Monday, December 24, 2018

Sand Table Updates

I built the sand table as a quick project for the Milwaukee MakerFaire, and it showed.  The electronics and cabling were thrown together and everything was held together with zip-ties.  Now that the table is at my house, I wanted to dress it up a little before the final-final upgrades that are being planned.

The electronics enclosure for the sand table was butt-ugly, just as it was when it was on my 3D printer, so I decided to do something about it.  I found a wonderful piece of junk on the hack-rack at the makerspace- an aluminum enclosure, anodized black, that contained two MeanWell 12V 10A power supplies.  I tested them and both worked fine (of course, they're MeanWells).  The box had a line cord jack, fuse holder and power switch ready to go, and there were a lot of ventilation holes in the box.

The first thing I did was test the sand table running from 12V.  There were no problems at all, even running at 500 mm/sec.  Switching the sand table from 24V to 12V operation meant I could get rid of the DC-DC converters that were powering the LED strips, so wiring was simplified and made more reliable.

I pulled one of the power supplies from the box and installed the SmoothieBoard controller in its place, then got to work on wire management.  When I bought the t-slot material used for the frame at the scrap yard, it came with the plastic slot covers, so I routed the wires in the slots and placed the covers over them. The result is much nicer looking, even though it is under the table and mostly hidden from view:

The new electronics enclosure.  No wires hanging all over to temp the cats to chew on them!

New electronics enclosure- all the cables enter the box through a hole near the top.  It's still pretty inconvenient to reach under the table to select a file to run.  The next version will probably have a wifi capable controller.

"A" motor and Y axis limit switch.

"B" motor and X axis limit switch (behind the motor).  Cables are routed in the slots in the table's frame and covered with the plastic strips that are made for the task.
The wires to the LED strips really need to be reworked so they don't exit along the back edge of the table, and I should add a connector so that they can be disconnected easily if the table has to be taken apart to be moved.

Since my original post on the sand table, Jeff Eberl and others have been making huge upgrades to Sandify and it now produces even more interesting patterns than before, and has many added convenience features such as allowing the gcode files to be saved under names you specify.  That makes it a lot easier to combine gcode pattern files into one big file.  It also stores the parameters in each gcode file so you can recreate it if needed.  You can also grab pattern files from several locations on the web, too.

Ms. Kitty enjoying one of the new patterns produced using Sandify.

Final upgrades being planned:

1) Switch to NEMA-17 motors for lower vibration, quieter operation.
2) Install a controller with 256:1 microstepping drivers to reduce noise.
3) Replace the big, ugly limit switches with something smaller and quieter (reed switches, maybe...)
4) Finally, get a nice looking glass-topped table and rebuild the whole mechanism into it so it looks a lot nicer and is more presentable and usable as a piece of furniture.  I'm watching Craig's list for a good deal on either a whole table or a nice glass top.

When I update anything else I'll make another post about it.  When I get NEMA-17 motors installed I'll post some video comparing the noise levels of the NEMA-23 and NEMA-17 motors, and then do it again when I install a higher microstepping controller board.

Monday, December 17, 2018

Comparing Steel Core and Glass Core Belt Stretch in UMMD's Z Axis

A recent debacle led me to replace the belt clamps, and while I was at it, the belts, in UMMD's Z axis.  I took the 10 mm wide steel core belts out and installed 9mm wide, glass core, Gates LL2MR09 belts.

In a previous post I had tested the stretch of the steel core belts under a print load up to 4 kg (plus the mass of the bed and it's support structure, another 3.5 kg).  I found that the steel core belts stretched about 42 um/kg of print load, which translates to worst case stretch in any 0.25 mm layer of 1.2 um- completely inconsequential.

Today I clamped my digital gauge to the printer's frame with the bed about mid way down the Z axis and loaded it up the same way- just stacked a few spools of filament on it.  Photos below show the resulting stretch:


Glass core belt stretch when loaded to about 4 kg.

0.58 mm/4kg = 0 .145 mm/kg which is 3.4 x the stretch I measured with the steel core belts.:

Steel core belt stretch.
The steel core belts stretch came to about 1.2 um maximum in any 0.25 mm thick layer (entire bed covered with a layer of PLA).  That means these glass core belts will stretch a maximum of about 4 um in any 0.25 mm layer.  It seems unlikely to cause any issues in a real print situation, but I'll probably put the steel core belts back on the machine with an improved belt clamp design.

Sunday, December 9, 2018

Another Interesting 3D printer Failure- How NOT to Design a Belt Clamp

UMMD has a belt driven Z axis using steel core polyurethane GT2 belts from China.  The belts are held in 3D printed clamps and use short pieces of the same belt to lock the belts into the clamps.

This is a test piece, but the clamps used in UMMD's Z axis use this technique to clamp the belt.
Several months ago I noticed the Z axis belts were flopping around a little, as if they had lost tension.  Initially I thought it was due to the top pulley mounts shifting under the constant pull of the belt tension, and later, after readjusting them, I thought maybe it was caused by broken steel wires inside the belts.

I recently retightened the belts and within a couple days, found they were flopping around again.  I decided it was time to take them off and see what was going on.

Here's what I found:

The polyurethane portion of the belt was sliding and stretching over the steel cables at its core.

The tension on the belt caused the end of the belt to stretch over the steel wires that run through it.  That's not good.  The same thing could happen to any belt held in a clamp the way I have designed the clamps in this machine.  I think it's better to use clamps that fold the belt over on itself to lock it- that way the belt is less likely to slide and stretch on the core.

Here's how the XY stage belts are clamped in UMMD.  The belt folds back over on its own teeth to lock it in place.  This seems to be a better design...

I'll replaced the belts with some glass core, 9mm wide Gates GT2 belts (the same type used in the XY stage), but I'll be redesigning and printing new clamps that will fold the ends over on themselves so that this won't become a problem again.

The new design, which is very similar to the design used in SoM's X axis, which has been working for about 5 years:

The space around the posts is just wide enough for the belt to fit, and the entry and exit slots are just wide enough for the folded belt to fit with the teeth interlocked.

A New Self-Locking Belt Clamp Design

Here's a quick, simple one, in case you're tired of my long-winded posts.

I am rebuilding the Y axis in SoM for belt drive (long-winded blog post will appear soon) after finally giving up on getting rid of the resonance in the ball screw drive.  To that end I needed to make a belt clamp that will mount on the carriage plate (SoM is a bed-flinger).

I decided to try a printed belt clamp because they seem to work well in SoM's X axis and in all three of UMMD's axes.

The drive pulley on the motor and the end pulley, on an aluminum mount I pulled from a piece of surplus junk, are at different heights (about 1 mm).  The belt has to be parallel to the guide rail on the side of the loop that attaches to the carriage plate.  I could either figure out a way to put the pulleys at the same height, or I could adjust the belt clamp design to hold the belt at one level above the carriage plate on the motor side and at a slightly higher level on the end-pulley side.  That was the easier way to go.

I played with a couple single piece designs, but in the end I decided to make two different but almost identical parts and screw them down on the carriage plate.

Here's what I came up with:
The printable belt clamps, one holds the belt 1mm higher than the other.  The belt enters the slot on the other side, folds over the steel pin, then goes back into the slot.  The slot is just high enough to fit the belt with the teeth interlocked, so the belt can't come loose.

A sectional view of one of the clamps.  Each clamp is held down with a single flat head screw.

The belt folds over in the slot and its teeth interlock.  I used a 3mm diameter steel pin (left over from the sand table project) to stop the belt from pulling back through the slot, though I suspect it would be OK to make the whole thing printed plastic.  Even if the pin broke off the plastic, it won't fit through the slot so it would still do its job.

Y axis belt clamps in place.  There's plenty of clearance for the belt above the clamps.

I pulled the upper part of the belt away to make the clamps more visible.
This design depends on the strength of the plastic on either side of the belt - if the slot is too tight the belt might produce enough force to split the plastic layers.  We'll see how long it lasts.  SoM's printed X axis belt clamp has been working fine for about 5 years...

Fusion360 design file is here.