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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

I think I should get my Mini printer working first before doing more to the Giant so I'm thinking of stopping construction of this for a little while.

This looks good for controlling my mains voltage 1200W bed heater :- GEREE 24V-380V 40A 250V SSR-40 DA Solid State Relay Module 3-32V DC To AC

Yes, I'm using the 10A version for mine (SSR-10 DA). My silicon heater is 750W so about 3A.

I had the Mini printer working quite well yesterday but today it refuses to connect to WiFi   So I'm back to the Giant while I wait for inspiration or get help.

From my experience with the Mini printer I have changed my mind about the mountings for the XY motors.  Access for threading up is paramount and vertically mounted motors would make access difficult so I'm going back to having the motors mounted horizontally just in front of the front rail.  This leaves the drums out in the clear and readily accessible.

A couple of photos - top view and underside view.  The drum is just representative of one possibility and was taken from the Pilot printer with the motor.  I shall probably be using a smaller drum in the final version.

In checking things I found this comment about the 400mm square heater pad :-

Quote

Description:

• Wonderful silicone heater for your MEGA 3D printer heatbed. Suitable for print plate of 400mm~500mm by 400mm~500mm.

They are saying this heater pad is suitable for a heatbed 500mm square which is what I have (haven't attempted to cut it down).  So it seems to me that I could go for a larger printer by just increasing the frame sizes a bit.  I already have 500mm available height.

I can reduce the cost of new V-Slot aluminium extrusion by using the front and back frame members on the sides and getting new front and back pieces.  I have 4 pieces I could cut myself but I think I would rather pay the price for accurately cut pieces for the main frame to be sure top and bottom measurements are identical as the X axis relies on that.  The Z carriage (alias XY frame) is less critical as I can adjust that to make the wheels fit the main frame uprights.  Since everything is made from the same type extrusion, everything expands and contracts at the same rate if the temperature changes, so two of the pieces of extrusion I already have can be used on the Z carriage, leaving the front and back members of the main frame (4).

Parts ordered.  The overall main frame size will be 740mm x 610mm x 1000mm (WxDxH).  This will give not only a bigger printing area but also space for two extruders plus Z probe. 

I now have my Mini printer working well and up to printing drums for the XY motors on this printer   I've printed a 20mm diameter one but might go for bigger.  Circumference for 20mm = 62.83mm so for 500mm range the number of turns = 2x500/62.83 ≈ 16, width of 8mm - that's alright   The one I've printed is fine so I'll just print another.  ...  Done.

Here is how I'm thinking of arranging the Z drive - as seen from above.  The cord is wound up on the axle and goes over pulleys front and back down to the Z carriage (XY frame).  I haven't shown the pulley brackets and some parts are not to scale.

Z axle bearing block.  Now printing.

First one printed perfectly so now printing the other.

Second Z axle bearing block printed fine

It's looking like the printable height may be something like 800mm or maybe more but I won't really know until I get the main build complete.  The design is beginning to come together

Now printing Z drive gears.

Designed and printed the three pulley support blocks for the Z cords.