Tennis Ball Mortar Bore and Powder Chamber Dimensions

[MKlein]

Does anyone have a scaled drawing of a 24lb coehorn for firing non concrete filled tennis balls. I think the bore diameter is 2 9/16.

 

[Double D]

Does anyone have a scaled drawing of a 24lb coehorn for firing non concrete filled tennis balls. I think the bore diameter is 2 9/16.

Did you look in the stickies? Tennis ball dimensions are there.

Dimension of a Tennis ball.
...the acceptable range for ball diameter (2.575 inches to 2.700 inches respectively for a Type 2 ball). The accuracy of calibration is 1/-0.005 inches.  http://www.itftennis.com/technical/research/lab/balls/size.asp


Order the Coehorn plans from AOP and scale down.  The tennis ball is close to pop can size.   Tennis balls filled with cement at are poor projectiles in my experience.  You can get some popcan/tennis ball size zinc projectiles form our group pour in the classifieds .  A bit expensive but can be used over and over agai,  They have a much longer life than pop cans or tennis balls,

Good project.

[MKlein]

Great place to get plans, I agree. Here is where I got the idea.
 

 
And lots of of great info on the Group buy-group pour topic.

[KABAR2]

The grenade launcher barrels are also tennis ball sized........

[Zulu]

The grenade launcher barrels are also tennis ball sized........

Kabar,
That's always been one of my favorites.  When are you going to do something with it?
Zulu

[KABAR2]

The grenade launcher barrels are also tennis ball sized........

Kabar,
That's always been one of my favorites.  When are you going to do something with it?
Zulu

Some time next year after I get moved...... Don't ask.....

[MKlein]

 
Ok, maybe they can try that powder chamber calculator on this.

[GGaskill]

I was thinking about a cylindrical chamber for the calculator but I will calculate yours by hand if we can get all the dimensions consistent.  Using the 8° number gives different dimensions than you specify on the other view so we need to sort that out first.

[MKlein]

Using the 8° number gives different dimensions than you specify on the other view so we need to sort that out first.

6° is on external taper and it came out to 8° on the powder chamber

It is still in design phase and any input would be great. I read that a lot of can mortars have 1.25 x 1.0 powder chamber but coehorns seem to keep the gomer style pretty consistent.
 
Tennis ball shouldn't kill any cows if they dart accross the field.

[GGaskill]

OK, for a first cut, I am going to simplify the chamber shape to a plain sharp cornered frustum of a cone, 1.125" deep, 1.25" large diameter and 1" small diameter.  The conical frustum calculator gives a volume of 1.123 in³ which is 18.403 cm³.  Using Lee's bulk density equation of V/.0719 = weight in grains of Fg black powder, we get 18.403/.0719=255.95 grains.

Neither of your chambers are exactly that size but 256 grains is certainly within 10%, I would say.  Probably the actual charge weight would be a little less than that.

[MKlein]

Thanks Mr Gaskill! I had to make a few dimensional changes because I noticed Double D pointing out in another post that someone didn't have over chamber dimension thick enough. I made a few changes and now I will try that fancy formula. They say if you don't use your head to think you will loose it.

[GGaskill]

I think you will find that chamber too large if all you shoot is tennis balls.  I shoot cement filled beer cans and zinc round balls from a mortar with a cylindrical chamber that is 1" X 1" to excessive ranges so you don't really need a chamber as large as yours unless you want to try noise making (but a mortar is not the best thing for noise making.)

[Double D]

I think you will find that chamber too large if all you shoot is tennis balls.  I shoot cement filled beer cans and zinc round balls from a mortar with a cylindrical chamber that is 1" X 1" to excessive ranges so you don't really need a chamber as large as yours unless you want to try noise making (but a mortar is not the best thing for noise making.)

As I am reminded periodically by the club president...   

[MKlein]

Here is my try at that fancy calculator.

OK, for a first cut, I am going to simplify the chamber shape to a plain sharp cornered frustum of a cone, 1.236" deep, 1.125" large diameter and .912" small diameter.  The conical frustum calculator gives a volume of 1.012 in³ which is 16.584 cm³ by using Volume Conversion Calculator Using Lee's bulk density equation of V/.0719 = weight in grains of Fg black powder, we get 16.584/.0719=230.654 grains. It would be a little less due to the .25 radius in the bottom. I might change my design so I can use a 7/8 2 flute ballnose endmill for the bottom of the powder chamber
 

[MKlein]

More cyfering fun   Might have to carve it out of wood
 

[MKlein]

OK, first I am going to divide the chamber into two different shapes.
1- Spherical Cap Shape, Volume= .20541 in³ as per Spherical Cap Calculator
2- Conical Frustum,  Volume= .50204 in³ as per  conical frustum calculator

Add volume of shapes together to get .7074 in³ then convert to 11.593 cm³ using the Volume Conversion Calculator 
Using Lee's bulk density equation of V/.0719 = weight in grains of Fg black powder, we get 11.593/.0719= 161.238 grains.
 
Dang I need a Beer now! My head hurts!

[MKlein]

Updates on 20121007 post

[GGaskill]

That should be fine although I am not sure the taper buys you anything.  I think I would just drill 1" and finish the bottom with a 1" ball nose end mill.  But that's me.

[MKlein]

   New Updated Version                                                                       Version 20130113                        The Full Plan Set is Free 

[MKlein]

Worked on this a bit, the patterns worked out great, perfecto fit!
 

 
More pics
http://www.flickr.com/photos/87356288@N06/sets/72157632778803415/

[JBAZ]

That is beautiful work!