You could use several technical values to determin the parts “quality”.
Just to name those that come to my mind right now:
Surface quality (smoothness)
Usually this is measured with very precise machines and is given in a variety of values.
You could measure the deepest groove with an infinitely thin needle, then (randomly) select a few ranges of groove depths that resemble the quality.
For example:
maximum 1mm deep grooves - quality: rough
maximum 0,5mm - quality: medium
maximum 0,2mm - quality: fine
maximum 0,1mm - quality: very fine
maximum 0,05mm - quality: superb
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Measurements (tolerances)
Print yourself some random objects of which you exactly know the measurements (ie from CAD files) and measure your prints with a caliper.
The customer could then select what level of tolerances is acceptable for his / her project.
For example:
maximum 2% alteration (ie 100mm +/-2mm) - quality: rough
…
instead of finding out appropriate tolerances you can also check out the general ISO tolerances (european standard): ISO 2768
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other tolerances
This is getting more and more complicated, but it would also be possible to judge the print quality with shape tolerances, for example by how much does a printed cube alterate from perfect rectangular corners, or how round is a printed cylinder and does it bend in any direction or is it perfectly symmetric to the center axis. Another thing would be: how flat is a printed side of a cube or whatever object ?
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In the end it comes down to these 3 options:
1) 3dHubs sticks with a very basic scale for quality (layer height) that gives a rough idea about how good the printed surface is.
2) The customer supplies you not only with an stl file, but also with drawings that include tolerances for EVERYTHING, every angle, radius, lengths, smoothness, shape, bore, …
In 99,99% the customer has no idea what tolerances would be appropriate and nobody would have a clue how to produce those parts and how to measure every kind of tolerance.
3) The customer chooses the average surface quality from pictures of prints (that are printed with different layer heights etc.) that you printed earlier.
Secondly the customer tells you what part has to fulfill what job (ie gears have to interact, slides have to fit into each other) and then the customer tells you: “make it fit”.
At that point it would be the task of the hub to judge how complex it will be to make certain parts fit (maybe 2 or more prints are needed to get the right fitting) and based on that judgement the price would need to be adjusted.
I usually ask a lot of questions about the models before printing them to be absolutely sure, that the customer gets what he wants, given that there are precise holes needed i often offer to print them smaller and then drill them out so that the diameter is at least a bit closer to perfect. When taking this further you could also take your 3d print and print it a bit too big and then machine everything on it to perfection (most likely CNC mills, lathes, drill presses and such are required if you want to go that far).
The judgement by previous prints is definitely the easiest for the customer, although it doesn’t allow objective ratings that could be taken as search criteria for finding the right hub.
Secondly the hub would be required to print a lot of “waste” stuff, just to show the customer all possible materials and quality levels. The waste-prints would have to show all (or nearly all) 3d printing features.
“3d printing features” means:
overhangs of a variety of angles
top and bottom surface
vertical walls
cylinders and holes in various sizes and shapes (for the holes)
bridges of different lengths
bridges and overhangs with removed support material (that might leave surface marks)
All those features should be shown in the pictures so that the customer definitely knows what he’ll get.
That should be everything for now
after half and hour of writing I need to get myself a coffee asap !
Cheers,
Marius Breuer