Heatbed assembly

Goal

By the end of this stage, your heatbed will look like this (Viewing Bottom / Heat-bed is shown upside-down)

id=”" align=”aligncenter” width=”300″ caption=”Assembled heatbed”[1]

Tools

• Wire strippers

• Pliers

• Scissors

• Pan head screwdriver

Step 1: Assemble the thermistor

Follow these instructions to prepare your thermistor.

Step 2: Assemble the heat element

id=”" align=”aligncenter” width=”300″ caption=”nichrome and thermistor wiring”[2]

To connect the Nichrome wire to the PTFE insulated wire, cut the insulation from a ferrule and slide the Nichrome and PTFE insulated wire in from each end, then crimp the two wires together. Cover the join in black heatshrink to electrically insulate the join.

The main failure point seems to be the contact areas between the nichrome and the connecting wires, so these need to be as good as possible.

• You can’t easily solder nichrome. Solder just doesn’t seem to flow onto the metal.

• You can tin the ends of the connecting wires with solder. This should help to prevent the rapid oxidation of the bare copper caused by the heat of the nichrome.

• You need to increase the contact area between the wires. Rather than placing them side by side in the crimp tube, I wrapped my nichrome wire tightly around a fine screwdriver to form a spiral and then inserted the connecting wire through a new crimp tube and into the spiral.

• Finally, you need a strong crimp tube that will grip the wires firmly and resist any movement as the bed moves back and forth.  The supplied wire ferrules are marginal for this application; a ring terminal with the terminal clipped off is a much better choice.
• If you really want to solder the nicrome, this is how to do it:
  Strip the Nicrome wire end.
  Lightly sand the wire with fine sand paper; it will ONLY tin the part you sanded!
  Immediately cover the sanded end with PLUMBING solder flux; this is Zinc Chloride based.
  Using either lead free solder (see below) or 60/40 plumbing solder, tin the end of the wire; it will have to get quite hot to tin.  You may also have to repeat the above steps more than once.
  Wipe immediately with a paper towel.
  Clean the wire end with rubbing alchohol and/or your favorite flux remover; this is CRUCIAL!  Zinc Chloride flux will eat up your electronics if you leave it on!
  Cut the terminal off of an un-insulated crimp on terminal (such as a ring terminal) that is a good fit for your wire; the wires should almost fill the barrel.  You can double over either the Nicrome or the copper to get it full.
  Crimp the terminal.
  Tug firmly on the wires; they MUST NOT pull out!
  Solder the joint using lead free electronics solder (and conventional flux if you don’t have flux cored solder).
  Clean it again, carefully!

Don’t use the eutectic tin lead solder (Sn63Pb37) for this application; its service temperature is too low.  Lead free silver tin solders will take 175C.  The best choice is made of Tin (95 to 96.5%) with the balance Silver.  Sn_96.5Ag_3.5 is going to do the best if you can find it; it does the best job wetting stainless steel.  Solder labelled “Sillver Bearing Solder” is Sn₉₆Ag₄ and will work very well.  The tin/silver/copper alloys should work OK here too.

Before laying out the Nichrome wire, it is HIGHLY advisable to cover the underside of the Aluminium bed in Kapton.  In fact, if you have access to fiberglass tape, a layer of that both under and over your Nichrome is a really good idea.  Oddly enough, fiberglass tape for electrical wiring applications is often available in the the electrical department of large hardware stores.  A good industrial supply place or electrican’s supply house will definitely have it.  3M/Scotch type 27 is a good choice (link below) and is rated for continuous service at 150C!  It is also called high temperature glass cloth tape.  Photos of this method and the soldered splice discussed above are in Thingiverse at: http://www.thingiverse.com/thing:13939

http://www.shop3m.com/80012020352.html?WT.mc_ev=clickthrough&WT.mc_id=shop3m-AtoZ-3M-Glass-Cloth-Tape

The side with holes beveled is the top; heater wires go on side without beveled holes.

This will reduce the risk of shorting the heater (and potentially other circuits) in the event that the Nichrome insulation fails. (The following images were taken without this extra insulation layer. The bed has subsequently been modified to include this).

id=”" align=”aligncenter” width=”300″ caption=”Nichrome wire temporarily taped”[3]

Note the Nichrome wire path. This is designed to give even heating of the build platform. The nichrome wire should not protrude outside the area of the Aluminium bed plate, and should not be closer than 5mm to any edge.

id=”" align=”aligncenter” width=”300″ caption=”Nichrome and thermistor fitted”[4]

Step 3: Insulate the heatbed

Tape around the perimeter of the heatbed to secure the heat shield.

Step 4: Heatbed wiring loom

The heater connecting wires should be high temperature, high current capacity wires (the thick red silicone insulated wire), cut to a length of 200mm. For the thermistor input wires, use some spare from the motor wires.

The Molex connectors are keyed to only allow connection in one orientation. The male crimp pins fit into the female connector only.  The pins are also keyed in the connector; the grooved side of the pin matches the key inside the pin hole.  They lock into place when you have inserted them far enough; give them a tug after you insert them to make sure they locked. Don’t try to fit the male pins into the male connector.

It is possible to remove a Molex pin from a connector after installation.  The easy way is to use a removal tool; it looks like a set of tweezers with very skinny points.  You slide it over the pin and into the matching slots on the connector body to make the latches retract, then pull the pin out.  You may be able to make the tool from sheet metal once you know that it needs to snake down those two tiny slots!

In case you get in trouble, these Molex connectors are the 0.165″ pitch Mini-Fit Jr. connector series.  The female pins are 39-01-0039-C and the male ones 39-01-0041-C.

Step 5: Mounting the heatbed

RP parts	Quantity
	1
	1
Hardware	Quantity
Bed mounting plate	2
#6×3/8″ self tapping screw	4
M3x30mm cap head screw	3
M3 nut	10
M3x16mm screw	4
M3 washer	10
Bed spring	3

Insert an M3 nut into the hexagonal recesses, two on the double spring anchor, and one on the single spring anchor. Screw the spring anchors to the frog using the M3x16mm screws, and an M3 nut on each screw.  The pressed-in nuts go on the BOTTOM of the spring anchors!  Use a little superglue on them if they are loose.

The Aluminium bed plate is to be screwed to the laser cut bed mounting plates using four #6×3/8″ self tapping screws.

(note from TheCase: had to re-drill the holes in the aluminum bed with a 9/64″ bit to allow the #6 screws to pass through)
(note from Rob: Same situation with mine (Huxley #20), used a 3.5mm drill bit to enlarge the holes. My 4 screws were in bag #13, not #2 as indicated by the packing list)

Now insert one M3x30mm screw through the centre hole of one of the bed mounting plates, with a washer either side of the laser cut part. Slide a bed spring onto the screw then secure with an M3 nut, screwing the latter on such that at least 5mm of thread protrude beyond it. Repeat this process with two screws on the other side.

NOTE: Ensure the number of springs you have on each side matches the position of the spring anchors. The bed heater and thermistor wires should protrude from the end nearest the Y axis motor.

Finally, attach your heatbed to the machine by screwing the M3x30mm screws into the captive M3 nuts in the spring anchors. The heatbed needs to clear the Y axis end brackets as it goes right over them during normal operation.

Next step

Hot end assembly