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1/3 scale Coehorn project.

10K views 70 replies 15 participants last post by  Fredstaple 
#1 · (Edited)
Actually this is a them, as I'm building a pair. One to keep and one as a gift for a friend.

I'd never built a mortar before when I started my first golfball project, so I just looked at my piece of stock and said "It will be this big." Turns out it was seriously overbuilt, with a chamber that could almost put a GB into low earth orbit. But it's been a lot of fun.




Over the years and other mortar projects of varying calibers, I've come back around to the golfball. This time in a more compact package.








I've had this on the boards for awhile now (a long while), and I've had the steel blanks sawed for months. I didn't want to start a thread until I had actually done something.

A few days ago I went to the shop and turned and faced one end of each. Then went back this morning and bored and chambered the first tube. It will probably be next weekend before I can get back up there to bore the other one.






In the meantime I will start making handle parts and assembling my soldering fixture.

I bought 3/16" brass flat for the handles, the brass rod I already had.




I will be constructing a bed for mine, the other will be sent with all the instructions and measurements and he can take it from there.

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#5 · (Edited)
Time to start center punching brass (handle brackets) and aluminum (soldering fixture) for drilling . . .




My printer ran out of ink and of course Walmart was out of the one I needed. I ordered online, so that delayed this for a few days.

If push had come to shove I could have scribed this all out, but the printed labels are easier on the old eyes.

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#7 · (Edited)
Soldering fixture . . .





Mocked up with stacked brass washers (roughly the thickness of the brass flat) and a scrap piece of brass rod.


Basically a variation of the fixture I used on the poolball mortar, on a smaller scale. For size comparison, both bases were cut from the same 1/4" x 1-1/2" aluminum flat bar, the smaller one is about 1/4" shorter . . .



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#8 ·
Chopped up and ready to stack and shape, not closely since the sanding pad on my angle grinder would make quick work of the excess . . .




Uh, Houston we have a problem ... my angle grinder took a large and probably irreparable dump partway through the shaping.

I somewhat cleaned up what was close on the diamond hone but the radius around the 3/16" hole (which I was going to do last) is still pretty much untouched.

Stuck here until I can either repair the grinder (doubtful, fire and smoke from the front end is usually a death rattle) or get another one . . .

 
#10 ·
Turned out the grinder as not as bad as I thought, wire rubbed through and shorted out against the armature. Probably my fault as I think I left it too slack when I had to repair the cord awhile back. I was more careful reassembling it this time.

Back on track.

Here is the soldering fixture disassembled for a final cleaning and deburring . . .




And reassembled next to the fixture I used for the poolball mortar . . .




And finally mocked up with a couple of the brass handle brackets . . .




Next step, bend some round stock.

Hope to get the breech profiled on at least one (hopefully both) of the mortars this weekend.

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#11 · (Edited)
Fist attempt on a grungy piece was close but not quite right, tweaked my measurements a little and bent a clean piece and it's a keeper . . .




Now I need three more.


PS: The reason the test piece has a long and a short leg is that there is a minimum length you need sticking out in order for it work in my makeshift bending rig. But then you have to cut that excess off once it's bent, otherwise it'll hit and not let the second side go all the way to 90°. Cut to length it just barely clears. Since I'm not using this piece I didn't bother to cut the other leg to length.

If these handles had been any smaller I would be going back to the drawing board.

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#12 ·
Propane torch was almost empty, so I only soldered this one for now. That's okay for now, I just wanted one finished handle that I could hold in my hand.

I'll pick up another cylinder this weekend.

Quite a difference between these and the handles I made for the poolball mortar . . .

 
#14 · (Edited)
Thanks Zulu and all who have complimented!

Was pushed for time this morning so I just profiled the one tube.
I think it turned out okay . . .




I did stop by the hardware store on the way back and get propane so I can get the rest of the handles together before I lost any of the pieces (again) . . .

 
#16 · (Edited)
Now all of the brass is done . . .




The trunnion straps could use a little more polish but they're good for now. I'll probably open the holes up slightly to make them easier to get on & off when it's all done, but for now I want them on-size.

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#19 · (Edited)
The finish dimensions of the bed are 10.563" long x 5.125" wide x 3.063" tall (scale rounded to the nearest 1/16th).

My current plan is to slab it up from the same red oak stair tread that I've used on the others. I have a thing for red oak.


And redheads, but that's a whole 'nuther story . . .

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(fixed typo in dimensions)
 
#24 ·
Haven't really felt up to going into the shop for the past week or so, but I finally made it up there this morning and sweated the trunnions onto the mortar tubes. Giving it a quick cleanup with a spray bottle and brush here in this photo, just enough to get them home so I can work on them in the AC . . .

 
#25 ·
A little preliminary cleaning with a stiff wire brush and some sandpaper.
Clean enough to set out, but still a way to go before I'm happy with the finish . . .




PS: This is the 'gift' one, it has priority in all processes from here on out. Mine will get done as it gets done.

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#27 · (Edited)
Soldering. Cleaned and fluxed the groove and trunnion. Laid 3 strips (about 6" total) of 1/16" diameter 56% silver solder in the groove, set the trunnion on top of it, and heated with an oxy/acetylene torch until the solder melted and the trunnion settled into the groove. It isn't going anywhere.

OBTW: I weighed this one before I put things away for the night. As it sits it weighs 6 lbs 7.6 oz.
No significant metal removal left other than drilling the vent so that's close to the finished weight.

I plan to take the old GB mortar off its base tomorrow and weigh it for comparison.

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#29 ·
I did weigh the old GB mortar this morning. My kitchen scale has an 11 pound limit, but will actually weigh up to almost 11½ pounds before it errors out. Despite this, the mortar was still too heavy for the scale. So I put it on the bathroom scale. Not nearly as accurate as the kitchen scale but it would have to do.

According to the scale the original GB mortar weighs 12 pounds. Roughly 5½ pounds more than the new one.

I think I mentioned it was slightly overbuilt.

:tango_face_wink:
 
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