Making Cannon Manifolds

This morning I did all the mill work for Nassau's cannon manifolds. The manifold is basically the bottom of the part of the cannon that rotates. It has two purposes (a) it redirects the CO2 to the barrels and (b) provides a way to adjust how the shot sits in the breach. This blog post will walk you through most of the process for making a manifold 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 werer taken with chip guards removed. I am not saying you should remove them. It is your responsibility to use your tools safely.

Copyright

Copyright 2010 by Brian K. Alexander, Jr. All rights are reserved.

No one has permission to copy the information contained here. Please feel free to link to this page but do copy its contents or embed it into your site.

 

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):

• Improving Accuracy of Mini-Lathe Tail-Stock
• Making Cannon Manifolds
• Making Cannon Breeches
• Making Accumulator End Caps
• Making Accumulator Bodies
• Making Ball-Valve Housings
• Making Parts for Ball-Valve Push Rod Assemblies

 

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	#9	0.3800" (7+30)	1.2500" (25+0)	Through
A2	#9	1.6850" (33+35)	0.4966" (9+46.5)	Through
A3	#9	1.6850" (33+35)	2.0034" (40+3.5)	Through
B1	#43 (4-40)	0.5107" (10+10.5)	0.8618" (17+12)	Through
B2	#43 (4-40)	0.5107" (10+10.5)	1.6382" (32+38)	Through
C1	0.25" End Mill	0.5107" (10+10.5) to 1.2500" (25+0)	0.8618" (17+12)	0.2000"
C2	0.25" End Mill	0.5107" (10+10.5) to 1.2500" (25+0)	0.8618" (17+12)	0.2000"
C3	0.25" End Mill	1.2500" (25+0)	1.2500" (25+0)	Through
D1	1" Forstner	1.2500" (25+0)	1.2500" (25+0)	0.2000"
E1	1-3/8" Forstner	1.2500" (see notes)	1.2500" (see notes)	~0.3000" (see notes)

 

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: Drilling Assembly Screw Holes

The first step is to drill the through holes for the three screws that will attach the manifold to the breaches (A1, A2 and A3).

Repeatable of Setup

If you look at the pictures you will notice that I have a scrap of PVC bolted to the side of my vice. This provides a firm stop for the top left corner (the one you marked). If you made the parts as I described earlier (See 'Materials Needed' above) then you can swap parts out without loosing position. This allows you to carefully move to each location and then drill each of the parts at that location. Trust me, this saves a lot of time when you have to make a lot of parts.

This will allow you to move to the position of A1 and then drill that hole in each of the parts. You can then move to A2 and drill that hole in all of the parts. (and so on)

Double Check Position

If you have a finished part then you can use it to double check your position. This is very important when you are making lots of parts at one time as one mistake will ruin all of the parts. In the picture below I have placed bolts in A1 and A2 to double check that I am correctly positioned at A3. This process will not correct small errors but it will correct if you skip the count on the lead screw (which is the most common mistake I make)

 

Step B: Drilling Riser Pin Holes

The next step is to drill the holes for the breach riser pins. (B1 and B2). Later these holes will be tapped with a 4-40 thread. These holes also benefit from the repeatability of setup. So you can drill each hole in all of the parts before moving to the next hole.

 

Step C: Mill Slots and Center Hole

The next step is to mill the two shallow (0.2" deep) slots and a through hole in the center of each manifold. These are all done using a 2 flute 0.25" square end mill. I use the z-axis fine feed knob to get the depth correct on the first part. I then use the z-axis travel stop to 'mark' that position. This allows me to simply use the z-axis course feed handle for the remaining parts. This allows these slots and the hole to also benefit from the repeatability of setup. So you can make each slot or hole in all of the parts before moving to the next slot/hole.

The center hole is needed to realign the mill in Step E (below).

I recommend drilling the through hole (C3) last and using the x-axis and y-axis lock levers to fix the table at the center (1.250",1.250") position. This is the same position you will use for the entire next step.

 

Step D: Bore Out CO² Chamber

In this step you will use a 1" forstner bit to bore out the hollow area that allows the CO² to reach the slots you milled in the previous steps. As was done in the previous step you can use the z-axis fine feed knob to get the depth correct on the first part. I then use the z-axis travel stop to 'mark' that position. This will allow you to simply use the z-axis course feed handle for the remaining parts. This gives you the benefit of the repeatability of setup so you can bore out all of the parts before moving on to the next step.

Few Tips About Forstner Bits

These bits will get very hot while cutting. It does not take long for them to get hot enough to melt the PVC. This will produce toxic fumes and result in a very poor finish and probably bind up the machine. So you will want to cut slow and allow the bit to cool down regularly.

I also strongly recommend that you lock the table in place with the y-axis and x-axis lock levers before cutting with the forstner bit.

 

Step E: Bore Out Race for Bearing

The next step is hard to explain. You want to use a 1-3/8" forstner bit to bore out what has been the bottom of the parts until now. Obviously you will need to flip them over. However, you don't want to re-zero everything an re-measure... (at least I didn't).

Quickly Reset Position

This is where we take advantage of the strait top edge of the parts. If you flip the parts over but leave the top edge as the top edge (basically flip the left and right edges) then you know that the y-axis position has not changed. Great, so we don't need to worry about the y-axis.

If you flip the stop we bolted to the vice (see 'Repeatability of Setup' above) to the other side of the vice then we still get the exact same benefit we did before we flipped the parts over. So once we get the x-axis set correctly for one part it will be correct for all of the parts. Basically, you are moving the stop so that the same corner of the parts can still contact it (the corner you marked earlier)

In the previous step we milled a 0.25" through hole in the center of the parts. You can slowly move the x-axis until that hole lines up with the 0.25" end mill. This very quickly lets you reset the x-axis to account for flipping the parts over. Once you know the new location you can lock the x-axis again.

Cutting Race

Now you can use a 1-3/8" forstner bit to bore out the hollow area that the rotation bearing will sit in. As was done in the previous steps you can use the z-axis fine feed knob to get the depth correct on the first part. I then use the z-axis travel stop to 'mark' that position. This will allow you to simply use the z-axis course feed handle for the remaining parts.

How Deep To Cut

This cut should create a through hole in the center of the part. So you are basically boring down to the point where you reach the air chamber you bored out in Step D (above). Unfortunately 0.5" stock is usually more than 0.5" thick. So you will probably have to bore down 0.3500". If you bore slowly on the first part you should hear a distinct pop as the forstner bit cuts far enough to break through the PVC to the hollow. You can set your z-axis lock lever at this point.

Optional: If Recessing Snap Ring

If you are going to recess the snap ring (Step G below) then you will need to add an additional 0.0250" to how deep you cut the race. This means you will generally need to cut down 0.3750".

 

Step F: Turn Parts

The outside of the manifold needs to be made round and 2.25" in diameter. There are a lot of ways you can do this: cutting and sanding, using a rotary table on the mill, or using a lathe are the most obvious to me. As I own a small lathe I opted to turn the parts on the lathe.

Before turning the parts it will save you some effort if you cut off some of the material on a band saw. You are not trying to get the size correct - just trying to round the part enough to have an easier time on the lathe. Otherwise you will spend a lot of time turning down the corners.

Little Details About Turning

If you are using a 7x10, 7x12 or 7x14 lathe then it probably came with a small 3" three jaw chuck. The jaws for this chuck will fit great inside the bearing race. Additionally, the jaws will then also be narrow enough to not be hit by the cutting tool when turning down to 2.25" (as seen in picture above right).

Discussing cutting tool angles is asking for arguments. So, I am not going to claim what I use is best. However, it works well for me. On PVC I usually use side and reliefs on the side and front of seven degrees. I have no (zero degrees - perfectly flat) top relief. I find that on parts this size setting the lathe to 200 RPM creates a finish almost as smooth as uncut surfaces from the factory (very nice)

 

Step F: Recess Bearing Snap Ring (optional)

I like the assembly screws to sit flush with the bottom of the manifold. The assembly screws press the bearing to the manifold. So for the screws to be flush the bearing's snap ring needs to be able to sit below the surface of the manifold. You can do this by cutting a 0.0500" recess around the bearing race. Getting the width is easy - go until you clip the assembly screw holes (A1, A2 and A3)

Remember: If you want to do this then you need to make certain you cut the bearing race a little deeper in Step E.

 

Finished!

All that remains is to clean out any flashing and to tap the riser pin holes (B1 and B2) with a 4-40 tap. Here is a picture of what you should have when finished: