Monday 1 September 2008

Screw Top Pot

A day before going on holiday I decided I needed a container to store the lens adapter for my camera. The lenses have caps on each end, but they require and adaptor which is a bit delicate: -



I knocked up an HDPE pot with a screw top lid that just the right size to hold it: -



The outside diameter of the adapter is about 48.5mm so I made the pot I.D. 50mm to allow some clearance. It actually shrank to 48.5mm, so it is a snug fit. Lucky it didn't shrink any more!



I made the thread using the helix tool in CoCreate. You draw a 2D profile and then use the helix tool to spin that round an axis, specifying the pitch. The dimensions were just a stab in the dark: I made the crest of the thread 0.8mm as that is two layers (Nyquist sampling theorem) and made the sides 1mm long 45° slopes, so that made the crest height 0.7mm. The minimum pitch with this profile would be about 3mm so I made it 3.5mm to give some clearance. I also made the the lid 1mm bigger radius so there is 0.3mm clearance from peak to corresponding valley.



A couple of things I missed which would make it easier to engage: -
  1. The thread starts abruptly, but it should have a tapered lead in.
  2. Both the pot and the lid should have a few millimetres with no thread to aid lining them up before the thread engages.
Despite this it works surprisingly well for a first attempt with an arbitrary profile and dimensions.



I am slowly homing in on getting rafts peel-able for HDPE. I made this with the first layer outline hotter than its infill :-
HDPE.raft_temp = 215
HDPE.first_outline_temp = 230
HDPE.first_layer_temp = 205
HDPE.layer_temp = 240
Most of it peeled with a little encouragement from a chisel at one side.

12 comments:

  1. wow cooler than ever.

    somewhere to put all those little buts you strip of anything you throw away. (At least I do anyway)

    sorted

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  2. Wow, it looks very neat! Very useful to make closable flasks of any size... there should be a 'flask wizard!' in which you only need to specify major properties and it generates a printable .stl :)

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  3. That's AWESOME! :) Can you post this in the object repository? Great work!

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  4. You won't get any awards from the Whitworth Society but it's an impressive achievement all the same.

    This project is exactly the kind of thing I'd want this kind of machine for....

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  5. Andy D,
    Yes when I get home.
    Andy from...,
    Yes he will be turning in his grave (clockwise of course). With this sort of fabrication there is no need for standardised threads.

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  6. Andy D.,
    The files are now in the wiki here

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  7. nophead,

    care to share g-code that works with HDPE, not important what object. I'm trying for weeks to get usable output using HDPE and all I'm getting is garbage :( (managed to get very good prints from ABS and PP .. but HDPE is just not working for some reason)

    some attempts:
    picasaweb dot google dot com /lh/view?uname=bogdan.kecman&tags=HDPE

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  8. Hello Bogdan,
    I don't use different g-code for different plastics. I do change the temperatures and feed rates but I don't use the values in the g-code. I only use the g-code to specify the path the extruder takes. I ignore all the other information in it.

    You are welcome to the files, the pot is 2MB and the lid 1MB, but any speed and temperature information in them will be rubbish so you can't just send them to a machine.

    It looks like your objects are not sticking to the base well enough. It is very difficult to get this just right with HDPE so it is held down and still peelable. I tend to have to use a chisel to get the raft off the object.

    To get the right amount of adhesion to the raft I vary the temperature of the first layer and / or the height of the first layer. To ensure the raft is at a consistent temp I cool it with a fan for about 1 minute.

    The parameters I use are:

    HDPE.raft_temp = 215
    HDPE.first_outline_temp = 230
    HDPE.first_layer_temp = 205
    HDPE.layer_temp = 240

    5C or 0.05mm first layer height make noticeable differences in the raft adhesion.

    I do the first layer at 4mm/s and the rest at 8mm/s.
    The filament is extruded through a 0.5mm aperture, swells to about 0.8mm and stretched back to 0.5mm. The layer height is 0.4mm giving a filament width of 0.6mm.

    You can better corner definition with HDPE by cooling the object with a fan while building, but you need a well insulated extruder.

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  9. Can you make a lid for an existing metal jug or do you know what I should search for to find someone who can make one? I have an old thermos who's lid had gotten a hole in it. I'd appreciate any help. thanks, Kitty

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  10. It may be possible if the thread is not too fine. The difficult thing would be measuring it and making something to fit. It may take a few iterations. Do you have a picture?

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  11. well, not a picture of mine exactly, but it's a Therm-a-jug such as is seen on ebay for 25 dollars or so. The threads are not at all fine. they are as thick as a pen refill or a bit thicker. There is about three or three and a half threads per 3/4 inches without measuring anything exactly.

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