I might be using the wrong term . that aside i was thinking of making one or a few if there are others looking . does any one have one they could show some photo’s of with measurements of the parts screw and cross bars etc. I’ve also herd Rian talk about them getting jammed or seized up destroying the clamp I’m thinking there must be a way to prevent that with better materials . well let me know what you think
Here is a picture of the one I have. The bar is 10.25" long and made from 0.625" square stock. It looks like the screw is 5/16" x 18 threads/in. Here are my comments on this particular design from a Mechanical Engineering standpoint. I like the nice long sleeve for the slider (marked at either end). A short slider can lead to binding. I would probably have welded the entire length of the coupler nut at the right side to the bar rather than just 1/2 of it. The left end is reversible and is held on by a socket head cap screw. The screw only engages about 4 threads (good for tension), but not enough for the bending moment that it will see in use. In addition, it appears to be a regular grade screw. I plan to replace it with a longer, grade 8 (high strength) screw. I would also add a sleeve to the end piece that slides over the main bar.
If I was building a larger bending jack I would do the following things in addition to the comments above.
- Use a rectangular bar to increase the stiffness in the plane of the screw and hooks. The bending moment is proportional to the in-plane thickness to the 3rd power - i.e. twice the thickness is eight times stiffer assuming the same out of plane thickness.
- Use a larger all-thread rod, 3/8"-16 or even larger for a big bending jack
- Weld the end hook to the bar for either tension or compression. Yes, I would have to have two jacks, but that would eliminate the weak point.
When I do some larger bends I pull out my Jorgensen bar clamps that use a piece of 3/4" pipe for the bar. They are capable of very high forces, but are difficult to use on bonsai since they lack the hooks of a typical bending jack. If you take this route, use the 3/4" pipe clamps that are far stiffer than 1/2" or the bar clamps that use a 3/16" x 3/4" bar (estimated size).
Thank you lots of valid insights you seem to know a lot about engineering . One of the thoughts I was having is if the screw has been getting jammed is to use a buttress thread instead of standard 60° threads . although it seems this one can clamp and spread so it would not work so well there.
A buttress thread or a square acme thread would better than the standard thread. However, the use of a coupler nut with far more than the minimum number of threads for strength (4-5) will resolve most of the binding issues. I actually think most of the binding issues are related to bar bending which puts a side loading of the threads. If it all stays aligned there should be minimal binding.
PS I taught Mech. Engr. for nearly 20 years so I have a decent background.