|Monthly Tech-Tip |
Objective: See if it is possible for a potter to create a duplicate of this bowl purchased at Ikea. It is porcelaneous stoneware, not white but very vitreous. It is a simple form with a transparent crystal clear glossy glaze.
At first, I thought my 3D printer was not large enough to make all the parts I needed. But could it produce sufficient precision that pieces could be printed in multiple parts, with flanges, and glued or clamped together? I wanted to know.
As with other projects, this all depends on one's ability to do the 3D drawings. I use Fusion 360, one of the best skills I have ever learned. Here are the steps to create the sketches (I revolved them to create the 3D parts):
I measured the aluminum cup-head carefully and drew its cross-section.
I drew the profile of the jigger mold using outside dimensions to match the inside of the cup-head and an inside contour to match the outside of the bowl (with 12% size increase to account for drying and firing shrinkage).
I mirrored the bowl's outside profile upward and offsetted it to get a mold to print in PLA filament (filling that with plaster creates the outside shape of the bowl). I adjusted the outer contours of the PLA print for a flat bottom and made all sections of the wall less than 45 degrees off vertical, this enables printing the form without supports.
By mirroring the inside contour of the cup-head upward I was able to create a cross section for the collars. For the 3D printed parts I revolved these in two stages in two directions. The first revolve is the entire cross section at 90 degrees each way, the second cuts the first at 89 degrees each way (thus creating the bottom and edge flanges).
After making a couple of iterations I realized that it was not actually necessary to print a multi-piece shell mold (with flanges). A one-piece shell works much better (the diameter is 18cm and my Prusa MK3+ can handle this). It stays perfectly round by virtue of a thicker shoulder section and being printed as one piece (the molds made using the multi-piece method were not precisely round so they rocked slightly in the cup-head). Additionally, there is no need to remove the 3D-printed shell from the mold, there are multiple advantages to leaving it in place.
To make them I roll clay 3/8" thick, cut a 19cm diameter disk, press it into the mold and then while turning I work it down into the foot ring and create an inside contour by hand. Then the jigger mold simply finishes the rim. It appears it might be practical to make a template that only forms the upper inch or so and the rim, hand finishing the inside affords the opportunity to add a throwing spiral.
1 The original drawing in Fusion 360 (aluminum cuphead, jigger mold with bowl-outside profile, bowl mold, jigger mold case and jigger template).
2 The original bowl purchased at Ikea with the 3D printed shell of its outside contours (12% bigger).
3 The plaster form of the outside shape (created from the shell mold).
4 Printing a pair of the working mold collars (they have flanges for clamping together).
5 The collar assembly (clamped, glued and soaped).
6 The brown outside form soaped and glued to the Arborite bat using sticky clay. The collar assembly was glued down.
7 Plaster has been poured and has set.
8 The finished mold (it fit into the cuphead almost perfectly!)
9 The jigger mold inserted into the cuphead ready for use.
This is much more durable and it drops very precisely into the cup head, a perfect fit. It was much easier to center and pour the plaster into it.
The jigger mold is shown in front, by projecting its inside contour horizontally outward and then offsetting it by 3mm (and smoothing) I was able to create the jigger template. The template is 1 cm thick with a 30-degree draft along the cutting edge. I mount this to a 1/8" steel plate holder that is attached firmly to the jigger arm. No mount holes are needed, I heat up screws on a propane burner and just push them through the holes in the steel plate into this 3D-printed template, even fine threaded machine screws hold it on very firmly.
The head is lathed from aluminum. It fits on a potter's wheel (adjust the inside diameter to fit the shaft). The plaster mold drops into it this cuphead and stays in place by friction. The only point of contact between the mold and aluminum should be at the top inside corner of the cuphead.
We have mechanical drawings for this (and the cup heads). This is very sturdy and useful. The arm is relatively short compared to industrial jigger wheels and is thus useful for only small shapes. There is an advantage: The template contacts vertical walls at a more perpendicular angle. But the disadvantage is that the trailing edge of the template hits the outside edge of the lip on taller shapes. The pointed bolts hold the arm securely and their tightness enables varying the friction of movement. They have enough length to also position the arm horizontally. We 3D print templates and block masters for making working molds that drop into the cuphead.
Made from 1/2" x 3" flat steel, this frame is heavy and very strong and solid. The two larger 3.5" long bolts are custom-made from 5/8in-11 threaded rod, they have a 45-degree cone tip and locking nuts. The small vertical stopper-bolt is made from 3/8" rod. It is 2" long with 1" dia washer welded on top and a locking nut. The small bolt setting determines the bottom point of arm travel. The pivot sockets on the jigger arm are 1" long and made from 1" steel rod, each has the 45-degree recess machined into it. The arm can be moved left or right and its tension set by adjusting the two larger bolts. The weighted collar on the back of the arm employs two pieces of 1" thick flat steel, it is heavy (important for convenience-of-operation). The triangular notch enables the arm to be lifted higher. The weight colar needs some sort of set screw to hold it in place. A number of corners need to be rounded or rubber-padded for safety reasons. If you would like this 3D file in Fusion 360 and STEP formats, it is available in the Files manager in your Insight-live.com account.
|By Tony Hansen|
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