To push paste out of syringe, we need threaded shaft. There are 2 ways:
Coupler with threaded rod
You can take this coupler 2.3mm to 3mm. Use 2.5 mm drill bit to enlarge hole, then tap M3 thread and insert 60mm M3 rod or screw with loctite.
Disadvantage is loosing 20mm of movement length (or increase length of device). Let's leave this for Plan B.
Gearbox with built-in long shaft
There are no ready mini stepper solutions with M3 shafts, but we can assemble it from two donors:
In may sound strange, but those parts are compatible!
See photo. Donors are on left side, result on right side. Total cost with delivery will be about 10$. Not ideal price, but acceptable, requires zero efforts and is the most compact.
If you decide to combine other components - check gearbox sizes, those should be equal (the most popular can have 10x12mm or 12x14mm plates).
If components go out of stock - we still have Plan B with coupler. Both solutions do not require complicated tools and can be reproduced by anyone.
Many hand made dispensers use 28BYJ-48 motors. Those are very cheap, but not enough small and not convenient to mount. We need something better:
Let's estimate desired gearbox ratio.
All small 10-15mm steppers usually have 18° step (20 steps per turn). Gearbox can be:
Desired portion size per single motor step is 0.006 mm³.
Let's calculate, what stepper with plastic gearbox 1:50 can give us:
(3.14 * 15² / 4) * 0.5 / 50 / 20 => 0.09
0.09 mm³ is very bad. It's 1.5x more than portion size and 15x more than desired precision. What can we do?
1cc syringe has dia about 5mm. Portion per step will decrease in 9x. Better, but not perfect. Let's use metal gearbox 1:295.
Estimate for 3cc syringe (9mm inner dia):
(3.14 * 9² / 4) * 0.5 / 295 / 20 => 0.005
Estimate for 1cc syringe (5mm inner dia):
(3.14 * 5² / 4) * 0.5 / 295 / 20 => 0.001
Of cause, we could use dynamic model (quick step forward and back) and cheat with paste viscosity. But that's much more complicated and result will depend on paste temperature. Also, it's not good idea to use micro stepping, because it's not precise with cheap motors and consumes more power.
That's all with boring maths, next time I'll try to post some pictures :)
For dispenser it's important to have soft control button. Because if you need to apply big force, needle may shift out of small pad. There 2 main parameters:
Human skin is "soft". To be convenient, button should have big height or big square. To be sure, buttons with low operational force, available at LCSC, were tested:
The most promising are:
EVQQ2B03W looks preferable - more compact.
Since it's not interesting to reinvent wheel, it worth to inspect existing projects, and decide what can be done better. Here I will comment the most interesting ones.
28BYJ-48 based dispensers
All projects above are opensource. But while those are very cheap and easy to do, BYJ motors are not convenient for compact design. If you have TS-80 or TS-100, you should understand what i mean.
Though that project is not opensource, it's very nice. Author is professional engineer and shares interesting technical details.
Let's try to formalize, what would be interesting to do (and why that combination of features will be new or unique).
Seems ok. Now we need to do some estimates about precision.