Van de Graaff Generator Update

The coming of winter and dry Canadian air brings snow and other awful things, but there's always a bright side:  it's perfect Van de Graaff generator (VDG) weather!

I wanted to play with my VDG a couple weeks ago and discovered that it was producing absolutely no sparks, even though the air in my house was so dry my skin was cracking.  I set about troubleshooting...

First I tried wiping the tube down with alcohol to make sure it hadn't accumulated some sort of conductive dust (?).  Nope, that wasn't it.  

Then I tried swapping the fancy brushes I had pulled from a laser printer for the old standard aluminum tape with the edges clipped to make many sharp points.  Nope.  I put the fancy brushes back.

Then I checked the ground wire to see if the cats had chewed through it.  They had definitely been working on it, but hadn't chewed all the way through it yet.

Finally I took the belt off and examined the pulleys.  The top pulley, covered with Teflon tape, felt sort of rubbery, as if the thin layer of Teflon had worn through.  The bottom roller, covered with aluminum tape, likewise felt sort of rubbery.  Hmmm.  Close examination of the belt found its surface getting sort of crumbly and sticky.  Bingo!  The failing belt had left residue on the Teflon and aluminum rollers so there wasn't any triboelectric effect working any more.

It makes sense, if you think about it- as the belt goes onto a roller it stretches to conform to the roller surface, and as it comes off the roller, it relaxes again.  The  belt is like a pencil eraser scrubbing on a piece of paper, leaving crumbs behind.

I replaced the Teflon and aluminum tapes on the top and bottom rollers, and stitched together a new belt from some fresh therapy-band material, put it all back together, and POW!  The machine was throwing long, painful sparks into my hand again.

Bottom roller covered with fresh aluminum tape, fresh belt, and fancy brush.

Top roller, fresh Teflon tape, fresh belt, and fancy brush.

If you have a VDG and it mysteriously stops working, it may be time to replace the belt and clean or replace the coatings on the rollers.

One more thing- the rims of the two Ikea serving bowls used to make the spherical terminal at the top of the generator have no lips so it's almost impossible to stand one on the other, edge to edge, and have the top one stay in place.  It's a two person job to hold the top bowl in place to tape the two together, but once taped, you can't access the inside of the sphere.  That means you can't show disbelievers the inner workings of the generator.  Who would believe that all it takes is a rubber band, a small motor, and a couple plastic rollers to generate >500 kV if they didn't see it with their own eyes?

I got some help from Jake (the knight who does battle with a giant Tesla Coil at the Milwaukee MakerFaire) at the Milwaukee Makerspace and we TIG welded three small stainless steel screws to the inside edge of the top bowl.  After welding I cut the heads off the screws with bolt cutters and ground what was left down to nubs, just a few mm long.  The screws rest just inside the rim of the bottom bowl and keep the top bowl in alignment, without tape.

top bowl with three screws welded to inside rim.

One of the screws welded to the rim of the top bowl. 

Now when I run the generator, after a few seconds of operation I can draw 600+ mm long sparks to my arm.  If I stay away from it and just let the VDG run, it fires sparks into the air with a menacing "SNAP" every few seconds!

Look ma!  No tape holding the bowls together!

Here are a few fresh pictures of the generator doing what it does best:

Son of MegaMax Gets a New Y-Axis

My second printer, Son of MegaMax (SoM) has been a workhorse at the Milwaukee Makerspace for over 3 years, but there have been a few things I didn't like about it, so I decided it was time to make some changes.

The PEI has been reglued onto the bed surface a couple times and the edges were starting to lift up again.  I also wanted to change from the 450W, 24V heater to a line powered heater that would get the bed up to temperature faster, and eliminate the giant industrial power supply that sounds like a vacuum cleaner.  I gave up on the ball screw drive that has been limiting speed due to a severe mechanical resonance and went back to belt drive.  I also wanted to convert to a kinematic mount for the bed plate.

SoM's bed heater, shortly after it was put into service.

Bed plate just removed from SoM.  Dark spots are scorched kapton where air bubbles got between the heater and the bed plate.  This is how heaters eventually fail...

The New Design

I spent some time modeling the changes I wanted to make and got busy in the machine shop at the makerspace.  My machining skills and time are very limited, so whenever possible, I try to make use of existing parts and materials that will require a minimal amount of machining to make them work.  If you've read any of my other blog posts, you'll know that one of the materials I use a lot is square aluminum tubing.  In UMMD, the pulley supports, motor mounts, and extruder carriage are all made from square aluminum tubing.  I used the same tubing to make the bed supports and motor mount for the new Y axis.

The Kelvin-type kinematic mount uses three leveling screws- one each for reference, pitch, and roll, just like the mount in UMMD.  The reference screw sits in a chamfered hole in the bed plate, while the pitch screw sits in a chamfered slot that allows the bed to expand when heated.  The expanding bed plate is free to slide against the roll screw that simply supports the bottom of the bed plate (no holes or slots).  Springs at each leveler hold the bed plate down on the leveling screws.

Occasionally people ask me why I didn't use a Maxwell-type kinematic mount instead of the Kelvin-type that I used.  The answer is simple: the Kelvin type mount only requires slots/holes to be milled/drilled in the Y direction, just like the motion of the milling machine table.  The Maxwell type mount would require milling three slots, 120 degrees apart.  That would require a rotary table which we have at the makerspace, but it's a real PITA to set it up on the machine.

This is the new carriage plate and bed support designed for kinematic mounting of the new bed plate.  The sphere head screws are the reference and pitch adjusters, and the screw on the left is the roll adjuster.

The tubes used to make the mounts were first cut to a few mm longer than needed, then I drilled the holes, and finally milled the edges to final dimensions.  The angled edges dimensions weren't critical, so I marked the angles on the tubes with a marking pen, then clamped the tube in a vice on the mill table, tilting the tube so that the line I drew was parallel to the top of the vice.  Pieces of wood stacked inside the tubes kept them from collapsing when they were squeezed in the vice.

The springs attach to bolts screwed into the plate on one end and the bed support tubes on the other end.

Springs and the screws that will be used to anchor the springs to the bed plate.  I drilled holes then cut the heads down.

The bed hold-down springs hook in the screw heads.

UMMD has a kinematic bed plate mount and it is extremely stable.  Of course, it moves the bed in the Z axis, not Y, like SoM, so we'll see if the concept holds up in a bed-flinger type printer.

Here's the carriage assembly.  The reference adjuster is on the right, pitch on the left, and roll adjuster at the top of the photo.  The plate that links the three bearing blocks is 2.5 mm thick aluminum that was cut on a band saw (no milling on this piece, though the milling machine was used to accurately drill the holes for the bearing blocks).

Heater mounted on the bottom of the bed plate. The left side tab will sit on the reference screw, the right side tab will sit on the pitch screw, and the tab at the top of the picture will sit on the roll screw. 

This is the reference ear of the bed.  The dark circle in the chamfered hole is where the spherical head of the reference screw contacts the plate.

This is the pitch ear of the bed plate.  The dark lines in the slot are where the spherical pitch adjuster screw head sits.

This is the reference adjuster screw assembly.  The pitch adjuster is identical.  The screw on the side is there to anchor the spring that will hold the bed plate down on the adjuster screw head.

This is the roll adjuster assembly.  The end of the screw supports the bed plate from below.

This is the reference end of the assembly.  Putting the carriage plate on the bearing blocks instead of on the leveler tubes keeps it far from the bed heater.  All the leveling screws are threaded into teflon blocks that won't melt or soften when the screws get hot.

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This is the pitch end of the assembly

This is the assembled roll adjuster.  The knob has 16 ridges, each of which represents 50 um of vertical displacement.

Motor Mount

The motor mount was made from a piece of 2 1/2" square aluminum tubing.  I had to raise it a bit by putting a piece of 1/4" thick aluminum under it so that the belt could easily be clamped to the carriage plate.  The motor mount is held down by two 5/16" carriage bolts that fit into the t-slots in the base plate.  Belt tension can be set by pulling on the motor mount, then tightening the bolts.  I used a 20 tooth pulley.

End Pulley

I had a piece of junk from something I took apart years ago that looked like just what I needed- a milled aluminum bracket with two bearings pressed into it.  The bearings have 1/4" bore, like the motor shaft, so I simply mounted a pulley on a 1/4" shaft that I also had from a junk tear-down.  The only problem was that the shaft height of the motor and the end pulley differed by about 1 mm.  That meant that the belt clamp would have to accommodate that difference.

Y axis end pulley

Belt Clamps

I wanted to mount the belt clamp on the carriage plate, so I calculated the necessary thicknesses of the clamp to keep the belt parallel to the Y axis guide rails.  On the motor side of the clamp, the belt would be 5.6 mm above the carriage plate, and on the end-pulley side, it would be 6.6 mm above the carriage plate.  So I designed a printable clamp with those distances in mind.

At first I designed a one-piece clamp, but thought about it and decided it would be less likely to become a source of backlash if I split it into two pieces.  That way when the bed reverses direction, the belt tension will always keep the clamps in position without introducing any backlash.

New Y axis belt clamps.  Splitting the clamp into two pieces reduced the possibility of backlash.  Steel pins secure the ends of the belt in the clamps and the belt teeth interlock in the slots in the clamps.

Electronics

The original heater was a 24V 450W unit, so it needed a big power supply- 24V at 31A.  That power supply had a fan that sounded like a vacuum cleaner.  Since I have switched to a line powered bed heater, I didn't need the high DC power so I replaced the main power supply with an LRS-220-24, a 24V 8A supply (still overkill) with no fan at all.  Complete silence!

The new, 750 W, line-powered bed heater is capable of getting much too hot, so I added a thermal cut-out for safety.  In UMMD I bolted the TCO to the bed plate, but decided it would be safer to have it attached to the heater.  That way, if the adhesive on the heater lets go (as the adhesive on UMMD's heater is doing now, after about 2 years of use), the TCO will stay with the heater and be able to do its job.  I used the same TCO that I used in UMMD, but in SoM I attached it to the heater using high temperature silicone.  UMMD will be getting modified as soon as I get around to reattaching the bed heater.  When the adhesive eventually lets go on this bed plate I'll reattach it with high temperature silicone.

Here's the heater with the TCO added- it's in the blob of blue goop next to the thermistor at the center of the bed.

Power to the bed is switched using a Crydom D1225 SSR.  It's wired through the 10A circuit breaker that serves as the power switch for the printer, and then goes through the TCO on bottom of the heater.  I used Anderson Power Pole connectors for the heater and thermistor connections to the controller.

Performance

I've been able to crank the acceleration up to 3000 mm/sec^2 and print at 100 mm/sec, and I'm not done tuning it yet.  That's a big improvement over the 40 mm/sec limit that was imposed by the resonance in the ball screw setup that used to drive the Y axis.  Print quality is excellent, as always.

A Few More Changes

I connected the power supply ground to the line input ground and also to the frame of the printer - that should have been in the original build.

SoM's lighting has always looked a little dim, so I added some of the same 24V white LED strips that I used on the top of UMMD.  Much better!

I replaced the Titan extruder with a BondTech BMG.  That means I need a new print cooling fan duct design, so I'll be working on that over the next few weeks.

The BMG mounted on SoM's extruder carriage- the hot end offset from center is different than the Titan, so the print cooling fan would no longer fit.  I'm redesigning that now...

UMMD Gets a New Extruder.... Again

My experiment with the Chinese made aluminum Titan extruder was interesting, but it didn't last.  While it had great potential, it came up short.  Some of the problems I found:

• The gap between the feeder tube and the drive gear made loading filament very fiddly, even with my added aluminum extension tube.  I find a similar problem with the original Titan.
• The pinch roller lever pivoting on the motor shaft wore out quickly, leaving black aluminum dust all over the inside of the extruder.
• The concavity of the drive gear was not centered over the filament feed tube
• The screw that passes through the drive gear and holds the extruder to the motor bends the front cover and puts a lot of pressure on the ball bearing that mounts in the cover.  The original Titan has the same problem.
• The pinch roller spring was much too strong which made printing with flexible filament difficult.

Aluminum Titan clone and XCR3D hot end mounted on UMMD.

After more research I decided to give the Bondtech BMG a try.  I have to say it seems very solidly made and there's no chance that screw pressure will distort the body of the extruder.  Loading filament is super easy- just hold it at the entrance of the extruder and tell the printer to extrude some filament.  The dual drive gears grab the filament and pull it into the extruder without any probing around to find the hole.  The filament path is just big enough for the filament and there is nowhere for the filament to flex out of the path.

The BMG extruder parts.  Very solid construction, and unlike the E3D Titan, tightening the screws doesn't cause any misalignment of bearings.

The filament path has two drive gears.  The pivot arm, on which one of the drive gears mounts, is removed in this photo.

When the Bondtech extruder arrived I discovered that it didn't come with a Bowden adapter, so I printed a fitting that would allow me to mount a hose fitting on the extruder.  Then I discovered that it wouldn't fit on the extruder carriage because the hose fitting interfered with the front of the carriage.  I also discovered that unlike the excellent hose fitting on the input side of the extruder, my hose fitting was junk and didn't grip the teflon tube very well.  I ordered the Bondtech Bowden adapter and it fit perfectly, allowed the extruder to mount on UMMD's carriage, and gripped the teflon tube tightly.

The BondTech BMG extruder mounted on UMMD.

I had one other, minor problem installing the BMG.  Only one set of mounting screws came with it and they were a little too short to go through the 5mm thick mounting plate on UMMD.  Fortunately
I had some longer screws handy and was able to get the extruder mounted.

I set the steps/mm to 415 in the config file and ran some test prints.  I still have more extruder tuning to do in the config file, but it's printing pretty well with the default setting.  I'll report on any problems or failures here.

Note- I'm still using the Chinese made XCR3D hot end.  Other than the crappy fan that I replaced  with a Sunon part that is specc'd to operate up to 70C, it has been performing well.

Update 1/26/19

I had an interesting failure.  Someone at the Makerspace was trying to print with the nozzle smashed against the bed surface (to get better first layer adhesion?! - I'll have to revise my training materials) and the extruder kept pushing filament.  It pushed so hard that it pushed the Teflon tube out of the Bowden adapter at the exit of the extruder.  The BMG extruder can really push, so I reduced the extruder motor current so that when a jam occurs, the extruder motor will skip steps instead of pushing the tubing out of the Bowden adapter.  Reduced current means reduced torque and reduced heat which is good because the chamber goes to 50C when printing ABS.  I guess it also means that if I were so inclined, I could use a smaller, lighter, lower torque motor for the extruder.

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.

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:

Unloaded and zeroed.

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.

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... it's been working trouble free for >2 years.

I replaced the belts with some glass core, 9mm wide Gates GT2 belts (the same type used in the XY stage), and then redesigned and printed new clamps that 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:

SoM's X axis belt clamp has been working for >5 years.

I used this printed gauge to check the slot widths for the belt material I had on-hand,

Gauge used to check slot widths needed to clamp different belts.  1.0-3.0 mm in 0.1 mm steps.

then designed the new belt clamp to fit the bed lifting brackets and the belt:

The new belt clamp for UMMD's Z axis.  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.

Note: the belt clamp file linked above is not the exact dimensions I used in UMMD- I was unable to locate the original file so I recreated an approximation of it that you can easily customize to fit your printer and belt.

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.