Wednesday, June 27, 2007


Making a hole in it

Now that I know that I can print thick objects I can start addressing other challenges in 3D printing using Fused Deposition Modelling with HDPE. Dealing with a 0.8 mm HDPE extrusion and a positioning system which can position at about 0.1 mm but which needs a running start to get rolling makes the printing of small features a bit more of a challenge than it might otherwise be.

I knew from previous work that I'd eventually confront these issues, so I'm not particularly upset or despondent. They are, however, hard work. Mercifully, I have the XML formatted transfer file between the Slice and Dice software and Tommelise's Control Panel. That lets me tweak objects without having to rewrite the Slice and Dice code every time I want to try something. That has been a godsend.

Ultimately, I want to design and print a better polymer pump for my Mk 1 extruder. One of the things that it would be nice to have in the pump is aligned 3 mm holes for the built-up bolts that are spring-loaded to keep the pump firmly connected to the HDPE feed filament. I decided to see if I could set a 3 mm hole in my test square prism.

Here are two printed layers of the prism with a centred 3 mm hole and no infill.

The hole models as an octagon. You can see that the beginning of the hole's boundary suffers from the lag time between when you turn on the polymer pump and start tracking the perimeter and when you get HDPE coming out of the extruder orifice. Obviously, I need to make my extrusion startup firmware code a bit more sophisticated.

(Read the full story)

Can you take feedback from the motor? Can you measure impedance in the motors' coils and use that to determine how much pressure the filament is exerting on it? If so, you might be able to determine whether or not the filament has fed through, or if it's still bunched up between the motor and the extruder barrel.

I don't know if the controller is capable of that in hardware. I strongly doubt the firmware is.
You can put a shaft encoder on the gearmotor that runs the extruder's polymer pump. You can also measure the current that goes into the gearmotor. Unfortunately, I suspect that it would take a separate PIC chip managing just the extruder to be able to handle all of that and that is a project that I'm just not up to doing just now.

I plan for the short to mid-term just doing a lot of measured performance runs with the extruder and then building those performance parameters into the control programme that runs the extruder.

You'll be seeing more how I plan to address that in my blog in the next week or so.
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