vise clamping force

I have a pretty broad/general question I am hoping to get some feedback on. How much force (in ft/lbs) is required to hold a billet of stock in a vise for machining?? Is there a calculation I can do for material type and material removal rates? More specifically, if I am trying to hold a 2" thick x 4" wide x 6" long piece of 6061 aluminum in a vise with "gripper jaws" (dovetail with serrations...) what could be an expected ft/lbs to hold said piece? Not serious hogging, more HS paths with moderate stock removal... the vise would be gripping on the 4" wide x 6" long (2" would be the height in this case).

Enough force to keep it from moving while cutting.

This isn't exactly what you asked but here is what I do, since it's hard to get an exact force you will be putting on a part while machining. I go to the Kurt website and look at what force the vice produce at what you have it torqued to. We use the D810 and with grippers it is capable of more force than what we have been able to put on a part, or get a part to move. We stand aluminum parts up 15" off the bed of the vise and machine on a 50 taper HMC in all directions, including sideways to the jaws where there is no support. If you are looking to find a bare minimum amount that you can get by with I am not much help. One other thing I consider is the vise clamping force and figure how many of what size screws that force would equal or how many clamps with what size studs/screws would produce that size force and it gives you a good feel for what you are working with.

6061Mike said:

Not serious hogging, more HS paths with moderate stock removal...

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Half your weight on the end of the wrench.
Unless you're a big guy, then a quarter.
(you're seriously overthinking things)

Put some leather gloves on and tie the part down with a half hearted effort, and then see if you can yank it out by hand. If not you should be ok when you tighten it normally.

It doesn't make any difference how many pounds it takes because no one has ever measured the force any way.

I use the same method when I have a frail part to cut, and I'm using an 1/8" end mill. So I wonder if I have tightend the vise enough. I try to pull it out,and if I feel it will take the cut, I cut it. I've had it fail a couple of times, but I've learned along the way. If I have fear, I just don't take deep cuts.

Regards,
Stan-

Boy, the math on that one would be horrendous. First you need to know exactly how much force, as a vector quantity, you are generating when cutting it. I say vector because the direction of that force is important. Then there is the coefficient of friction between the vise jaws and the material you are holding. Every combination of materials, surface finishes, lubricants and contaminants present, and probably other factors would factor in that. Next, you have to consider the mechanism you are using to tighten the vise jaws. Screw, hydraulic, or what? More mechanical factors and coefficients of friction and whatever. And how about vibration?

Your best solution is to clamp it reasonably tight in a vise that looks large enough and start cutting. If it moves, try tightening more. If that fails, get a bigger vise or take lighter cuts. Some experience helps a lot.

Frankly, I have almost never had a part move in the vise while machining and in those circumstances the set-up was obviously not the best. But I had to try it for other reasons.

This is one place where the old adage, "get a bigger hammer" (vise actually) really works. Please don't really hammer on the vise.

6061Mike said:

. . .How much force (in ft/lbs) is required to hold a billet of stock . . .

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"Billet" seems to be escaping everywhere. Probably requires great force to restrain it from near ubiquity; more than mere stock.

More seriously depends on squareness of the material, the material, surface finish, cleanliness, the type and condition of the vise, depth of clamping, the type and condition of the jaws, the feed, climb cutting, depth of cut, the rake and condition of the cutting tool, lubrication, chatter, etc. etc. I doubt that even if you measured both the clamping force and the coefficients of friction for your jaws and material that you'd do much better than common sense and experience on this one. Pay attention to the variables, own a decent vise, and you should be fine. With a handle like "6061" I suspect you've already figured most of this out.

On edit: EPAIII beat me to most of this. I can add that the proper (or at least Wiki) definition of billet for aluminum is stock that comes in a square or round cross-section up to about 6x6" Pretty ease to hold in a properly sized vise or vises unless trying to stand a super tall section. The OP's not-quite-billet-perfect piece should do fine even in a 4" Kurt.

there are threads on a certain cnc forum that have mostly been sending folks to go google it... i have tried and while i've found some interesting papers, they aren't really noob level material and hard to read.

for an adaquate safety factor, estimate the torque required to turn the tool, then calculate the force on the tool according to the radius, then multiply by 5*.

in practice, the amount of force depends on the depth of cut because most tools are helical. if the depth of cut is 1.5 times the diameter of any average end mill, then the force is the same as the cutting torque plus something like 50%.. but this depends on the relief angle, and so does its direction.
if you have a single point fly cutter then the cutting forces are interrupted and you have to multiply the torque (aka horsepower) by the percent of contact area, but determining the average direction of force applied to the work by the cutting tool is rather self evident, the block of metal is pushed in the direction the cutting edge sweeps across it.

in a perfect world, you could build a milling vice out of granite, and the vice itself would absorb all of the cutting tool's vibration and the average force exerted on the table, by the granite vice, would be nearly zero.. this would require a great deal of math to calculate how much metal to take off.. but it could, in theory be done.
--this is why climb milling is preferable.

*this should be regarded as lawyer boiler plate.
you can probably get away with twice the force calcualated from spindle load torque and tool diameter.

6061Mike said:

I have a pretty broad/general question I am hoping to get some feedback on. How much force (in ft/lbs) is required to hold a billet of stock in a vise for machining?? Is there a calculation I can do for material type and material removal rates? More specifically, if I am trying to hold a 2" thick x 4" wide x 6" long piece of 6061 aluminum in a vise with "gripper jaws" (dovetail with serrations...) what could be an expected ft/lbs to hold said piece? Not serious hogging, more HS paths with moderate stock removal... the vise would be gripping on the 4" wide x 6" long (2" would be the height in this case).

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You're kidding right?

Here is a vise test

What's the clamping force of a vise? Hack a hydraulic cylinder to find out! - YouTube

I have a challenge for you, put a large piece of aluminum in a vise on a bridgeport. Put decent force on the vise handle, maybe 1/3 your weight. Then use a large (1") endmill running moderate speeds and take as heavy as a cut as you think the BP will manage. See if you can get that aluminum block to move. I doubt you will.

You need to beat the living shit out of the vise handle with a mallet. Then measure

Larry Dickman said:

You need to beat the living shit out of the vise handle with a mallet.

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Don't forget the cheater bar!

Most of my work holding has teeth or a knife edge on top because I rarely have more than .05 to hold onto a part with, and even then I occasionally have the corner actually shear away under the knife edge. That is why I switched to vacuum fixtures. But the real question is how much pressure you can put on the part without distorting or damaging it. If you have a solid chunk of CRS you only need to face off, go ahead and crank down on it. If, like me, your get a lot of flat aluminum plates with a lot of material missing from the middle, you need to be very gentile and creative in your clamping to keep the part from bending and squeezing out or deforming when you remove it from the vice. Knowing how much clamping force it takes to counter the cutting forces is pretty irrelevant when that amount of force would ruin the part. I would say creative work holding is about a third of my job.

Edit: How the hell does nnwonng get past the spellchecker?

Can't clamp something? Use a tooling plate and hold it down. Not that complected.

So first you're asking about force, and then somehow transitioning into torque(or trying to-there is no such thing as "ft/lbs" or "feet per pound"-you mean ft-lbs or foot-pounds, which is how torque is specified..

I think this thread has answered the OPS question well enough that I don't feel bad posting this tangential link http://en.m.wikipedia.org/wiki/Foot-pound_(energy)

will gilmore said:

I think this thread has answered the OPS question well enough that I don't feel bad posting this tangential link Foot-pound (energy) - Wikipedia, the free encyclopedia

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Yes, work is also measured in foot-pounds.

When talking about work, it is the force applied to an object(in pounds) multiplied by the distance the object is moved(you can push on something all day, but if you don;t move it, you are not doing work, in the scientific sense).

When talking about torque, it is the force applied, multiplied by the moment arm(distance from center of rotation).

It's unfortunate that they are the same units; many people say "pound-feet" when talking about torque, as a way to differentiate.

But in either case, it is a multiplication of the force by the length, never a division or foot per pound as is denoted by ft/lb.

Clamping force goes pear shaped on different jobs. Experience is the only thing that helps.

I was fly cutting six 4" X 6" plates at the same time. Plates were burned and had a slight kerf I was machining off. I had plates loaded in the kurt vise. I cranked the vise down with all 200 lbs of force I could.

Long story short I relived the clamping pressure via fly cut, and started to move the plates. . . So now you have a bit more experience.

6061Mike said:

I have a pretty broad/general question I am hoping to get some feedback on. How much force (in ft/lbs) is required to hold a billet of stock in a vise for machining?? Is there a calculation I can do for material type and material removal rates? More specifically, if I am trying to hold a 2" thick x 4" wide x 6" long piece of 6061 aluminum in a vise with "gripper jaws" (dovetail with serrations...) what could be an expected ft/lbs to hold said piece? Not serious hogging, more HS paths with moderate stock removal... the vise would be gripping on the 4" wide x 6" long (2" would be the height in this case).

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.
1) the less area you are gripping the more the metal will crush or deform rather than take any more pressure. i get that often when gripping less than 0.125 depth on small parts maybe 1.0" long.
.125 x 40,000 = 5000 lbs, compression strength on various materials varies just saying over a certain point and it will leave marks
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2) if item is not centered in vise or a block not put on left side to balance part on right side, this uneven vise pressure can easily cause one side to lift up when milling. holding force reduction hard to say could easily be less than 50% when not clamped in center of vise
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3) a tall item in a vise has more leverage. if part is 1.0" wide and sticking up 6.0" out of vise and only held in vise 0.125" there is a more of a chance it will move in vise
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4) dovetail, knurled surface jaws. if part is only partially sunk into part, further vibration can cause metal to deform more and vise clamping pressure to go down
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5) part is 1" thick or high and held in vise 0.125" and measures 6x6" and is machined down to 0.200. as more metal is removed part will bend more more vise clamping pressure and pressure will go down. part humps up in center from vise clamping pressure, thus why before finish cuts vise is loosened and reclamped lightly
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6) 1 hp = 33,000 ft lbs per minute so at 100 sfpm = 330 lbs
at 10 sfpm = 3300 lbs
......... had a metal shaper with 1/2 hp motor, if pulleys set to run slow that 1/2hp motor could easily push many 100's of lbs......... so slow sfpm = high forces
....... do not underestimate the slow flycutter, whack whack whack like hitting a item with a hammer and parts can easily start moving much easier than a end mill going 1000 rpm