I didn't understand the value of a good vise, or know how to identify one, when I started my build. Your vise will be used over and over and over again. If you have a cheap one (as I still do), you will be constantly paying for that "bargain" in terms of time spent making up for it's shortcomings.
Truth be told, I didn't have the budget to buy a good vise when I started. And, you shouldn't delay your project significantly if you don't funds for a quality vise either. However, if you do have the option, get a quality vise. You'll thank yourself many times later on.
Here are the problems you can expect with a cheap one like my Harbor Freight example.
1) Poor casting quality with basic machining of important surfaces not acceptable:
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Vise #1.JPG
2) Unacceptably soft vise materials:
In the photo above, you'll see the built-in anvil as you look back towards the yellow towel dispenser. The basic castings on this vise are a soft cast iron. When you hammer on a piece lying on the anvil, it very often causes an indentation. It's surface is not hardened in any manner.
3) Most of the parts are out of square and uneven with each other. That's a big issue in practice.
In the photo below you can see where I needed to shim the removable jaws to get them even close to level (and square) with the removable jaw on the opposite side. It's still not right now, just better than when purchased. There are several shims on both sides of the vise jaws. The removable jaws themselves are poorly machined, and not flat and square themselves.
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Vise #2.JPG
Here is a specific example of the problems that causes, where I'm using a vise accessory, a small bending brake, meant for a vise. My out of level, out of square jaws causes me to shim the accessory to get the two halves to line up correctly. They are not as "solid" as they should be because of that.
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In the photo above, also notice the poor quality and definition of the pipe jaws cast as a basic part of the two halves (movable and fixed). They are not really usable, and certainly not very secure when holding round objects.
4) Poor machining and fit-up means sloppy operations:
In this photo you can see the bottom surface of the slide (the movable part of the vise) has a barely acceptable ground finish to it. It should be very flat and uniform without a lot of divots and grinding/machining marks on the machined surfaces. Operation of the lead screw (the big screw that moves the vise jaw) should be smooth and uniform. There will be a
little free play in the slide as you try to move it in and out by hand, but not much. If there's a lot of play, it's a poorly machined lead screw and that's not good.
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Vise #3.JPG
That slide should itself ride on a flat, smooth surface. In the photo below, you can see the slide runs on an unmachined, bumpy, cast iron surface. That makes it hard to operate, but more importantly, allows the slide to move around easily when you're clamping work into the vise jaws. This makes uniform clamping hard to achieve. In addition, when the vise jaws are not square, the slide will be forced to move to compensate under high clamping forces.
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Vise #4.JPG
With a lot of fiddling and shimming, I have the vise in decent operating shape, but not what it really should be. Had I purchased a quality vise, the above problems would not have occurred. Here's what it looks like now:
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You now see some of the problems with cheap vise, and consequently things to check for when you buy yours, whether it is new or used. One of the easiest tests to perform is to take a piece of paper and inch or two wider than the vise jaws and slowly clamp down on it. It should be uniformly held across the vise jaws and one side or the other (or the middle) should not be loose when the other side is held tightly. Likewise as you loosen the jaws, the paper should not tear as you pull upward on it because one side is free and the other is held tightly.
Well, what does a quality vise look like? There are lots of them out there, but here's one example of a high-quality vise:
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Vise #6.JPG
When you go to purchase a vise (new or used) take some paper, a small straight edge, and a good quality combination or machinists square with you. You're going to want to check everything, especially the jaws. Don't be afraid to take it apart either. It's easy to do, and you'll not be able to see how well it's designed and built unless you do so.
Cheers,