I must say I'm not entirely sure I see the point of playing a historical trombone that is 3-D printed (kind of drastically goes against the idea, no?), but then again, the 3D-printed cornetti are fairly successful, so what do I know!
Couple things about your instrument specifically though. You say
Quote from: Pieter on Jul 19, 2017, 01:48PM The bell is a copy of one made in 1594 in Nürnberg by Altötting Schnitzer - at least if the measurements in the sources I found are correct.
There is no such instrument (or maker, for that matter). Altötting is the name of a Bavarian town where a trombone by Anton Schnitzer was found (or more accurately, rediscovered - as it had been on display for quite some time, unnoticed by scholars for decades). Moreover, the trombone found in Altötting is dated 1576 (the garland engraving reads "MACHT ANTONI SCHNITZER A MDLXXVI")
There is in fact a Schnitzer trombone dated 1594, but that one is in Edinburgh. Neither of those two instruments have a decorative ball on the bell like yours does, however. The only Schnitzer trombone that has one (actually, the only trombone of any maker, AFAIK) is a third instrument, which is date 1579 and is in Verona (Egger makes a copy of that one). Not that it makes a huge difference, as far as I know, all four known Schnitzer trombone bells have the same basic shape and taper (inconsistencies in fabrication methods and ulterior alterations notwithstanding). But I'm just confused about which instrument it is you're copying.
Anyway, all four of them have the bell in 4th position, not 3rd, and all have only one (flat and hinged) bell brace (as is typical on most trombones until at least a couple centuries later). I don't see why you want a wider bell section - the reason modern instruments are wider is because the larger bell flare requires that space. I wouldn't make it wider than the original. I also wouldn't make the bell ferrule into a ball - it's not on the original, probably doesn't change the horn and just looks kind of silly, especially if you leave the other one in. In fact you probably don't need that ferrule at all since I assume the bell itself is printed as one piece?
A few purists will disagree, but I don't have an issue with tuning slides. Most of us have one on our instruments; many, many aspects of modern day to day Historically informed performance are not historic at all and require a flexibility on tuning that they didn't necessarily need in their day-to-day playing. Is that thing in the back a counterweight? I would suggest making a version without it. There is no need for a counterweight on a sackbut as it *should* be nose-heavy (especially with the historical longer bell) and be played somewhat towards the ground. Easthetically, it kind of ruins the beauty and purity in the simplicity of the design of the sackbut.
For the bell measurements, it's hard to tell from a picture and without knowing the numbers you used, but at first sight, it looks like it might be too flared (either the end of the flare gets too wide, or the taper is wrong). But it might just as well be correct - as I said, hard to tell, also the absence of a garland can fool the eye.
There is a very detailed article in the Historic Brass Society Journal by Hannes Vereecke on the 1579 Verona trombone, which includes quite a few useful measurements (among other things, it has a graph of the taper of the entire bell, and measurements of wall thickness of every part of the instrument). Initial bore of the slide is 10mm.
http://www.historicbrass.org/Portals/0/Documents/Journal/2011/HBSJ_2011_JL01_002_Vereecke_NN1.pdf
It is also possible to order full-scale technical drawings of the 1594 instrument from Edinburgh.
I would use 10 or 11mm for the next one.
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