[smokediver576]

Okay, newbie question here. I'm trying to make my setups more repeatable (reduction in witness marks due to over/under tightening the vise). What kind of inch pounds (or Ft/Lbs) do you need for:

Average tightening on a vise: ????


Average tightening on T-Slot nuts: ????

Average tightening on fixture plate bolts going into the T-Slot nuts: ????

Thanks,
Awall

[SCzEngrgGroup]

How on earth do you define "average"? Every setup is different, and how tight it needs to be depends on how the part is being held, the part shape and material, what machining operations you're performing, and how aggressively your' doing it, among other things. Those factors alone can easily account for a range of 10:1.

My experience is "witness marks" are much more a function of how you hold the parts, rather than how hard. Screws, properly used, won't leave marks (i.e. - use washers, or put the screws where the marks can't be seen on the finished part). A vise, when tightened, will almost always leave some kind of mark, unless you either garb the partly lightly (which is NEVER a good idea!), or use aluminum soft jaws. IMHO, well-designed, dedicated fixtures are the best way to make mark-free parts.

Regards,
Ray L.

[smokediver576]

Hey Ray,

Agree on your points. Yes, I know it's a wild shot but I thought on T-slot nuts that there would be some kind of spec.

As for the witness marks let me explain further. I have a part that is 1" tall, is roughed out, then contour cut, then it has a 2mm corner rounding. I then flip the part over (held in a custom fixture - held with 1/4" of the part), deck off the top excess and then corner round that side. Depending on the pressure of the vise, I can have one part end up perfect. Too tight and I get witness marks on the fixed jaw side, too loose and I get witness marks on the movable jaw side.

I'll try and get a picture of the part and how it's held for your advise.

Thanks,
Awall

[Bryan Turner]

Awall,

I don't think there's a standard, but if you're making the same part over and over you should measure what strength you're clamping to and use the same setting for each part. My parts usually have an aggressive rough, then a pretty light second op. I use TalonGrip jaws for the aggressive cuts:

Attachment 351250

Then when I flip it over (into custom-cut soft jaws), the clamping pressure is pretty light:

Attachment 351252

The requirements for clamping are pretty simple though, you just need to keep the part from moving along all axis -- no actual force is needed!

The axis are +/-X, +/-Y, +/-Z, and rotation in X,Y,Z. If you have the workpiece flat on the table you've already covered several of them:
- can't move -Z
- can't rotate in X or Y

A common strategy might then be:
- butt up against a rail on the upper edge (locates Y position, cant move +Y, nor rotate in Z)
- slid against a pin on the left side (locates X position, can't move -X)

Then you need only work out a strategy for restraining movement in (+Z, +X, and -Y):
- A screw in a hole through the part (restrains all remaining axis, and does not need much clamping force)
- A toe-clamp on the bottom-right (can't move +X or -Y, and restrains +Z in the region of the clamp, but the upper-right might still lift)

So you can see that a soft-jaw does all the axis at once with the exception of +Z.. That is handled by the clamping force & friction against the jaw face, thus the witness marks.

Some other options to consider:
- Glue the part to a sacrificial piece, then clamp the sacrificial piece. Heat or solvent to remove the glue after.
- Heavy double-sided tape ("carpet tape", best for very thin parts).
- Low-melting point alloy in a tray; melt the alloy (< 450*F), place the part in the molten tray with locators. Let the alloy cool, locking the part in place. With compatible alloys, you can make the tray out of aluminum, with locators milled into it already. Works best for parts with lots of delicate bits sticking out one side, or difficult-to clamp geometries; the alloy will surround the delicate parts and solidify, protecting them from being crushed or deformed.

--Bryan

[Steve Seebold]

I wouldn't try to hold 2 round parts that big in a vise jaw. I would make a Mitee Bite fixture. If you try to hold 2 parts that big and there's a couple of thousandths difference, you stand a good chance of one part coming out of the vise. Mitee Bites will eliminate that possibility.

[talonosi]

I wouldn't try to hold 2 round parts that big in a vise jaw. I would make a Mitee Bite fixture. If you try to hold 2 parts that big and there's a couple of thousandths difference, you stand a good chance of one part coming out of the vise. Mitee Bites will eliminate that possibility.

Steve,

What particular miteebites would you use for that fixture? I may have a similar fixturing need shortly.

Thanks