kept the screw head out of the counterbores. It's a tight connection, but we'll see how it holds up over time. This entire assembly is a critical one for the accuracy and durability of the machine, so if problems are going to come up, this will be an area that is on the list of "the usual suspects".
Pictures 14 and 15 tell the story even better.