Extruder/Mendel

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Revision as of 16:35, 31 December 2009 by Grogyan (talk | contribs) (Added note about aternative if no lathe is accessible, and a couple of other small notes)
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Mendel Build Documentation



Mendel uses a thermoplast extruder based on RepRap's Thermoplast Extruder Version 2.0. The principle is exactly the same, but a redesign has condensed the RP parts into one component (click here for instructions on how to get hold of RepRapped parts). The non-RP parts remain the same, therefore please refer to the Thermoplast Extruder Version 2.0 documentation to assemble the non-RP components. The rest of this page will deal with the new assembly only.

BOM

Assembly qty: 1

Name Qty/assembly Total Qty Type
Thermal-barrier and nozzle 1 1 Assembly
pinch-wheel-bracket-NEMA17_604-bearing_1off.par 1 1 RP
m3-capx25.par 4 4 Fastener
m3-washer.par 4 4 Fastener
624-bearing.par 1 1 Bearing
m4-nylock.par 1 1 Fastener
m4-washer.par 5 5 Fastener
m4x16-cap.par 1 1 Fastener
m4x40-cap.par 2 2 Fastener
stepper-motor-nema17-fl42sth47-1684A-01.par 1 1 Motor

Thermal-barrier and nozzle

These are the only parts of RepRap that need to be machined. The DXF files for these drawings created by the free QCad CAD system are in the folder mendel/mechanics/solid-models/extruders/pinch-wheel of your RepRap download.

Thermal-barrier.png

Here are the dimensions of the thermal barrier. It is made from 16mm diameter PTFE rod.

If you don't have access to a lathe, you can try this method

Start by facing off the ends. Then touch a centre drill on, and follow it right through with a 3.5 mm drill. Use a woodpecker cycle to clear the swarf.

Next mark 15 mm from the tip of a 5 mm drill with a fine felt-tipped pen (the tapping drill for M6), and drill to your mark.

Now mark 15 mm from the end of an M6 tap, turn off the power to your lathe, and put the tap in the tail stock. Turning the chuck by hand, cut the M6 thread to your mark.

The valleys cut in the end of the thermal barrier need to cover slightly less than the top 10mm of its length. They are just there to key into some glue, and so don't need to be very precise, nor do you need exactly two. I have made them perfectly well by putting the cylinder in the chuck of a Black and Decker drill and then just running a triangular file against it.



Brass-barrel.png




This is the barrel. It is made from M6-threaded brass rod (or an M6 brass screw with the head cut off).

This next step is a lot easier if done with a lathe. Though if you don't have access to one, you can drill the rod all the way through with a 3.5mm dia drill on a drill press and vice, then drill a 0.5mm dia hole into a dome nut, be sure you use a centre drill first to make sure everything stays concentric/true.

Start by facing off the ends. Then touch a very small centre drill on. Drill the 0.5 mm hole for the nozzle about 3 mm deep. Go slowly and carefully, and withdraw the drill frequently to clear the swarf. An 0.5 mm drill is very fragile.

Next turn and clamp the cross-slide at 45o and turn the connical end. Take it far enough to get rid of the mark left by the centre drill and to take away a bit of the 0.5 mm hole. You will probably find that the 0.5 mm hole closes up - clear it by hand with the 0.5 mm drill held in a small hand chuck.

Holding both in your hands, offer up a 3.5 mm drill beside the brass. Get its pointed end about 2 mm from the tip of the cone you turned, and move it about so that it is as near the tip as possible while still leaving a bit of brass to hold the tip on. Mark where the other end of the brass comes with the felt-tipped pen, and once more drill to your mark. The 0.5 mm hole you made should meet up with the 3.5 mm hole, but you may again need to clear it using the 0.5 mm drill by hand.


RP parts

Extruder-printed-parts.png


Gluing and wiring

Ptfe-glue-1.jpg

The PTFE thermal barrier is attached to the reprapped part with epoxy glue. Any strong epoxy will do; Araldite rapid is good.

You will need a 3.5 mm diameter rod to put down the holes to exclude the glue and get everything central. I didn't have a 3.5 mm rod, so I made one from 3 mm rod and some Scotch tape (blue in the picture).


Ptfe-glue-2.jpg

Put a very small amount of silicone grease on the rod to stop the glue sticking to it and offer up the parts together. Set up some blocks so you can stand the assembly with the thermal barrier pointing vertically upwards for the glue to set.


Ptfe-glue-3.jpg

Mix up the glue, take out the thermal barrier, and paint the glue round the walls of the reprapped part with a small screwdriver. Take care not to get glue on the flat bottom of the hole.

Paint the glue round the valleys in the thermal barrier, and assemble the parts. If you twist the thermal barrier round on the rod that runs through it this will help the glue to spread. Add more glue if there are gaps - you will find that if you add it at the hole in the side, then twist the thermal barrier a bit, that will entrain the glue and move it between the two parts.

Put a piece of tape over the hole in the side.

Stand the device with the thermal barrier pointing upwards for the glue to set.

When the glue is rubbery, but not fully set, pull out the rod. Run a 3.5 mm drill down the hole by hand to clear away any glue that has got in.


Heater-wire.jpg

Cut a 6-ohm length of fibreglass-insulated nichrome wire and bare the ends.

Cut a 200 mm length of 4-way ribbon cable and bare the four wires at one of its ends. Bare one pair by about 15 mm the other pair by about 5 mm.

Twist one of the long bare ends onto one end of the nichrome, and secure it by crushing a bootlace ferrule on it, or any connector crimp can be modified instead of a ferrule. You can't solder the nichrome connections because:

  1. Solder doesn't wet well to nichrome, you can but you will need a very hot soldering iron, and
  2. The nichrome will get too hot for the connection during normal use and may eventually fail.

Put heatshrink over the bootlace ferrule.

Now do the other end identically. Think carefully about what order and orientation to put the bootlace ferrule and heatshrink on before you twist the wires...


Heater-wire-wound.jpg



Wiring-finished.jpg


Assembly

  1. Secure bearing using M4x16 cap, and glue nozzle assembly into pinch wheel bracket. Follow Thermoplast Extruder Version 2.0 documentation for gluing procedure. Note orientation of copper barrel tang.
  2. Loosely attach motor. Motor wires should run upwards. Set distance between motor shaft and bearing to 2.5 mm – use the round shank of a 2.5 mm drill bit poked down the filament hole as a spacer. Lock motor into position. Load M4x40 caps for ready for next step.

Extruder-assembly.png

Extruder-finished.jpg


Back to Mendel mechanical construction root.