Thursday, July 27, 2017

UMMD CoreXY 3D Printer Frame

Some of the basic design goals for this printer included:

  • being able to fit the printer in my car (a Prius)
  • being light enough that I could handle loading and unloading it myself
  • fit through doorways
  • completely enclosed for printing ABS
  • stable, solid construction so I don't have to make bed leveling or other adjustments
UMMD's frame is made from used, 40x40 mm aluminum t-slot extrusion that was purchased from a local scrap yard for $1 per lb.  The frame was designed to fit the XY stage and the Z axis, then the t-slot pieces were cut a few mm longer than needed and milled square to final, matching lengths in the machine shop at the makerspace.  The axial holes were tapped with 5/16-18 threads and the frame members were screwed directly to each other using 5/16-18 button head cap screws and washers, the same way that Son of MegaMax was built.  When the frame members are cut and milled this way, they form square joints when screwed together as verified by measuring the diagonals.

UMMD's frame made from 40x40 mm t-slot aluminum.  The XY stage subframe is an integral part of the main frame.  The Z axis frame attaches to the XY stage at the top and to the main frame at the bottom.  The Z axis shown is rev 1 which was eventually replaced with the rev 3 design.

Bracing the Frame

The frame tended to wobble slightly in the fore-aft direction because of the relatively large moving mass in the Y axis and the height of the machine.  1/4" aluminum plate corner braces were added to the sides of the printer and the problem was solved.

Corner bracess were added to the sides of the printer's frame to boost fore-aft rigidity.  The braces are 1/4" aluminum plate, held in place with 5/16-18 carriage bolts that just fit the 8mm wide slots in the T-slot frame members.  The green wire at the top connects the printer's frame to the power line ground.

The corner braces were added to the sides of the printer only- it was rigid enough in the other direction without the braces.  A shorter machine shouldn't need the braces at all.

Casters and Feet

UMMD is tall and pretty heavy as 3D printers go.  I wanted to be able to move it without help, so two casters were added to the front legs and leveling feet were placed at the back.  Moving the machine is as easy as tilting it forward, rolling it to its new place and standing it back upright.  The leveling feet adjust to compensate for uneven floors.

One of two casters made from skate wheels.  The bracket is 1/4" thick aluminum.  Casters are on the front legs of the printer.

One of two leveling feet at the back of the printer.  The axial hole in the frame member is tapped for 5/16-18 threads.  The feet had black rubber that will mark floors, so I printed TPU "shoes" to cover them.
It works pretty well.  I am able to load the printer into my car by myself and then take it out and move it to where it needs to go.

The skate wheels came from skates purchased for $3 at a local Thrift store.  Unfortunately, they are a little small for moving the printer up and down stairs, so I may be swapping them for larger wheels in the future.

Enclosure Paneling

A lot of people building coreXY printers say their machine is "enclosed", but what they really mean is that it has side panels.  You'll see a lot of printer designs with pulleys and guide rails mounted directly on the top frame members, making full enclosure impossible without building a big box around the whole printer (like I did for my first printer, MegaMax).

You can't print ABS with just side panels.  ABS printing has to be done in a 45-50°C (or more) enclosure or prints will delaminate.  UMMD was designed from the start to be completely enclosed to allow printing with ABS filament.  Electronics and motors are located outside the enclosure because heat and electronics are a bad combo.

Rigid panels screwed to the sides of the printer can increase the frame's rigidity, but they also tend to increase the printer's weight, which can make transport a problem.  UMMD was going to be heavy enough without adding a bunch of relatively heavy panels to the enclosure.

My last printer, Son of MegaMax, used some PIR foam panels jam-fit into the frame, held in place only by the tightness of their fit.  It worked well, and was light weight, but the panels were easily damaged which didn't look especially nice, and they often came loose during transport.  I wanted something a little more secure and aesthetically pleasing.

I started looking for material that would provide thermal insulation and fit into the 8 mm wide slots in UMMD's frame.  I looked at doubling up coroplast in the slots which would provide thermal insulation, but it was a little too floppy and I wasn't sure how well it would hold up when exposed to 50°C inside the printer.  I discovered dual layer polycarbonate (DLPC) on a walk through a local home improvement store.  Greenhouses are often paneled with DLPC that allows light transmission and provides some thermal insulation.  It turns out the stuff is readily available in 8 mm thick sheets, which matches the width of the slots in the 40x40 mm frame members.  It is very light but very tough, and won't mind 50°C at all.

I spotted someone selling a 4' x 8' panel of 8 mm DLPC for $40 via Craig's List, so I bought it.  After the first piece was mounted in the frame I knew this was exactly what I wanted.  DLPC allows light into and out of the machine, and provides thermal insulation.  One thing I hadn't anticipated was the nice lighting effect produced by the multiple reflections of light off the PC panels.

Dual Layer PC panels mounted in the printer's frame reflect light adding a nice visual effect.  In this picture the UV LED bars are reflected in the back and side panels.  The PC panels also transmit a lot of the light so the room around the printer is bathed in a UV glow.
The printer has DLPC panels in the sides back and bottom.  The top of the printer is a 1/4" sheet of Sintra, a foamed PVC product, to which I have added some aluminum tubing stiffeners.  The front of the printer will have two access panels- an upper one to allow access to the XY mechanism and a lower one to allow access to the print.  The upper panel is 1/4" abrasion resistant polycarbonate and drops into the slots in the frame.  I haven't yet decided on the way that a clear PC panel will be attached for the lower front access.  I'll probably use magnetic tape as I did for Son of MegaMax.

The upper front cover is 1/4" abrasion resistant PC that drops into the slots in the frame.  It has been notched for the belts and bolts.  The upper rear cover is DLPC that has been notched for the bolts in a similar way.

Since the DLPC panels are captive in the printer's frame, the only way I can access the back and sides of the printer's mechanism for service or modification is to lay the printer down, remove one of the aluminum frame members, then slide the DLPC panel out of the frame.  The corner bolsters on the sides of the printer add to the complexity of removing the side panels.  Fortunately, the way the printer is built, maintenance should be infrequent, and I can reach most of it through the easily opened front panels.  The DLPC panels are a loose fit in the slots and rattle if I tap on them with my fingers, but they don't make any noise when the machine is printing.

The bottom of the printer has two DLPC panels, one toward the front (top) and one toward the back of the printer.  There fit into the slots in the frame at the front and back and are also held in place by printed clips that snap (well, OK, pound) into the frame slots.
The top cover has to do several things.  It has to support the electronics and filament spool holder, provide thermal insulation, and hopefully look decent in the process.  It also has to allow access to the extruder carriage from the front and rear of the machine.  The upper-front cover is a piece of clear PC and the upper rear panel is a piece of the DLPC, the tops of which are cut flush with the top of the printer's frame.  Both fit into the vertical slots in the frame members.  I wanted to be able to close the printer for printing ABS, so the top cover had to fit over the top edges of those two panels, but allow for their easy removal.

I printed some long T-strips that fit into the slots in the top frame members and screwed the top cover to it.  The cover can slide forward and backward allowing the upper front and rear covers to be removed for extruder carriage access, and can be locked in place by two 5/16-18 screws at the top front corners.  Normally the screws will only be used when transporting the printer or leaving it set up as a demo at a Maker Faire.

Installing Electronics

For some reason, people tend to think of their printers in terms of the mechanical stuff and forget about the electronics until after the mechanical stuff is already built.  The result is an undersized frame and a struggle to shoehorn the electronics into it anywhere they can be made to fit.

Electronics can go inside or outside the printer's frame.  Putting it outside tends to increase the overall size of the printer, so many people put the electronics for their CoreXY printer (or any other type that has a bed moving in the Z axis) at the bottom of the machine, under the bed.  That helps make use of dead space inside the printer's frame.  If the machine sits on a desktop, this puts the electronics at an easily reachable height for wiring and repairs.

There are two potential problems with putting electronics inside the frame at the bottom of the machine.  If you need to service the electronics, you have to get the bed up and out of the way to do it.  If the drive to the Z motor isn't working and the bed is at the bottom of the Z axis, you've got a problem.  Also, if the enclosure is to be kept warm for printing materials like ABS, the heat will eventually cause problems for electronics mounted inside the enclosure.

Putting electronics inside the bottom of the enclosure in UMMD was out of the question from the start.  I could have used a drawer at the bottom of the printer, as I did for my last printer, Son of MegaMax, but that would have required very long cables to most of the electrical connections that are up near the top of the printer.  UMMD is tall and stands on the floor, just like me.  I hate bending over to work on things, and it hurts my back, so I planned to put the electronics at the top of the printer, outside the enclosure instead of at the bottom.  The worm gear drive in UMMD's Z axis guarantees that I can't manually move the bed up or down.  If the electronics aren't working, the only ways to move the bed are to manually loosen the grub screws on the drive pulleys or completely remove the bed and its support from the printer.  It just made more sense to put the electronics close to the XY stage.

More details about the electronics will be provided in another post, soon...

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