Can Coehorn trunnion attachment

[Cannoneer]

More than a few of these pop can coehorns have been sold on an online auction site recently, and I'd appreciate some opinions on the way the trunnion is attached. They're made of 1018 steel that's CNC machined, and the seller says that the trunnions are pressure fit.
Wouldn't the vibrations from repeated firings tend to loosen the trunnion eventually?

ebay photos

[Double D]

That trunnion needs welded in place

[GGaskill]

That trunnion needs welded in place.

I agree; at least the ears need to be welded to the trunnion so they can't spread.  But I would weld all the easily accessible joints.

It would be interesting to know how deep the chamber goes and whether it has a caliber of metal between the trunnion and chamber.

[dan610324]

Im not so sure that it need to be welded
as it is a press fit I think it would stay in place without welding

the force from firing it will just push the mortar barrel down against the trunnions

but of course you will be 100% sure if you weld it also

[Double D]

That is and awful small amount of metal holding that trunnion.

[Cat Whisperer]

I like the curves.

On my design, I use 2 1/4-20 AHCS's to secure the trunion. 

I'd find someone GOOD at TIG or MIG and weld it - make it easy to clean up.

[willdj79]

Hi everyone,

I am the maker of the coehorn mortars. I make the trunnion .0015 press fit. I make the trunnion first then freeze it. By the time i am ready to install it it has shrunk .0005-.001. I then beat it in with a dead blow hammer. I preferred to not weld it because i think it looks better and more clean with less hand work. In my 15 years of tool & die experience i feel it would be very unlikely to come apart considering the force is going against the bar and not the other way(But will consider putting 2 1/4-20 SHCS outward from center and cover with weld). As for the meat between the bottom of the powder chamber and the edge of the trunnion is about 3/4"+. The bottom of the powder chamber is not flat but has the 118 degree of a 1 1/4 drill. The touch hole is at the very bottom of the chamber so looking at the picture you can visualize the material between the two. The reason there is a gap that doesn't totally surround the trunnion is where the center drill hole for the live center gets cut out.  I drill and ream the trunnion hole and leave a .060 web from the bottom and then turn the radius. These are semi-cnc'd on a bridgeport ez-path sd combination lathe and the trunnion hole is machined on a horizonal boring mill (total time start to finish 5.5 hours ). I appreciate your feed back and have been an avid cannoneer since i started in this trade. Will Jones

[little seacoast]

Will, that is a nice looking Coehorn you've made.  I can't speak to the weld/no weld aspect of it but I would not have any qualms about firing it. 

[willdj79]

Will, that is a nice looking Coehorn you've made.  I can't speak to the weld/no weld aspect of it but I would not have any qualms about firing it. 

Thanks. I understand the other members concern about form over function, and I agree. I myself feel comfortable with it because I know the type of fit I have on it and haven't had any problems with the ones I have made for myself, but I also don't want anyone that buys a cannon off of me worry about the integrity of trunnion being attached to the barrel. So on mortars I make in the future I will add a couple hidden bolts to ensure that it is permantely attached.   

[Double D]

Wildj79.

Glad to have you join us.  You build a very nice lucking mortar. It is very apparent you know you way around the machine shop.

Absence any other safety guidelines we on this board recommend the Safety  guidelines of the American Artillery Association and the  North South Skirmish Association as minumum standards for safe cannon construction.  You can click on this link to our sticky at the top of the forum and find a  synopsis of those guidelines Safe Loads and Construction. You can click on the links in that sticky and you will be taken to the organizations website sites for additional details.

One of their rules is that for safety the walls surrounding the powder chamber must be a least one caliber thick.  In your case since the powder chamber is 1 1/4"  the thickness of the walls must also be 1 1/4" thick.  That includes the thickness of the bottom of the chamber.

The rules are not specific about the fastening of trunnions, but I don't believe the either of the groups would certify this mortar as safe with this trunnion.

That narrow web at the back does not have the strength to resist the flexing of the trunnion bar.  The focus of the flex is right on that narrow web.   Over a period of time that trunnion is going to come loose.

Places I have shot would not allow an un welded  or non-solid trunnions on larger cannons.  My own cannon was refused with threaded and interference fit trunnion.  

You express a desire to not weld for cosmetic purposes.  I understand that, it looks more pleasing to the eye, but it compromises safety.  So use you marketing skills. Weld that trunnion bar in place.  Tell your customer how you build the trunnion, just as you  have you told us, but then tell them for there added safety you weld the trunnion.  

[willdj79]

Wildj79.

Glad to have you join us.  You build a very nice lucking mortar. It is very apparent you know you way around the machine shop.

Absence any other safety guidelines we on this board recommend the Safety  guidelines of the American Artillery Association and the  North South Skirmish Association as minumum standards for safe cannon construction.  You can click on this link to our sticky at the top of the forum and find a  synopsis of those guidelines Safe Loads and Construction. You can click on the links in that sticky and you will be taken to the organizations website sites for additional details.

One of their rules is that for safety the walls surrounding the powder chamber must be a least one caliber thick.  In your case since the powder chamber is 1 1/4"  the thickness of the walls must also be 1 1/4" thick.  That includes the thickness of the bottom of the chamber.

The rules are not specific about the fastening of trunnions, but I don't believe the either of the groups would certify this mortar as safe with this trunnion.

That narrow web at the back does not have the strength to resist the flexing of the trunnion bar.  The focus of the flex is right on that narrow web.   Over a period of time that trunnion is going to come loose.

Places I have shot would not allow an un welded  or non-solid trunnions on larger cannons.  My own cannon was refused with threaded and interference fit trunnion.  

You express a desire to not weld for cosmetic purposes.  I understand that, it looks more pleasing to the eye, but it compromises safety.  So use you marketing skills. Weld that trunnion bar in place.  Tell your customer how you build the trunnion, just as you  have you told us, but then tell them for there added safety you weld the trunnion.  

I appreciate your advise on building practices. I want to incorporate as much safety in to my work as possible even if it does rob a little beauty. I will adjust the the numbers so they are more in spec with the standards and lay a bead with tig around the trunnion on my future builds. What are your thoughts on the actual dimensions of my powder chamber as to what would be considered normal? I am going to change the diameter to 1 1/8" and keep the same depth of 1 3/4" so my minor diameter of the barrel which is 3 1/2" would be one caliber thick, and move my trunnion from being flush with back of mortar out .328 which would give me 1.176 thickness between the chamber and trunnion. I think that the weld going up on the side of the trunnion would look better that way.

[Cat Whisperer]

Wildj79 --

WELCOME to the board.

You'll hear a thorough discussion of all aspects. 

Again, I like the curves.  Some folks are traditionalists, others are shooters.

We'd love to see more of your work and what you like to shoot.

[Double D]

 
 
I appreciate your advise on building practices. I want to incorporate as much safety in to my work as possible even if it does rob a little beauty. I will adjust the the numbers so they are more in spec with the standards and lay a bead with tig around the trunnion on my future builds. What are your thoughts on the actual dimensions of my powder chamber as to what would be considered normal? I am going to change the diameter to 1 1/8" and keep the same depth of 1 3/4" so my minor diameter of the barrel which is 3 1/2" would be one caliber thick, and move my trunnion from being flush with back of mortar out .328 which would give me 1.176 thickness between the chamber and trunnion. I think that the weld going up on the side of the trunnion would look better that way.

When building these guns there is a lot to recommend to looking at the original designs when you build.  I not talking about small details like bolt hole patterns or color of paint.  I talking about engineering and structural design.   Build in the same ratio as the originals and you should be pretty safe.

One feature I have noticed on original rear trunnion mounted mortars is a fillet.  I just looked at every original mortar drawing I have and everyone has a fillet.  I am not sure why the are there, but they are. 

You can compute the dimensions for your powder chamber by using the guidelines and chart in the Safe loads and Construction sticky.  Look Maximum recommended powder charges are determined using bore diameter.  In the case of a mortar the diameter of the powder chamber is the bore diameter. For your 1 1/4" chamber, refer to the graph and you will see that that the maximum recommended safe load for this diameter is about 290 grains of black powder. 

Weigh out 290 grains of black powder.  Drill a 1 1/4" diameter hole in a block of wood deep enough to hold the 290 grains of powder level with the top.  Measure the depth of the hole and this is the depth you want for your powder chamber.






 

[GGaskill]

Fillets on joints of cast pieces make taking the pattern out of the sand mold easier and less likely to damage the sand.  And there is always the improvement of the product by lessening stress risers.

[Cannoneer]

wildj79,

I've seen your new model pop can Coehorn with the trunnion now welded to the tube; it looks good,Will! I think that people with some knowledge of machined reproduction artillery will be of the opinion that the weld makes for a more desirable mortar.

[willdj79]

wildj79,

I've seen your new model pop can Coehorn with the trunnion now welded to the tube; it looks good,Will! I think that people with some knowledge of machined reproduction artillery will be of the opinion that the weld makes for a more desirable mortar.

Thanks Boom J
I have to say that I wasn't crazy about the weld at first but now that I have done it I think your right it looks even better. I guess I get that from the years of working in injection mold repair that ingrained in me to make weld disappear such as on highly polished mold cavities. I also like how I was able to have the trunnion bar more exposed which seems to look more authentic. I know that the original cast mortars had more of a square support, I'm still wondering how I could incorporate that.

[GGaskill]

Your welding is a thing of beauty.  The squarish trunnion supports you refer to are called rimbases (same as on cannon barrels) and they serve the same purpose, that is they keep the barrel centered on the carriage/mount.  They are easily added to a casting but more difficult with fabricated mortars.  If you have a milling machine, it wouldn't be too much difficulty to add them.  See Reply #10 and also Reply #14 for a previous discussion of this very subject.

[willdj79]

Thanks for the information on the rimbase, it does look a little difficult to match up a square block on one radius and then on the trunnion radius. I guess i could take a piece of 4 1/2" round and drill a whole the width of my tunnion slot on the barrel, cut a concave radius to match the rear of the barrel then drill it through the side the diameter of the trunnion. Then cut the round stock on end off center on both sides which would give me a close fit to weld.. That would give me enough to make rimbases on two barrels. I am also thinking of just turning down each end an 1/8" so the shoulder will keep it centered on a sled style carriage.

[GGaskill]

It sounds like you are thinking about making the rimbases a tight fit on the curved surface of the mortar. 

I would take the opposite tack and mill flats on the mortar to accept flat rimbases welded on.  You would only need to cut a curve on the bottom of the rimbase that matched the radius of the trunnion.  Then fill any gaps on the barrel with weld and grind it down smooth.  If you  look at this drawing of a Coehorn, the rimbase stands about 1/16" above the surface and is about 1/8" thick at the top.  Mill a 1/16" flat in the right place and set the 1/8" thick rimbase in place and weld.  It would take a lot of filling to weld up the void behind the 1/8" outer face so I would fill it with Bondo and paint the whole thing bronze.

Your choice, of course.