Every now and then someone floats the idea of building a fly press. The problem always seems to come back to making an appropriate lead screw and nut at a reasonable cost. As I understand it, part of the problem is that square threads are difficult and expensive to cut. (I believe another problem is that most lathes can't produce a sufficiently aggressive lead, but leave that aside for a moment.) What about buttress threads? Could they work? They're supposed to be considerably easier to produce, and the really serious load on a fly press lead screw is in one axial direction -- which is where buttress threads are supposed to shine, right?

I realize the the force is typically only supposed to be applied in one direction, and it would take some force in the wrong direction (for a buttress thread) to lift the weight of the ram, screw, fly, etc. But would it be enough to cause a real problem?

The problem is not the thread shape, but the lead. It is hard to even call it a thread as it is actually a multiple lead helix. As a thread the pitch would be .166 thread per inch = 6" travel in 1 rotation. My Dad's old Monarch lathe will only get as low as 1.5 threads per inch. It is more of a helical milling operation to make a fly press screw.

Ditto what BigGunDoctor said - most of the threads on fly presses are 2 or 4 start and long lead pitch. In addition, most machining texts have abandoned recommending square threads in favor of a 10 degree per side modified form or even something approaching an Acme.

However (when I'm in a whimsical mood), I've daydreamed about milling 4 straight threads on a mill and twisting the shaft evenly to get the right form then casting a box nut in brass around the thread. Then I look on Ebay at surplus screw presses and lose interest...

I do recall a large press located in a factory where I worked in college that had a very coarse single start thread. It was quite slow to drive even with a 100 lb flywheel on top; would mash your finger flat as a sheet of paper if you had it in harm's way but the movement down was no faster than spinning the handle on an average bench vise.

I realize the lead is a problem, but I know a guy with a large lathe. If he's to be believed, the bed is around twenty feet. (I haven't seen the thing, yet.) I thought I might at least ask him about whether it could handle something like this, but it seemed like being able to use buttress threads might make it a slightly simpler, more attractive project.

I was thinking of casting the nut in bronze. Yeah, whimsical.

Square threads are not that hard to make, and I usually see buttress threads in two applications; quick acting vises, and toothpaste tube caps. For this application, square, buttress, or acme threads would work fine.

lol at this whole thread. if you want a fly press screw made... you know who to contact.
B)

I don't think the problem is in the screw---you can *buy* such screws if you hit the thomas register. The problem is building the frame to take the massive spike in pressure as the screw bottoms out and "bounces" back up---very different from the slow climb of a hydraulic press and great for cracking welds!

To do it right would probably be more expensive than buying a used one on the used machinery market.

I'm happy to be able to provide so much amusement for y'all. :P