No-Lathe Mortar... Part 2 - PICS OF THE FINISHED PRODUCT ON P2

[StealthyBlagga]

OK, so you guys convinced me that the no-lathe DOM golfball mortar concept is not a good one.

Now I'm thinking of scaling down to a 0.75" bore (shooting musket balls), which I should be able to drill out of 2" barstock using my drill press. To close the breech, I am thinking of using a 1/2" bolt threaded into the back end of the barrel - I figure that interupted screw breeches work OK for modern artillery, so an uninterupted screw should work here. If the bolt were the right length, it would leave space for a powder chamber.  I'm thinking about a 3" barrel length, and I'll weld the trunion bar directly to the bolt (will help unscrew it for cleaning too).

What do you guys think of this new concept ?

[GGaskill]

If you are going to use barstock, why not simply drill a 1/2" pilot hole leaving at least a half inch wall at the rear, then enlarge the forward part of the hole to 3/4" for the bore.  No need to bother with the screw.  And simply weld the trunnion to the rear of the barrel with a good fillet.

And you could safely have a 1" bore since the chamber qualifies for the "wall thickness = powder chamber diameter or greater" rule.

[Cat Whisperer]

Doing a powder chamber at the bottom is an excellent idea.  (One looses about half the strength by adding threads.)

Keep PLENTY of material behind it if you're going to weld - to keep the heat-affected area out of where the stresses are.

[StealthyBlagga]

If you are going to use barstock, why not simply drill a 1/2" pilot hole leaving at least a half inch wall at the rear, then enlarge the forward part of the hole to 3/4" for the bore.  No need to bother with the screw.  And simply weld the trunnion to the rear of the barrel with a good fillet.

And you could safely have a 1" bore since the chamber qualifies for the "wall thickness = powder chamber diameter or greater" rule.

Thanks for the quick feedback guys. I was thinking of doing as you suggest, but wasn't sure if it would be better to have a removable breech plug for cleaning purposes. I guess most cannon and mortars are cleaned from the muzzle only, so your suggestion makes more sense. The reason I am thinking of .75" caliber is simply easy availability of projectile (musket balls).

Two other questions:

1) What would be the minimum, maximum and ideal barrel length you would recommend for such a mortar barrel ? Of that, how thick should I leave the breech if I'm welding the trunnion in place ?
2) What size would I drill the touch hole ? I would prefer to use cannon fuse (something I can light and retreat from)... is this viable on such a small model ?

[Rickk]

1/8 inch fuse is the most common, but 2.2 mm and 1/16 are available as well.

I have a .50 caliber CVA cannon that snugly takes a 1/8 inch fuse. The hole is only slightly bigger than .125. There is a chamfer on the exterior to make it easier to get the fuse started. I know that 3/16 is a more traditional sized hole for 1/8 fuse, but you may be better off with the smaller hole considering your bore size.

If you make it something like 17/64, you can use 1/16", 2.2 mm, and still be able to squeeze 1/8" in there in a pinch.

As far as a removable breech verses solid bored breech... the removable breech is where alll the junk is going to accumulate. If there is no threaded or welded breech, you will have less places for the crud to hide and cleaning will probably be easier.

[GGaskill]

1) What would be the minimum, maximum and ideal barrel length you would recommend for such a mortar barrel ?

There are three general categories of muzzle loading artillery.  Mortars have bore lengths of about twice the shot diameter (two calibers), howitzer bores are 6 to 7 calibers deep, and guns are 12 to 15 calibers deep.  Your choice.  For a 3/4" bore, I personally would lean toward a howitzer unless you would be satisfied with a relatively small piece.  But I have a lathe.

Of that, how thick should I leave the breech if I'm welding the trunnion in place ?

The rear of the breech should be at least as thick as the breech walls.  Just don't get real heavy on the heat input while welding.  Make a pass and let it cool before making the next one.

2) What size would I drill the touch hole ? I would prefer to use cannon fuse (something I can light and retreat from)... is this viable on such a small model ?

For 1/8" fuse, I use a .136" drill; others will suggest other sizes.  You want the fuse remnants to come out easily but you also want to retain as much pressure in the bore as you can.  Smaller fuse would be better than larger.

[Double D]

Two rules of thumb apply.

The walls of the breech should be no less than one caliber thick in all directions.

The diameter of the fuse or vent hole should be  1 1/2 calibers.

[StealthyBlagga]

The diameter of the fuse or vent hole should be  1 1/2 calibers.

I'm not sure I understand this part - please could you elaborate ?

[jeeper1]

1/8th diameter fuse= 3/16ths fuse hole.

[Double D]

Two rules of thumb apply.

The walls of the breech should be no less than one caliber thick in all directions.

The diameter of the fuse or vent hole should be  1 1/2 calibers.

If your fuse is  1/8 inch in diameter then its caliber is  1/8th inch.  1 1/2 calibers is  1/8 plus 1/16th (half of 1/8)  or 3/16th inch.

The navy had a gun that they used on board their ships for many  years called the 3 inch  50 cal.  That meant the barrel had a 3 inch bore and was 50 calibers long or 150 inches long.  (3 inches times 50 = 150 inches long)

If you want to make a cannon for a 4.5 diameter ball the breech walls have to be one caliber thick or 4.5 inches thick  or the breech has to be 13.5 inches in diameter.

[StealthyBlagga]

OK, so now the touch hole size is clear. Any suggestion about powder chamber depth (given the 1/2" diameter) ?

[Double D]

Yes look up the maximum load for a 1/2 inch boreon Switlik's chart and drill a hole deep enough to hold that much 1FG.

[StealthyBlagga]

Yes look up the maximum load for a 1/2 inch boreon Switlik's chart and drill a hole deep enough to hold that much 1FG.

Thanks DD. For my proposed chamber diameter of 0.5" (1.25cm), Switlik maximum is 40grains (2.6grams). Assuming a black powder density of 1.04grams/cc, I need a chamber capacity of 2.5cc... this suggests a chamber depth of 2cm, or 0.8". Does this sound about right ? [ the calculation I used: volume = pi * (radius * radius) * depth. This works out as 2.5cc = 3.14159 * (0.625cm * 0.625cm) * 2cm ]

In light of everyone's input above, and in the hope I can get good performance with 1/16" cannon fuse, I have revised my barrel design as follows:



Let  me know if you have any last-minute comments before I start cutting metal. Thanks everyone for holding my hand through this.

[jeeper1]

Personal opinion, the trunnion is too short and should be the same diameter as the bore.

[dominick]

If you're going to use a drill press to drill the hole you may want to tack-weld  [heavy tacks] the 2" round to a good size plate and then clamp the plate to the drill press table.  This will lock the piece to the table better than a vise and produce a smoother bore.  Run the drill speed as slow as the drill press will allow and use plenty of oil.  Just a suggestion.   Dom   

[StealthyBlagga]

If you're going to use a drill press to drill the hole you may want to tack-weld  [heavy tacks] the 2" round to a good size plate and then clamp the plate to the drill press table.  This will lock the piece to the table better than a vise and produce a smoother bore.  Run the drill speed as slow as the drill press will allow and use plenty of oil.  Just a suggestion.   Dom   

Great minds think alike  ... exactly what I was planning to do. Thanks for the validation.

[Cat Whisperer]

A barrel 2 calibers long (inside length of twice the diameter of the bore) is traditional for a mortar.

This will give you good flash, smoke and bang.

A longer bore, 3 or 4 calibers, will give you greater efficiency, less smoke and bang, and greater range - perhaps more range than you want.

For example, a golf-ball mortar of 2 calibers will go 100-200 yards with 30-60 gr of ffg or fffg.  A ONE caliber long mortar of the same bore diameter will get maximum range out to 12 feet with about the same altitude.

[StealthyBlagga]

OK, so I started making the barrel for this mortar. Using a drill press, I have created the powder chamber and the bore - no problems. However, the bore is a bit rougher than I'd like; it looks like a brushed metal finish, with fine machine marks around the inner circumferance of the bore. Nothing unusual given the way I drilled it out, but I'm wondering how smooth it needs to be ? I have tried spinning a slotted mandrel with "fine" emery cloth, but the marks are still there. Should I keep going to a mirror finish, or can the design tolerate some minor roughness ? If it matters,  the bore is 0.75" and I plan to shoot 0.73" musket balls (which will hopefully be a loose fit). What do you guys think ?

[GGaskill]

I drill my small bores, then ream them to final size.  Even that is not a mirror smooth finish.  It you want a really smooth finish, try some flex-hones.

[jeeper1]

A rough bore gives plenty of places for corrosion to start. A smoother bore is better.

[Cat Whisperer]

A rough bore gives plenty of places for corrosion to start. A smoother bore is better.

Agree.  More surface area and better pockets for corrosive elements to get stuck.

BUT, regarding pressure, you have windage.  No problems there.

[StealthyBlagga]

BUT, regarding pressure, you have windage.  No problems there.

Thanks - thats what I needed to know. I don't think I will shoot it so much, so cleaning/corrosion are less of a concern than are "making a bomb". I did not want to keep in spinning the emery cloth to the point where I distort the bore and cause other problems.

[StealthyBlagga]

OK, after all you input, here is the final design (with the thicker trunnion, which I agree is more appropriate):



The barrel is drilled (chamber, bore and touch hole) and I am in the process of welding on the trunnion. I'll post pics when the project is completed.

[Rickk]

I'll be tossing pieces of white oak into my firewood pile during my carronade construction probably bigger than your base will be, so if you need a piece of 3 inch thick anything, lemme know. If I have a scrap piece of suitable size I'll send it to ya.

Rick

[Cat Whisperer]

StealthyBlagga -

I would round the bottom corner (inside corner) of the powder chamber - the sharp corner will cause stresses to be concentrated at the corner.  Rounding the corner spreads the stresses out.  You could easily do the rounding by gently grinding the edge from sharp corner to a radius of the drill used for the powder chamber.

[StealthyBlagga]

StealthyBlagga -

I would round the bottom corner (inside corner) of the powder chamber - the sharp corner will cause stresses to be concentrated at the corner.  Rounding the corner spreads the stresses out.  You could easily do the rounding by gently grinding the edge from sharp corner to a radius of the drill used for the powder chamber.

Thanks CW. Unfortunately, the barrel is already drilled and the trunnion welded on. so drilling the powder chamber deeper now will be a real PITA. Do you think my design, with the >90 degree angles, is a safety hazard ? Would I be OK just staying well below the Switlik maximum ? I'd really prefer not to mess with the bore now, but I certainly don't want to be unsafe either.

[GGaskill]

Your primary problem now is holding the finished mortar in the drill press.  It would be easy to run a ball nose 1/2" end mill down there and round out the bottom.  It wouldn't need to go more than a tenth of an inch deeper to have a full hemispheric bottom.  You would still have plenty of material at the bottom.

In fact, if you like, you can send it to me and I will do the above for only the cost of return shipping.

[Cat Whisperer]

It's a golf-ball mortar.  Golf balls are light, hence pressures are light.  Lead/zinc/iron spheres are another story.

SO, rounding the inside corners is insurance by making it stronger.

But even so, George has made a GENEROUS offer - I'd take him up on it!

[StealthyBlagga]

Alright, alright, you old grandmothers  ... I bit the bullet and radiused the bottom of the powder chamber as CW recommended. Thanks for saving me from my own laziness. I'll post some pics of the barrel pre-paint so you can see how crappy my welding is.

[GGaskill]

I'll post some pics of the barrel pre-paint so you can see how crappy my welding is.

No need to spare the grinder.