Based on my experiences on the Fisher delta and the D-Box, I developed a new printer, the HXM (hexagon minimum) which is using the Fisher components. It was designed to be used with the Fisher mechanical kit, but with RRP closure, it does have lost some interest as it is now. The delicate part is to find another compact hotend which could be fitted in the effector.
Its really usable space is diameter 170mm, 242mm height at center and 223 mm height at 170mm diameter. Base size is 320x370mm, with 500mm between plates. That is roughly twice the really usable volume of the Fisher and the same as a Kossel.
My aim was to:
Rods are used to give good dimensional accuracy
It is using the Spool on top support
Extruder installed to ease access and minimize bowden tube length (420 mm)
I have done like on the D-Box, with the kinematic positioning system moved from the bed to the effector. Globally the effector weight is increased by 15g (including balls), which is compensated by the lighter arms.
The arms are made as on the D-Box, with balls in printed cups, maintained with tensioning wires. I was initially thinking that there was no need for very high tension, so I used nylon wire instead of high grade Dyneema, and this is simpler to install and with elasticity of nylon, there is no need for springs. Tension is just made by adding knots on the wires. However, further tests showed that good tension creates friction in the ball cups which dampens vibration and have totally eliminated the banding I had on large diameter parts, so I increased the nylon wire size to 16kg (35 lb).
There is no end stops for simplicity and reliability (while they could be installed). I stall the motors on the top support. That makes quite a bad noise while stalling, but precision is not that bad if you use a not too low current. Enclosing the printer significantly reduced the stalling noise.
The difficulty of delta printer is accurate geometry positioning, so I use threaded rods to make the triangles. On the bottom, they shall be cut after assembly (at least one to use the maximum diameter without conflict with the fan part duct).
The assembly is faster than the Fisher, but you have to do some woodwork before (quite easy) and modify and complete the wiring (fairly tedious). Buy 2 new motor cables.
Plastic parts are printed in PETG, and so, all components have been reinforced compared to the Fisher, because the PETG stiffness just above half of the PLA stiffness.
I have used two high quality Sunon 20x20x10 fans (5m3/h),supplied in 5V, which are extremely silent. They are not that powerful, but that seems to work. Hotend cooling is not perfect, but seems not worse than the Fisher, however, plastic parts are more exposed to heat (insulation is done with a plumbing fiber gasket).
This fan setup works, however you shall be very cautious while wiring them. Check and recheck polarity on the controlled fan, a mistake will burn the board (sending 19V on the 5V input cannot be nice). It is safer to use 12V fans, but it needs a DC/DC converter. That DC/DC converter is more or less required for the fumes recycling fan, anyway, so that is now what I recommend.
- I simplified the already designed spool on top system, with no bearings neither any mechanical part. Its works better !
- I improved a bit the already designed geared extruder, with stiffer bearing holders. As I already experienced on the Fisher, a good position of the extruder is fairly difficult to establish. I tested 6 positions before settling on the final one. But this allow to use the original bowden tube of the Fisher (I have to lengthen mine, which was reduced with my spool on top installation, but this extra-length is the original cutted part...).
Does it work ?
What are the drawbacks and compromises ?
- The initial fixation of the hotend on its support was improved and better cooling flow reduced the bottom hotend temperature to around 52°C with hotend temperature at 280°C. Screw temperature is approximately 10°C below low fin temperature, so this is quite acceptable for PETG support.
- For weight and dimensions reasons, I have not used magnets to maintain the hotend like on the D-Box, instead I used rubbers cuts in bike tire tubes. This is less practical to install, but I find a reasonably easy way to do it. So, in case of mechanical trouble, that is not the hotend which is ejected, but the arms which pops out (I tested involuntarily during commissioning...).
- You have to order a few stuff (2 fans, a diode, cables and connectors, some M3 screws, a larger buildtak, 25cm of bowden tube for filament inlet, a DC/DC converter) and buy parts in DIY stores (wood, to be cut by the store for accuracy, M4 bolts and nuts, wood screws, M5 rods, M3 rods, rubber supports, nylon wire, aluminium sheet for board shielding, windows seals), for a total which may not exceed 80 euros for an enclosed printer.
- Alas, like on the Fisher, you will not escape to do some manual calibration as the offset on the calibration points is not constant, because on the center, you are using 3 belts and near columns, mainly one belt is used, so the offset is larger on the periphery (this is in reverse of the Fisher, where offset is larger at center due to spring loads). But with recent version of DC42 fork, it is a bit easier to do, but a procedure have to be written.
- The effector plug installation is really messy. On the D-box, I wired directly to the board and find it much more simple, with minimal drawback as the route is direct and so wire replacement easy. The best installation may be to use a sleeve instead of zip-ties to group independent cables.