CAD, CAM, and CNC: a Bolt Action Pen

I am a sophomore at Cambridge Ridge and Latin High School in Cambridge, Massachusetts.

Being really into EDC and knowing that there was CNC machines at my school, I decided that it would be pretty cool to design and eventually machine my very own bolt-action pen. This project introduced me to CNC machines and bridged the gap between our older mills and fusion360.

Supplies

Materials

• Small bolt (6-32)
• Ink cartridge (I used the fisher space pen cartridge)
• Small spring
• 0.5in x 5in rod stock (aluminum or brass)
• 0.25in x 1in rod stock (aluminum or brass)

Machines

• CNC/conventional mill
• CNC/conventional lathe

Tools

• OD turning & facing tools
• small tap (same threading as bolt)
• ID drill for thread
• aircraft drill for long hole (7/32in x 6in long)
• small end mill for slot (3/16 in)
• medium end mill for reference surface (3/8 in)
• small drill for pen tip hole (#39)
• v block to hold pen while machining slot
• caliper for measuring dimensions
• edge finder for locating work coordinate system (WCS)
• (optional) knurling tool for pen grip

Software

• Fusion 360 CAD & CAM

Step 1: Attaining Inspiration

You should start by getting inspiration from other bolt action pens already on the market. I found the Boker Plus cal .50 to be particularly interesting because of it's one piece construction. I later made the chamber a similar design to allow for more flexibility during manufacturing and general ease of construction.

Step 2: Designing the Chamber

Start by importing the components, in my case that's going to be a fisher space pen cartridge and a 6-32 bolt. Follow along with the pictures above!

1. Create a cylinder centered about the axis (I went with 0.4in, personal preference) and create a long hole through one side for the piston, and a shorter one slightly larger for the pen tip to go through. (I made a variety of test holes in a spare piece of aluminum to test the fit with the ten tip, and ended up finding that 0.098 in worked best)(I made the large hole 7/32 in, didn't want any friction between the refill and chamber)
2. Find the distance that the ink has to travel to stick out a comfortable amount (I found it to be 0.3in)
3. Make two circles equal or greater to the size of your small end mill (3/16in), make the distance between those circles a little more than the stick-out distance you previously found (I added 0.5mm in my original metric design, which translates to about 0.02 in), and make a rectangle with sides tangent to the circle that extends to the side of the pen. Extrude this sketch through the pen to get half a slot.
4. Rotate the part 90 degrees and use the same two circles as before, but now separate them by the distance you added to the last one (again, about 0.02in). Now repeat the extrude that you had done before, selecting this 2d shape and extruding it through the pen, leaving you with a full slot!
5. Add a chamfer to facilitate writing, and you are done designing the chamber!!!

Step 3: Designing the Piston

Similarly to the previous step, Follow along with the pictures above! ;)

1. Make a cylinder the size of the large hole, spanning the distance between the back of the chamber and ink cartridge in its contracted position.
2. Add a vertical tapped hole to this cylinder in such a way that when the bolt (6-32) is in the thread, it is tangent (in contact) to the top of the slot of the chamber. Make sure the hole is deep enough to fit your bolt, but not too deep that it digs into the chamber.

You've finished the design! Congrats!!!

Step 4: Technical Drawing (Optional)

If you only have access to manual/DRO machines, are doing part of the process manually, or handing the process of manufacturing off to a friend, you are going to want to create a technical drawing. I do not yet feel that I am at a point where I know enough to explain how it works, so if you want to make your own you should watch this short video or learn from Autodesk in the first unit named "blueprint basics" of their course "Introduction to CAD, CAM, and Practical CNC Machining for Milling". If you want to skip the whole design process and get straight to machining, use the technical drawing I put together below.

Step 5: CAM Toolpaths

Feeds, Speeds, WCS, Machines, Post-Processing, and tooling are going to depend on what you have access to. Remember to follow along with the pictures above!

Chamber

• OP1 This is where we define the stock as 0.5in x 5in. (I drilled this hole with a 7/32in, 6in long drill bit) However, I decided to drill this hole manually with the tailstock but still programmed it so that it would show in-process stock and the progress.
• OP2 In the setup, be sure to click "continue rest machining" to continue machining the same stock. Similarly to the last operation, I decided to drill this hole manually with the tailstock but programmed it as a #39 drill bit
• OP3 Now, rather than machining the slot, we must machine a small flat surface completely across the part with a 3/8in endmill to act as a reference surface when we clamp it in the vise along with the v-block. I went down 0.02in. When clamping OP4 & OP5, you'll have this flat side against one of the v-block's edges, and then rotate it 90 degrees to have the flat side tangent to the other flat side (perpendicular to the first one). It may be easier to do this manually on a Bridgeport, but do as you like.
• OP4 For the first half of the slot, we are going to be using the 2d "slot" toolpath to cut away the top of the slot. I used a 3/16 flat end mill, as well as "multiple depths" to clear away chips and leave a cleaner final product (reduces chatter).
• OP5 While it may not be the case for you, Fusion360 seemed to refuse the contours I was trying to machine, so I instead opted to use the 2d "trace" operation with sideways compensation set to left. (This second half uses the same 3/16 flat end mill.)
• OP6 Op 6 consists of 3 different toolpaths; facing, roughing, and finishing. I did all of OP6 & OP7 with a right-hand, diamond-shaped, general turning tool. I started with the facing, then proceeded to do the roughing pass (back is set to 1/4in offset from chuck front), and a finishing pass with the same parameters as the roughing, just slower and less depth of cut (including back being set to 1/4in offset from chuck front).
• OP7 Similarly to the previous operation, Op 7 consists of 3 different toolpaths; facing, roughing, and finishing. Starting with the facing, I used "multiple passes" to remove the excess material left in front of the pen, then proceeded to do a roughing pass (back is set to 1/4in offset from chuck front), and a finishing pass with the same parameters as the roughing, just slower and less depth of cut (including back being set to 1/4in offset from chuck front).
• (Optional) While not as easy to program in Fusion 360, if you have access to a knurling tool, it leaves a great finish and makes the pen a lot more functional.

Piston

• OP1 This is where we define the stock as 0.25in x 1in. Using the same turning tool I had used for the chamber's OP6 & OP7, I started by doing a small finishing pass (back is set as 0.01in from the front of the vise), and a facing operation to bring it to dimension.
• OP2 Similarly to the previous operation, OP2 consists of doing a small finishing pass and another facing operation to bring the cylinder to its final dimension.
• OP3 The final operation, OP3 consists of a drill operation and a tapping cycle. Rather than having to browse through Fusion360's tool libraries, I simply used the "hole recognition" operation which did almost all of the work by itself.

We have finished the programming, Congrats!

Step 6: Machining

I machined this pen out of brass and aluminum using a prototrak dpm2 CNC mill and 1660sx CNC lathe at school. Enjoy some pictures of the machining process!

Step 7: Conclusion

While still a work in progress, I think that these pens turned out sweet! This project has taught me so much about machining and design, and the education that I received goes way beyond anything that I could have ever hoped for.

Thanks for reading through to the end, if you have any questions feel free to reach out!