Author Topic: Trunnions - how to? (Read 1726 times)

Cat Whisperer · « **on:** August 25, 2005, 12:36:52 AM »

Cutting the pockets and affixing the trunnions are one of the most challenging things in making a viable cannon or mortar.

How about some lessons from the masters? I'll kick in a few pix from this amateur's attempts.

Powder keg · « **Reply #1 on:** August 25, 2005, 12:47:45 AM »

If possible I like to use hidden studs between the barrel and the trunions. On smaller bronze barrles I have soldered the trunions on with good results. I'll post some pictures tonight.

Cat Whisperer · « **Reply #2 on:** August 25, 2005, 02:30:50 AM »

I'm looking at some ideas on fixturing for both cutting pockets etc and holding for welding/brazing. As soon as I build one I'll post pix too.

GGaskill · « **Reply #3 on:** August 25, 2005, 09:53:04 AM »

All of the fabricated guns I have built (steel ones, not the cast bronze ones) have had welded on trunnions that were set in pockets milled in the sides of the barrels. I cut the pockets to a depth that left at least a 1/2" wall for the barrel which I consider to be more than adequate thickness that far down the barrel.

The recently updated http://www.graybeardoutdoors.com/phpbb2/viewtopic.php?t=53901">Building a 24 PDR thread has a picture of the pockets and http://68.190.219.22:443/images/MillTrunnPocket.jpg" target="_blank">here is a picture of the setup for that process.

On the Coehorn mortars, I mill a round bottomed slot across the back and weld the trunnion in that, again being sure to have sufficient thickness of metal at the back of the chamber. On the M1861 style mortars, I profile the inside of the trunnions and weld them to the sides of the mortar since the welds can be much heavier (lots of filleting on the original castings) and the pockets would be more difficult to make due to the greater diameters.

Double D · « **Reply #4 on:** August 25, 2005, 10:47:10 AM »

I just realized how long it's been since I did one of these.

I used the method recommended by the late William Green. Starting with the unturned round barrelstock, I drilled a pilot hole at the trunnion location to about 3/4 of the depth of the finished barrel wall. This pilot hole is drilled to a standard size to be tapped later. The hole is opened up to trunnion base diameter and the bottom squared to about half the barrel wall depth. The pilot hole is then tapped.

I do one thing different than Green at this point. I make a false trunnion and screw it in place. The false trunnion is a lose fit and when in place should be about the height of the unturned metal. When turned in it should be hand tight. This fills the hole for turning. I had some concerns about the false trunnions coming loose during turning but never had one do it.

After the profiling is complete remove the false trunnion.

The trunnion is made with a thread post in the bottom for the pilot hole. The side of the trunnion should be a tight fit in the hole. I turn the trunnions in with my action wrench that I normally use for pulling rifle actions from gun barrels. I make sure the square bottom of the trunnion bottoms out in the hole not the stud. I have shot my cannons with no more than this for attachement, but just to be safe I weld the trunnion in place.

Powder keg · « **Reply #5 on:** August 25, 2005, 07:46:53 PM »

I've also used epoxy like JB weld to attach trunnions onto smaller cannon barrles. Back you naysayers!! :blaster: If done correctly epoxy does a good job. General motors uses it to secure the doors on many of their vehicles (since the 80's I think) Some fighter jets have the wings Glued on. lots stronger than Weld, rivets, bolts, or even the material to be bonded in some cases.

The secret is to do it right. I mill a flat bottom pocket (end mill will do even though they do not cut a flat bottomed hole) into the barrel to hold the trunnion. This pocket takes the sheer away from the glue joint. When building the trunnions the end that engages the barrel (rimbase) needs the bottom hollowed out. I have been going .035" - .045" deep. This is so when you assemble the barrel/trunnions all the glue dosen't get mooshed out of the joint. There is a "cavity" between the two pieces to hold some glue. Also the surfaces have to be prepared properly. I use some 100 grit emery paper and "rough" the surfaces to be bonded. A small piece glued to a dowel chucked in a drill works good. After the surface is rough I clean it with some paint thinner to get rid of any grease or oil. Now the barrel pieces can be assembled. I clamp them togather and wipe off any excess glue and let dry for 24 hours before un-clamping. Done.

I've done this with smaller (1/4 scale) barrles with good results. Larger barrles I tend to use blind studs. But the epoxy should work good on them also. I am not a wordmiester, so if my discription is lacking please let me know and I'll try to elaborate. Thanks for your time, Wes

Cat Whisperer · « **Reply #6 on:** August 26, 2005, 12:56:11 AM »

You've described well an industry-proven process. When you get into the metalurgy of each of the types of metal you will find specific products (Loctite has over 1000 of them) that are tailored to specific combinations.

Brake cleaner is an excellent product for getting surfaces VERY clean too.

Cat Whisperer · « **Reply #7 on:** August 26, 2005, 01:04:32 AM »

Quote from: GGaskill

All of the ....

The recently updated http://www.graybeardoutdoors.com/phpbb2/viewtopic.php?t=53901">Building a 24 PDR thread has a picture of the pockets and http://68.190.219.22:8080/images/MillTrunnPocket.jpg">here is a picture of the setup for that process.

.... due to the greater diameters.

George - THANKS! Great information specific to the several styles and lots of information hidden away in the pictures! (I really like the 2x4 bracing!)

GGaskill · « **Reply #8 on:** August 26, 2005, 07:30:36 AM »

There is a little play in that collet indexer so I clamp the material against the table when cutting holes large enough to cause the material to move.

I also had an inspiration while working on this process. The saddle casting is broken out around the Y axis clamp on my mill which prevents me from being able to lock the Y axis when making a cut. It dawned on me that if I blocked the motion of the table toward the column, I could run in the lead screw and hold it in place using the lead screw as the clamp. So I made a variable length block from a clamp bolt with hex couplings on the ends. The procedure worked perfectly. I suppose this causes extra wear on the lead screw but I don't think I will use this machine enough for that to matter.

Powder keg · « **Reply #9 on:** August 26, 2005, 09:34:29 AM »

I liked the clamps also. Great Idea! I'm sure I'll use it someday. I'm glad I'm not the only one that can mess up a mill table. Seems their never big enough to hold all of the tools I'm using. Thanks for sharing George.

Articifer Tom · « **Reply #10 on:** August 26, 2005, 05:28:18 PM »

With sub calibering and scale sizing don't forget about propoderance when locating the trunnions. This will ruin a project quick if you cannot keep your muzzle up! :oops:

Powder keg · « **Reply #11 on:** August 26, 2005, 06:40:39 PM »

propoderance ? I had to look that one up :oops: I always worry for some reason that a barrel I build will not tip the right way. (and you said he couldn't hit the ground) Before I mill the trunnion pockets I test where it will balance and move the trunnions ahead of that balance point. Very good point. Thanks for bringing it up!!

GGaskill · « **Reply #12 on:** August 26, 2005, 07:58:59 PM »

"Preponderance--The weight needed to balance a piece when suspended freely from its trunnions. Preponderance is typically at the breech, to seat the gun against the elevating screw."

from CivilWarArtillery.org's http://cwartillery.org/artdict.html" target="_blank">Dictionary of Artillery Terms.

Cat Whisperer · « **Reply #13 on:** August 27, 2005, 01:34:43 AM »

Quote from: GGaskill

Preponderance--The weight needed to balance a piece when suspended freely from its trunnions. Preponderance is typically at the breech, to seat the gun against the elevating screw.

Agree, unless you're using an equilibrator.

:grin:

kappullen · « **Reply #14 on:** August 27, 2005, 01:42:45 AM »

I don't pocket the barrel out to fit the trunnions into.
I'm not saying that there is anything wrong with that, but generally bore the trunnions to
fit the arc of the barrel.

Then I tap a little hole in the barrel to fix the trunnions for welding.
Then a couple starter tacks, and build a nice fillet.

Here is a woodruff gun welded and ground.

Making a barrel with offset trunions like Georges takes a lot of machining to the

trunions to fit, but allows a large area for weld.

I used a double vee block affair to align the trunnions on the three pounder.

My setup for pattern making (boring for trunnion) looked very similar to Georges picture.

An angle plate with a couple (4) sawed out vee blocks were in order to stiiffen things up.

Ready for the trunnions.

However you do it, happy machining.

kap

Cat Whisperer · « **Reply #15 on:** August 27, 2005, 01:50:20 AM »

WOW -

You're a better man than I, Gunga Din!

Powder keg · « **Reply #16 on:** August 27, 2005, 04:32:58 AM »

Hay Kap, Nice work!!! Is that the rail gun I read elswhere that you were building? Please try to post pictures of the foundry work. I'd like to see your settup. Again nice job.