I made some progress on this. Below is some additional information for anyone else interested in doing something similar. I will try and post some pictures once everything is assembled and working.
1. Instead of a glass capillary, it is far easier to use stainless steel. The nibs on technical pens are stainless steel tubes with a needle in the center. You can find pre-cut stainless steel tubing from suppliers of HPLC equipment that can be used to make a custom “nib” connected to a pump.
Stainless tubing is available as small as 0.36 mm OD in size. The ends of this type of tubing can be electrolytically cut and polished by the supplier. It is not terribly expensive. I bought some 0.8 mm OD (1/32″) tubing, and the polished end glides across the paper smoothly. The tubing that I purchased has a 0.25 mm (0.01″) ID.
2. If you use 1/32″ tubing, as I am doing, the OD of the tubing is close to that of a 22 gauge needle. You can get a 22 gauge needle, cut off the sharp tip and join it to a 1/32″ tubing using a standard tube union with 1/32″ fittings and ferrules. That way, you can link the syringe output to tubing that goes to the plotter.
3. PEEK plastic tubing is also available in 1/32″ OD size. This tubing is quite flexible and is what I am using to join the syringe pump to the plotter. It can flex easily as the plotter arm moves around.
4. I am joining the PEEK tubing to a stainless steel “nib” tip using a 1/32″ bulkhead union. That bulkhead will be mounted on a bracket that replaces the bracket on the Axidraw that holds the pen. I’m waiting for my bracket to be 3D printed. I decided to use plastic to make it as light as possible. My bulkhead union weighs 12 grams, and I do not want to overload the servo on the Axidraw.
5. I found some information on the flow rate coming out of a technical pen. These guys wrote long lines on paper and used a gravimetric method to determine how much ink is being put down:
For a 0.5 mm technical pen drawn across paper at a speed of 2 cm/s (this corresponds to 25% speed on the Axidraw if I am not mistaken), the flow rate is about 500 nanoliters per second. Syringe pumps are easily capable of these types of flow rates.
I have yet to assemble everything and test it. Syringe pumps use stepper motors, so pulsing may be a problem at very low flow rates. I have some ideas to get around that if it is a problem, though.
- This reply was modified 11 months ago by Matt.