Suspended laser bed? [Spoiler alert: No...]

Here’s a sadly blurry picture showing the SC8UU blocks, printed end fix blocks, and suspension blocks as attached.

Note that the holes in the SC8UU blocks aren’t on a 20mm pitch so I can only use two of the holes to attach it to the bed frame. That’s OK.

Installing those rods which I had ground to length within 0.5mm (and probably better) made it clear that one of the corners had slipped down about 4mm in all the disassembly and reassembly I had to do, so I fixed that too.

3 Likes

I finally realized that this configuration interferes with where I had expected to run the drag chain. So now I’m playing with new ideas on that. As a result, I discovered that you can get 7mm x 7mm drag chain, and I think it will fit where I need it. :crossed_fingers:

2 Likes

My next task has been to find a way to attach the UHMWPE string to the Z torsion rod.

It’s harder than it seems.

I made plastic clamps to hold the string against the rod and with 15mm of tightly clamped plastic, it slipped with 30 lbs of force; even with 50mm of very tightly clamped plastic, it still slips at around 70 lbs of force. Then I tried installing it, and even with the clamp forced open I can’t get it installed on the torsion rod because it’s too tight. PE is almost as slippery as PTFE (“poor man’s teflon”) and UWMWPE though much stronger than steel and much less stretchy is very slippery and hard to fix in place.

So now I am starting over on a new design. This is one of the trickiest design challenges I’ve come up against so far, and I thought it was going to be fairly easy. :frowning:

2 Likes

Everything is a journey, nice you are keeping up this diary. Ive not gone with a laser as far as you have, but you will definitely help others.

Keep up the good work - its hard enough making your own thing, let alone the documentation you are putting into it. Fair play.

2 Likes

You are definitely further with a laser than I am, since you actually own a working laser cutter. I just have parts and ideas that I sure hope are going to become a working laser soon!

I’ve switched from M3 to M5 hardware, and I added a hole for an M5 bolt through the body to act kind of like a “dog” — I plan to run the cords through the split and then lock two nuts together around the cords on the far side of the split, and use that to fix the cords to the shaft so that as it turns it lifts the whole bed level.

Imagine two M5 flat-head screws in these two holes in the side:

…screwed into M5 nylock nuts on the other side, with printed hex sides that act as a wrench:

(Yes, the outside is really narrower; that’s to aid in disassembly if necessary.)

The hole in the top is tapped M5 and a 40mm M5 socket-head screw goes through it. Two washers (including a lock washer on the outside, I think) lock around the UHMWPE cord:

Anyway, that’s printing now; hopefully I find this evening that it works. :crossed_fingers:

2 Likes

The first one took 1.5 hours to print. The move from M3 to M5 hardware created larger unsupported overhangs that printed worse, but sere still good enough to use.

The main difficulty I had was that the hole down the side is 30mm tall, and I don’t have an M5 tap with quite 30mm of cutting area. I ended up just pushing a few threads out of the way at one end to have threads all the way through, and it worked fine. PETG is a bit of a pain to tap. It really gums up in the flutes, and even with spiral flute taps I can only tap about 5–10mm at a time without clogging the flutes and having to back out and re-cut.

I haven’t tried a strength test. It looks good enough to print a second. At this point I plan to just install them both and test them.

The UHMWPE I’m using is impregnated with graphite, which is great for being slippery and even more self-lubricating, but also is… great at being extra slippery and hard to fix to the rod! Also the graphite gets all over my hands while I’m working with it.

The v bearings I bought are at best just barely big enough for the four strands (it’s doubled) that have to run through them. While I’m trying to make this work, I’m not sure I’d recommend it to the next person. I think it would have been smarter to use a couple of stepper motors to drive M8*2 lead screw, one motor at each end, and just have sensors/stops to tram them to deal with them getting slightly out of sync. At this point, there’s nothing actually preventing me from switching except curiosity as to whether this approach will work.

I note that my solution for the problem of terrible KP08 bearings would generalize to holding lead screws on the sides near the corners of the frame of the bed. So if this suspension approach ultimately fails, I’ll have some unused 8mm rod and exactly the eight extra 608 bearings I would need to move to a more conventional design.

2 Likes

I printed the second and tried an installation. I was able to lift the bed by turning the torsion rod, but it wasn’t perfectly level, and adjustment was extremely difficult. Part of this was because I had expected to be able to adjust it with the cord pulled through the slot underneath the clamp-closing screws, but the cords got a bit pinched between the clamp and the rod and hung up subtly, so one side was under slightly less tension than the other.

The side hex recesses turned out not to be tight enough to hold nylock nuts solidly, so I had to test with regular nuts.

I used an ULP screw head for the dog screw that holds the thread, but it still seemed likely to tangle the cord around the head as it turns.

The next design iteration buries the head of a standard M5 socket head screw to avoid tangling, which also solves my problem with short M5 taps by reducing the length of the tapped segment. I have added a hole for the cords to go through from the head side to the far side, and because I don’t know which side will work better, I added a hole on each side. I left enough plastic to be strong enough not to break. I reduced the nut clearance, hoping that might make it possible to use nylock nuts, but I don’t think it will matter in the end.

Ultimately, even if this works, I expect that it will be less accurate than stacked belts, so for anyone following along, I’d recommend single-start trapezoidal lead screws.

3 Likes

My thought would have some sort of turnbuckle for leveling and just tie the line to the buckle with a fisherman’s knot?

I haven’t figured out a way that any form of turnbuckle could fit…

This is whipping around inches from the laser tube, so anything that might come loose and hit it would be an expensive mistake. Especially since I went all out and bought a Reci… :slight_smile:

3 Likes

Here it is, printed, with just the dog screw installed:

Now with the fixing screws installed:

Installed on the Z torsion rod:

My son helped me tighten it as evenly as possible, line everything up, and then crank the bed up and down.

  • The bed moves smoothly up and down, even if the corners are out of tram by over a centimeter.

  • Trammed as well as we could, the corners were out of tram by over a centimeter. This means that getting the tension even on all four ends in each bundle wasn’t feasible with this design, and I don’t have ideas for how to make it secure and adjustable that make sense to me.

  • The cord wrapped around the tension rod nicely and tracked along the rod in a spiral without wrapping over itself. So if we had been able to do fine adjustments, it would have worked nicely.

  • The suspension blocks were great. The line stayed put. They are clearly very strong, but also very easy to put the cord onto and the cord stays put. I really like this design.

  • This last version of the tension rod cord fixture didn’t slip on the rod. Might finally have that solved, just in time to abandon the project.

Oh well, this was a fun idea to play with!

I’ll switch to trapezoidal rod, but I haven’t worked out the belt path(s). I might try making my own closed-loop belt. A thread on reprap.org recommends Black Witch neoprene glue. I have some IC-2000 rubberized CA that is also recommended for joining belts. I don’t have any experience forming a scarf joint in timing belts but why should I let that stop me?

Also, while I use KFL08 pillow blocks on my 3D printer where the thrust loading is low (the bed doesn’t weigh very much) I hesitate to use them here where the loading will be much higher. My experience with their KP08 cousins doesn’t make me expect some random KFL08 to be high quality anyway. So now I’m pondering making something with needle or taper bearings for support, and 608 bearings to just hold them in place.

3 Likes

I chose the right time to figure this out.

Zyltech is my in-the-US source for single-start lead screw. And through tomorrow, they have a 21% discount on almost everything with “2021” coupon code, so I saved $6.68 on my order. :tada:

3 Likes

UPS crushed the box with the lead screws and chucked it into the garage where it was hard to see and was in danger of being run over, but the contents look OK.

I bought some tiny thrust bearings for the bottoms of the lead screws, and will use the same trick of 3D-printed bearing blocks, but these will have pockets for the thrust bearings as well.

3 Likes

I augmented my design for the pillow blocks to add these offset bearing blocks. They are asymmetric so that the edge of the bearing block section is lined up with the edge of the 2040 v-slot on which it is mounted, and the mounting holes in the flanges line up with the slot below them (centered 10mm in from the edge).

Here’s a bottom/top pair; the bottom one is thicker because it includes the thrust bearing.

Looking from the top, you can see the cute little 8mm thrust bearing at the bottom. There is nothing but grease holding it all together! The 608 bearings are held in firmly with crush ribs, but the 8mm top race easily sticks to the rod riding on it and can come out through the hole in the 608 bearing, so being careful not to lose the loose bearing race is important. The fact that they can come out should make it easier to re-grease them if there is ever evidence that they need more grease…

I next need to model parts into which to install the trapezoidal lead screw nuts, that hold up the bed.

3 Likes

The offset holes in the base are offset to make assembly possible. A lead screw runs through the middle of that hole, and if the mounting screws were aligned with the axis of the lead screw I wouldn’t be able to attach the assembly to the bed frame because the screw would be in the way — but I can’t attach the assembly to the bed frame first because then there would be no way to insert the screw.

I will assemble the nut in this block, then the lead screw through the nut, a timing pully on the bottom of the screw, and then put the top and bottom bearing blocks, and then slide the whole asesmbly into the side of the main frame and attach this new block to the bed frame.

After doing that at all four corners, I can tram the bed frame to the head by turning the corner pieces individually. Then I can run belt around all the pulleys and a motor, glue a closed loop, tighten all the pulleys, and I should have a working bed.

I do expect to have to adjust the locations of the guide rods to fit the lead screws right by the corners.

3 Likes

Having the holes on two axes meant overhangs, and I wondered whether I would get away with the overhangs.

Reader, I did not.

I hate cleaning up supports on PETG, but it looks like I’m going to suck it up and print with supports. I was able to confirm that the cutout in the tab is sufficient for installing the nut inside its cage.

Since I’ll be re-printing, I might as well make four mounting holes instead of two in order to give myself more options for mounting the part.

3 Likes

So, it turns out that I really was remembering correctly how horrible it is to clean out PETG supports. Boy howdy. But I guess I’ll be doing it three more times.

Critically-sharp focus hard to achieve on cell phone pictures, but:

3 Likes

I printed the remaining four and put them together this morning, and I am not satisfied with this design.

I redesigned it to align critical holes and faces with print direction, and made teardrop-shaped large holes for mounting screw heads to print without supports in this orientation:

This is upside down relative to how it will be installed. The flange of the nut bears against the inset face, and that carries the entire load of the bed (across four screws) down to the thrust washers. This will not only print without supports, but also has the layers aligned to carry the load in the strongest orientation, and is simply more plastic and will be stronger.

(I know, four lead screws isn’t exact constraint design, but then a 2060 1.5 meters across and 850mm deep isn’t exactly perfectly stiff either.)

3 Likes

The teardrop counter-“bores” work well enough. It’s a bit ugly inside but it doesn’t impede function. The M5x8 mounting screws inside those counterbores are captured by the lead screw but only barely, and can still be turned with a driver with the lead screw in place. This is pretty much exactly what I wanted. The mounting holes aren’t perfectly shaped but there is no reason they need to be. I didn’t use heat-set inserts for the M3 screws holding the flange; there is no tension on them; they just need to keep the flange from rotating, so I just tapped the plastic directly. It’s not a strong fit but it’s plenty good enough. I’m printing the last of the set of four now.

3 Likes

Last night I cut the lead screws to length, which sounds trivial but took a while.

Measuring 338mm accurately (which should give me 0.5mm of clearance) on a screw was tricky, because my largest calipers go to 300mm. I guess I could have stacked calipers :open_mouth: but instead I measured the first screw against a long rule and turned it until I had a land that was aligned with 338mm and marked the land with a sharpie. Then I put it in a 5/16 collet (I don’t have metric 5C collets, but fortunately that’s close enough to 8mm) on my lathe, and used a razor blade along the edge of a cutoff tool to locate the cut to the right place on the sharpie mark.

Stainless work-hardens, so I had to be a little bit aggressive in feed rate. It still screamed a bit during the interrupted cut until it got through the thread, and I was using a carbide cut-off tool that deformed the edge of the thread instead of a HSS cut-off. I didn’t need the end to engage with a nut, it just needed to be able to be threaded through the nut without damaging the nut, so I filed off the first ⅓ or so of a turn of thread where it was deformed. Then I ran the lathe in reverse to relieve only the outer few turns of thread.

Then I filed it lightly to fit stacked into the top and bottom bearing blocks I made.

I used the first screw as a reference to cut each of the other screws, in order to avoid stacking errors. I used a square reference for the end, lightly engaged threads side by side, and used the reference screw to mark the next screw.

Now all the screws are threaded into the nuts, with pulleys installed, waiting for my next shipment to drop-in T-nuts to arrive so that I can install them. (The alternative would be to take apart the frame a little bit for the nth time to fit slide-in T-nuts, and I suppose I really should do that anyway, but I sure don’t feel like it.)

Next design task is an adjustable motor mount with idlers to drive the Z belt. So far, I have been trying to design this such that it could be built by someone with a 3D printer, common tools including a dremel, and easily-available parts. I’m trying to come up with a design for the motor mount that fits those design constraints. (I could have cut off the lead screws with a dremel, so even though I did use my lathe mostly for the fun of it, this fits my design constraints…)

3 Likes

Tonight I mounted all four lead screws. It turns out I could have used all four screw holes in each of the bed stage nut holders:

The M5x8 pan-head screws I used just barely fit behind the pulley, and everything fits together snug. Obviously, once there is a belt around the pully the bed frame will not go this far down and the screw head won’t matter anyway. I’ll have to have a stop of some sort then.

Then I got two of the four guide rods mounted:

The tops are all loosely mounted until I get them all started so that I can get them lined up.

Note that the guide rod holders are thicker than the flanges on the printed bearing blocks and use M5x10 screws.

2 Likes