Welded Trunnion(s) Distortion

[gunsonwheels]

I have perused some of the links /threads on the site about locating and welding trunnions.  I subsequently bought a 2.25" shell mill to mill the trunnion face on a half scale barrel.  The plan is to machine the face/socket, turn the trunnions to mate into and over the barrel facing, grind bevels for the weld and weld the OD of the rimbases with a good 3/8 to 1/2" weld.  The trunnions will be screwed onto the barrel to hold them for welding. 
 
Based on the thread(s) about how to do this it looks like several folks on the board have done it.   My question is has anyone who has done it redone their barrel setup in an indexing head and dialed the trunnions to see how straight they ended up after the welding?
 
And did that person preheat before welding to try to minimize/reduce the distortion?  And was externally boring the trunnions necessary to bring them into the same axis? 
 
If externally boring the trunnions is necessary then it would be best to oversize them with sufficient material to enable them to bored-to-net.
 
GOW/George

[armorer77]

When I weld trunnions , they are firmly clamped into a fixture . Welding is done in spots 90 degrees apart until the trunnions are 25 to 35 % welded . Then pour in the fller to bring them up to size . Clamp hard , move around alot .  Ed

[gunsonwheels]

Thanks Ed!!!
 
George

[GGaskill]

When I weld trunnions (TIG only), I mill the pockets first, turn the rimbases to fit and tack in place before completing the weld.  Usually I will put the bottom trunnion in place unwelded and the top one in place and clamp the whole assembly to the welding table.  Completely weld the upper side, then turn around (probably let the barrel cool) and do the same on the other side.

Most trunnions I have welded are on bore center but even the below center ones came out well because they were guided by the flat bottoms of the pockets and clamped heavily.

[gunsonwheels]

George & Ed,
 
Have you, or do you, ever intentionally set the trunnions a "tad" below center on a piece that is supposed to be right on bore center?
 
The reason I ask is I used to shoot against a fellow (Reid) who had a cast tube that just happened to end up with the trunnions a bit above bore center.  Result was the tube would start to pin-wheel on firing and the base would come up off the elevation screw and mess up his ability to hit anything.  When I shot against him he had about three or four heavy rubber bungees wrapped around the underside of the trail stock and running up around the cascabel.  It worked for him...
 
Because of my experience with Reid and also because I don't believe anything can be "dead nuts exactly on", when I fixtured the trunnions for welding on my gun, I purposefully set them about .050" below bore center so any pin-wheeling effect would cause the barrel to rotate down against the elevation screw instead of it raising up and off of the screw.  While it worked to keep the barrel stable until the projo cleared the barrel, heavy loads did cause a lot of down-ward force on the screw (folded the first trailstock up like a piece of paper - wood grain ran at a 45 degree angle in that area which is an absolute NO-NO for the force direction).
 
Assuming there is no "exactly on bore center", have you mounted a few thou low so the screw can hold the barrel instead of it pin-wheeling up off of it??
 
GOW/George (another one)     

[GGaskill]

I generally put my trunnion centers a tiny bit below center to make sure they don't end up above center.  Maybe .030" or so.  Not enough to be visibly not on center.  Although the preponderance should take care of a tiny bit above center.

[armorer77]

I mount my trunnions on center , give or take .0003" If there is a better way , I would be happy to learn about it . Ed

[gunsonwheels]

Ah come on Ed... three tenths...     if that ends up high I doubt the barrel could dip before the projo cleared the muzzle anyway unless the barrel mass was way low in relation to the projo.... 
 
I appreciate both of your inputs.  I still favor intentionally mounting low but just enough to be absolutely sure I am not high.  That's .005"-.010" for my level of precision.  Three tenths  ....    tool and die makers always were kind of a proud lot           But having three of Ed's pieces and living with their quality, the three tenths doesn't suprise me.
 
Thanks again for the inputs!!!
 
GOW

[armorer77]

Sorry , 1 too many zero's there .  .003"   

[de_lok]

The best way I have found to attach trunnions is to machine a flatbottom tre-pan slot pocket in the tube and a matching pocket in the trunnion(female tube, male trunnion). This gives you the full advantage of milling a pocket to set the trunnion in and still keep the full thickness of the tube wall the drill and tap for set screw to "screw" the trunnion in place without the mess and hassle of welding. With a good setup on a decent mill and carefull attention to tolerance you can get an almost airtight fit with perfect allignment. then if you must, you can heat the barrel and fill in the seam with silver solder. A picture is worth a thousand words, so I will try to get a pic posted in the next day or soof my trunnion method..............   Most of the cannons I have made have not been "historicly correct" to keep cost down, but they have always been over engineered to be safe, durable and still look nice

[dominick]

Hi George,
 
If the top and bottom of the barrel is identical looking [in most cases it is], I weld the trunnions first, clamp and weld and then check to see if they are off center in height.  If the trunnions are off center, I flip the barrel to the high side so the trunnions are below center and then drill the vent last. 
 
Dom

[de_lok]

A pic of the tre-pan method I use.

[gunsonwheels]

de_lok:
 
Thanks for the pic and explanation.
 
I too prefer locating the trunnion in the same manner but using the faced surface on the barrel to set the trunnion against and to locate its depth into the barrel.  The circular protrusion I use to locate it with respect to height and length on the barrel (I use the same precision bored hole in the trunnion mating to the protrusion and the set screw to hold everything in place).  I also do not cut as deep into the barrel to enable a bevel both of the fore and aft edges of the trunnion's rimbase, and the fore and aft edges of the cut recess into the barrel for a weld fillet.  Also by not cutting as deep into the barrel, the bottom surface of the trunnion and the slope of the barrel's diameter provides and nice natural bevel for the weld fillet at the top and bottom of the trunnion.  A little extra weld and grinding a nice blend between the barrel and the rimbases and it is both structurally sound and looks nice though not authentic with the sharp corner between the two as on the origninals.  Again I too locate the trunnion with a set-screw (put JB weld on the end and then screw into the barrel to compress the epoxy - it fills the tapped hole solidly to prevent any potential for blowout at that point) but only to hold it securely in place while tig tacking around the clock of the trunnions rimbase prior to final welding.  I preheat to 350 degrees F. before welding to both reduce distortion and to reduce the risk of hardening the parent metal of the barrel which I have to assume is high enough in carbon to either air harden the barrel or fatigue crack at the weld or both.
 
I'll be doing another one in a few weeks to couple months and get some pics and post... although it looks just like yours except cut shallower and with the bevels for and aft on the rimbase faces and the recess cut into the barrel at those locations.
 
I am not arrogant enough to presume my method any better than yours... in fact when I consider my working with unknown parent metals, your's may well be very superior.  I believe it is good to expose others to a number of methods and their several variations so as they embark on attaching trunnions, they can pick their own poison method.   
 
GOW/George