Clamps are key shop components for robot work. Uses vary widely, including holding stuff down to the bench for sawing or filing, holding pieces together for drilling, and holding parts on the drill press or mill.
There is a wide variety of sizes and quality. Here are some 4 inch clamps from Home Depot. The blue one is made by Irwin, at $8. The clamping force is not specified, and it just says it is made of "steel". The black one next to it is the same size in drop forged steel made by Jorgensen, and is $20. The difference is the material and the clamping capacity. The Jorgensen clamps at 6200 pounds; the Irwin does not specify clamping force . The third one, also made by Jorgensen but cast, not drop forged, clamps at 2400 pounds, has square threads, and goes for $13. For our purposes, drop forged clamps are probably overkill, but I like the cheaper Jorgensen with the square threads. Square threads make a significant difference in how much effort you need to put into tightening the clamp.
Deep reach C clamps
These let you clamp to benches, drill press tables, the band saw table, etc, with a lot of distance between the back of the clamp and the clamping surfaces. Careful on this, because they need to be very strong to support this distance and still clamp tight. Three different depths shown below. The deeper, the less rigid, so take some measurements on the machines before deciding.
Bar clamps can open up very wide, but are not generally as strong as a good C clamp. they are, however, a very good clamp to have around because of the ability to clamp over a large distance. Two bar clamps are shown here, one with a deep reach.
How do you check clamps for rigidity? How far can you turn the screw after the clamping surfaces meet? My 4" cast clamps with coarse square threads are tight after about 10 to 15 degrees. My lighter duty ductile iron 3" medium deep reach clamps with finer triangular threads are almost tight after 720 degrees. Big difference. Which is more rigid? For the deep reach C clamps, you might want to go with higher quality drop forged construction, and stick with less costly cast clamps for normal reach. But don't get me wrong. The smaller/lighter clamps are very useful. They just don't clamp as hard.
Machine tool work holding
For clamping work to a mill or drill press table, you can use c-clamps, but this is time consuming and awkward. A good drill press vise along with clamping accessories is a very worthwhile investment.
Drill press vise
For precision drilling, a drill press vise with step jaws is fundamental. Good ones can be very expensive, but there is a good range between junk and top of the line. The main things I recommend is step jaws and a way to easily clamp to the table with standard clamping kits, described below. The reason for having step jaws is so that you can hold work at the top of the vise with a "floor" that keeps it parallel to the machine's table. If your vise does not have a precision step, then you must rely on a spacer under the work, or hold it at the bottom of the jaws, which can get in the way of the work you are doing. Some illustrations:
Screw style vise with lug style clamping sites and step jaw. Note jaw also has V grooves for holding round objects. About $200 for 3" jaws
Pump action slide bar style with step jaw. About $165 for 3" jaws. No V grooves for holding round objects, but this style has an open bottom for drilling through without having to worry about drilling into the vise.
Inexpensive light duty cam action slide bar without step jaw. Slots for clamping. About $50 for 3" jaws.
Tool Maker's Vise
Though drill press vises are very useful, there is a tendency for the moving jaw to move slightly upward when the work is clamped. In general use, this is not a major issue, but it does result in the hole being drilled slightly off of perpendicular. Essentially, whether or not the jaws end up parallel depends on where the work is being clamped. An alternative type of vise is a tool maker's vise, which clamps in a way that forces the jaws to stay parallel. Typically this is done by forcing the moving jaw diagonally onto a precision ground bed at the same time the work is being clamped. Tool maker's vises are a bit harder to set up, but keep everything square and parallel. The example in the picture has slots along the sides and ends for clamping to the table using t-slot clamping sets as described below.
T-Slot Nuts and/or bolts, flange nuts and studs, strap clamps, step blocks
These come in different sizes, so you have to check the size of the T-slots where you want to use them. They fit into the table slots on the drill press and are great for holding down a drill press vise, other clamping fixtures, or the item you are drilling. This stuff is pretty inexpensive, and is much more effective than using c-clamps. As part of this type of clamping system, there are clamping bars that can be used for holding work or fixtures that do not have clamping tabs or slots. Many of these have steps for use with step blocks as shown below.
You can also get a clamping set that has a lot of of these components for just over $100
These screw down on the table through the slots, and are good for holding work too large for the drill press vise. They are more convenient and flexible than C-clamps because they use the table for part of the clamp and can be positioned along the table slots, not just the edge of the table. These are $20 to $100 depending on size and capacity.
X-Y vise. Good ones are very expensive, but even cheap ones are sometimes handy for positioning multiple holes on the drill press. These have a lot of play and backlash, and are not precision tools, but for the right price, they can be useful if you pay attention to how far the work moves with each turn of the positioning screws. (Using these for milling slots and such on the drill press is tempting, but not a good idea. Remember that on most drill presses the chuck is not solidly attached, and depends a lot on downward drilling forces to keep it tight on its Jacobs or Morse taper.)
This vise is $60 at Harbor Freight.
Toolmakers Parallel Clamps
These look like miniature Jorgensen woodworking clamps, but they work differently. You back off the rear screw, tighten with the middle one and then finish with leverage by tightening the rear screw. These come as small as just over and inch and up to about four inches. These are incredibly useful for holding small stuff, clamping two or three parts together for drilling, etc. A pair of top of the line Starrett clamps with 1.25 inch max jaw opening is $45. But you can get lower end ones that work well enough for as little as $5 for 1.25" up to $23 for 4.5"
Kant Twist Clamps
This come in a variety of sizes, hold tightly, and work well in close quarters. They are not always a good substitute for a toolmaker's parallel clamp, but they are much easier to use and almost as secure. The freely rotating pads at the ends of the jaws align easily with the surface of the clamped item, and there is a v-groove on one side for holding small cylindrical objects. The rotating pads do, however, make these mostly useless if you need to clamp very close to an edge. There is a variety of jaw styles and other features. These vary in price from about $15 for a shallow one inch opening to around $45 for a six inch or $185 for a 12 inch.
For drilling round items, it is good to have a way of holding them. The typical thing for this is a V block. The kind with a clamp is nice, but you can improvise with just a block. The examples below come in matched pairs for holding round stock at two points on a machine table. Price range $29.50 to $56.60 for capacity up to 1.5" These can get really expensive for larger work, but for larger work there is usually another workaround.