Saturday, November 7, 2009

Veritas Twin Screw Vise... My modifications and supplemental user's manual

Heads up to the reader: This will be one of those technical entries that's up for the sake of posterity, but will be incredibly boring and irrelevant to anyone without this particular piece of equipment. So feel free to skip this one and move on to the other postings, which will talk more about chairs, and shop stuff.

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This afternoon I was in the shop working on chairs and trying to mount something in the vise. But the problem I've been having with this particular vise is that both screws have been turning independently of each other.

Mechanically, the root cause for this is grub screws that aren't able to bite into the shaft of one of the screws. The left handle is pinned through the end of the screw shaft, but behind that, there's a gear that drives/is driven by a chain. The gear on this side clamps to the shaft using 2 grub screws, oriented at 90 degrees to each other. The chain goes around this gear, linking the left screw to the right screw. The right side has a similar gear, but the gear is hooked directly to the handle by way of a retractable pin. The pin is there to allow the two screws to operate independently, within a small range. All things being equal, this arrangement would work out fine. But, all things are not typically equal, and the issue here is that the retracting pin is a stronger mechanical interface than 2 grub screws. (Thin as that pin happens to be, it's still a better lockup than friction.)

The other root cause of my issues is that I'm simply stronger than the average guy, and I've been known to really crank down on the vise from time to time. The result is typically the same... something's in the vise, and both sides need to be individually tightened... and as a result, the grub screws scrape a little bit around the shaft. Repeat this issue a few times, and there's a small groove around the shaft, requiring that I remove the chain cover, and re-tighten the screws. There's an access window to do this, but it's a dumb idea. My logic is this: The screws should really only be tightened when the vise is actually aligned. And the best way to align the vise is to clamp it shut, tighten both sides equally, and then secure the screws where they are.

Well, eventually I tired of all this nonsense, and simply let both screws turn independently. There are certain advantages to using the vise this way, but the truth is that really, it's just better to use the thing as it's designed to be used.

So, I pulled it apart again today, and fixed the problem once and for all.

Famous last words...? Let's simply say that I have modified the vise to alleviate my issues, and pending further notice, it's working a lot better... but there are specific techniques that bear mentioning.

After removing the cover and aligning the vise per usual, I took one of the grub screws out. Using a transfer punch, I marked the location of the hole on the shaft. I then disassembled everything on the end of the shaft... Handle yoke, gear, and Jaw plate all came off. I then drilled an indentation into the surface of the shaft, to make a socket that the grub screw can mate with, and that will give more purchase to the screw. I put the gear back on, tightened the screw in place, and marked for a second indentation for the second screw. Removed the gear, drilled the shaft, and reassembled everything.

What I should have done in retrospect is used thread lock, too... but I'll do that another time if the gear develops any slop.

What I also should have done is taken better pictures, but it didn't occur to me until afterward that this was worth putting up, so the best I could do was to remove one of the grub screws. You should be able to see the socket I drilled out by looking through the hole where I removed the screw... if you click on the picture above.

In theory, this modification will give the grub screws a mechanical hold, and not just a friction hold, on the shaft. And, in theory, this end will now have a much stronger lock than the small sliding pin on the other handle. The pin is fairly small, and I've heard stories of them breaking... which is probably why a spare pin is included.

So... new vise operating procedures.

Now that the gear for the left screw has such a stronger lock on the shaft, I think it's a safer bet to use that one to tighten the vise with. The risk is that I'll tighten too hard on the right handle, and shear the pin.

So, from now on, I'll do most of the holding work towards the left of the jaw, and tighten with the left handle. The right handle should be able to follow along perfectly well, and it still has the crank handle for quickly running the jaw in and out.

Last technique, one I figured out while running each screw independently... It's possible to exert a lot more gripping force on a board (or anything else) outside of the left hand screw. (As opposed to using the space between the screws) I set up the object to be held, to the left of the left hand screw, and tighten the vise down normally. Then, I thread the right hand screw counterclockwise to move the right side of the jaw outwards. Basically, I'm using the width of the jaw as a long lever, pivoted on the left hand screw. This technique is NOT good for massive movements, and certainly not good for holding tapered work, because the design of this particular vise doesn't really allow for that. But disengaging the sliding pin on the right handle will allow the gear on that side to disengage completely, so the right hand screw can operate independently, and lever the right side of the vise outwards just enough to add a lot more pressure to whatever's being held.

(A more visual explanation is here)

Other modifications:

I made a few small design changes to the way I installed the handles. First, rather than drilling all the way through for the crank handle, I installed a threaded insert in the handle. I just think it looks cleaner. I had to shorten the crank handle bolt accordingly, but that was pretty simple.


Second handle modification. I noticed right away that using the crank handle means locking the handle in place so that it can't roll. There are brass thumbscrews that come threaded into each handle yoke, but I knew right away that these would have issues keeping the handle from spinning if they were only being used on the surface of the wooden handle, with the end result of a scratched up, screwed up looking handle, and an ongoing difficulty in using that crank handle. So, I drilled a hole in the center of each handle, and inserted a brass shelf pin socket. I think they're made by Vertex, or one of the other premium brass hardware suppliers. These sockets have holes drilled in them that are 1/4" in diameter, which fits the brass thmb screws very well. They do a great job of keeping the handles secure, and they keep the crank handle from rotating the right handle around. AND they look a lot nicer than simple holes drilled in the wood, which would most likely wallow out over time.

2 comments:

raney said...

James -

Really smart mod. I had the same issues with my twinscrew as well - over time, the grub screws just get too loose to function well. My solution was to replace it - I think yours was probably the more sensible approach. Funny - I had talked to a lot of people about the vise, and I seemed to be the only one who had this issue... and for what it's worth, I'm not generally a heavy-torquer.

Seth said...

Re: not adding threadlock - Henkel sells a few wicking threadlockers under the Loctite brand. That way you can add the threadlocker after you've assembled whatever it is.