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Stiffness of jaws on the back of a Kurt vise

swarf_rat

Titanium
Joined
Feb 24, 2004
Location
Napa, CA
I had a plan to mount the jaws on the back of the Kurt 3600 to hold aluminum plate for machining. The plate is too wide to hold the normal way. I moved the jaws to the back of the cast jaws, and then indicated the movement of the fixed jaw as I tightened the vise. Its pretty easy to get 0.001 of movement, even with a little speed handle. It seems to be due to stretch in the jaw fixing bolts.

When clamped in the normal way the bolts are unloaded, they just locate the jaw until pressure is applied. Once you clamp up above the cast jaw, the bolts become stressed, whether the removable jaw is on the front or the back. On the back, the leverage is more: the fulcrum is the lower edge of the removable and the leverage about 2:1 with any clearance at all of the work over the casting. On the front, the fulcrum is the top of the cast jaw, and the leverage about 2:3. The removable jaw movement is about half with the same torque on the screw. The cast jaw does not move the needle on a 0.0005 indicator. There does not appear to be appreciable flex in the removable jaw itself, rather it is rocking and stretching the bolts. A quick calculation of the stretch on two 1/2-13 seems to confirm the plausibility of this conclusion.

So the question: is there a way to make or fix a removable jaw that is stiffer than the standard installation? I've come up with two obvious ideas: one is to make a thick "L" shaped jaw which lays up and over the cast fixed jaw, then machine a step in it to grip the work. The leg resting on the top of the cast jaw will resist the rocking back of the removable, and convert some or most of the tension in the fixing bolts to shear. The other is to deface the vise by drilling and tapping holes and maybe a key slot, so that a jaw can be bolted on top of the cast fixed jaw. This would be loaded mostly in shear, and I think would be pretty stiff. I wonder why Kurt doesn't supply them that way, seems like an obvious benefit. Is there any other way to attack it?

There is no doubt the same problem on the moving jaw, however that is without consequence as long as it doesn't come off. I need the fixed jaw to stay still, so I can depend on it for work location.
 
You may want to put your indicator in the master jaw to see how much it moves. They aren't as rigid as people think.
 
I usually use a torque wrench for final clamping torque. 120 inch pounds is snug, 240 is tight and so on. Having a specific repeatable torque value helps prevent surprises. I started doing this when I used to machine a lot of plastic and the clamping force deformed the part. There were some bores that I programed as ellipse and they sprung back round after clamping.

blah blah blah... My point is, if you use a torque wrench, you may find your feature position is repeatable, even with movement, as long as the movement is repeatable. Even if your jaw were 100% non movable, the part itself still moves. Ever notice you always need a little bit of a positive Y tweak to get a feature to land perfectly in the right spot?...
 
Depending on the tolerance of work you are presenting to the vise, you can get pretty repeatable results by always tightening the vise the same amount. We have used this method for years on plate work using a standard torque wrench. They do repeat pretty well, but they definitely flex quite a bit too. I had to calm down one of my guys a year or so ago...this whole moving jaw thing was too much for him, on a plate that just had a bunch of holes in it with +/- 1/32" tolerance (can't use a decimal number on that tolerance, just looks wrong!) He had tenth indicators on the back jaws of three vises and was worried about the flex...he was watching all three indicators spin like a top as each vise was snugged up. No coffeee for him that day!

Back to your question at hand, I believe Kurt makes a jaw that does something like you described, bolts onto the normal face, over the top, and has another step "up". I know I have seen this before, pretty sure it was Kurt that has it, in aluminum IIRC.

Steve
 
I never got that far into checking the flex with indicators and such, but I agree with NTM on tweaking Y. For example I was boring out a hole in a brass half nut once and test fitting the pin that needed to fit snugly in the bore...when the bore was right on the numbers, the pin still didn't fit in the bore until I loosened the vise. I must have been squeezing the part a little too hard and making an oval shaped hole.
 
You may want to put your indicator in the master jaw to see how much it moves. They aren't as rigid as people think.

True of a Kurt D series, but the V is pretty damn rigid.

The cast jaw does not move the needle on a 0.0005 indicator.



I did try several re-tightenings of the vise and it is fairly repeatable with the removable jaw flopping around, but not as good as clamping down between the cast jaws which will repeat to about a tenth or so with some care. Torquing the floppy jaw maybe you can get 5 tenths pretty reliably. Problem is the errors all add up, got to get rid of them where ever I can.

Has anyone bolted a jaw or stop on top of the castings?
 
IF you think about it, bolt stretch is probably what is happening. Try longer bolts and really socking them down. Beyond that it is the detail of the bottom of the counterbore and the bottom of the bolt head. If everything was clean and ground and perfect, and teh bolt is torqued to its limit, then you have done all you can do.

True of a Kurt D series, but the V is pretty damn rigid.





I did try several re-tightenings of the vise and it is fairly repeatable with the removable jaw flopping around, but not as good as clamping down between the cast jaws which will repeat to about a tenth or so with some care. Torquing the floppy jaw maybe you can get 5 tenths pretty reliably. Problem is the errors all add up, got to get rid of them where ever I can.

Has anyone bolted a jaw or stop on top of the castings?
 
I can usually avoid any problems by using big, heavy washers with longer bolts. Flip the jaws with the counter bore to the vise.
I think most of the deflection is in bottom of counter bore. It is very thin. I actually broke it off once before I started to use washers.
Jerry
 
I have thought about a secondary jaw fastening system. Maybe make the jaws wider and through bolt them to a backup plate on the other side of the casting. You could get the through bolts up closer to the load as well. I did try a much heavier jaw (2" thick) and the result was about the same. Given the size of the bolts, a couple thousand pounds of load on them is going to stretch them a few tenths. Longer bolts will stretch more. I did find these top jaws from Snapjaws, a bit along the lines I was thinking.
 
I don't think my Kurts are rock solid even with the jaws on the front (normal position). There is a little flex.

But, could you simply make a new rear jaw block with an integral step built into it? Isn't the jaw block just bolted down from below?
 
Here is a quick and dirty sketch of the sort of jaw I am contemplating. To get the reach required I would bolt the moving removable jaw on the other side of the moving jaw.

Kurt3600X_T.jpg
 
I don't think my Kurts are rock solid even with the jaws on the front (normal position). There is a little flex.

But, could you simply make a new rear jaw block with an integral step built into it? Isn't the jaw block just bolted down from below?
This is true of a D series Kurt, the V series has the rear jaw cast as part of the frame. I sold all my D series vises because the rear jaw is so floppy, probably because of the bolts. On a V series, it is pretty hard to measure any movement of the rear jaw - but also not possible to replace it with a substitute.
 
I would try what Jerryzak says. I do not believe you are correct that longer bolts will stretch more. Longer bolts will have more thread engagement, allowing more torque and more pre-stretch[you only care about stretch after the bolt is torqued]. The big fixture washers will be squarer than the bottom of that counterbore.


I have thought about a secondary jaw fastening system. Maybe make the jaws wider and through bolt them to a backup plate on the other side of the casting. You could get the through bolts up closer to the load as well. I did try a much heavier jaw (2" thick) and the result was about the same. Given the size of the bolts, a couple thousand pounds of load on them is going to stretch them a few tenths. Longer bolts will stretch more. I did find these top jaws from Snapjaws, a bit along the lines I was thinking.
 
Could you use one of the D series vises? The fixed Jaw is removable, and keyed to bed. Two bolts hold it down. When tightened The key and bolts are in shear. With the D you could get a second fixed jaw and machine it however you want. Assuming it is soft enough (I think only the bed is hardened) just face it down leaving a step jaw on the back, or supply your own hunk of steel. Is the D rigid enough for you? I would exepct <.0005 of flex, stiff but not rock solid.
 
If I still had any D series vises I would try it. Kurt actually used to make the DU68 fixture vise intended to be used just like that.

As I said above, I have tried a thick jaw with longer bolts. It moved at least as much. In theory there are two sources of movement in the bolts: distortion of the thread and stretch of the bolt. In either case once the clearances are gone and you have entered the elastic range, the preload of the bolt does not matter. An additional load causes additional strain, proportional to the Young's modulus and inversely proportional to the sectional area of the bolt. The displacement is proportional to the strain times the bolt length, longer grip length bolts will stretch further under the same load. What I am observing is roughly obeying the theory. If thread engagement is insufficient to support the load in the elastic range then more would help, but its easy to get 3x the diameter which should be more than enough.

I wouldn't mind attacking the whole problem differently, but haven't found another solution. I need to clamp aluminum plate from about 6" to about 12" across, up to 24 inches wide, and between 3/8 and 1 1/2 inches thick. They must be supported at least 2 inches above the table, and I have to machine through them over nearly the entire area in various places. I need to be able to flip the plate and machine the other side with location good enough to avoid a noticeable step on the thru features - from experience that means a few tenths maximum error. I have been able to do that on smaller pieces in the Kurt V3600 vises. Larger pieces I have bolted onto table rests and relocated on the flip, but it is very time consuming. If I can't get the vises to stay still for the larger pieces I may have to settle for holding the work that way but relocating on each flip - faster than bolting the pieces down and less waste, but still not as fast as using the fixed jaw to locate. On larger pieces I might end up having to do that anyway, because any rotational error in the vise alignment is multiplied 2x in the features when you flip. If your vise is off alignment by a tenth in 6 inches, then when you flip a 24" wide piece you could be off by a thou in your features.
 
Re bolt stretch, something like 75-80% of the load is taken by the first 3 threads. So longer bolts won't help.

Depending on the exact geometry, the 'L' shaped jaw you've shown could lift the workpiece as it flexes. And raising things higher above the bed will just increase the moment, flexing the vise more.

I think I'd be looking at step blocks bolted directly to the table, with 'something' on the opposite side that resembles a vise jaw to clamp them. The guts of a toolmakers vise are fairly easy to whip up.

Oh, and I have a couple D675's here I've been wanting to get rid of ;-)
 
How about drilling and tapping a third bolt in the back jaw?

At some point it is not about the bolts and you are flexing the vise and at that point maybe you are tightening the vise too tight!

If you don't feel you can fix this issue on the back jaw why not make and extended movable jaw? Then you don't care if it moves.
 
I don't believe that is correct, I think 3 threads hold most of the load, but which 3 threads vary with the torque applied. I have never seen a high torque application without somewhat extended thread depth.


Re bolt stretch, something like 75-80% of the load is taken by the first 3 threads. So longer bolts won't help.

Depending on the exact geometry, the 'L' shaped jaw you've shown could lift the workpiece as it flexes. And raising things higher above the bed will just increase the moment, flexing the vise more.

I think I'd be looking at step blocks bolted directly to the table, with 'something' on the opposite side that resembles a vise jaw to clamp them. The guts of a toolmakers vise are fairly easy to whip up.

Oh, and I have a couple D675's here I've been wanting to get rid of ;-)
 








 
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