Leg vise toe-in

[Gene Pavlovsky]

Good points, Andrey. I will keep this in mind with regards to screw installation.

As for the linear bearing, I wonder if anyone had done any mechanical analysis of this solution. It seems to me that someone just had an idea to try it (because it kinda made sense to them), posted about it on the internet (and mentioned that it worked well for them), and then naturally a bunch of people repeated that (with many saying it didn't work well for them). Considering that the leg vise can develop a lot of force, any bending of the shaft and play in the linear bearing will result in the chop's bottom moving inside more than desired. So to me it seems that to counteract that (and to provide some amount of toe-in), the linear bearing and it's shaft shouldn't be installed exactly parallel, the shaft's far end should be slightly angled towards the floor. But which angle to use is a question, and once the angle is chosen, it would be difficult to adjust it later.

I've found about another version of the cross guide - X Link by Hovarter. It mentions more clearly how toe-in is achieved "This is easily accomplished with the included shims which are added behind the wear plate in the vise jaw.", supporting what was mentioned earlier by Joshua. Their product looks even more fancy than the one by Benchcrafted, incorporating a spherical bearing to allow for less-than perfect precision when drilling the pivot holes.

I consider my original question (how does Crisscross achieve toe-in) is resolved, but welcome any further discussion on the topic of how toe-in can be planned for, when installing some other kind of automatic parallel guide.

I am currently quite attracted to a ratchet system parallel guide. I found examples by Will Myers and Mike Davis.
Article by Will Myers: https://eclecticmechanicals.com/2018/10/28/ratcheting-parallel-guide/
Video of it in operation: https://www.youtube.com/watch?v=0cji8ymNjJQ

[Joshua Lucas]

Gene, I learned parametric CAD in school so I'm not the best person to ask for advice on how to learn it, but I think I've heard that Autodesk has a good series of YouTube videos on getting started with Fusion360. Fusion has a lot of features, most of which you'll never use for woodworking design, so try not to get too overwhelmed. If you can draw a sketch and extrude it you're good for 99% of furniture design.

In case you weren't aware, Fusion is free for hobbyists, you just have to sign up for a hobbyist license. Autodesk changes the process it seems like every few months so I can't tell you the exact process, but it wasn't hard when I did it.

I actually did do a simulation in Fusion of the wooden crisscross, but Fusion doesn't support simulation of wood so I tried it with a couple materials with similar mechanical properties, ABS plastic and MDF. ABS is less stiff than most wood, but it is less brittle, while MDF is almost the opposite - more brittle, but similar stiffness. Both seemed to perform acceptably with a clamping force of 500 lbs (IIRC), although the MDF was close to splitting where the dowels go through the arms. I think the ABS probably could have taken quite a bit more before the bending got unacceptable. I think a hard, strong wood like hickory or European hornbeam would work pretty well. Also, if I were really building it I would exclude the sliding blocks. They made simulating in Fusion easier but they actually decrease the overall strength of the mechanism.

I like the ratcheting design too, but I decided to go with the chain more for ease of construction than anything else. If someone made a kit for the ratchet I'd likely go with that. And the crisscross would probably be my first choice if my bench design were compatible with it.

[Gene Pavlovsky]

Ok Joshua I will try their YouTube videos. Yes I've heard about the hobbyist license, will apply for that one.

Interesting about the stress simulation, too bad they don't support wood. Perhaps they will add such support if enough of us woodworkers ask them for it

The ratchet option sounds attractive and I'm thinking more and more about it. One con I see (or just imagine?) in the ratchet design, is that if you have to work on multiple workpieces of the same thickness (e.g. plane multiple same-thickness boards on one or more edges), each time you open the vise, the ratchet might move by a tooth or more, in that case when you close the vise again, you have to release the ratchet pawl again to set it to the position (hopefully the same position). The traditional pinned parallel guide, on the other hand, provides repeatable clamping. The adjustment step (distance between adjacent settings) of the ratcheting guides that I've seen also seems to be a bit on the large side, compared to what the pinned guide offers (those are often made with 3 rows of holes). This may or may not be a problem.
Another issue is the lack of available kits, or standard parts that can be used to make it. Sawing the sawtooth rack by hand / using an angle grinder, plus hand filing, seems like a lot of work. Of course sometimes it's fun to create something that works, using only basic materials, but there is also a lot of fun when you can adapt existing hardware to implement an idea. What is easily available (and affordable), is rack-and-pinion hardware for sliding gates. I'm seeing a couple of options here.
Both options include a gear rack that is installed on the parallel guide (instead of the linear ratchet rack). A matching gear is attached to the bench's leg to mesh with the rack. A ratchet and pawl set (commercially available) is obtained, an axle joins the gear and ratchet together. A suitable pawl release lever is rigged.
The second option is simpler and requires less precision. Only the gear rack is needed. A small piece of it is cut off, and is attached upside down to the bottom of a "brake block" which is installed on the back of the leg (or in a mortise on the front of the leg, same as the pawl in Will Myers' ratcheting solution). The brake block can be lifted by a release lever (foot- and/or hand- operated) to adjust the vise opening, or locked in the down position, which locks the gear rack. This seems like something that can be made using standard parts, and it would also be not too hard to start offering kits based on this idea. What do you think?
But yeah, a simple pin in the hole sounds much simpler

[Andrey Kharitonkin]

Good points, Andrey. I will keep this in mind with regards to screw installation.

As for the linear bearing, I wonder if anyone had done any mechanical analysis of this solution. It seems to me that someone just had an idea to try it (because it kinda made sense to them), posted about it on the internet (and mentioned that it worked well for them), and then naturally a bunch of people repeated that (with many saying it didn't work well for them). Considering that the leg vise can develop a lot of force, any bending of the shaft and play in the linear bearing will result in the chop's bottom moving inside more than desired. So to me it seems that to counteract that (and to provide some amount of toe-in), the linear bearing and it's shaft shouldn't be installed exactly parallel, the shaft's far end should be slightly angled towards the floor. But which angle to use is a question, and once the angle is chosen, it would be difficult to adjust it later.

Yes, that's why prototyping is very important. In fact, it is very useful for any vise installation. I did prototype from pine also for Veritas Twin-Screw Vise and I'm very glad I did.

And yes, shaft is installed at an angle, to compensate for flex of the shaft and the jaw at the desired holding force. Toe-in is required only for that anyway. It also depends on how far the jaw is open as it flex more easily then. Cross guide is better in this regard as it doesn't flex and does not depend on how much you open it, and you can apply any force. I want to try linear bearings guide because I liked the idea and because I could completely detach it from the workbench and then use workbench in a English workbench way, by clamping to its side. There are no sticking out parts when it is unscrewed completely.

[Gene Pavlovsky]

Yes, that's why prototyping is very important. In fact, it is very useful for any vise installation. I did prototype from pine also for Veritas Twin-Screw Vise and I'm very glad I did.

...

I want to try linear bearings guide because I liked the idea and because I could completely detach it from the workbench and then use workbench in a English workbench way, by clamping to its side. There are no sticking out parts when it is unscrewed completely.

I think I will heed the advice and do a prototype first (especially if I try to experiment with the gear rack approach, which I haven't encountered attempted/documented so far).
Is there any other solution that does not permit to remove the vise, with nothing left to stick out? Sure, a cross guide needs a bit more effort to detach (removing one pin holding the cross to the bench's leg), and other solutions may also need some extra effort. The ratchet rack (or gear rack) solution seems as easy to remove as a linear bearing.