Making Cannon Breeches

Posted Tue, 02/16/2010 - 20:48 by Brian

This evening I made the breeches for Nassau's cannons. The breech is where rounds are fed from the magazine into the bottom of the cannon's barrels. This blog post will walk you through most of the process for making a breech on a mini-mill and mini-lathe. These steps have been simplified from what someone might do who wanted everything as precise as possible. I am more interested in making parts quickly - this means minimizing setup and maximizing the number of parts that can be made each time a measurement is taken.

Disclaimer

This worked for me - it might not work for you. I do not describe how to use these tools safely. For example, the pictures were taken with chip guards removed. I am not saying you should remove them. It is your responsibility to use your tools safely.

Copyright

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Part of Series:

This is one post of a series describing in some detail how I made the cannons for my battleship SMS Nassau. Here are links to the entire series (including this post):

Holes and Slots

Below is a chart of all of the holes and slots you will need to create using your mill.

Note About Coordinates

I find it easiest to treat the upper left corner of the part x=0 and y=0; with x increasing to the right and y increasing as you move forward (down). All my coordinates use this system. I also have a funny notation that I use for dimensions... I represent dimensions in terms of how many lead screw turns plus number of thousandths (rounded to nearest half thousandth). My mill's lead screws advance 0.050" each turn. So I often would represent 0.8618" as 17+12... for 17 turns of the lead screw plus 12 thousandths(0.0118 rounded to 0.012).

I do this because it provides all the information I need when working at the mill. I count off whole turns of the lead screw and then the number of thousandths over. I don't know if it is something weird in my brain that likes this... in case it will help anyone else I provide coordinates in both systems.

Id	Drill	X	Y	Depth
A1	#21 (10-32)	0.3800" (7+30)	1.2500" (25+0)	Through
A2	#21 (10-32)	1.6850" (33+35)	0.4966" (9+46.5)	Through
A3	#21 (10-32)	1.6850" (33+35)	2.0034" (40+3.5)	Through
B1	1/4" Drill	0.5107" (10+10.5)	0.8618" (17+12)	Through
B2	1/4" Drill	0.5107" (10+10.5)	1.6382" (32+38)	Through
C1	3/8" End Mill	0.5107" (10+10.5)	0.8618" (17+12)	0.1000"
C2	3/8" End Mill	0.5107" (10+10.5)	1.6382" (32+38)	0.1000"

Material Needed

The first thing to do is to cut a bunch of 2.5" square and 0.5" thick blanks of PVC. I recommend making the parts slightly trapezoidal - with the top left corner and bottom right corner sticking the farthest left and right. It is super important that the top and bottom edges are very strait and perpendicular to each other. I started with a 2.5" x 0.5" by 6' stick of PVC and cut off the number of 2.5" parts I need (this morning I needed five)

You will want to mark the top left hand corner on the top face of each blank. This will allow you to change parts up later.

Step A: Drill & Tab Assembly Screw Holes

The first step is to drill and tap the three holes that allow the manifold to be screwed to the breech. These holes will need to accept a 10-32 threaded bolt. So you will drill through with a #21 drill bit and then tap each hole. This step benefits from repeatability of setup (see below). so you can drill and tap each hole in each part befor moving to the next hole (A1, A2 and A3).

Repeatability of Setup & Double-Checking Position

My earlier blog post about making manifolds had some pointers on how to save time and effort, and to minimize mistakes. The processes I describe in that post are generally applicable here. I recommend reading that post.

Step B: Drill Barrels

The next step is very simple, just drill 1/4" holes for each barrel. This step benefits from repeatability of setup.

Step C: Drill Risers

Unlike most Big Guns cannons this design does not rely on pressed in risers. However, there needs to be a recess to align the barrel risers to the breech. This is done by boring out a 1/10" (0.1000") deep recess where the risers meet the breach. This step benefits from repeatability of setup. The following picture shows the two recesses around each barrel hole.

Step D: Drill Feeds

This is the hardest step in making these parts. You need to drill a 1/4" feed hole which will slope from the top of the breech (which will be the magazine in the assembled cannon) down to the barrel holes you drilled in step B. In order to do this easily and accurately it is very important that the top edge of your material is strait.

Repeatability of Setup - Almost

If you are very very very careful and accurate you might get some repeatability of setup for this. You probably will not for the X-axis position. Once you set the Y-axis position for each hole you can lock that axis. Once you know how far down you milled for the first part you can place the z-axis stop at that position. However, you will probably have to adjust the X-axis each time you change a part.

Mark Each Part

I am not going to explain the math but you need to mark a line on each part that is 1.9681" from the back edge of the barrel holes. This is the point that the feed holes exit the part. This line will allow you to quickly reset the X-axis when changing parts. Obviously you can not mark it this accurately. Just get it as close as you can easily. I placed 1/4" dowels in each barrel hole and used some digital calipers to measure.

This picture was taken from an eariler run of cannons then the rest of the pictures in this post. That is why the plastic is a different color.

Mount at Angle

With the mill's table positioned so that the Y-axis is correct for the barrel hole you will us a fifteen degree angle block to clamp the part.

Set X-Axis

Chuck up your 1/4" end mill and adjust the Z-Axis and X-Axis so that the closest cutting edge of the end mill is just barely in contact with the line you marked earlier. You can use the course feed to set the Z-axis.

This should only take you a few seconds for each part.

Mill/Drill Feed Hole

Ideally, you will have a 1/4" end mill that is long enough to bore all the way down to the barrel hole. Long end mills are expensive (mine cost $20) so you will probably not have one long enough. Don't despair. You can mill down as far as you can and then replace the end mill with a 1/4" drill bit. The end mill will give a nice flat surface to drill into.

Do not attempt to drill this hole without first milling a starter surface. The drill bit will wander enough to ruin the part. If you are going to make a lot of cannons I really recommend forking out the money for the longer two-flute end mill. It makes a lot nicer finish and is a lot faster.

Step E: Turn Parts

These parts mate up to the manifolds so they need to be round and 2.25" in diameter. Just like we did with the manifolds we will rough out each part on the band saw before turning on the lathe. I used a manifold to mark roughly how much to cut.