We need a design for a load cell that we can fit into a typical 3D printer toolhead to use as a probe. (This other thread is mainly about the software required for probing.)
There is prior art for this idea:
Duet 3D sells the Smart Effector for delta printers. It uses SMD resistors as strain gauges.
The Prusa XL which looks to be using an aluminum C beam load cell
The BIQU B1 SE PLUS uses a straing gauge on a simple aluminum flexure similar to whats commonly found in bathroom scales.
This thread is to provide a place to discuss and develop this idea further. Ideally we would end up with an open source design that some interested party can manufacture and that open source printer designers (Voron, Rat Rig etc.) can integrate into their tool heads.
Btw: for the Renkforce RF1000 3D printer, there is a 3D model available which includes the load cell construction. It can be downloaded from the German-language forum rf1000.de, here is the direct link (download button only appears after registration+login):
You can also find a FreeCAD version of the model with my personal modifications here (no login required):
I can imagine, the design can be miniaturised a bit by using smaller load cells. In the original it will probably not fit for most printersā¦
It should also be mentioned that I see two fundamental distinct possibilities to use load cells in the toolhead. Either the load cells can measure any force applied to the hotend, including the force applied by the extruder motor, or they cannot āseeā the force from the extruder motor. This has the following consequences (both positive and negative):
With measuring the extruder motor forces one can do interesting things besides probing the print bed. E.g. one can measure the force while printing the first layer, which might increase if the hotend elongates for thermal reasons, to compensate the first layer height (this has been successfully implemented at the RF1000).
If the extruder force is applied to the load cells, those must be stiff enough so that the force wonāt press down the hotend too much which would reduce the layer height depending on the current extrusion force.
The RF1000 design can measure the extrusion force and is stiff enough so that it usually does not create problems, but we have seen such effects already if the required extrusion force gets too big (bad hotend, sticky material, high printing speeds - usually a combination of at least 2 of these). As a conclusion I would recommend to make the design either at least as stiff as done at the RF1000 (with 2 load cells rated for 5kg, TAL220B type) or such that the extrusion force is not seen by the load cells. That can be achieved by either putting the motor also onto the load cells for direct drive printers. For Bowden printers the force wonāt be applied to the load cells without artificially separating the sleeve from the hotend somehow.
Personally, I donāt see measuring the extrusion force as a killer feature, but others in the RF1000 community will disagree with this statement I am even thinking of changing the construction of my printer such that the motor sits also on the load cells, just to allow easier disassembly/reassembly of the entire toolhead. I am currently not using any feature which makes use of the extrusion force measurement (but I am frequently removing the toolhead, since the printer doubles as a small CNC mill).
One of the nice things we can see from this is what range of force values we need to support. On the high end it seems that 5KG would be the most that an extrusion system could want, with most typically operating under 1KG at sane temps and flow rates. On the low end the Prusa XL was stopping a probe at 50g.
Iām in the camp that primarily wants probing and the other applications are less interesting. The first layer is the hard part of FDM 3D printing. If we can make a significant dent in that issue Iād be very happy. So if I had to pick some things I want I would say:
Very compact so it fits inside the volume of existing toolhead designs between the extruder and the hot end
Sensitive in x, y & z directions @ 50g of load: I want to do x/y probing on my tool changer
Stiff enough that it wont significantly deform under extrusion loads ( maybe 0.05mm @ 1Kg ? )
Safe to operate at 70c in an enclosure
Has some hope of being cheaply mass manufactured
Some less important things:
Thermally stability: we can reset the detection algorithm on probing to deal with this
Linear response to load: you can imagine a sensor that has a non-linear response being just as useful for probing as a linear one. In fact it might be more useful if most of its sensitivity is near 0 force.
Maximum load of 5KG: why are you printing like that in the first place? Iām not building this for your speedboat rig.
Iām very intrigued by the PCB designs with SMD resistors. Given how cheap & easy they would be to manufacture I think we have to prove they wont work before going on to more complex solutions.
50g extrusion force is close nothing. With a E3Dv6 hotend, I typically end up with extrusion forces around 1kg at normal printing speeds and easy materials. If one starts tuning for higher speeds or starts to use other materials (e.g. PETG has a tendency to stick to all-metal heat breaks), extrusion forces of say 2-3 kg are pretty normal. Hence I would say 5kg is a reasonable maximum design load for an average printer, since you need some headroom between a typical scenario and the maximum acceptable situation. My printer has a pair of 5kg load cells, so the maximum acceptable force is even higher (but not twice as high, as the forces are not evenly distributed).
I would recommend considering available off-the-shelf load cells before getting into the business of designing your own. You can buy a variety of differently shaped load cells for not so much money, you just have to think about how to get them where you need them. Maybe for first tests and gaining experiences I would recommend even to live with a potential reduction of the maximum print size (if the load cells are too big and will hit some other components at the edges of the axes). This will help you understand what to expect and what to optimise your custom design for.
If you are not planning on measuring the extrusion forces, things will also be more relaxed.
Just want to chip in my two cents to the mechanical design. If we want to go for off-the-shelf cheap load cells, those are quite bulky. Hence orienting them for direct measurement will not allow sticking to conventional toolhead packageing.
Hence, hereās an idea: how about using a lever to change the direction of the forces registered by the load cell.
If we go for a levered design, we could amplify forces mechanically, which could increase the sensitivity of the measurements. Besides, a lever allows limiting forces on the load cell as it can be designed as natural breaking point. A Lever would also be most sensitive around cero load.
If you think this may be of interest, I could work on a design. Iām currently on business trip and thus could look into this in a couple of weeks. Regarding xyz measurements, this is probably very tricky to achieve. The other design requirements should be doable.
My custom Rapido heat sink is mounted to the underside of the tongue. You could also mount the standard Rapido (or other brand) heat sink in the same fashion.
I am even thinking of changing the construction of my printer such that the motor sits also on the load cells, just to allow easier disassembly/reassembly of the entire toolhead. I am currently not using any feature which makes use of the extrusion force measurement (but I am frequently removing the toolhead, since the printer doubles as a small CNC mill).
