When my son & I built our Roubo bench, we added a parallel guide to the bottom of the leg vice. Information on an alternative, the St. Peter's Cross, was discussed when the Benchcrafted prototype was recently introduced.
In haste we made an attempt to make one using 25x6mm steel flat bar.
1st we cut grooves in both the inside of the leg vice & the bench leg to accommodate the steel bars.
Halfway up we notched a space to fit the nut and bolt (6mm) which hinged the two bars in the centre, thus forming the cross.
We rounded the ends of the bars to prevent them from catching the wood:
We fitted the cross with 6mm threaded rod on the two top pieces and the bottom pieces were not attached in any way.
Our St. Peter's Cross didn't work for the following two reasons. 1.) The steel flat bar is only 6mm thick, so the vice has play from left to right when being loosened or tightened. This seems like a minor problem that may be fixed by thickening the ends of the two bottom pieces (with a short bolt & nut I guess), thus making the fit inside the wooden grooves snug.
2.) The second problem is the major one. When the vice is tightened, the cross ensures that the vice moves in parallel to the bench leg. But once the piece of wood being held in the vice is engaged, then the bottom of the vice keeps on moving in. In other words the leg vice does not remain parallel when engaging the workpiece. And as far as I understand, this is the main purpose of the Cross. It seems as if the two bottom pieces of the cross are slipping and riding up the groove. This then puts the vice out of parallel, which loosens the grip on the workpiece being held. I attach a short video to demonstrate this. So far I have no solution, so if anyone can help, it would be appreciated and shared. (Please note the video was taken sideways in order to better fit the long leg vice. So the bottom of the vice is on the right of the picture-that's where you should pay attention).