Part of my day job involves managing the glassblowing studio at Ringling College of Art & Design and I have been wanting to make something for the hot shop with my CNC. After some thought and a little research, I settled on making some hot glass stamps. I found out they’re called prunts and they’re usually used to stamp designs into blobs of hot glass that have been applied to a vessel or other object.

Raspberry designs stamped onto a vessel.

Design

I wanted to start out by making a traditional prunt design, so I chose a raspberry motif like the one shown above. I saw a lot of these in my research, and it also seemed pretty simple to make from a modeling standpoint. I modeled up the design in Fusion 360, by creating a pattern of overlapping spheres cut out of the top of a cylinder. As a part of the overall design, I’m going to attach the prunt to a handle with a bit of threaded rod. Additionally, I can use the threaded hole for holding the stock in the mill.

Preparing My Stock

I cut off some hunks of 1/2” diameter 360 brass that I got at the local metal supplier. Despite my best efforts, they didn’t come out square. Before milling them, I needed at least one face square to the rod’s length. If I had a metal lathe, that would’ve been a super easy operation, but I don’t have one (yet). I figured that if I could get close to having a flat face, I could face the other side on the mill, flip it, and then face it again and have parallel, mostly square faces. I do have access to a wood lathe, so I chucked up my blank and tried to do a quick-n-dirty facing operation. Surprisingly, it worked! While I was at the lathe, I also put a dimple in the face to mark the center for drilling later.

"Facing" on the wood lathe.

Back at the CNC mill, I put the lathed side down and and held it in place with a generous bead of hot glue around the base. Once I had faced the top, I flipped it and re-faced the bottom. With two fresh, clean, parallel faces, I drilled and tapped a hole in the side I had dimpled earlier.

Freshly faced.

Workholding

Since I plan on attaching the prunt to a handle with a length of threaded rod, I decided to use the threaded hole on the back for workholding as well. I created a fixture for this using some 1/4” aluminum and mounted the blank to the fixture with a flathead screw. I needed to add a few washers underneath the stock because I didn’t have a shorter screw. I then bolted the whole thing to the T-slot bed of my Bantam Tools mill.

All buckled in and ready to go.

Software Setup

Screenshot

Setting up the fixture in the control program was a bit of a challenge. In the Bantam Tools software, it is only possible to set up stock in reference to the spoilboard or the fixturing bracket. Because I’m using neither of these in my setup, I had to calculate the stock location as if the spoilboard was there. It’s just some arithmetic, but it would be handy to be able to set an arbitrary origin. Also, for some reason that I still have not figured out, my math for the X axis would not add up, so I had to eyeball it. Had this been a job that needed more precision, I’m not sure what I would need to do to calculate the stock position.

CAM

I used an adaptive clearing tool path with a 1/8” ball mill to remove the bulk of the material, leaving 0.1mm of radial and axial stock. Following that was a spiral tool path for finishing using a 1/16” ball mill. Total machining time was less than ten minutes—which felt a little silly after all the preparation I had done—but hey, that’s machining in a nutshell.

I’ll probably do a longer post on this further down the road, but I’ve been experimenting with doing full-depth adaptive tool paths in Fusion 360 and having a lot of success. The feed and speed recommendations provided on the Bantam tools website don’t get into ideal tool engagement for full-depth cutting, they just give a standard depth of cut. I’d like to treat that more fully in a future post.

Milling and Finishing Touches

Like I said, this job took less than ten minutes and it was done. Though the feeds and speeds from my tool libraries were technically successful, they were also a uncomfortably aggressive. I wish I had turned it down a notch and gone with a slower feed rate. Nevertheless, I was super impressed and happy with how it turned out!

Almost there!

I gave the brass a quick scrub with a Scotchbrite pad, turned a handle out of a piece of cherry from the scrap bin, and assembled the prunt. I’m not gonna lie, I think it looks fantastic and I can’t wait to stamp raspberries all over some hot glass. I have a few ideas for other, less traditional, prunt designs that I’m going to make, and I feel like I’ve got a good idea what to expect and how to improve that process when I do.

Finished.

As always, thanks for reading and please contact me below if you have any questions or comments.