PDA

View Full Version : Solid jaw flex



spope14
03-05-2005, 12:04 AM
OK, here it is. last evening, teaching my night class on milling, a very disturbing trend occurs. Trammed the head on my mills, checked vise squareness, and solid jaw perpendicular to spindle on my bridgeports. I have to hold .002 over 2 inches.

Darned if I can't hold perpendicular on the parts!!! Or paralllel betwen top and bottom surfaces. i am using the first milled surface as my 2nd and third surface reference off the soild jaw. doing it all right.....this is new.

the flex is at the top of the jaw, and indicating on the back side of the entire solid jaw assy (behind the vise where you do not clamp, the column side), it shows flex of 1/2 the front ammt.

jaws are tight. soild jaw to vise base is quite tight.

I check spindle to vise jaw perpendicular with a 1x2x3 block clamped in the jaws. However, something is amiss......

Bottom line, the entire solid jaw on more than one vise is flexing .0015 to .003. I have six bridgeports, using fair quality vises, a Bridgeport whole cast vise, and a kurt 5 inch.

Tonight, went in and tightened screws holding the soild jaw to the base. Got four to .001 or better with moderate clamping pressure.

Using a brass rod on the movable jaw to hold the part in, but does not matter, does this anywy, even face to face clamped.

It is the .002 issue that is killing me. .005 over 2 inches has never been a problem, and even .002 in the past was a football field.

Looking for ideas, and questions. probably questions at this point so I may describe the issue better to your understanding as you might see to ask

Is this a common occurance?

precisionworks
03-05-2005, 02:23 AM
Scott,

Can't say how common it is, but it happens on all my vises (six at last count). Even the best vises, Kurts or Kurt clones, do this to some degree. It is most noticeable when you stack a number of identical parts (precision ground) and clamp down. The part on the bottom is held fast. Next one up is not bad. Next one up starts to get loose and you can almost remove the top one with hand pressure.

Kurt (and clones) use a sliding wedge "pull down" design where every one pound of jaw pressure produces one half pound of downward force, therin lies the problem. Only half as much force is directed vertically which allows a slight jaw lift.

My "cure" may or may not be right, but it works. Do what you've done -- tighten everything down snug, fixed jaws, moveable jaws, wedge contact adjustment, everything. Clamp a parallel in the bottom fourth or bottom third of the vise jaw. Indicate the top of the vise jaw to determine how many thous of tilt you have. Remove the back (moveable) jaw and mount it on a magnetic sine vise on the surface grinder. You want to grind the back side in such a way that, when reinstalled, the jaw tilts in by the same amount that it tilted back. Reinstall and reindicate. If you held your mouth right, the jaws will come into parallel when fully tightened.

MikeHenry
03-05-2005, 01:43 PM
This is a long shot, but you might check to be sure that there is no interference between the table, vise base and mounting hardware.

One of the bolts and T-nuts will interfere with the base or key on my vise if the T-nut is not positioned just right, which raises the base very slightly on that end. It took me quite a while to figure that out and still catches me every once in a while. The result is a vise that can rock just a little bit, maybe a thou or two.

Mike

wierdscience
03-05-2005, 08:16 PM
I had a Kurt that was cracked under the paint in one corner of the fixed jaw keyway.It would flex noticeably.

Could it be the vise base is no longer straight or maybe has been streched over the years?

ahidley
03-09-2005, 01:28 PM
I have a kurt clone, not a kurt.... But with it came instructions.... Yea, who reads instructions?????? *L* Anyways it said something about adjusting a collar on the shaft (the one used to tighten the jaws). This was how the "wedge action" was increased or decreased on the vise. (As mentioned above 1lb clamp=1/2lb wedge action.. I believe its really adjustable)