Uncommon Press

Like some of my previous posts, this project was another senior project at my university. The team was tasked with designing and building a recreation of a 18th century wooden printing press. Most of the wood and iron parts were done at a local living museum. Only the most critical metal components were done in our shop. At the core of this press is a screw that generates the pressing force as you pull on the handle. When these presses were built in the 18th century, screw cutting lathes were not a thing, and as such the screw would have been made using a mix of files, saws and chisels. While the team wanted as faithful a recreation of a press as possible, we drew the line at trying to file threads by hand. The mating brass nut would have been cast in place around the finished screw (similar to babbit bearings), and the team originally wanted to do this.

The finished press on opening day

At first this didn't pose a huge issue. While the central screw was exotic, we didn't think it was impossible. The thread is a 2 and 5/16 inch major diameter, 3/16 square thread form, 3 start, 2 inch lead. The most unique part was the very large lead. Originally one of the full time machinists intended to do this thread on our larger ProtoTrak lathe, but we quickly found out between minimum spindle speed and maximum carriage federate, it would be impossible. At this point everyone else in the shop dusted their hands and backed away from the part. I was the only one who was willing to give it a shot it one of our newer machines.

Finished spindle and nut

Close up of the thread

Down the barrel of the finished nut

We had just received delivery of a demo machine, a Tsugami dual spindle y-axis lathe. I ended up coding everything for both the thread and the nut by hand. The external thread was very straightforward to program, but the threading tool gave me a lot of trouble. The idea was to use inserted acme threading tools reground for a square profile but the tools kept cracking off due to how much relief for the large thread lead. After some trial and error (and snapping 12mm cobalt tool blanks in half on the machine during testing) i found a tool geometry that worked. I ran the external thread for both the final part, as well as a shorter, lighter version for me to use as a thread gauge.

Raw stock next to my "thread gauge"

Lucky for me all of these parts were bespoke, so I only had to make one of each nut and thread and they only had to fit each other. With the external thread complete, I moved on to the internal thread. To this day the brass stock for this nut was the most expensive piece of material I have worked with; 600USD (with half of the cost being in overnight shipping). Unlike the external thread where I could cut the entire thread form with a single tool, the internal thread would require a tool 1/2 inch wide, which would cause chatter no matter how rigid the lathe is. I decided make each internal grove with 5 separate passes with a thin tool on the end of a boring bar. The tool was a smaller version of the external threading tool bolted into the end of a large and stubby boring bar that I also made.

Testing thread contact pattern by smoking the thread

Again, I manually wrote the program for the internal threads, mostly because I had no idea how I would achieve my desired approach in CAM. This meant for making one set of internal threads, there was 15 independent threading cycles that had to run. Now, because I decided to tweak the nut to fit the screw, I would have to change each of those 15 threading cycles to change the fit. To eliminate this, I parameterized the code. The z thread length and major diameter are both driven by variables at the beginning of the program, so I could easily tweak the program at the machine by changing a single value; not 15 values. I chose to parameterize the z thread length because as I was testing the program, I didn't want to use a ton of material.

Snippet of parameterized code. The numbers in the comments are the z starting locations of each pass

The strange thread wasn't the only odd feature on the screw. It also included a tapered, rectangular hole. We dont have and EDM capacity at our shop and the team didn't have money to have it sent out. Again, I stepped up the challenge and said I would do it. I pre-drilled to remove most of the material before welding our main band saw's blade into the hole. I first squared out the hole with the band saw, before angling the saw table and band filing a 1 degree taper on all walls. The fact that this technique left a rough finish on the walls was beneficial to the design team, who didn't want perfect faces and surface finishes. On many of the pieces we went back with files and sand paper to roughen up the parts to make them look more handmade.

The 1 degree tapered square hole

The job was a lot of fun overall and the printing press is now on display at my university. I grew to like that Tsugami lathe but sadly its been moved out to be replaced with some other machines. I also did some miscellaneous parts and welding for the press, one part can be seen below.

A 5 part series of articles about the building of this press can be found here. 

Machined oil pan