Author Topic: GB Coehorn project (Read 6323 times)

Terry C. · « **on:** July 24, 2007, 01:28:11 PM »

As I said in the 'eprouvette' thread, I've decided to build the Coehorn mortar first. My mortar will be built using my own seat-of-the-pants plan, based on several other plans posted before. The basic dimensions of the tube are a length of 6" and a major diameter of 3½". Bore is golf-ball sized and the chamber is 1" x 1¼" with a hemispherical (as close as I can get) bottom.

Here is a view showing the proportions of my mortar:

The stock is a chrome-plated ram from the track adjustment cylinder for a Timberjack harvester. It is 100mm in diameter and a little over 8" long, so there was a lot of stock to remove. The material is (as far as I know) 1045. There is a hardened layer underneath the chrome about .050" deep, but once this is removed the stock machines easily.

Yesterday I managed to saw off the excess, drill and bore the stock, and turn the face. All of this without a steady rest (it was nowhere to be found) so it was a little hairy at times. I had to go slow and make sure that the chuck kept a good grip on the stock. Unfortunately, I didn't have my camera with me yesterday. I wasn't expecting to get it into the lathe until this morning.

I did take my camera with me today, so here it is before any further machining:

The piece next to it is the other (identical) ram, when the tube is fully machined I'll do another comparison shot to show how much metal was removed. At this point it's only been reduced in diameter enough to remove the chrome and case layer from all but the portion of the stock that was in the chuck when I was turning the back end flat. I had intended to let the chrome run in the steady rest when I turned it around for drilling, but as I said the rest has disappeared.

I tried to take a photo of the bore and chamber, but there wasn't much light down the hole. I need to put a flap wheel on my die grinder and polish out the area where the radius meets the outer wall. I didn't come out far enough with my homemade radius cutter. The lighting (or lack of) makes the steps look worse in this photo than they really are. I'll polish them out best I can but I don't think they are really hurting anything in this area.

You can't really see the inside of the chamber, but it came out well. There is a slight 'dimple' in the center from the pilot drill, but otherwise it has a nice, reasonably hemispherical shape.

The bore finished at 1.725". I was shooting for 1.730" but this is close enough. Another cut would have been too much:

So now it's back into the lathe.

At this point all of the chrome is gone. The tube started to lose some weight, A LOT of weight:

Lotta glare here, but that's because I was polishing out surfaces as I went. The tube is starting to take shape:

I got as far as cutting the next step to depth, working all the way back to the chuck jaws (no more pics, it was time to get out'a there). Tomorrow hopefully I'll find time to turn it around and radius the breech end. It'll be a freehand radius so it might not be perfect, but I have high hopes. If all goes well, the next post will be a fully turned tube.

Stay tuned...

m223 · « **Reply #1 on:** July 24, 2007, 03:10:03 PM »

I love lathes, your tube is shaping up nicely! I hope you find your steady rest, you probably wont need it when you flip your stock to do the radius. If you center drill the end you have in the chuck you can just flip it around and use a live center. The reinforcing ring on your mortar wont let you get a very good bite with the chuck. My friend at work found this out the hard way. Like I said it looks nice! Thanks for sharing. Tracy

lance · « **Reply #2 on:** July 24, 2007, 03:21:36 PM »

looking good Terry C. you know if this stuff keeps going on, golf balls might get expensive one day......

Rickk · « **Reply #3 on:** July 24, 2007, 03:24:44 PM »

9 cents apiece on ebay for me... stock up now while the getting is good :-)

m223 · « **Reply #4 on:** July 24, 2007, 03:35:16 PM »

I have 4700 bright yellow ones for sale. Oh wait no I don't, My bad.

lance · « **Reply #5 on:** July 24, 2007, 03:38:07 PM »

i knew Tracy [m223] was the great golf ball caper bandit...........

Terry C. · « **Reply #6 on:** July 24, 2007, 05:33:15 PM »

I shouldn't need the steady rest tomorrow, but I sure could have used it during the boring.

I would like to have it firing by this weekend but it depends on my workload.

I've got a couple dozen balls stashed somewhere around here, if I can find them. I've been planning this project for a LONG time.

m223 · « **Reply #7 on:** July 25, 2007, 01:28:25 AM »

Try not to get in to big of a rush, better to get it like you want it rather than hurrying it up at the last minute. I know its hard, I have to rein myself in all the time. Wish I would of had some restraint yesterday as I got in a hurry, and over looked a simple detail. Result, I ruined both my side boards for my golf ball mortar. I hate wood. No offense to those of you who have the talent and equipment to bring out its beauty. Tracy

Terry C. · « **Reply #8 on:** July 25, 2007, 11:34:04 AM »

Problem is, everything I do is by definition a "rush job."

I'm expected to get what I need done without it interfering with my workload or tying up shop resources.

Case in point: Today we were shorthanded and I had very little time to work on the mortar. Not even enough time to take a picture of the tube while it was in the lathe, it would have held me up and I wouldn't have finished this step.

I did get a couple of quick pics when it was done. Lots of glare from the sun but as I said before, I'm polishing out surfaces as I go. It'll save me a lot of work later.

The fully turned tube is dwarfed by the stock:

My freehanded radius came out a bit blunt on the end, but once I got it to this point I was happy to leave well enough alone. If I'd tried to 'improve' it I ran the risk of getting it all out of whack. I'm tickled that it came out this good:

I'm not sure if I'll get to the trunnion tomorrow, I may let it rest for a day. Working on it every spare minute in between paying jobs has run me ragged. But now that it's not competing for lathe time I'm not so rushed.

GGaskill · « **Reply #9 on:** July 25, 2007, 12:12:20 PM »

The breech end of a US Coehorn has a 6.5° taper on it which will help with your bluntness. It starts at the end of the fillet to the largest diameter. Also you can use a live center when working on the breech because the center hole goes where the trunnion eventually will be.

Rickk · « **Reply #10 on:** July 25, 2007, 12:17:02 PM »

Most of it will be burried in the wood, so it won't be a visual issue.

lance · « **Reply #11 on:** July 25, 2007, 03:51:01 PM »

looks good to me for a "rush job" like the old saying:want to get something done,give it to a busy man.

Terry C. · « **Reply #12 on:** July 26, 2007, 12:28:08 PM »

Well, I lied. Looks like tomorrow, Monday and the next several days after that are going to be killers.

Today, on the other hand, we were waiting around for parts to show up. So I took the time to fit and weld the trunnion to the tube, and drill the vent.

Not having a mill, cutting a recess for the trunnion was slow and nerve-racking. I started with an angle grinder, then finished with flap wheels in my die grinder. All the while checking that the trunnion sat level and centered.

Here is the tube, ready for the trunnion to be clamped into place:

Now the mortar is finally home!

Here a few quick shots. The mortar is hot and dirty and so am I, so these will have to do until we are both in a more presentable state:

A couple of changes were made on-the-fly. The minor diameter between the bands is smaller than the drawing (by about 1/16") because of a gouge in what should have been the final cut. Stuff happens. This discouraged me and I thought that the tube would not look right, but I like the way it looks.

I also made the trunnion 3/16" longer than the plan. Ive decided on building a sled base and the longer trunnion will allow me to use thicker wood.

Rickk · « **Reply #13 on:** July 26, 2007, 01:17:00 PM »

Unless you let someone near it with calipers, plans of an original, and a calculator, I suspect that no one will notice anything.

It look really good to me.

Black paint (epoxy paint works really good) or something else in mind?

EL Caz 66 · « **Reply #14 on:** July 26, 2007, 01:40:06 PM »

Not Bad! Not bad at all...

m223 · « **Reply #15 on:** July 26, 2007, 02:09:12 PM »

Terry, That looks great. It really is amazing how much work one can bang out during a lunch break, I know what you mean about getting frazzled though. Your radius turned out pretty darn good! There is definitely an art to doing it freehand, for me its the funnest part of the build. I wish I could as excited about the base I have to build. I think I might like doing a hand gonne , after all how hard is it to mess up a pole? I'll let you know when I do. Keep up the good work, Tracy

GGaskill · « **Reply #16 on:** July 26, 2007, 02:52:11 PM »

Since Coehorns were bronze, give RustOleum 7275 Burnished Brass a look. The Coehorn below is painted with it.

lance · « **Reply #17 on:** July 26, 2007, 04:40:53 PM »

Terry C, really like the way you tackled the trunnion groove. my kind of guy, i have a lathe but no mill. well i do have all those files, they used to be called milling machines over a hundred years ago.

Rickk · « **Reply #18 on:** July 26, 2007, 04:45:38 PM »

Terry,

Do you know about the two bosses that are usually down where the trunnion and the base connect (one on either side)?

Rick

GGaskill · « **Reply #19 on:** July 26, 2007, 04:52:25 PM »

Do you know about the two bosses that are usually down where the trunnion and the base connect (one on either side)?

Those are actually called rimbases, same as on howitzer and cannon barrels. There is a thread here someplace discussing adding them to fabricated Coehorns.

Terry C. · « **Reply #20 on:** July 27, 2007, 05:04:57 PM »

Yeah, I know about the rimbases. And I considered early on carving out a pair to weld to this mortar.

In the end I decided it would just complicate an already rushed build. I'm not ruling out adding them later, though.

Last night I got out a can of Fg and my digital scale. With a piece of brazing rod stuck down through the vent (to represent the fuse) I first filled the chamber as full as I could get it, shaking the mortar to settle the powder. With the chamber level full it held 246 grains.

With a 200 grain charge dumped in the chamber (no shaking this time) there was enough room to seat a thick cardboard or cork wad over the charge. Then I filled the chamber with just enough powder to cover the rod (fuse). It took 26 grains to completely cover the rod.

What do you think of this shape for the sled base? It's based (loosely) on the drawing of the cast iron sled below it. The sled as it's shown here would be 12" long:

lance · « **Reply #21 on:** July 27, 2007, 05:24:25 PM »

i like the shape, you going steel plate? or wood? either would look great.

Rickk · « **Reply #22 on:** July 27, 2007, 06:51:17 PM »

I'm about to do a sled that is roughly the same thing.

The one issue I need to deal with is limiting the max angle, as that is generally required in formal competition. I am trying to figure out how to limit the max angle, and also shield the fuse hole (also required) and still make it look something like the sketches posted.

I can do it with an "add on" piece... I will go that route unless someone else has a more permenant and non-ubtrusive better idea

Rick

GGaskill · « **Reply #23 on:** July 27, 2007, 07:13:30 PM »

The one issue I need to deal with is limiting the max angle, as that is generally required in formal competition.

Depending on how far away from a perfect model you are willing to get, you could drill one side of the trunnion for a pin, then mill a slot in the capsquare that limits rearward travel of the barrel. It would be pretty unobtrusive if done right.

You could even put the pin on the bottom side of the trunnion with the slot in the sled piece although you probably should use a crosspin through the sled as a stop.

Or you could locate the pin such that it bears on the underside of the capsquare at maximum elevation.

Double D · « **Reply #24 on:** July 27, 2007, 07:50:58 PM »

Quote from: Rickk on July 27, 2007, 06:51:17 PM

I'm about to do a sled that is roughly the same thing.

The one issue I need to deal with is limiting the max angle, as that is generally required in formal competition. I am trying to figure out how to limit the max angle, and also shield the fuse hole (also required) and still make it look something like the sketches posted.

I can do it with an "add on" piece... I will go that route unless someone else has a more permenant and non-ubtrusive better idea

Rick

First make the barrel scale to the base. Terry's base i slightly over size in relation ship to the base. This is something he can adjust in his cad program if he wanted to reduce the mount elevation avaialble.

The 1844 did not have trunnion caps. If you are shooting some where that requires restriction of max angle they are most likely are going to require trunnion caps also. So you can get tight fitting trinnion caps. Might be nice to see the actual wording of the rule to get som clues as to what they are trying to achieve...my guess barrel flip.

The 1844 also has a brass cross bar that the Trunnion sit in and this should control max elevation also.

Fuse hole shield is pretty simple

GGaskill · « **Reply #25 on:** July 27, 2007, 08:58:25 PM »

Since there seems to be a great interest in the sled style mortars, I have redrawn an 1844 ten inch mortar scaled for golf balls. Click the image for a clear version. By the way, the thickness of the lifting lug is .509".

m223 · « **Reply #26 on:** July 28, 2007, 05:50:51 AM »

Terry, Like D.D. said you may want to scale the sled back just a little bit, Other than that I think that base will look great, and probably more authentic than the one I am piecing together. I really wanted to do something like that with metal side plates, but the set up and operation time on the mill proved to be prohibitive. Either way you do it I'm sure it will be fine. Best regards Tracy

Terry C. · « **Reply #27 on:** July 28, 2007, 08:55:12 AM »

Quote

Terry's base i slightly over size in relation ship to the base.

If this supposed to read "slightly over size in relation ship to the barrel" then yes it does look a little large.

I came about these dimensions by scaling the the overall length of the barrels. I tried to go by diameter but the base came out tiny! I doubt I'll get perfect scale on two such different barrel shapes.

This base is slightly smaller than the version sized to the scale of the barrel length, as I cut it back to an even 12". It looks larger than an identical-sized image of the original, until you overlay the two. Part of this is an optical illusion created by the fact that it is one unbroken shape with no details, and part of it is due to the fact that it does not have the stepped bottom of the original, it is full-length all the way down to the ground line.

If you think it looks grossly oversized then I may scale it down a bit, but I'd rather have it a little large for stability. And it's a lot easier to cut it down than make it bigger.

Quote

I am trying to figure out how to limit the max angle, and also shield the fuse hole

What is the max angle allowed? I hadn't considered shooting this mortar in competition, but I don't want to totally rule it out. Are there any hard and fast rules on this, or is it something that would be at the discretion of the rangemaster?

Does a vent shield have to be physically attached to the sled? Or can it be something that is held in place by it's shape or weight?

Rickk · « **Reply #28 on:** July 28, 2007, 11:29:33 AM »

From the American Artillery Association rules:

"Mortar beds shall permanently restrict barrels from raising above 75 degrees from the horizontal"
"Vent shield, fixed or weighted to remain in place, must be used on all pieces"

On a solid base the barrel is generally fixed. On the sled type, it isn't. I have seen rope used... I guess that is a possibility. I was also thinking about a bolt-on piece that would block the barrel from to rear to prevent the angle from exceeding 75 degrees and also incorporate a vent shield. It would be removable... I'm not sure what their interpretation of "permanent" is. I would think that nothing is "permanent" given a cutoff wheel and some time.

http://www.americanartillery.org/aaa/mrt.html

Rickk · « **Reply #29 on:** July 29, 2007, 03:49:16 AM »

It occured to me last night that a small metal tang could be welded oto the bottom and a bracket of some sort can go underneath the barrel with a stop on it that will catch the tang at 75 degrees. That would be farily unobtrusive and the bracket would add strength the the carriage at the same time.

On your coehorn the tang would see quite a bit of stress due to the fact that the trunnion is right at the bottom... leverage and all. Mine is a dictator style barrel, so the trunnions are closer to the center of the length of the barrel. The forces on the tang would be less.