K.I.S.S. bowling ball mortar

[Double D]

Nag, nag, nag...man you guys are a tough crowd...I have mentioned now and again I am a putting together the material to build a bowling ball mortar.   I keep getting emails and PM's saying hurry up and start the build. We want to see it.  Events of the past year have put me off pace a bit.

Some years ago when we had the golf ball mortar making contest  I called my entry the K.I.S.S. mortar---Keep It Simple Stupid!  I followed that theme all the way through the build.  The design was to keep the machining to the minimum-simple. Simple square turning, boring, and drilling on the lathe and the rest drill and tap in the drill press.   Of course my drawing followed the K.I.S.S. principal and was done up on a dry erase board.




Last year I upscaled the design and made the  K.I.S.S. the can popcan mortar.  It is not the neatest cannon I own---my Dom Dictator by far is the neatest.  But this  little K.I.S.S. the can popcan mortar is one of the most fun cannons I own.  It's small enough to easily carry around and set up. yet delivers a large projectile down range.

K.I.S.S. Golfball Mortar



The K.I.S.S. the can popcan mortar  is just an up sized K.I.S.S. Golf ball mortar.



Like most of us the idea of a bowling ball mortar intrigues me.  I had it in the back of my mind to build one for a long while.  I have been keeping my eye out for a pressure vessel, but like most of use can't seem to locate the  right one.

I'm not sure when I came up with the idea to use seamless tubing--perhaps during the Technical Conference-braai-barbecue after CBCCI.  The discussion at the time centered  around getting seamless to build the K.I.S.S. the can popcan mortar. 

After he returned home from CBCCI GeorgeG went on a scrounging mission for me and he found the material for the  K.I.S.S. the can popcan mortar.  George mentioned while at the scrap yard he saw some large seamless tubing that would make into a bowling ball mortar..  That really got the wheels spinning. 

In the fall of 2010 after some thought I had George acquire the  large steel for  me .   Friend Southpaw and his wife were headed south Snowbirding for the winter.  They connected up with George in southern Cal  and load up the tube that weighed some where around 150 lbs and the trunnion piece  that weighed some where around 110 lbs in the back of their Volkswagen station wagon.   Ernie was pulling the VW behind his motorhome  and he said every bump he hit made the VW drag bottom.  When he got home with the steel, he sold the VW and the motor home burned up---hey not my fault. 

So here is the  bowling ball steel collection.

 

I have already made a modification to the plan and changed the trunnion to 3 3/4 inch...George to the rescue again and the UPS lady delivered a 65 lb piece of steel to me yesterday.

I do not have a lathe big enough to handle this project and I will have to contract the work out.  That means I am going to have  provide working drawings of the project for who ever does this work for me.  That need was certainly part of the motivation to return to college this past fall and take CAD classes.  I have made 5 drawings so far.  I still need to make the  detail drawing for Assembly and the vent detail drawing.  Since I am going to do the vent and assembly myself those drawings are less  a priority to me.  I do want to have a complete  set to post here for guys.

Here is what I have so far.













I still have to take my final exams next week and drag the travel trailer home Havre.  After that I will look around for someone ito do the work fore me.

Since I am depending on others to do the work for me, I have no control over when the work gets done. I also have to budget  this work...there are limits to how far the monthly Social Security check goes.   Work will be done one piece at a time.

I haven't even started looking at a base. The plan is to make a British traveling carriage for an 8 inch mortar.

The goal is to have this done in time for CBCCIII.

I will  document this project as I go along as best I can.

[GGaskill]

I would be happier with a light press fit, maybe .002" overlap, which isn't much for large pieces like that, unless you want to be able to disassemble it.

[Double D]

I would be happier with a light press fit, maybe .002" overlap, which isn't much for large pieces like that, unless you want to be able to disassemble it.

Press fit isn't Simple in pieces this big and is simply not needed.  The breech plug is held in the bore by the trunnion which bolts on to the bottom of the breech  plug pulling the plug into the seat in the barrel.  Remember in the mortar, the breech is the cannon. The barrel is just an expansion chamber.  The goal isn't hold the breech plug in the barrel.  The breech plug is held by the trunnion.  The goal is find a way to attach the barrel to the trunnion and breech-simply.

 The idea is also to be able to disassemble the mortar periodically for inspection.

In the K.I.S.S. system the only measurement that comes close to being critical is the bore diameter which must be large enough to fit the projectile.  The lower portion of the  barrel does not have to be machined at  all.  The  breech plug is made to slip fit down barrel.   The MiF concept (Make-it-Fit) is the key part of the K.I.S.S.  The upper part of the breech plug is a few thousands under bore size.    The lower portion of the breech plug is not critical and can be machined smaller to fit through the  the lower portion the barrel that is not machined  and may not be concentric.

I know this MiF concept is like fingernails on a chalk board to you Engineer, mass production, CNC types and sure wouldn't work in your environment...but its how the Millwright-Job shop guys work every day.  Study the work of the famous gunsmith John  Browning and you will find that is how he worked.

[Cat Whisperer]

You've come a long way since P&P-01.

WHen I did essentially the same design indepenantly when we were doing that, I adjusted the length of the powder chamber and the location of the trunnion such that it would draw the powder chamber tightly into the barrel for a rigid lock-up.  Not sure that it was needed nor if it improved anything; but it MIGHT improve the seal a LITTLE bit.

I think also that THE critical thing that you included here is the radius inside the powder chamber - to lower stress risers.  We see that all the time in repetitive flexing of motor shafts - it just takes a little extra to provide much more strength.  A twisted off motor shaft is nothing to the result of cracks in the powder chamber.

[Double D]

You've come a long way since P&P-01.

WHen I did essentially the same design indepenantly when we were doing that, I adjusted the length of the powder chamber and the location of the trunnion such that it would draw the powder chamber tightly into the barrel for a rigid lock-up.  Not sure that it was needed nor if it improved anything; but it MIGHT improve the seal a LITTLE bit.

That is exactly how the design works, the lower portion of the breech plug is a few thousands shorter than the related section in the barrel.

I think also that THE critical thing that you included here is the radius inside the powder chamber - to lower stress risers.  We see that all the time in repetitive flexing of motor shafts - it just takes a little extra to provide much more strength.  A twisted off motor shaft is nothing to the result of cracks in the powder chamber.

The call out for a stress riser is an Engineering/Production/CNC feature.  A Millwright or Job shop would put the radius there as part of practice.  A radius is non critical but required and is identified in the chamber detail drawing.

[subdjoe]

((pulls on armor, then fireproof suit))

Hey!  You could get a job building for Mythbusters!!

((dashes off and dives into bombproof))

[Double D]

((pulls on armor, then fireproof suit))

Hey!  You could get a job building for Mythbusters!!

((dashes off and dives into bombproof))

Well they could certainly use the help...but they are  you know, what you call experts.

[smokemjoe]

  Thats one heck of a trunnion , I wish I had that back ground to shoot my BB mortars into, Your not to bad of a looking old boy. getter done,   Joe

[Double D]

The trunnion in the picture is not what I will be using.  That is the 5 inch piece.  I have down sized to 3.74 inch. 

Those are the Sweetgrass Hills as seen from Southpaws shop.  We have shot at them but haven't hit them...yet!

And, I am much uglier in person.

[seacoastartillery]

     I really hate to admit it, but I am one of those folks who have been nagging you to get off the dime and POST IT!  Well, you have an excellent beginning, Douglas.  Mike and I are both very interested to see how this progresses.  We won't give you too much grief if it takes quite a while to get the machine shop work done.  We are very familiar with the cost and schedule of going large.  The Paixhans Monster Mortar was quite an experience.  While making it was fun, we will not be doing another any time soon.

     I looked at your excellent and clearly done drawings.  I would just mention a second, critical fit, dimensioned area, besides the bore dia.   You and Cat Whisperer have already brought it to light, but it's important enough for a third shot.  The "cinch-up" of the forward edge of the trunnion and the bottom edge of the tube, which keeps the whole assy. tight, depends on making sure that the length of the smaller dia. of the breech plug is less than the length of the smaller I.D. of the tube.  If all surfaces including the ends of both are to be carefully machined, then a delta of .005"  (4.70"- 4.695"= .005") is probably O.K. in a perfect world.  Because the world is not perfect, machining practice demands tolerance for every dimension, so you may want to allow more of a length difference there.  .020" or even .030" would not be excessive, especially if the tube end or the breech plug end or both are to be used "as is".

    When you tighten those big trunnion to breech plug bolts up, shake the assy violently.  If there are no rattles, you have a good 'lock- up'!  Plus, there is an added BONUS!  "Misfires",  where your errant bowling ball goes through houses, barns, equipment sheds or elk herds are much less likely when you are firing your cannon balls from a NO-Rattle Mortar!!   

Tracy

[dominick]

Douglas,  Have you considered milling a flat on the trunnion where it bolts to the plug?  Dom

[Double D]

Thanks Tracy for the comments, even in K.I.S.S Design, ends are faced.

When the mortar is ready for assembly I will let you know, so you can come up and "shake it " for me.  You can also bring your 3/4 inch Allen key

[Double D]

Douglas,  Have you considered milling a flat on the trunnion where it bolts to the plug?  Dom

Yes I did in the original design, but I decided it was contrary to the K.I.S.S. principle and unneeded.  I thought about it again when I made the K.I.S.S. the can mortar and came to the same conclusion--not  K.I.S.S. and not needed.  The Trunnion is there to hold the breech plug in anchored against the seat in the.  The trunnion doesn't actually contact the breech  plug at all, only the barrel.  If you milled a flat it would increase  surface contact, but to what end.  The is nothing to be gained.

[smokemjoe]

Douglas,  Have you considered milling a flat on the trunnion where it bolts to the plug?  Dom

Doug,   Thats what I done when I did my 2 BB mortars, It sure helps to kept everthing square.
   If we can shoot at that distant why Ill be there in no time. Ill bring all the balls,+ 170
   Joe

[Cat Whisperer]

Douglas,  Have you considered milling a flat on the trunnion where it bolts to the plug?  Dom

Doug,   Thats what I done when I did my 2 BB mortars, It sure helps to kept everthing square.
   If we can shoot at that distant why Ill be there in no time. Ill bring all the balls,+ 170
   Joe

I've done that on several mortars I've made too.  It makes for a large surface area of contact rather than a line of contact.  That spreads the forces instead of concentrating them.

[Double D]

Douglas,  Have you considered milling a flat on the trunnion where it bolts to the plug?  Dom

Doug,   Thats what I done when I did my 2 BB mortars, It sure helps to kept everthing square.
   If we can shoot at that distant why Ill be there in no time. Ill bring all the balls,+ 170
   Joe

I've done that on several mortars I've made too.  It makes for a large surface area of contact rather than a line of contact.  That spreads the forces instead of concentrating them.

But in this case are there forces that need dispersed?

The trunnion is bolted tight against the bottom barrel so the barrel  and trunnion act as one.  The  recoil forces are mitigated by the 350 lb more less weight of the gun and the full length  3.75" diameter trunnion seated in the trunnion recess.  Where the trunnion seats in the base,  is an energy transmission point. 

The point of contact where the trunnion  contacts the barrel is narrow, but it is not flat narrow against a flat but a round narrow surface against the flat.  The resistance to the force is radial not linear. The force if anything is compressive.

And don't forget there are two 1 inch cap screws to absorb energy also. 

[Cat Whisperer]

The forces from firing the round are axial (to the bore).  If you ignore vibration the only thing to worry about are the forces concentrated along the line of contact will deform the metal a bit over time.  With vibration and minimal contact between the trunnion and the bottom of the tube and powder chamber you'll see transient forces on the bolts.  Two choices in increasing the contact area - milling a flat on the trunion or a round groove in the base of the tube/powder chamber.

Is it critical?  Good question.  I spend a lot of time "over building" thnigs (in manufacturing) so that they will last through years of PRODUCTION abuse.  Loosly speaking I have two levels of quality - engineering quality (it does the job) and production quality (it does the job for a long time under repeated use/abuse).

[Double D]

First let me say I am not unconcerned about the issue brought up in this discussion.   I am indeed concerned.  I just do not believe the that forces at play here are significant enough  to warrant the  extra machining step.

We had this same discussion when the orignal K.I.S.S.  was made.  And, yes there is a  big different between a bowling ball mortar and a golf ball mortar. There has been no displacement  observed in  4 of 6 golf ball mortars and one pop mortar I have made...yet

I also do understand the  theroy of the mechanical advantage of a wider foot print.  The theroy is one thing, but there is  nothing presented yet, beyond wider is better, to show that the  footprint of this design is inadequate. 

I can assure you it is something I will monitor.  If one of you engineer types know how to compute the energy factor tell me what numbers you need. Please show me I am wrong now before I get started.

With my limited knowledge best I can do is the recoil factors for the barrel.

Barrel weight 350 lb 
Projectile weight  16 lbs (112000 grn)
Charge  7000 grns
Velocity 350 fps

Recoil velocity 17.5 fps
Recoil Energy  1664 ft lbs
Momentum 6125 ft lbs

Source http://kwk.us/recoil.html



 

[smokemjoe]

   DD : Your have enought steel size and wt. to be safe. With 7,000 grs, almost 1 lb. of powder, What range  do you think you can put the ball out to, Thanks- Joe

[Double D]

The goal it to shoot in Mile bowling ball shoot.

[Cat Whisperer]

...
I also do understand the  theroy of the mechanical advantage of a wider foot print.  The theroy is one thing, but there is  nothing presented yet, beyond wider is better, to show that the  footprint of this design is inadequate. 
...

What I've shared I may not know how to quantify - it is however what I've learned from experience from being a tooling engineer working with 50 to 120 ton presses that repeat the same operation all day long all week long all year long.  Numbers and formulas give you a starting point - experience shows you where to beef it up. 

I see two potential failure areas here.  One is very well addressed - the thickness of material around the powder chamber and the inside radius - ignore either and someday down the road you will have cracks and potentially kaboom.  The other may or may not ever occur - I certainly couldn't afford to shoot it enough (at 1 lb or even 1/4 lb of powder at a shot) to where a bolt would give way.  And it certainly would be much less risky for those standing around than the powder chamber coming unglued.

[Double D]

What I've shared I may not know how to quantify - it is however what I've learned from experience from being a tooling engineer working with 50 to 120 ton presses that repeat the same operation all day long all week long all year long.  Numbers and formulas give you a starting point - experience shows you where to beef it up. 

I see two potential failure areas here.  One is very well addressed - the thickness of material around the powder chamber and the inside radius - ignore either and someday down the road you will have cracks and potentially kaboom.  The other may or may not ever occur - I certainly couldn't afford to shoot it enough (at 1 lb or even 1/4 lb of powder at a shot) to where a bolt would give way.  And it certainly would be much less risky for those standing around than the powder chamber coming unglued.

The stress riser issue is a concern of mine also and is addressed in the drawing.

Tim do you think the bolt is the weak point?

For the rest of you, don't get me wrong, I am questioning Tim's opinion.  But I respect his experience and superior knowledge and I am listening to what he says. You should also.

[Cat Whisperer]

One looses one half the strength when the shaft is threaded (tensile strength).

The threads are more or less sharp, certainly the place where the bolt will crack or shear (two different failure modes).

Pinning the assembly handles the radial (to the bore) stresses from vibration, leaving the major function of the bolts as holding the parts together.

Putting the trunnion through the assembly is a strong alternative (as I did on my golf-ball mortar).
It puts large areas of trunion and barrel/powder chamber in contact spreading the forces.  (I offset the holes slightly to draw the two together when the trunnion was inserted and more when the setscrew pressing on the trunnion from the bottom of the powder chamber piece pressed it towards the muzzle.  (Over built - golf ball mortar is both low pressure and torn apart after every firing.)

Milling a somewhat shallow groove in the bottom of the powder chamber/barrel will lock the two together from forces/vibration radial to the bore axis (except in the directions axial to the trunnion). 

Milling a flat on the trunnion provides a large area - increasing the stiction (resistance to initial movement - radial to the bore) and decreasing the forces per unit area (axial to the bore). 
With a flat area of contact there is more strength resisting side-to-side movement of the trunion (perpendicular to the trunion).  (Same concept as using large washer around a bolt to spread out the forces.)

So, in a word, yes the bolts are the weak link because of weak support at the points where it is weakest.

[Zulu]

DD,
Do you have drawings for your "British traveling carriage"?  What size wheels will it take?  I'm not familiar with it.
Zulu

[Double D]

British Traveling carriage.

[Zulu]

DD,
 It was pretty hard to scale on the screen but those wheels look bigger than 4' diameter.  Are you going that big?
Zulu

[Double D]

They are 49 inches.

[GGaskill]

First, as the title says, I dropped out of engineering school to play my small part in Lyndon Johnson's little exercise in Southeast Asia and when I finished with that, I went into computer programming for a living rather than continuing with engineering school.  So keep that in mind when reading the following.  However, I think it will be generally correct even if lacking in some of the details.

It is my understanding that the KISS mortar (henceforth, mortar) will be assembled with the trunnion bearing on the tube and with 1" bolts pulling the breech plug (henceforth, plug) against the internal lip but not in contact with the trunnion.  What follows is based on that assumption.  Also, one critical piece of information is lacking; you guessed it, chamber pressure.  I am going to assume that the chamber pressure will be 20,000 psi and that that will fall to around 3000 psi as it expands out of the chamber area and will fall even more as the bowling ball moves up the tube.  I am also going to round off numbers since we are estimating the chamber pressure.

The cross-sectional area of the chamber is 9.62 sq in (3.5² / 4 * pi) so the force it would see to the rear would be 9.62 * 20000 = 192K lbs (!!!).  The cross-sectional area of the bore is 60.75 sq in (8.795² / 4 * pi) so the force to the rear a little later in the firing cycle will be 60.75 * 3000 = 182K lbs (interesting they are similar.)  As the ball moves forward and the powder gasses expand and leak out through the windage, the pressure will go down and likewise the force so we need not be concerned about other points in the cycle.

The internal lip is .3" wide and its area is 7.94 sq in.  Since the trunnion is not bearing on the plug, the lip will transfer the force from the plug to the tube.  The stress here would be 192000 / 7.94 = 24200 psi which is well within the elastic range of the material.

The width of the tube at the point it contacts the trunnion is 1.25" (10.75 - 8.25 = 2.5, 2.5 / 2 = 1.25) but the other dimension of the contact point is theoretically zero (because a line has no width) so the area is zero.  Now in the real world, since everything is flexible to some degree, the trunnion and the tube wall will distort until there is enough area to resist the force applied.  Let's again assume that the steel has a compressive strength of 60000 psi; therefore it will need approximately 3.2 sq in to resist the back thrust.  Dividing by 2 because of two sides, that is 1.6 sq in on each side.  Dividing that by the 1.25" width of the tube wall gives 1.28" of width for the contact points.  It would take .113" of compression for the trunnion contact to be 1.28" wide.  That is probably enough to cause plastic (permanent) deformation.

Now we can put a flat on the trunnion to give us that much contact to start with (.125" depth would give us 1.346" of width) or we can make the plug bear on the trunnion and relieve the tube from carrying the back thrust, although a little flat would still be desireable.

In the spirit of KISS, you could shim either the plug or the tube for the tightness to pull the pieces together while making the plug bear on the trunnion.  I would probably go for full contact between trunnion and plug and shim the tube for tightness but I suppose you could millwright the tube edges with a file to get good contact.  Full contact between plug and trunnion seems highly desireable to me.

OK, so much for that; remember the title.  If any PE wishes to challenge any part of this analysis, feel free to do so.  And remember, the numbers are based on the chamber pressure assumptions we made at the beginning so they may not be absolutely correct although the general idea is pretty good.

[Cat Whisperer]

George -  I agree.

[Double D]

Okay George, what does that all mean....is my design going to fail?