Leg vise screw hole / parallel guide clearances

Scott H

Scott
User
I'm working on my first real workbench (more or less a Roubo style), and I'm in the middle of working on my leg vise. The leg vise is a traditional pin board/parallel guide style. I have not had the opportunity to use a leg vise in person and while I have tried to refer to every book/video/set of plans I can get my hands on, there are still a few things I don't totally understand. I'd like to really get it all clear in my head before I start removing material in my workbench leg and vise chop.

I have a Yost 18" vise screw and the screw major diameter is about 1.163". (The picture shows the nut backwards to how I am planning on installing it. Chris Schwarz's book and the actual countersinks in the casting itself suggest it is fine the other way, and that seems like much less work.)

So here are my main questions. Help on any of them would be appreciated.
  • Does the screw ever touch the wall of the leg hole in use? Does the screw rest on the bottom of the hole? Or is it just supported by the nut on the back of the leg & never touches the leg hole? (I see some hardware has a bushing or sleeve bearing type thing on the front of the leg but I don't have one of those --- if it's critical I guess I could make one out of delrin or UHMW or something?)
  • If the screw never touches the leg hole and the nut is supporting it, is it normal that the screw sags a small amount from true horizontal when it's open because of the clearances in the threads in the nut vs the screw?
  • Does the screw ever touch the wall of the hole in the chop? (I am pretty sure the answer here is "no")
  • Does the parallel guide ever touch the top/bottom walls of the mortise in the leg in use? Am I supposed to tweak it until that never happens?
Basically I just really want to make sure I don't make a vise that's so unconstrained it's sloppier than it could be, or that is so constrained it can't toe-in enough for some workpiece thicknesses without the vise screw chewing into the wooden parts. I think if I understand where there is meant to be clearance, I can work out the particular amounts with some geometry.

Thanks for reading!
 

Graywolf

Board of Directors, Vice President
Richard
Corporate Member
Myself I have a wooden screw. The clearances I went with was approximately 1/16” over all. I could have made it tighter but it worked. I think if something rubs some, it’s not the end of the world. If it’s to loose then you will be unhappy with the function of the vise.
 

Phil S

Board of Directors, President
Phil Soper
Staff member
Corporate Member
I have a criss-cross leg vise here in NE Raleigh. You are welcome to come over and test drive
 

pop-pop

Man with many vises
User
Scott, I sent you a PM offering a leg vise tutorial since we are both in Raleigh. I have made several and mostly understand the nuances.
 

pop-pop

Man with many vises
User
So here are my main questions. Help on any of them would be appreciated.
  • Does the screw ever touch the wall of the leg hole in use? Does the screw rest on the bottom of the hole? Or is it just supported by the nut on the back of the leg & never touches the leg hole? (I see some hardware has a bushing or sleeve bearing type thing on the front of the leg but I don't have one of those --- if it's critical I guess I could make one out of delrin or UHMW or something?)
  • If the screw never touches the leg hole and the nut is supporting it, is it normal that the screw sags a small amount from true horizontal when it's open because of the clearances in the threads in the nut vs the screw?
  • Does the screw ever touch the wall of the hole in the chop? (I am pretty sure the answer here is "no")
  • Does the parallel guide ever touch the top/bottom walls of the mortise in the leg in use? Am I supposed to tweak it until that never happens?
You are worrying about the right things. Rather than answer each question individually, let me explain things in a general way from what I have learned over the past decade.

First, a chop weighs several pounds or more. Something has to keep the chop from falling to the floor. The choices are the parallel guide, the screw, or sharing the weight between the two. I immediately discount sharing the chop weight because the fits are too finicky especially over time.

Supporting the chop by the parallel guide involves getting the fit on the top and bottom of the leg mortise just right so that it slides freely without binding. The mortise side wall fit should be a little sloppy to slide freely. The parallel guide hole that you put the pivot pin into controls the toe-in. A thin piece of slippery plastic (like UHMW poly) lining the bottom of the leg mortise can be helpful.

My design preference is what I call a "hanging chop" in which the vise screw supports the total weight of the chop. The benefits are the leg mortise fit to the parallel guide can be sloppy (like 1/32' to 1/16" clearance) and the parallel guide to chop joint can actually be a pivot. For the parallel guide, I usually used three rows of 1/2" holes 1" apart thus giving 1/3" adjustability. Note that this way the only job of the parallel guide is to control the toe-in by which hole the operator puts the pin into.

Next, how to support the weight. One way is use two nuts, a nut recessed into the front of the bench leg and another nut at the back (which can be recessed or not). I prefer the back nut to be in a wooden nut block which gives some error forgiveness.

My preference evolved from two nuts to a nut and a radial ball bearing. The bearing is recessed into the front of the bench leg and is a loose fit on the vise screw (note in your case a 30mm(1.18") bore bearing would work). The bearing only supplies vertical support for the chop weight and is held by a retaining ring. Again, house the nut in a wooden block and fasten it last after you have checked for smooth operation and removed any vertical slack.

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NOTW

Notw
Senior User
I went similar to @pop-pop but with different materials. I used the Yost vise vise screw and the nut that comes with it I mortised partially into the leg on the backside. On the front side I used a piece of 1/2" Delrin with a 30mm hole (30mm was the closet drill bit i could find to the size of the Yost screw). I tried it both ways, with and without the Delrin and the Delrin makes a huge difference, without the Delrin the weight of the chop would cause the screw to bind on the leg.
IMG_4437.JPGIMG_4435.JPG
 

pop-pop

Man with many vises
User
I went similar to @pop-pop but with different materials. I used the Yost vise vise screw and the nut that comes with it I mortised partially into the leg on the backside. On the front side I used a piece of 1/2" Delrin with a 30mm hole (30mm was the closet drill bit i could find to the size of the Yost screw). I tried it both ways, with and without the Delrin and the Delrin makes a huge difference, without the Delrin the weight of the chop would cause the screw to bind on the leg.
View attachment 197094View attachment 197095
Has the sharpish corner on the thread crest chewed up the Delrin any?

I tried a bronze bushing once and the Acme screw gnawed at it. A 30mm bore radial bearing is only $4.29 and it’s hard to get any piece of slippery plastic that cheaply.
 

NOTW

Notw
Senior User
Not too bad, this is after about about 2ish month of use. the bearing is cheaper, i got a 6" x 6" x 1/2" piece of Delrin from McMaster Carr for $15 and it is enough to do this 4 times if needed.
IMG_4442[1].JPG
 

Scott H

Scott
User
Thanks for all the input so far everyone. I appreciate the offers to test drive too.

The hanging chop approach makes a lot of sense to me, especially with a parallel guide that can pivot at the chop end. I can totally visualize how that functions as a mechanical system. I can get a 6206-2RS bearing which feels like a good size for my bench leg pretty inexpensively.

@pop-pop A few random questions if you don't mind:

Is the retaining ring just digging into the wood? Or did you route an actual groove in there?

For testing out the nut block position do you just clamp it in place and test the vise action with a workpiece thicker than your clamp jaw, and sink screws when you're happy?

I am interested in the idea of a pivot between the chop and the parallel guide because it means the top/bottom parallel guide clearance doesn't need to increase the more your jaw opens. If you get a chance to take a picture of that I would love to see it. I'm not sure how I'm going to get a hole for a pin in the side of the chop accurately though, I am running into the limits of my bench top drill press pretty heavily on this workbench. Not a lot of plunge depth and not a lot of table to chuck distance even before I added an actually-flat table.
 

pop-pop

Man with many vises
User
@pop-pop A few random questions if you don't mind:

Is the retaining ring just digging into the wood? Or did you route an actual groove in there?

At first I made a groove with a slotting cutter in a trim router and a turned wooden washer to enlarge the bearing and decided that was too much bother. Since there is nil axial thrust on the ring, a friction fit works just fine.

For testing out the nut block position do you just clamp it in place and test the vise action with a workpiece thicker than your clamp jaw, and sink screws when you're happy?

Yes. At this point in the construction you are looking for any binding and that the screw is perpendicular to the bench leg.

I am interested in the idea of a pivot between the chop and the parallel guide because it means the top/bottom parallel guide clearance doesn't need to increase the more your jaw opens. If you get a chance to take a picture of that I would love to see it. I'm not sure how I'm going to get a hole for a pin in the side of the chop accurately though, I am running into the limits of my bench top drill press pretty heavily on this workbench. Not a lot of plunge depth and not a lot of table to chuck distance even before I added an actually-flat table.

At this point I don't have one to photograph. I recently converted my last parallel guide to an X-cross mechanism.
Think of the parallel guide as a tenon going into a mortise in the chop. A large radius on the end of the parallel guide seats against the vertical wall of the mortise. Then pin that joint with an oversize hole in the parallel guide.
Remember this is a sloppy joint to avoid binding. For the pin hole in the chop, precisely lay out and drill half way from both sides as carefully as you can. Then with a long drill bit in a hand-held drill, re-drill all the way through and you'll be OK. BTW, even Paul Sellers uses this method.

Edit: I found this CAD file which should show my thinking:
parallel_guide.jpg
 
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Scott H

Scott
User
I spent some time making a prototype leg vise out of some 2x4 legs I had discarded from an earlier project. (Basically 2 legs w/ short stretchers and another 2x4 for a jaw.) I think I've been overthinking this because I did a traditional design (no bearing, fixed pin board/chop joint) as quick and dirty as possible and the leg vise functioned fine -- once I rasped out the pin board mortise in the leg to stop it binding side to side. The plywood I used for the pin board is cheap and not uniform thickness. In the end the amount of sloppiness it had did not really bother me. The screw does sag from level significantly though since it's only supported by the nut.

That being said I did add a bearing this morning and the screw seems to glide a bit more easily throughout the whole travel, and does not sag anywhere near as much. The nut block system does make it easier to get the screw aligned. I didn't obsess over it for the prototype but it seems like for the real thing you could fine tune it pretty well. I definitely learned a lot for when I do the real thing on my real workbench.

@pop-pop - May I ask if you had a way to remove the bearing from the leg if you ever need to change it? I made mine a press fit at first and I had a heck of a time getting it out again. Once I got it out I ended up drilling holes around the periphery of the bearing mortise so I could put a punch through from behind and tap on the outer race, which works when you only have to put a punch through a 2x4 and I can unscrew the nut block, but when you're talking about final legs that are 3" thick even for my small workbench, plus the nut block... Not sure what to do there.

IMG-8705.jpg IMG-8706.jpg IMG-8707.jpg
 

pop-pop

Man with many vises
User
@pop-pop - May I ask if you had a way to remove the bearing from the leg if you ever need to change it? I made mine a press fit at first and I had a heck of a time getting it out again. Once I got it out I ended up drilling holes around the periphery of the bearing mortise so I could put a punch through from behind and tap on the outer race, which works when you only have to put a punch through a 2x4 and I can unscrew the nut block, but when you're talking about final legs that are 3" thick even for my small workbench, plus the nut block... Not sure what to do there.

View attachment 197196 View attachment 197198 View attachment 197197
A cardinal rule for a successful leg vise is avoiding any binding. Generally, everything must be a nice slip fit.

I have always used inch series bearings and the Forstner bit corresponding to the outer race diameter gave the right fit. In your case, a 30mm bearing would have a 47mm outer diameter which is probably not a standard Forstner dia. In that case, I would use an adjustable circle cutter in a drill press to form the outer diameter of the bearing hole and the hog out the interior waste with a smaller Forstner.

TIP: The drill bit in a circle cutter tends to “worry” the hole it creates reducing accuracy. I replace the drill bit with a 1/4” brass rod with a rounded nose. Drill the work with a 15/64” bit. Lube the brass rod with paraffin and chuck the circle cutter in place of the 15/64” bit and form your hole O.D.
image.jpg
 
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