Gina

Done it

Cord threading is a tricky job! Found I hadn't allowed enough length on one of the cords so had to replace it. Then I got it crossed over in spite of plenty of light. This job is showing my age - shaky hands and poorer eyesight, but I'm lucky compared with some of my friends Have no fear, I shall conquer it!!

I put the camera outdoors last night but there was a very heavy dew and the lens misted up almost immediately so I brought it back in. It definitely needs a dew heater as I suspected. Next job will be to finish adding the components for that to the HAT and fitting the resistor string that produces the heat.

XY cords threaded but need slight adjustment - job for tomorrow - I'm off to bed So tomorrow the first job is to fix the drum settings and then it will be time to take some parts from my Pilot printer. The print bed parts, Mega/RAMPS and LCD unit.

I've started threading up the XY cords and think I may have discovered a flaw in my design using the Core XY system. With the Y carriages at the front ie. Y = 0, the X carriage can be in any position. This means that as the X position is moved from 0 to 200mm the cord winds up onto the right-hand drum and there will be a small error due to the geometry of the cord. Minimal error would be obtained if the cord is at right angles to the X axis when X is centred ie. X = 100mm. Now to look at the geometry. Taking the calculation above the total cord range on each drum is 4mm corresponding to 400mm (diagonal movement, 200 X and 200 Y). The movement we are considering is 100mm from the reference. meaning 1mm of cord sideways movement. The minimum distance from pulley to drum will be about 20mm. Referring to the diagram below (not to scale), the horizontal line represents the case where the X = 100mm, the origin represents X = 0 and the hypotenuse of the triangle is the cord run with X = 0 (or 200mm). The error is the difference between the adjacent side (horizontal) and the hypotenuse (angled). In this case the vertical side, the movement of cord on the drum, will be 1mm and the adjacent side 20mm. Applying Pythagoras, "the square of the hypotenuse is equal to the sum of the squares of the other two sides", the hypotenuse is sqrt of (1^2 + 20^2) = √(1 + 400) = √(401) = 20.025 giving an error of 0.025mm. This is negligible, so my worries were unfounded But I had to check.

On further looking at arrangements of XY motors I've concluded that the only practical option is motors fitted underneath the frame with the flat mounting plates. I have just one of these in stock so will need to order another but I'll wait to see what else I shall want. That's the XY drive arrangements sorted out and the Y carriages are probably alright, so I've moved on to thinking about the Z drive. Whether I can get away with three cords or need four depends on how far the extruder motor sticks out the back of the X carriage. I think one cord in the middle of the back rail will be alright as long it wont catch on anything sticking out too far behind the X carriage. I guess the next stage should therefore be the X carriage.

This project is taking too long so I think I'll go back to a fixed dome. I've been getting to darn clever for my own good!!

Yes, and I agree that if printed well they mesh better and run more smoothly. But they take more designing and they don't tolerate printing defects. They also need aligning properly. Plain spur gears are more tolerant of various problems.

This printer will not be going in the living room when in use - there is now enough room in my workshop - so I can do away with the fume casing and just ventilate the room (for ABS or ASA) whilst controlling it from my living room. Control will be via WLAN using Firefox on the client desktop and the Duet Wi-Fi to control the printer. In addition I plan to have a camera to watch over the printing process, probably an RPi with Raspberry camera (or USB camera). This could be accessed with KStars/Ekos/INDI as I'm using with my all sky camera. Using the printer without its own fume cabinet will save a considerable amount of work both the cabinet itself with door etc. and the need to avoid things sticking out past the frame.

One of the NEMA 11 motors has arrived and quite a bit smaller than the NEMA 14 and looks good for the cover/dome motor. Mind you, I am wondering if a removable dome is worth the effort. If the moon is up the stars disappear anyway so reflections don't really matter.

With the ready made motor mounting plate the drum diameter works out at about 25mm with the current pulley position on the Y carriage. This seems a reasonable size for the print bed area of this printer though I might need bigger motors to give me more torque. That remains to be seen. This printer would seem to be a good candidate for the Duet Wi-iFi control board with its higher power motor drivers, particularly running off 24v. The circumference of the drums would be 78.5mm and with CoreXY and 400mm square print bed giving a maximum movement of 800mm for either A or B motor, the number of turns required would be 800 / 78.5 ≈ 10. With the cord I'm using this would be just under 5mm which is fine. It can be arranged that the cord position is level with the pulley groove when close up then with the Y carriage furthest away the angle would be only 5mm in 400mm causing negligible error.

If the drum was a straight cylinder well onto the motor shaft it would be limited to 18mm but I could use a larger drum with a smaller hub with the drum overlapping the front frame member. There is a ready made motor mounting plate available from ooznest that I'm using for the Z motor mounting in my Mini printer that would save me some effort. This is shown in the photos below. The second photo shows that the pulley and therefore the bottom of the drum, is well above the level of the front rail and the drum could overlap the rail.

Now to the motors... There would seem to be three possible positions for the motors as shown in the photos below. The first two show the motor mounted horizontally outside the frame, attached to the front part. This would mean bringing the door forward with respect to the main frame and not very convenient. The other two photos show a motor mounted vertically either above or below the XY frame. Above would not be easy but below would depend on the amount the nozzle is below the XY frame or mean the print bed raising up a bit (far from impossible). The motors would be outside the print bed area. The underneath method would limit the size of drum but otherwise seems the best option.

Here are a couple of photos of the Y carriage on the frame and holding the X rail. This is one of the back corners. I've also shown one of the cords.

Braided fishing line is exactly what I'm using - this is rated at 80lb There is 100lb rating I believe but it was out of stock when I bought this.

Added the extra parts to the main frame and also now have the Y carriages a perfect fit on the XY frame (alias Z carriage). I have succeeded in moving the cords and pulleys within the outline of the frame - it just remains to determine the arrangement of the motors. My original idea may not be practical but we'll see. What has been determined is that the motors will be at the front and the pulleys at the back. This arrangement will maximise the Y range. Here is a screenshot of the Y carriage model in SketchUp. I found the arm holding the V-slot wheel was not sufficiently rigid so I changed the design to make it thicker. The position of the attached pulley was moved to make for a better print - the front and back cords to the X carriage will be attached to the front and back of the X carriage frame rather than to one side. I adjusted the thickness of the part on the end of the X rail so that the V-slot wheels exactly fitted the Y rails. The hole takes a screw into a tapped hole in each end of the X rail. I'll take and upload a photo shortly.

The shaft couplers arrived today and they seem to be an adequately good fit on the motor shafts with less than 0.1mm play so I shall try these as the XY drums. Now to calculate how many turns of cord it would take to cover the full range. Worst case is from (0,0) to (200,200) or the other diagonal, making 400mm on the A or B drums. Circumference is 16xPi ≈ 50mm so no. of turns ≈ 8. The diameter of the cord is 0.48mm so 8 turns is > 4mm and the gap between the holes for the grub screws is 6mm so that should work out alright

Meanwhile, the ooznest parts have arrived So I can build the rest of the main frame and the Z drive as the polished bar that I'm using to wind the cord onto, is also in the box. Lots of other things to make too though

No, it's worse Filament too soft - probably needs a lower temperature or needs longer to cool. A bit later... Dropped extrusion temperature by 10°C to 230°C and it's better. I'll try dropping it a bit more.

Produced what should have been a more practical version but it didn't print well in ABS so I'll try PETG which is my favourite filament second to ASA which seems to print pretty much perfectly.

This would seem to be pretty much it Just the odd tenth of a mm adjustment on the extrusion fit and a couple of mm on the end thickness to get the correct overall length for the X rail. The original sketch was lost on my old Windows desktop that died on me - I only back up important stuff. SketchUp models can be redrawn and rarely want repeating once the item has been printed.

Second attempt

Y carriage. First attempt...

I think I have the answer

This is a lot trickier than I thought!! It's not so easy rearranging things so that they all fit into a moving frame. With the fixed XY plane, the motors and pulleys could be outside the XY movement system.