Tightening a vise

[Mr Ron]

What is a good gage for tightening a milling vise. I tend to overtighten, I think, but not sure if I am tightening too much or not enough. Obviously if ia part flies out of the vise, it's not tight enough, but so far I have not had that happen. I mill mostly aluminum, so I would think vise pressure wouldn't be too high.

[SteveM]

Kurt, on their website, shows clamping forces given the torque applied to the handle.

They say that, on the D688, with 80 ft-lb of torque on the handle, the vise is clamping with (GASP!) 7,342 pounds of force.
https://www.kurtworkholding.com/manual- ... -1205.html

My guess is that if they are listing that torque and pressure, they think that clamping that hard is within the limits of the vise.

Now the limits of what is being clamped? That's another story.

Steve

[warmstrong1955]

Just for reference, a 3/8"-16 grade 8 capscrew torqued to 35 ft/lbs has a clamp load of 7,000 lbs.


Bill

[whateg0]

I doubt I ever get near 80 ft-lbs of torque on my vise. I don't really know how to describe when I think it's tight. I guess it's kind of like torquing lug nuts without a torque wrench, you just use your calibrated wrist. Bad example, I know.

Not really an answer, but something to think about, if you are clamping something with a small contact area, that 7,000 lbs can translate to an extremely high pressure. That's how hardened jaws get dimpled by a ball bearing. Just as an example, say a part that is 1/4" diameter is clamped with the ends against the jaw, and that 7000 lbs of force is applied. That's 140,000 psi!

Dave

[pete]

Reliable torque numbers such as with a proper torque wrench also gets you repeatability on multiple parts of the same type. Lot's of places doing fairly accurate milled parts seem to use that method. Even a Kurts fixed jaw can flex enough to change a referance coordinate on tight tollerance parts. Would that be a concern on something like a Bridgeport and using the dials on a well worn machine? Maybe not. To mention the obvious, how tight the vise should be tightened would be a function of the part shape and it's depth. With no parallels used and a full jaw depth part then a decent torque number could be used compared to say a 1/2" thick plate sitting on those parallels at the top of the jaws. Kurts numbers would be pretty close for any of there vises because they produce consistant quality. A smaller bondo filled one with barn sweepings material used for the castings, poorly machined nut and male thread then not a hope. My first Chinese mill vises would show the rear jaws flexing enough under even moderate tightening pressures to affect the part holding. If you've never had a part move then maybe your using a bit too much. Seating the part firmly on the parallels then checking the top of the part with an indicator for movement as the final tightening is done might show you should ease off a bit or that your not affecting the part at all. But if a part then moves on you? I never mentioned that. I'm positive we've all looked at a part and knew for sure that light cuts and feeds would be a smart thing to do just from previous experience because the vise couldn't be tightened as much as we'd like.

Like a lot of things I don't think there's any hard numbers anyone can use and expect to get reliable results every time just like Whateg0 said. It's probably something that can only be learned with that experience and eye ball estimates for what the part will take against the guesstimated cutting forces. There's too many variables adding to the problem such as each materials coeffcient of friction and how hard your pushing the cutting tools or even tool sharpness. Then you can add the material removal possibly affecting the parts rigidity at some point. There's at least a few more variables but you get the idea. I've never had a part move while flycutting it, a few have moved with end mills though. Pocketing an internal shape will add a lot of drag to try and pull the material up when touching the XY transition point than profiling the O.D. will. They do make down cut or opposite helix endmills if you really need them. A big fat drill will do the same on the break through. I've not had a drill move a part at least on a mill, so I'm guessing right for those so far. You can also use your mills hold down set along with it's milling vise if there's any doubts about the part moving. Imo the only thing worse than having a part move is drilling or milling the table or vise. Doing that once should be enough for anyone to ruin there day. There's more than a few YT videos showing crashes and parts moving. They even sound expen$ive.

[Mr Ron]

Thanks. There seems to be a never ending question about seemingly simple actions that we do and don't think about. There are so many things to remember when operating a mill that affect your setup; vise torque, backlash, speed, removal rate to name a few. Machines are similar to playing a musical instrument; practice makes perfect.

[pete]

A standard machinist's apprenticeship is 4 years. That's with working full time at it and learning from hopefully some other highly talented journeymen machinist's in the same shop with long years already put in. Then you go back to school every year for more testing and book learning with some practical tests. At the end of it all you get a certificate if you pass that's about good enough to get you an entry level position somewhere else. And if you care at all you'll spend the rest of your career learning. Even then there's various skill levels and some are exceptional. If it were easy then the fry cook at Micky D's could do it. Trying to learn and gain as much experience as possible to get even half way competent at the hobby level seems to take forever and it probably does. The more I learn then the more I have to respect those who do this everyday who have to hit those tight time and number limits. I've never seen it here, but I have to shudder a bit when I see a few on the hobby forums calling thenselves machinist's. That's more than a bit insulting to those who are.

Yep musical instruments that can bite and cost you a whole lot if you hit the wrong note and bad enough.

[tornitore45]

When I started this hobby I read lots of books. I know my machines that I could tear them down and reassemble them blindfolded. I have added some very useful modifications. I have nobody to mentor me. I have learned the hard way what is the wrong way to approach every single operation. There are so many things the books do not tell. There is a lot to be done by feel.
For example milling a thin part on edge do not keep the end mill axis on center and cut across, the part will vibrate and sing, move off axis to just cover the part with the end mill tip, that way most of the force is pushing the long axes of the part.
How many time something went wrong minutes before you thought "This is not really a good way of doing this".

[Harold_V]

Even a Kurts fixed jaw can flex enough to change a referance coordinate on tight tollerance parts.

I've related this story before, but the very reason I purchased my first Kurt was because of what you mentioned. I had a nearly new Bridgeport vise, which I had purchased new, and was trying to hold 5/8" square brass parts while machining each face. They had to be held within a half thou of being square, and the BP vise couldn't repeat, nor did it yield square parts.

I ended up tilting the head of the mill to compensate for the out of square condition, but the vise was still incapable of holding the tight tolerance. At this point in my life I had never heard of a Kurt vise, but one was recommended to me by the salesman at the tool supply house I frequented. I reluctantly bought one, thinking it would make no difference, but it proved me wrong.

The first thing I had to do was to sweep in the head, so it would, once again, cut the parts square to the vise. Once done, I was pleasantly surprised to discover that the vise was capable of repeating, so running the balance of the parts, which were both read and write heads for IBM computers, ran without issue. To be fair, the parts were leaded brass, and the cuts I had to take were relatively light, so a low level of clamping pressure was required. Consistency of tightening was more important than how tightly the vise was closed.

All I did to these parts was to rough out the center, then machine them square, and on size. Only about .015" was removed from the parts in bringing them square, with all sides machined to assure they were square. Once square, and on size, the parts were then finished machined with an NC mill by the shop from which they were contracted. I make mention of the NC to give readers an idea of how long ago this was. All of this occurred in the late 60's, long before there were CNC's on the market. I worked with this shop on a huge array of projects, and had a good relationship with them. As a result, I have at least one of the heads in question, parts that were scrapped by them in finish machining, as well as one of the blanks that were prepared at my hand.

H

[Harold_V]

For example milling a thin part on edge do not keep the end mill axis on center and cut across, the part will vibrate and sing, move off axis to just cover the part with the end mill tip, that way most of the force is pushing the long axes of the part.

Correct. Also, which edge of the end mill is critical as well. Rotation of the end mill should push the part away from the end mill. If the other side is chosen, it almost guarantees trouble.

How many time something went wrong minutes before you thought "This is not really a good way of doing this".

Brings to mind the young fellow who worked at Huetter's Mill, in Murray Utah.

Huetter (the founder of the mill) was one of my folk's neighbors. He had long since passed, but his son ran the mill, which employed about a dozen people. I did maintenance for them on their power tools, and they, in turn, made things I wanted made. A great working relationship.

One of the people employed there was a young dude, strong as an ox, and almost as smart. This is about him, and a decision he made.

They had a huge Onsrud router, powered with something like a 10 horse motor. Typical of routers, it ran at mind boggling speed. This young guy, who will remain nameless, made the fateful decision to hold, by hand, an item he intended to route. At lighting speed, it removed a thumb from one of his hands as the cutter grabbed the part.

I fully expect that, after the fact, he thought that "This is not really a good way of doing this".

The thumb was re-attached, but it was much shorter than it was prior to the accident.

H

[tornitore45]

How many time something went wrong minutes before you thought "This is not really a good way of doing this".

Actually I meant to say "Minutes After" as to underscore the fact that there is little voice in you that tells you are about to screw up.

[SteveHGraham]

One of my many pet peeves involves overtightening things. I bought a Harley new, and the dealer did the first oil change for nothing. When it came time for the second oil change, it was nearly impossible to get the filter off. Some idiot had used a wrench to install it. I had to make my own tool to remove it.

HD puts oil filters in places where it's not possible to grip them with generic wrenches, and just coincidentally, they make and sell expensive tools for removing them. That's another pet peeve. When you pay $18000 for a $9000 bike, you ought to get a little more consideration.

A well-known Youtube machinist uses a breaker bar to tighten his 4-jaw chuck, which is a kind of vise. He got rid of his old chuck because it was bell-mouthed. I wonder why. He knows a trillion times what I do about machining, but it's hard for me to believe his method is correct.

When the rest of us buy used stuff, we don't know how many times things have been overtightened by people who smugly thought they were doing things "better."

I would never use a hammer or breaker bar to tighten a vise. I give the handle a good crank and let it go at that. I've never had a part move. I don't think it's that hard to hold most parts in place. For me, the hardest thing is getting parts to stay where they should be while the vise is tightened, and even that is not very hard.