The face vise screw mechanism is all DIY. Here are the piece parts. The hand-wheel has been kicking around my basement for 15 years. I remember buying it on Ebay for a project I never completed. It was too nice to throw away, so it waited and waited until now to find a purpose. The acme screw and nut I picked up on Ebay more recently. I cut the screw to length and drilled the hole that holds the hand-wheel setscrew. I found a 5 inch brass plate 1/2 inch thick also on Ebay which seemed ideal for this application. No idea what it was used for originally. I drilled and tapped 4 screw holes so it could be mounted to the chop. The leather washer protects the brass from the steel hand-wheel. For a garter, I put in a 1 inch metal set collar that is held in place with a setscrew. It sits in a recess in the chop behind the plate and pulls the chop back when releasing the clamping forces.
The nut for the screw is welded to a pipe flange and a 1” black iron pipe is used as a guide to keep the screw from sagging. As a side note, welding galvanized is not a good health idea and stainless needs fancier gear. I am not a great welder, but I picked up a HF flux welder and this is my second project using it. I overdid the weld and then used an angle grinder to get it back down to a reasonable shape and also to round over the hex corners. Welders would laugh, but for the forces involved in wood working it will hold. The biggest challenge was to keep the flange flat as I ground things away. If welding is out, cutting a hex hole for the nut in the leg and screwing down the flange over it would also probably work (maybe some JB weld (no affiliation)?). I cut the sides of the pipe flange off a little with a cut off wheel on the angle grinder to allow closer mounting of the chain mechanism. The nut sits in a recessed hole on the back of the front leg. I made a small wooden plate with a larger hole to accommodate the weld area. This eliminated the need to make a larger recess in the leg. The other end of the pipe is held in place on the back leg using a wooden bracket. I considered a second flange, but decided that cutting and threading the pipe to exact length was too complicated and wood is much cheaper. The bracket also helps to align the screw. There is some play in the flange and locking down the other end stabilizes everything and also can be used to make sure the linear bearing shaft and screw are on the same plane.
The linear bearing consists of a few piece parts purchased on Ebay. The shaft and bearing are both one inch. I could not find a cheap 1 inch version of the flanged shaft support block, so I picked up an aluminum 25mm one, loosened it and drilled it out to an inch. I also cut off one of the legs of the flange (no functional reason I thought it looked wrong when used this way). Both the acme screw and the bearing shaft are as long as I could fit between the legs. This gives me a lot of clamping space, but more realistically, I thought it would help keep the chop from sagging by providing some balancing weight at normal working distances and give the pipe something to hold on to.
My biggest concern was alignment of the screw and bearing. If out of wack, the bearing starts racking and locks up. This was my original intent in adding the linear bearing. Keeping it parallel should keep things smooth, non racking and non-sagging. Drilling the holes for the linear bearing seemed to have the most potential for problems. I finally decided on the following steps. I did all the drilling before assembly, so I could work with the leg and chop on the drill press. I used carriage bolts to keep the leg in place so I could put it on and take it off as needed. I drilled 1/8” oversized holes in the leg and chop, and 1/16” oversized hole in the brass plate for the screw. I then drilled a precise hole for the linear bearing in the leg and installed the bearing. I centered and mounted the pipe flange with the nut. With the leg installed in the bench, I mounted the hand-wheel assembly on the chop and screwed it into the leg. I made sure the chop was hanging down the same way it would in use. This meant the brass plate on the chop was touching the screw at about the 11 o’clock position due to the 15 degree angle. I then clamped the chop to the leg and used the linear bearing hole in the leg to align the hole for the shaft in the chop. I then drilled a precise 1” hole 2/3 of the way through the chop on the drill press. In spite of this there was a little drag at the brass plate as the chop moved. A little file work on the plate at the touching point eliminated this. I then screwed in the pipe and used the bracket on the opposite leg to make sure that the bearing shaft and screw were equidistant and aligned. Still not sure why but the bearing hole I drilled in the chop was not perfectly square to the chop face and one side of vise closed before the other by about 1/16”. I took some wood off of the side of the hole in the chop to get it to line up better. This created a slight side to side taper in the hole. The bottom of the hole did not move and was still at the correct position for the screw to bearing hole distance in the leg. As a last step, I was going to clamp the chop tight and epoxy the shaft in at the correct angle. As mentioned earlier, the chain mechanism eliminated the need for this and leaving it loose with a slight taper appears to work well. Not sure if a pinned parallel guide has the same issues or is inherently a little looser so alignment is not such a big deal.
I was a little surprised by how little info is available on how a face vise works. Which holes should be oversize? How do you keep it from sagging? What are the alignment issues? The conclusions I reached are as follows (but I could also be wrong since this is a sample of one). The screw is held precisely in the leg by the nut only. The hole in the wood should be oversized to eliminate friction. The parallel guide in the leg should only allow perpendicular movement with as little friction as possible. Both the screw and the guide need to be in the same plane. You can’t have one swinging left and the other right for example or up vs down. On the chop side, the screw holes are loose (friction again) but the chop rests on the screw at a single point which affects the height and possibly the angle of the chop. The parallel guide should be tight and square to the chop face vertically, but some looseness in side to side movement in conjunction with the looseness at the screw seems to help even out the clamping pressure. I wonder, if you built the chop with a slot at the nut and a hinge at the linear guide, if this would allow you to clamp odd shapes better hmmm.
I am still a little uncertain about the sagging. I have two places that prevent sagging, so they need to be aligned to prevent racking. In other designs, I am not sure if the sagging is eliminated at the screw, the parallel guide, both, or either. One conclusion, regardless of how it works, is to leave the chop long until everything is done and settled down before cutting it flush to your table top.