Before I am hung, drawn and quartered, for operating a lathe without guards, here is the proof that I have been sensible.
Before I get onto a brief reflection about scale, the photo below shows 2 cannon barrels.
The big one was what impelled me to converting a manual lathe into a CNC lathe. That time consuming, costly, and ultimately very satisfying project, started because the CNC lathe which I used to turn the big barrel could only handle the job by doing it in two stages…. doing the breech first then the muzzle. That was due to the big barrel being too long for the lathe, at 300mm (12″).
The small barrel was a test for the CNC converted lathe just finished, being the first complicated shape which I have made. To save on material, I made it at exactly half the scale of the big one, ie 150mm long (6″).
Comparing the two barrels reminded me, that if an object is twice as big as another, in all 3 dimensions (height, width, depth), it is 8 times as heavy. And any projectile, and weight of black powder, would also be 8 times the weight. But the wall thickness of the explosion chamber is only TWICE as thick.
My point is, that if scale is maintained, the smaller the cannon, steam engine, boiler, whatever….. the less likely it is to explode.
Not that these cannons will ever be fired. Just hypothetically.