In the age of internet, some people think it’s weird not looking up if something exists. I myself I’m in the leage of “buy the tools if possible”, because I don’t like spending time creating the tool I need AND then more time to do what I intended to do in the first place.
However, in this case, it didn’t cross my mind that it could be possible that what I needed already existed (not in a egotistical way, I just didn’t thought much about it). Anyways, I wanted a mechanical paste dispenser.
Easy enough, a screw, a syringe, and something in between to hold everything together, right? Well, normal metric screws do not bear well with pressure, they tend to bind, so I figured that, at least as a test, I could go by with a micro fine pitch screw. As I didn’t had such a screw (or tap, for that matter) at hand, I had to make my own:
That’s like half a millimetre or less pitch screw in a 8mm diameter shaft. That would need a keyway guide AND machining the end to accept a standard plunger tip:
Since I had made it at home, I also had to make a tap for the nut/actuator wheel, wich was simply done by putting a cone on a threaded shaft and some relief cuts so a sharp edge with all the threads would engage on the nut material:
The point here, tough, is that the shaft was 8mm, so in order for the tap to work, I had to make an undersize hole, probably in the order of 7.8mm or so in the nut, but I don’t happen to have a drill in such diameters at hand AND they are usually expensive. However, if I freezed a drill bit to -12ºC, I could achieve somewhat the same effect:
Super cold alcohol+water mix to freeze the drill bit.
And there we go!
Shaft threads haven’t been deburred yet on this photo.
So, did it work? Well, more or less it did:
However, the nut bind easily because of the shape of the screw threads AND the friction against the backplate. As expected, I stripped the nut threads, as acrylic is not the best material to machine high load/friction parts. But the concept was there and somewhat solid, now it needed a real trapezoidal screw, a technic material for the nut and some bearings.
As before, a tap is needed for this screw:
With that I could thread a delrin wheel and machine the leftover screw end to accept the plunger:
Also machined two flats on the screw, as I did not like the keyway idea in the first prototype:
With all that done, a needle bearing (and some acrylic machining I did not photograph) there it was:
The bottom plate captures the syringe in place, and the top bearings both grab and guide the axle so the gear can do it’s job. The needle bearing removes stiction between the top plate and the gear.
It is made to be easily removable/serviceable:
Note how fast stops the paste extrusion. (nothing exceptional, just that it does XD! )
It will have a ratchett mechanism to actuate the wheel and a small nut on top to make the needle bearing captive, but it sorta works now, wich was the objective.