RepRap Builder

August 2013

Mounting the Linear Bearings and Assembling the Extruder




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August 2013

Fitting the linear bearings and constructing the extruder

Once the smooth rods had arrived and had been cut to the required lengths and also had been bevelled, the elements with the linear bearings could be placed onto the smooth rods. However, prior to mounting the bearings these had to be fixed into the plastic printed parts (see image 3).  It is important that the mounting holders for the linear bearings are preheated with hot air before the bearings are pressed in. Without preheating the bearing holders they may break or you may force them into a loose fit which creates unwanted play (equals inaccuracy) in your RepRap.


At about the same time I had the extruder assembled, which contains two roller bearings and a hobbed bolt (image 1). The hobbed bolt is at one side mounted in a roller bearing and at the other side connected to the larger gear and it is used for transport of the printing material, a 3 mm diameter ABS filament. The filament enters the extruder at the top of the extruder (image 3, lower set of parts, item in the center) and is pressed by a second roller bearing against the hobbed bolt. The pressure is adjustable by two long bolts, which each have a compression spring. The notches in the hobbed bolt will grip into the filament and provide non-slip transport pressing it to the heated surface of the extruder. See also the schematic drawing in image 2. The hobbed bolt is located in the center of the large gear, next to the filament.


Hobbed bolt
Image 1: Hobbed bolt (© FRS 2013)



Extrudercogs
                    Image 2: principle of extruder (image: http://reprap.org)


parts with linear bearings 1
Image 3: assembly of linear bearings (© FRS 2013)



Linear bearings have been assembled as shown in image 3. Right top shows four linear bearings in holder for mounting the (sliding) heated bed; top left and middle show two mounting blocks for the Z-axis. The hexagonal vertical openings will contain an 8 mm nut inside the hexagonal channel and one nut outside the block (resting on the newspaper) with a compressed spring inside the hexagonal channel in order to compensate for slack in the 8mm threaded rods that run through the channels for the Z-axis movement. The three items in the lower position show from left to right: the extruder base plate with linear bearings (X-axis), the extruder (middle) and the stepper motor with pinion gear for powering the feeder of the extruder. Note that all gears have fishbone gearing.



parts with linear bearings 2
Image 4: Same items seen from the opposite direction (© FRS 2013)


In image 4 the same items can be seen from the opposite direction. The pinion gear on top of the stepper motor clearly shows the fishbone gearing (i.e. shaped like this: >>). The stepper motor will be connected to the extruder (attaching holes visible at the right of the extruder).


When all items with linear bearings had been placed onto their mooth rods the rods were put and fixed into the frame.
Then all smooth rods needed to be adjusted; the rods for movement of the heating bed (Y-axis) had to be placed exactly parallel and in such a way that the bed was exactly in the center between the sides of the frame. This could be done with help of the guides that I had fabricated from the old, useless smooth rods.


Both the smooth rods for the Z-axis, moving the extruder in the vertical plane, had to be fixed and adjusted exactly 90° verical to the bottom (Y-axis rods). This was easy to do with help of the Digi-Pas DWL-200 digital leveler and by using the alternate zero function the RepRap frame needed not to be leveled before.


The same applied to adjusting the smooth rods that carry the extruder (the X-axis). By putting a single straight (extra) rod at 90° over the two bottom rods (i.e. in parallel with the X-axis) this line was taken as a reference and the X-axis rods were leveled exactly in parallel and at level by individually turning by hand the two stepper motors for the Z-axis. When arriving at level the adjustment was secured by fixing the screws of both the helicoil couplings (image 5), which had been loosened prior to adjusting.


helicoil
Image 5: helicoil couplings

Constructing the Y-axis platform and heated bed


Heated bed
Image 6: Heated bed printed circuit board  (image: http://reprap.org)

Once everything was leveled and had been fastened building the heated bed could start.

The heated bed consists of several layers on top of each other, separated by spacers. The carriage is made of 6 mm thick plywood, attached on the linear bearing holders (image 4, the four lower identical items in the center of the picture). The bed support is also made of 6 mm thick plywood and is attached with spacers on the carriage. The heated bed has been attached on top of the bed support on spring operated spacers, i.e. the heated bed is pressed upwards by compression springs. This construction prevents breakage of the heated bed when the extruder accidentally dives into the bed, e.g. when the Z-axis endstop fails. The heated bed, consisting of an etched circuit board that acts as a heating element when powered with current, is covered with a glass plate, which has been covered with Kapton tape (heat resisting polyimide tape). Underneath the heated bed a thermistor has been placed for measuring and controlling the heated bed temperature. The thermistor makes contact with the glass plate by using heat conducting paste. The glass plate has been chosen because the heated bed is never totally flat whereas a glass plate guarantees flattness at all working temperatures. Kapton tape provides good fixing of an ABS printed object to the heated bed and therefore prevents warping of the object during printing.

Some deviations that I have introduced compared to the original design are: plywood instead of MDF ( medium density fibreboard), double wiring, a mirror instead of ordinary glass and metal pulleys to replace the printed ABS pulleys.

Commonly, MDF is used for both the carriage and the bed support as can be seen in the photo in the header of this page. In this photo we can see a lasercut carriage plate of MDF. The disadvantage in my opinion is that MDF is heavier than plywood (more mass to transport for the Y-axis motor) and that a thicker plate will have to be used to prevent warping of the plates. By using plywood I gain print height (lower thickness of plate layers) and higher printing accuracy due to less weight. During printing relatively high accelerating speeds of the carriages will occur when the direction of movement changes and than it is advantageous for accuracy when the mass is low.

The heated bed (image 6) has a resistance of about 1.2Ω and therefore at 12V will draw a current of 10 Amps or possibly even more whilst heating. The wiring for the RepRap is generally 0.5 mm˛. For safety reasons I have preferred to use double wiring for powering the heated bed.

Because the printed circuit board (PCB) that is used for heating the bed is rather thin and never seems to keep its flatness during heating it is common use to put a glass plate on top of it to obtain total flatness. A standard glass plate is 3 or 4 mm thick and unfortunately glass is a perfect heat insulator. For my RepRap I have chosen to use a mirror of 2 mm thick (less insulation). By accident I found mirror tiles (for glueing onto a bath room wall) in the dimensions 200x200 mm, which is exactly the dimension required for the heated bed.
Probably an extra advantage of a mirror is that it has an aluminium (reflective) backing, which is useful for rapid heat transfer over the total of the surface. This is only the case for convection heat and not for radiation heat. For radiation heat the aluminium reflective layer will bounce back the IR-rays!

Finally, after mounting the stepper motors for X- and Y-axis and attaching the belts the RepRap was ready for a testrun.


In a further attempt to improve print accuracy the printed ABS T5 pulleys had been replaced with aluminium T2.5 pulleys and the corresponding T2.5 belt had been attached to the X- and Y-carriages.


Recent publications mention that T-type belts are not really a good choice as they might introduce backlash when the stepper motor is reversing. The theory is that T-type belts, with a trapezoidal tooth profile, do not fit fully in the profile of the pulley. When the stepper motor is reversed the belt may slightly 'jump' and backlash is introduced. To prevent backlash a GT2 pulley and belt, with semi-round profile, should be used. I am not in favour of this theory because my T2.5 belt certainly has a tight fit and -most important- I use a pulley with 16 teeth. The more teeth on a pulley the better the flexing and the fit of the belt.
Below, image 7 shows the dimensions of a T2.5 belt and image 8 shows the dimensons of a GT2 belt. It should be clear from the images that a better performance can not be expected from just a change in belt profile.

T2.5 beltGt2 belt
    Image 7: T2.5 belt (image: http://www.beltingonline.com)     Image 8: GT2 belt (image: http://openlab.com.au)






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Last Updated on: Mon Nov 10 21:35:36 2014