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Biceps, a two headed 3D printer

Posted by leadinglights 
Biceps, a two headed 3D printer
July 18, 2019 12:33PM
This is sort of a build log of the last stages in the development of a twin headed printer with a heated enclosure, vacuum hold-down build stage, kinematic mounted build stage and two water-cooled print heads with automatic filament purge and nozzle wipe between each head change.
The printer is a cartesian type with the build stage moving in both X and Y while the print heads move only in the Z direction. Print heads are swapped out so that only trivial adjustments of X, Y and Z need be done by the printer firmware.
This printer has existed in many stages, some parts have been made and used while others exist only in the imagination of TurboCAD. The frame has been used as an engraving/light milling machine and as a proof of concept for extremely sensitive piezoelectric sensors. The name of the printer is Biceps, Latin for "Two headed".

A view of the printer showing the toggle heads. The DC motor in this picture has been subsequently replaced with a stepper motor. A video of the toggle operating can be seen on [www.youtube.com]

Present project status.
Many parts are working well. The framework is good and accurate although, if I knew then what I know now, I would have increased the ground rods from 10mm and 8mm diameter to 12mm and 10mm diameter to reduce the small sag and elasticity. The vacuum build stage is now able to reliably hold a vacuum and the kinematic is able to return after a disturbance to within 3 microns. The piezo sensors now also have a repeatability of better than 3 microns.
The head changing mechanism was accurate until I found that 3D printed gears are a bit fragile so is now being redesigned to use Huco Acetal gears - this would be easier if they were available in the sizes that I want.
The heated enclosure is the only part that exists still only in CAD

Apologies for this CAD rendering but the heated enclosure does not yet exist.

The history – Polyceps (many heads)
The dual headed mechanism is the final fall back position from what started as a seven headed monster including four print heads, two pick and place heads and a head to spray liquids such as adhesives or dyes during the print. The frame only of this printer was ever built: Converting the CAD blocks into realizable specifications came up against strength of materials, availability of gears and bearings and many other problems – a less gung-ho approach was obviously needed. The existing frame, which after all is simply an XYZ mechanism, was used in all subsequent developments.

Milling a vacuum hold down heated bed for a delta on the 3D printer frame.

A PCB machined on the printer. 3Mm square IC with 6 pins. 0.2mm isolation lines demonstrating that the XY mechanism has very little play.

Re-casting my ideas to a five head printer I did get as far as building a mechanism with one motor for all five extruders, a second motor to switch between active heads and a solenoid to lift the heads for position changing. Mechanical fragility and repeatability caused major problems while software to align the gears so that they would mesh correctly when changed finally killed this one.

Five extruders made for the above printer. These have been used for all subsequent iterations including Biceps.

A second iteration of the five head design used five identical extruders with three of them feeding the hotends through short Bowden tubes. As the software was beyond my capabilities I paid for some development work. The less said about this the soonest mended - suffice it to say, if I can’t program it myself in assembler then it ain’t going to happen! For a while the frame was used as milling machine to make vacuum stages for other printers, PCBs and other light duties.

CAD rendering of one of the many 5 head combinations tried. This one has two direct drive extruders for Bowden sensitive materials. The other three feed filament through 250mm long Bowden tubes

After the project had languished for a couple of years I needed the space and I had to choose what to throw out. In the end I sent my trusty Prusa type printer 'Caliban' to what I hope is a good home and resurrected Polyceps as Biceps. The hope here is to get a very rapid head change along with sufficient stability that filament in the inactive head can be changed and the nozzle purged and wiped to allow the nozzle to be presented clean after changing filament. Other technologies would be carried through including the water-cooled hotends and the vacuum hold-down used on my Delta printer. The software problems go away as M and G codes are easy to understand as the codes I need already exist and I don’t need to rely on anybody else for this bit

Early attempts at a change mechanism used a DC motor with a gearbox but repeatability prove very poor and resulted in a redesign using a NEMA11 stepper motor. With the new motor and gearbox nozzle returns to the same position within measurable limits (ca 3 microns)

The kinematic mechanism and piezo sensors did pretty much everything they should and the extruders and watercooling are now well proven in earlier trials

Present work

The head changing mechanism gear problem mentioned above is ongoing. New gears have been ordered but will need to have some teeth removed to fit into the framework.

The vacuum hold down is proven but is a lash-up. The vacuum pump is now a small one which switches on only as necessary to keep a good vacuum – about 1 to 2 seconds of pumping per minute. A solenoid which is to be normally open and inexpensive seems to be unavailable – several that solenoids that I have bought are not satisfactory so I am making a new one.

Vacuum, heated bed power lines, thermistors and power and signal to the piezo signal conditioner board have to be got to the build stage (bed) . Variations of cable drag chains have been tried under the bed but this seems to be complex, heavy and snag prone. The most satisfactory answer seems to be simply a conduit hanging down to one side of the bed.

The Heated enclosure so far exists only as some speculatively purchased plastic sheets to check for warping when hot. The purge and wipe exists at the moment only as a guess that there is enough room and a hope that it wont be too hard.

If anybody has any use for any of the ideas here they are most welcome to them. I will endevour to occasionally come back to this thread to report on any progress.


Edited 1 time(s). Last edit at 07/18/2019 02:49PM by leadinglights.
Re: Biceps, a two headed 3D printer
July 24, 2019 08:51AM
Quite impressive to say the least. Obviously you have a high degree of mechanical and electrical expertise. Is the purpose of having a vacuum hold down to allow a quick change out of the build plate?
Re: Biceps, a two headed 3D printer
July 24, 2019 09:14AM
Shank man
.................................................................................................................... Is the purpose of having a vacuum hold down to allow a quick change out of the build plate?

That is the main reason. Video at [www.youtube.com]
There are several less important reasons such as not having to worry about it if the print surface sticks too firmly- just cut off the excess.
The vacuum hold down works beautifully on PLA but not too good on ABS due to the bad edge curl with ABS. This is the reason that this has a heated enclosure.

Re: Biceps, a two headed 3D printer
August 07, 2019 04:35PM
Since my previous post there have been some good successes along with some false starts which have taken a lot of time to resolve.

On the piezo sensors I am now almost at the point of declaring that 1 gram of pressure and 1 micron of post contact travel had been achieved. Repeatability is a bit more like 3 microns although that is of the order of variation caused by random breezes causing local expansion/contraction.

View below is of the XY carriage showing the three piezo mounts and the signal conditioning PCB. Also visible is the black silicone tube of the vacuum connection and the heater which consists of Nichrome wire in a PTFE tube laid in a track machined on the lower surface of the build plate.

One of my most import changes was to make the vacuum hold-down more practical: I have been using a vacuum pump with a 1/4HP motor running continuously and have intended to fit something with a much smaller vacuum pump that only runs when necessary to maintain the vacuum. The first problem I found with my new pump is that most plastic films squirm about like one of those "fortune telling fish" found in cheap Christmas crackers when put on a 100°C plate. With the big old pump this was not a problem as the film was pulled down with a snap and had no chance to squirm but the little pump did not move enough air to do this. The solution was to pull a vacuum in a reservoir and only connect this to the hot plate as plastic film is put on the plate.

Another problem was in getting the vacuum, the heater power and thermistor wires and the wires for the piezo sensors to the XY carriage. The original design was to have two drag chains, one from the static framework to an intermediate carriage and one from there to the XY carriage. This approach was problematic in almost every respect but the killer was that drag chains on sufficient size to carry the vacuum tube as well as the wires took up more space than I had allowed. Using a single drag chain from the frame to the XY carriage was rejected when I found that the largely unconstrained chain swung about in an unpredictable manner.

The solution to this problem was to simply have the wires descending from a slider block to one edge of the XY carriage. The longest part of the run in stiffened with a thin wall stainless steel tube which also forms part of the vacuum line. This was less than half of the moving mass of either of the cable chain arrangements although I will admit it is a bit ugly

The view below shows the vacuum and cable connections. The perspex cylinder on the left is the vacuum reservoir with a vacustat (diaphragm and microswitch) on the bottom.

I am still having a bit of a problem as I haven't been able to source a good normally open solenoid valve with very low flow restriction – at the moment I am providing a manual valve action by kinking a section of the vacuum tube to empty the reservoir until the new plastic film is fitted. Once I have that sorted out I will use it to update the vacuum hold-down I have on my old Delta printer

Re: Biceps, a two headed 3D printer
August 24, 2019 06:01AM
I found a new and interesting problem with the kinematic mount. When the bed was heated it would expand and each of the screws would move outwards along its slot - away from the common centre. The problem came in that the friction between the head of the adjusting screw and the chamfered slot gave rise to stick-slip behaviour during heat-up resulting in the middle of the bed rising and falling by up to 50µm. The slot is shown below

The immediate solution was to tap the bed with the nozzle or a convenient hand tool - like tapping a sticky gauge. Lubricating the chamfer with extreme pressure grease (Millers MPF2) seems to work but only time will tell how long the lubricating film lasts.

Re: Biceps, a two headed 3D printer
October 17, 2019 11:48AM
Another report on the Biceps printer.

The fix for the slight distortion caused by the friction of the parts of the kinematic mount (see previous post above) seems to be good. Having said that, having to re-lube an almost inaccessible part - albeit only every year or so, is a bit of a drag so I am looking at slight design changes to incorporate a low friction part.

The vacuum does exactly what it should, silently and without fuss. When I get a chance I will modify my other printers to use the same vacuum work holder.

The bed leveling conditioner is giving the most problems. Although it is very sensitive it was intended that the conditioner would discriminate bad contact signals such as external mechanical or electrical noise or the signal from a nozzle with plastic or dirt on it. The problem comes with rejecting noise: Mechanical noise from things dropping on the bed can be detected and it is even possible to tune the response so that it rejects almost all external noise - but this can only be done with one part of the bed - typically the centre. Blunting the response characteristic so that all good contacts will be reported increases the number of false positives and the sensitivity to general mechanical noise being a lot less good than hoped.
The main part of the duty of the conditioner, reporting dirty nozzles, seems to be pretty reliable at detecting the plastic deformation signal with more than about 25µm of molten crud or 50µm of dry plastic. Even at this though it is not as great as hoped as some surfaces, notably blue paper, have more variation of height and compliance than any reasonable dirty nozzle limit.

Of the other main new bits of Biceps, the twin head toggle mechanism is redesigned and waiting for the conditioner to be resolved one way or another. I have managed to narrow down the reasonably achievable temperature of the mini-enclosure to better than 70°C but struggling at 80°C

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