Machine vices all flex when tightened - can't machine square
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  1. #1
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    Default Machine vices all flex when tightened - can't machine square

    So I have a fantastic condition Schaublin 13 mill which is good to within 2 tenths on all axes and the spindle has no measurable run-out but my machine vices just don't seem capable of holding work so that I can machine faces square to each other. The vices in question are a 4" Abwood a 3"precision toolmakers and a very chunky 6" import jobee (if I could see how to post pictures I would) anyhow if I put a dial gauge on the fixed back jaw of any of them when I tighten the work in the vice I see the top edge of the fixed jaw flex back by around a thou or more so naturally that means the workpiece pretty much rotates out of square as it follows the fixed jaw flexing away. I am using a piece of welding rod between the movable jaw and the work and the work is on parallels but despite knocking down with a soft hammer this means that the parallel nearest the movable jaw is as loose as the proverbial dong in a bowler hat. I would point out that I am not being stupid and tightening these vices with a yard long lever I just use the supplied handle reasonably. HELP

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    To post photos click Advanced, Manage Attachments, then Add Files, Browse to add one photo, then Upload, then Browse to add another. When you have a maximum of 5 photos then click Insert Inline. You can Browse for photos on your hard drive.

    Need photos. My machine vises all have solid bodies, as in the fixed jaw carrier is not bolted to the vise bed, it is one solid casting. When I discovered what you are talking about with a newish Kurt vise I spent almost a day grinding, fitting, and torquing down bolts to get rid of the movement and couldn't. Kurt 3600s are my vise of choice for accuracy when milling.

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    All vices will flex to some degree, if you can't knock the work down on parallels you will have to raise it up on higher parallels until it can be knocked down, not ideal but that's reality.

    Tony

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    Everything moves.

    Just because the back jaw flexes doesn't mean the part will be out of square.

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    You could perhaps use less vise pressure for finish cuts.

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    Shim the top of the fixed jaw between the hard jaw and the vise itself as necessary.

    Then you have to make up your mind what you want to do with your facing cut. If you want the piece square to the back jaw, grip as much of the workpiece as possible and only use a parallel next to the fixed jaw. If you want parallel to the bottom, grab as little of the workpiece as you think safe and use two parallels. Tap down to ensure the workpiece is setting firmly on both parallels.

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    If the stock is close to square already, it should seat on both parallels with a hammer. The thing is, that clamping an already out of square piece must result in vise flexure. This is actually a good thing, because we do want our vises to accomodate a slight amount of imperfection in order to achieve better contact.

    Moving a part 4 times to make a square is going to result in some accumulation of error, too. Can't be helped.

    In reasonably sized parts, I'd recommend using a Talon Grip (or similar) system, and machining the part on all three sides (even up to 5 sides) in one setup. At least front and back will be parallel, although this is limited by tool flexure to some degree, as well as machine tram.

    You need to have a really good reason to want to bang out perfectly squared reference surfaces, because it can be difficult to achieve. It can be similar to guys dicking around with their lathes, getting it to machine perfectly equal diameters at a 3 foot spacing with a .001" skim cut, but in actual part production, the effects of the super careful levelling goes out the window because of machine flexure.

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    A solution to this problem I have found is that the stationary jaw is more rigid than the moving or can be made so. Register the work against the stationary jaw and vice bed, then place something like folded paper or such between the work and the moving jaw. The paper will take up the play, keep the work tight and square to the fixed jaw.

    Tom

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    This should help.

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    And this, everything is rubber, I get .002 on the back jaw of my 6" abwood if I clamp tight an high.

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    Okay well thanks for the various comments and suggestions, so I want to use the fixed jaw as the reference and therefore it cant be knocked down onto both parallels and as you can see from the attached photos that means over just 2" in the 123 block it is out by 2.5 thou, notwithstanding that I am a bit obsessive about accuracy, I am a concert harp maker/repairer and what you see in the second photo is the mechanism that I am about to start making from scratch for an own design concert harp so I really need the internal parts to come off the machine a heap better than how it stands. I did try the shimming trick which did improve things to an almost good enough on that 123 block using a 1 thou shim but that seems a bit crazy trying to find the right combination of shim stock for each different size piece I hold in the vice. I was just surprised that what I thought was a pretty beefy little vice would have that much movement in the fixed jaw.
    Attached Thumbnails Attached Thumbnails 20190911_192249_resized.jpg   20180130_164727_resized.jpg  

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    That vise doesn't look very beefy to me, but it's all relative.

    If the vise isn't working don't use a vise. I make a lot of parts and by far the majority are made in fixtures.

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    I stand corrected I should have said reasonably beefy although judging by the comments about even the top end vices, I have to accept that I will need to account for flex/movement whatever vice, unless I make suitably designed fixtures which I was hoping to avoid because there is a multitude of different shapes to hold and I am only going to make less than 5 of each part required

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    My solution is to use a 2 station vise converted to single station use. Both jaws are equal and clamp towards each other, and neither jaw flexes. A little large for a small machine but it definitely doesn't flex because the strength of the body doesn't come into play (much).

    333-1600.jpg

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    The rule in the metal shop is everything is made of RUBBER. And you, the machinist, must account for that fact.

    Is that vise in the photo your "very chunky 6" import jobee"? In other words, is it your heaviest vise?

    Anyway, there are two things that I see. First, the vise in the photo is not all that heavy. The fixed jaw seems to be an integral part of the base casting and it is not that thick (deep). Also the base itself does not appear to be very thick in the vertical direction. So, being made of RUBBER, it seems that it can flex a fair amount. Most vises that are sold as milling vises will have a base that is about twice as thick and the fixed jaw will be a separate piece which is larger in it's thickness to prevent any flexing.

    The second thing is the swivel base. The vise is sitting on a circular area of that swivel base and is held down by two bolts on each side of it. This is a fairly standard design for a swivel base, but it does not provide the best rigidity for the base of the vise. That vise base is hanging out, over the edge of that circular track of the swivel base and those unsupported ends can flex a lot more than the center section that is over the more solid support of the swivel base. I would remove the vise from the swivel base when that feature is not actually needed for a job and allow the base of the vise to sit flat on the mill's table. When the two hold down bolts are tightened, this will provide additional stiffening of the ends of the vise and it will flex a lot less. You can apply a fair amount of torque to those two hold down bolts without any danger of damaging the table or the vise. Oversized Tee nuts can be helpful here. Try to select a Tee slot on the mill's table that will keep all of the back end of the vise solidly on the table and not overhanging it in the rear.

    That thin vise base needs all the help it can get and I think if you remove the swivel base you will see a big improvement in the amount of flexing of the fixed jaw.



    Quote Originally Posted by choppernigel View Post
    Okay well thanks for the various comments and suggestions, so I want to use the fixed jaw as the reference and therefore it cant be knocked down onto both parallels and as you can see from the attached photos that means over just 2" in the 123 block it is out by 2.5 thou, notwithstanding that I am a bit obsessive about accuracy, I am a concert harp maker/repairer and what you see in the second photo is the mechanism that I am about to start making from scratch for an own design concert harp so I really need the internal parts to come off the machine a heap better than how it stands. I did try the shimming trick which did improve things to an almost good enough on that 123 block using a 1 thou shim but that seems a bit crazy trying to find the right combination of shim stock for each different size piece I hold in the vice. I was just surprised that what I thought was a pretty beefy little vice would have that much movement in the fixed jaw.

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    Don't get me wrong in my answer above. I have used a "screwless", precision vise for making many parts on the mill and they can work just fine. But I have used them bolted down directly on the mill's table, not on any kind of swivel base. The mill's table works to stiffen them just as it will with your vise in the photo. The trick with mounting them is to have the hold downs around the center of the vise, not at one or both ends. HOLD THAT CENTER DOWN! And the mill's table holds the ends UP.

    How the vise is mounted is just as important as the vise itself.

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    Out of curiosity, how does the classic solid base/jaw Bridgeport vise work?

    Tom

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    Yes, get rid of the swivel base. The vise is even notched to rest parallel to the table. I'd be curious of the results.

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    I see gib screws in moving jaw. Ever check that jaw for lift during clamping? Ever try snugging up gib screws so the vise screw is just able to move that jaw?

    As said above - lose the swivel base = more rigidity
    Attached Thumbnails Attached Thumbnails wiggly-vise.jpg  

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    This is a Stevens sub plate torque limiting vise mounted on a 2" thick steel plate, being used in an old jigborer. This vice has considerable less flex then a 6" Kurt.
    The repeatable torque setting is controlled by an adjustable multi plate clutch on closing, opening is done with a sprag clutch bypassing the settable clutch.



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