Problems getting a good seating with an Abwood machine vise

[JasonBModerator @jasonb]

Tapping forces the part that was raised by the jaw lift back down onto the parallel, not to do with dirt or burrs

Pink on the left shows the jaw and work have lifted when tightened. Tap the work down Green arrow and that pushes it back onto the parallel closing the gap caused as it raised.

The “Bounce” mentioned a few posts above is caused by the jaw being pushed down by the hammer at the same time the work is pushed down and then it bounces back up lifting the work with it. The paper trick allows the work some slippage against the jaw so when hit it reduces the chances of the jaw also moving down and then bouncing back up.

As you say if you don’t suffer lift not much point in tapping down. I do it on the K4, you can see from my table above why I do it as work can lift 5 thou at the moving jaw

[JasonBModerator @jasonb]

No Online Thesis here but a practical demonstration.

With the jaws open the workpiece sits on the parallel with no gaps

Tighten the vice jaw and if there is any lift in the jaw it also lifts the work, here I can pass a 0.1mm feeler gauge under the work when the jaw is tightened.

Hit it with your implement of choice and there should no longer be a gap between work and parallel. Nothing to do with Burrs or crud where it should not be.

A simple test on any vice even one where you think there is no lift is can you easily remove the parallel once the vice is tightened, if work is firmly down then it is hard to pull the parallels out.

[Clive FosterParticipant @clivefoster55965]

Like Ian I do wonder how the process of tapping down actually works. Jasons explanation ad drawings are excellent but the various forces involved just don’t feel right to me. Probably a failure of imagination on my part. I know it does something because the shift is “measurable” in so far as a parallel becomes tight after tapping when it was moveable, not loose but stiffly shiftable, before. I frequently find that one side of a parallel becomes tight before the other two. Dunno how much movement is needed to make up the difference between just loose enough to move with a moderate push and proper tight though. Can’t be much.

What I really don’t understand is that the vice jaws need to be pretty tight before tapping down works and, for me at least, the tighter the jaw the lighter the tap needed. Bonking too hard at final tightening makes the parallel loose again. I generally use one of the 32 mm ( 1/14″) nylon head Thor dead blow hammers with a very light grip. Basically flicking the handle in a partially opened fist rather than holding it. Which seems to work well. Even if I haven’t got a clue why.

Concerning the relatively small vice opening of Abwood vices my understanding is that good design practice for vices having simple dovetail guides for the moving jaw is for an approximately square capacity at maximum opening with the moving jaw at least twice as long as the opening. Clearly for any given length of moving jaw the greater the opening the less support the jaw has from the dovetails at maximum opening and the less stable the system is so jaw lift is more likely. Abwood vices generally seem to be a bit under square with maximum opening less than jaw width. Presumably seeking better stability at the cost of capacity within practical constraints on size and weight. Big vices are very heavy!

Much as I love the wide opening capability of my Vertex VJ400 110 x 180 mm  4 1/2″ x 7 1/2″) vices http://www.rotagriponline.com/index.php?page=shop.product_details&product_id=19436&flypage=shop.flypage&pop=0&keyword=vice&option=com_virtuemart&Itemid=29  due to having three positions for the nut retaining pin moving jaw stability is seriously compromised once opened beyond the basic 4 1/2″ square. For me the extra effort needed to get things settled down at the wide end when I occasionally need to hold a larger part is a good trade off against a much bigger vice whose capacity would mostly be wasted.

Wider opening vices fitted with shorter moving jaws to keep the size within bounds need at least some form of anti-lift device and, preferably some way of getting the closing forces more in line with the workpiece centre line.

Kurt Acculock and clone devices still need a decent length of moving jaw for stability because the closing forces are out of line with the job but the pull down effect helps control jaw lift.

There have been numerous breeds of vice with a short closing screw held in some form of easily adjusted locator pushing centrally on a short moving jaw. The Taylor et al rack type being an older British example whilst Gerardi and clones are basically a modern update for the CNC era. All seem to use some sort of angled rear face on the jaws so there is downward force derived from the closing action. It would seem that this design means the jaws can’t be fixed in dead tight so some uncontrolled movement is possible. In practice it all seems to work just fine, even on the once common lever action production vices with shaped jaws to fit a partially made component.

The side rack used by Chick to pull the moving jaw closed is interesting as the forces are applied both centrally to the moving jaw and in front of it. Which geometrically seems a good way to combat lift. Never quite figured out how a Chick keeps its jaws in line or why it doesn’t jam up before going real tight tho’. A simplified Chick style device might make an interesting project using a laser cut rack to get the really tricky part done easily and accurately.

Clive

[Charles LamontParticipant @charleslamont71117]

Taping down works if you do it right (even if you don’t understand the mechanism). But to get work surfaces parallel I was taught to use ‘pull-downs’. These are strips of steel flat with beveled edges, and they are used as shown in the diagram. Admittedly, it is tricky to get the right amount of tilt so that they hold with friction against the vice jaw and the job. When set properly the clamping is actually quite secure even though it looks iffy.  I find that using just one at the moving jaw is normally sufficient and does not require five hands. I prefer the type on the left.

[JasonB]

On JasonB Said:

No Online Thesis here but a practical demonstration.

With the jaws open the workpiece sits on the parallel with no gaps

Tighten the vice jaw and if there is any lift in the jaw it also lifts the work, here I can pass a 0.1mm feeler gauge under the work when the jaw is tightened.

Hit it with your implement of choice and there should no longer be a gap between work and parallel. Nothing to do with Burrs or crud where it should not be.

A simple test on any vice even one where you think there is no lift is can you easily remove the parallel once the vice is tightened, if work is firmly down then it is hard to pull the parallels out.

An interesting demonstration that does show the result of tapping down. To me though it begs the question, why did the workpiece rise up in the first place?

My assumption would be that the moving jaw has risen because of play or looseness in the dovetail gibs combined with the force provided by the screw being below the point at which the job (at the top of the jaws) is being resisted.

Tightening up gibs and repeating the demonstration might give an unmeasurable amount of lift (I’m not inferring that you don’t maintain your equipment)

If the same (1-2-4) block was gripped without sitting up on parallels then I’m not sure you would experience any jaw lift as the block itself would self-align the opposing jaw faces.

I will do some test on my precision vice and report back

Ian P

Edit: I ‘Quoted’ Jason’s posting but it seems quoting does not include his video

 

[JasonBModerator @jasonb]

It is certainly more of  a problem with thin work being held at the top of the jaws. Charles’ pull downs would unlikely work with just 1/16 “of material in the jaws.

I would say it goes up as there is nothing to restrain it in that direction, can’t go down as base of vice restrains the jaw and parallel restrains the work.

Simple example is to place your hands palms down on a table. touch the ends of your middle fingers together and then apply a bit more pressure and your fingers will bend backwards and go up where they meet

The K4 does not have any gibs and is not a dovetail design, flat base and plates that fit under it to hold the jaw down.

[DiogenesParticipant @diogenes]

Inventively clocking screw vices as you tighten them is one of those things like clocking the head of a mill to the table as you push and pull it all about – half of you thinks it would have been better not to know..

Clive’s post raises an interesting question, though – what’s a good rule of thumb for (say, side-)   milling unsupported spans of work, in terms of ‘X’ times the ?thickness?

 

[Howard LewisParticipant @howardlewis46836]

Are you concerned that the face on which work sits in the vaice ia not parallell to the table?

Or is your problem that the work “rocks” across the surface, contacting one edge, but not he other?

If the seating surface (The upper surface of the vice bas is not parallel to the table, you can always remove the vice jaws (To maximise space, open the jaws fully, and take a skim across the base to just clean it up.

If the problem is that of the work m”rocking”, the only solution is to tap the work down when firmly gripped, before tightening the jaws fully, and checking.

Howard

[JasonBModerator @jasonb]

Greensands problem is that his parallels only sit on the narrow base. If he tries to seat something down by tapping it then the ends which are unsupported may go down too far and as they do they lift the opposite end. If just hit in the middle a thin part is lilely to remain up at the ends even if down on the parallels in the middle.

[Martin ConnellyParticipant @martinconnelly55370]

Ian, when using vices the moving jaw presses on the workpiece and, unless the vice has a design to push the moving jaw down, the force usually causes some lift of the jaw and the workpiece as it is the only way it is able to go. Tapping is applying enough force to the top of the workpiece to overcome the friction between the jaws and the workpiece to get it back down on the parallels or vice bed. The jaws do not move when the workpiece is tapped so the workpiece settles down on the support, machine bed, vice bed or parallels it is being tapped down onto. The harder a vice is clamped the more lift is likely to occur. The more slack there is in the ways the more likelihood that  jaw lift is going to occur.

Adding a soft material between the workpiece and the jaw such as some paper or a piece of copper wire reduces the lift of workpiece, also a cylindrical or spherical piece of material between the workpiece and the movable jaw is a good way to ensure a flat face on the workpiece is flush against the fixed jaw.

So you may never need to tap a part down but it has a positive effect on the accuracy of machined parts for most people.

Martin C

See a lot of people beat me to this post. I think I was only looking at the first page of replies.

[Howard LewisParticipant @howardlewis46836]

FWIW, As Apprentices at Rolls Royce, we were taught always to tap down the work.

Since the base of the vice is narrow, tapping down at the ends will cause the work to rock. the obvious thing is only to tap down over the base of the vice, to prevent rocking.

As said, the risk is that thin material may seat in the middle where it has been tapped down, leaving the ends high.

Maybe the solution here would be to check with a parallel, and the tap down, lightly at the ends, before rechceking that the middle section has not rocked and tilted.

If this is the case, increasing accuracy may well be an iterative process of tapping down in the middle, checking, tapping down at the ends, checking and repeating until an acceptable degree of accurate levelling is achieved.

If you want a high degree of accurate levelling, there  is no short cut, just care with repeated checks and corrective action.

Howard

 

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