Eprouvette

[Terry C.]

Has anyone ever fired a golfball-sized eprouvette mortar?

I have a piece of steel that will soon be going into the lathe. Coehorn patterns are abundant, and that was my original intention. But I like to make things a little out-of-the-ordinary and the eprouvette is something you don't see everyday.

Unlike the plans for the original, which had a very small powder chamber, I plan to put in a chamber of 1" x 1¼" comparable to the Coehorn pattern GB mortars. This may be the sticking point as I'm not sure how the eprouvette with it's smallish flat base will behave with a hefty charge propelling a golfball.

My plans (still in the works) are basically reworked from the Coehorn that I had originally intended to build. Tube shape was changed slightly to more resemble the DuPont eprouvette, but bore and chamber dimensions were not altered.

The tube is 5¾" long x 3½" in diameter. The base is 5½" square at the bottom (for now, it may be reduced in size later) x 1" thick. The rounded end of the tube will sit in a hole in the base, and be welded from the bottom.

The material for the tube is a section of 100mm diameter chromed hydraulic cylinder rod, made of 1045 if I am correct (that's the most common material for these rams). It will clean up nicely at 3½".

The base will be cut from a piece of 1" x 8" cold-rolled flat bar.

Opinions, concerns, warnings, flames?

[Terry C.]

Here is another image showing the relationship of the bore and chamber to the barrel.

[lance]

if your just shooting golf balls, i've found that any charge over 100 grains is wasted powder. Beaufort Naval Armorers makes those eprouveete golf ball mortars, top of board where to find cannons. usually a bunch at auction on gunbroker.com

[Terry C.]

I know about the BNA and SBR eprouvettes.

The point is to design and make it myself.


And I have plenty of powder. 

[lance]

yeah, more powder is fun

[GGaskill]

As long as you restrict yourself to golf balls, the recoil will probably be too small to cause stability problems.  However, I would be sorely tempted to shoot 1 lb spherical sinkers at times and that could cause problems since the shot would be about ten times heavier.

Golf ball stats:
1.   Weight: The weight of the ball shall not be greater than 1.620 ounces avoirdupois (45.93 gm).
2.   Size: The diameter of the ball shall not be less than 1.680 inches (42.67 mm).

[intoodeep]

If you are only shooting  GB's you should be fine. However, if you plan to shoot something heavier you can tap holes in to the bottom to allow it to be bolted to a larger platform.  Or, you could make an removable base like the one below.

[Cat Whisperer]

Here's a hand-held one for comparison in size:



Material:  4140

Weight:  7 lbs

OD:  2.7"

ID:  1.690

Powder chamber: about 5/8 dia x 1/2" deep

Charge/range  30-60 gr fffg for 100-200 yards at about 45 degrees

Recoil about like shooting a 38 spcl

OVERBUILT  (that's good)

I like the idea of putting it on a base and the contours of these posted looks good too!

[Terry C.]

Well, something about the base was just 'not right' and I couldn't put my finger on it. Then I studied all of the images again and it finally hit me, the bases are not square.

I designed my base square (and a bit oversized it turns out), and it just never looked right. Reducing the base dimensions to 4½" x 5" at the bottom puts the whole thing better into proprtion with the images of the eprouvettes made by BNA ad SBR.

My eprouvette was never intended to be a dead-on copy, rather a simplified interpretation, but certain proportions are important. If the base looks wrong, the whole mortar looks wrong.


By my calculations, my powder chamber will hold about 200 grains of powder. Maybe that's too much, but that's about par with most of the Coehorn plans that I've seen posted here. I'd rather have reserve capacity than a too-small chamber.

By comparison, the original DuPunt plan (full-size) has a powder chamber that holds about 320 grains and it throws a 24 pound ball. Of course, being a testing instrument keeping the range short was to their advantage. The plan is sorely lacking in details, though, and the few dimensions it gives are barely readable.


A firing fixture like the one above will be a must-have accessory, I'm sure. That's why I incorporated the beveled base of the original into my plans. I probably will succumb to the temptation to fire heavier balls.

[Terry C.]

Here is the only drawing I've been able to find for an eprouvette.

[GGaskill]

In your side elevation, draw a centerline down the bore through the base.  You want a significant amount of base behind the centerline to prevent tipping over.

[intoodeep]

Terry C,

 Below is a photo of my trio. Let me know of you need any details from them. Two of them are GB size the other is a 2".

[CU_Cannon]

Here is a full scale drawing that I made a while back.  I used the same plan that was posted above.  I just never had the chance to build it. 

[Terry C.]

intoodeep, can you give me some idea of the chamber size in the GB bore eprouvettes? powder capacity?

If they are true to scale, based on the originals, then they must be tiny!

CU_Cannon, thanks for that image, I have it saved.


It's good news and bad news on this project.

First the bad news: I had hoped to have the machining on this project done by this weekend. Sunday is my birthday and I really wanted the tube to be, if not complete, at least to the point that it could be test-fired. First the work piled up, meaning that the lathe could not be tied up. Then it became obvious that some of the tooling I will need is missing. Gonna require some scavenging and horse-tradin' but I'm sure I can get my hands on what I'll need.

It's just not going to get done this weekend.

But the good news is: Instead of the one piece of stock I was expecting, I have two. Each chrome-plated ram is 100mm in diameter and a little over eight inches long. They aren't pitted nearly as bad as I was led to believe (one looks almost like new). Doesn't matter, the chrome is coming off anyway.

Best part is that they were FREE!

The 1045 is going to be slow to drill and bore but the test cuts I made yielded a really good finish.


If and when I do get the stock back in the lathe, I plan to drill and bore both pieces for golfballs. One will be a Coehorn, the other will be the eprouvette.

I already have steel to make the trunnion for the Coehorn, and the base for the eprouvette.

[intoodeep]

intoodeep, can you give me some idea of the chamber size in the GB bore eprouvettes? powder capacity?

If they are true to scale, based on the originals, then they must be tiny!

 They are pretty tiny. Mine measures out to be approx. 1/2"w x 5/8"d. Hope this helps.

[CU_Cannon]

1045 is pretty easy to work with.  I haven’t found much difference between 1045 and 1018. 

I believe the chambers were intentionally small.  The purpose of the eprouvette mortar was to test powder, not to achieve maximum range.  I guess you could tell a lot more about the power from a small charge vs. a large one. 

[m223]

Hi gents, I really like your projects thanks for sharing. Intodeep  that's pretty slick how you dove tailed your bases to fit the heavier mount. C.U. maybe for your birthday someone will by you a lathe. I feel your pain, I'm limited to using works equipment also. Nothing is ever where you can find it , lathes are left with chips all over them. Then you have a limited time to set up ,and work. Looks like we both need a lathe. Good luck with your project and HAPPY early BIRTHDAY!  Tracy

[intoodeep]

Intodeep  that's pretty slick how you dove tailed your bases to fit the heavier mount.

 Thanks, but, I must confess.  Jim B. @ BNA made the base. It works real smooth. It just slides in and out and really holds the Eprouvette in place.

[Terry C.]

If and when I do get the stock back in the lathe, I plan to drill and bore both pieces for golfballs. One will be a Coehorn, the other will be the eprouvette.

Looks like I was a little over-ambitious, the logistics just won't support my intentions. It's going to have to be one or the other for now. There's just no way I can justify enough lathe time to build two mortars at once.

So I've decided to put the eprouvette on hold until later, and concentrate on the Coehorn. In the meantime I can further evaluate where I want to go with the design as far as chamber size and base dimensions.

I've already started work on the Coehorn, and have some pics to post, so I'll be starting a new thread when I get them resized and uploaded.

We'll get back to this one later. Maybe after the boss gets to pop off a few rounds with the GB mortar, he'll be more inclined to give up the machinery...