clamping things flat in a vise

[beanbag]

I have some bar stock that's 12" x 1.5 x .25 and I need to face that down to .2. I can clamp it in the vise, but the problem is that if the piece is thin, the moveable vice jaw tends to lift up a tiny bit. I can try tightening that screw that hooks it to the threaded rod that closes the vise, and I can hit the piece with a mallet to try to seat it, but I don't trust it to be reliable.

Another option is that I could clamp it down to the table, but the surface is kind of beat up, and I don't really trust that either.

Are there any other good options? I want to keep it parallel to .001.

Edit: Another way I thought of was to stand the piece vertically in the vise and side mill with .75 DOC, flip over and repeat.

[Black_Moons]

the standard proceedure is to put it up on parallels and give it taps while you tighten the vise, then insure the parallels are still 'held' by the work. if they slip out, the work lifted.

That said, its worth noting that clamping things with excessive force can deform them, and taking off the skin from one side is a sure fire way to deform cold rolled steel (less so with hot rolled). (you may need to take several passes and flip it over beween passes, if flatness is of utmost consern)

If your table is beat up, you should 'stone' the surface with a proper stone to take off any nicks. Generaly the plus part of cast iron is it doesnt raise up (much) when impacted, so all you have is dents.

Tightening the screw on the threaded rod will likey just bend the threaded rod.
This is why people buy those $500 kurt and $300 glacern vises
They just.. clamp down parallel, more or less

[Reed]

another method

You should fix your table, not much else to say on that.

Take a flat say of the dimensions of 13 x 3 x 1 and fixture it to the table so as to allow you to mill the top surface flat for a pass 2" wide the length of it. There are a couple of ways to do this, like mill steps on sides for clamps so the 2" part is above clamps, use 2" square tubing bolted to the table from the ends and skim the top, etc.

Then clamp your piece on it with sufficient clamps that you can remove a clamp at a time as you mill your piece to the required .2". I'm assuming you will need to turn it over to mill both sides. Then admire the warping.

Just fix your table. :-)

- Reed

[Dr Stan]

With material that thin you're going to have twisting, bending, & warping problems trying to reduce the thickness. So, a couple or three suggestions:

1) find someone with a tub (Blanchard) grinder to grind both sides. You'll still have warping issues, but probably not as bad.

2) buy some 3/16 over sized tool steel flat stock. Check with the vendor and find out the exact thickness.

3) make the part out of 3/16 material and shim it to .200"

Just had a couple other thoughts:

1) redesign the part or assembly so you could use 3/16" or 1/4" thick material

2) buy some 5mm thick material. that will get you real close (.1967")

[Carld]

If your using cold roll it will warp. If you use hot roll it may not warp. You could heat it to remove stress and have better luck. When you clamp it in a vise it will bow up in the center the length of the part and the ends hanging out of the vise will jump around as you machine it.

After you have stress relieved the piece you could epoxy it to a thick bar that is clamped to the mill table and then machine it. DON'T epoxy the parts and then clamp in down. The stress will/may pop the parts apart and DON'T get it hot while machining it, use a mist coolant. Flycutting would do a good job on this part.

[oldtiffie]

Some vise!!

I have some bar stock that's 12" x 1.5 x .25 and I need to face that down to .2.

That's going to be some vise that will clamp that 12" long piece.

[Ken_Shea]

Can this part tolerate any holes in it?
If it can, drill and countersink both sides (so it can be flipped), mount that to a block of metal clamped to the table that has matching holes drilled and tapped, also once that block is clamped to the table it should be faced so all starts out flat prior to securing the piece to be faced to .2

However you manage to do this, and you will, it's going to be a task to hold the .001.

[Ken_Shea]

That's going to be some vise that will clamp that 12" long piece.

Was thinking the same thing, even a 6" would have 3" sticking out each end, more then enough to sing you a song when machining.

[lazlo]

With material that thin you're going to have twisting, bending, & warping problems trying to reduce the thickness.

Mitee-Bites or thin toe clamps can hold pieces that thin, but it's not a lot of gripping power, so you're on the hairy edge of the milling cutter lifting and flinging it, which is scary (I have a picture I should post ).

How about double-sided tape? Carpet tape would work if you don't need high accuracy -- you could definitely get it within a thou...

[oldtiffie]

Flat out

Here is the OP heading:

clamping things flat in a vise

and here is the text:

I have some bar stock that's 12" x 1.5 x .25 and I need to face that down to .2. I can clamp it in the vise, but the problem is that if the piece is thin, the moveable vice jaw tends to lift up a tiny bit. I can try tightening that screw that hooks it to the threaded rod that closes the vise, and I can hit the piece with a mallet to try to seat it, but I don't trust it to be reliable.

Another option is that I could clamp it down to the table, but the surface is kind of beat up, and I don't really trust that either.

Are there any other good options? I want to keep it parallel to .001.

Edit: Another way I thought of was to stand the piece vertically in the vise and side mill with .75 DOC, flip over and repeat.

The OP (seemingly) specified a tolerance of +/- 0.001" all round - ie for size/thickness as well as parallel and flatness.

Even if he got it milled to that level of parallelism, I'd be very surprised if it was flat (ie not warped/twisted).

The bar stock material is not given but I'd assume hot-rolled. If that is the case, the surfaces will or may not be flat (enough) and will need addressing.

Getting a 12" x 1 1/2" x 1 1/2" bar milled to those limits is no mean job either - and its a lot stiffer (and a whole lot less "springy") than the OP's 12" x 1 1/2" x 1/4" bar.

[Rustybolt]

If it is that critical, have it ground. If you have any friends witrha large surface grinder, now is the time to statrt buying beer.

[mochinist]

Mitee-Bites or thin toe clamps can hold pieces that thin, but it's not a lot of gripping power, so you're on the hairy edge of the milling cutter lifting and flinging it, which is scary (I have a picture I should post ).

How about double-sided tape? Carpet tape would work if you don't need high accuracy -- you could definitely get it within a thou...

I tried some mitee-grip for the first time a couple days ago and was way happy with it over using double sided tape.

The mitee grip is a mesh that has some sort of wax in it, you put your part on it, in between a subplate, clamp and heat it to 180 to 200 deg(Fahrenheit) and then do the same to remove the part from the sub plate. The residue was easy to remove while the part was still warm using a little alcohol on a rag.

subplate I made and mitee-grip mesh, it measured .005" thick

clamped on the subplate and being heated up on the electric griddle, took about 5 minutes

part just got faced, I used a 2.5" face mill and took .020" per pass. material is t6 6061 aluminum by the way.


part was supposed to end up at .065" not bad, did another 19 with the same results

[oldtiffie]

Mitee-grip - Google result

Thanks mochinist.

First I've seen it - as I recall.

Very useful.

Check here:
http://www.google.com.au/#hl=en&sour...2f5f94750842e8

[metalmagpie]

You have a 6" Kurt vise? My suggestion would be to go borrow another one and mount it right beside yours and use two sets of parallels the same height. Flycut one side, debur, then flip it over and flycut the other side to leave about .005" excess height. Now remove the part and examine it. Did it warp? Can it be straightened? If it's still flat, the best way to hit your spec would now be
to move over to the surface grinder and grind to size. Even on a small one this would only take a few passes.

If you're working on a mill-drill with an imported vise with movable jaw that lifts, you're going to have to stone your table, then clamp the part to the table leaving some exposed, mill the exposed part, clamp the exposed part, remove the other clamps, and proceed until you have milled one side. Then flip and repeat to mill the other side. Just do the best you can. If it warps, straighten it. It's just a piece of metal.

If you do need to straighten it, here is a procedure which was sent to me to straighten a round shaft. It can easily be adapted:

"I have straightened many shafts. Here's the method:

Saw a couple vee blocks out of aluminum.

Support the shaft on these blocks in your press.

Mount a 1 inch travel indicator beneath the shaft under the bend.

Rotate the shaft such that the high spot is up.

Press against the shaft with a piece of aluminum between the ram and the shaft.

Deflect the shaft about .100".

Rotate the shaft and see if it has gotten any straighter.

Once the shaft starts to become straighter lessen the deflection increments. It may take a .250 deflection to make the shaft move .001 but only .260 to make the shaft move .002.

Use the indicator to monitor the deflection and be careful once the shaft
starts to straighten because it will be easy to over bend it. The aluminum vee
blocks will indent to fit the shaft and will work better as the pressure rises.
This indenting also protects the shaft. Steel vee blocks will damage the shaft.
It takes a long time to tell you this but straightening the shaft will be
quick."

The guy who sent me this owns a machine shop and has made his living as a machinist his whole life. Not some pinhead with a bright idea, this is actual usable info.

MM

[reggie_obe]

As stated if your mill table is so beat that you can't reliable clamp parts down on it, then how reliable could your vise be? I'd spend some time, stoning off the burrs and high spots off the table and use parallels and toe clamps. Toe clamps which pull the work down like these: http://www.northwesterntools.com/clmp_toe.htm are a good investment.