How to choose the correct bit for a screwdriver

Question

I'm constantly using a screw driver for small electronics and around the house, but I find that multiple bits seem to work. Of course, something bigger or something small requires diferrent amounts of pressure when screwing or unscrewing something, but do any of you guys use a specific method when finding the right bit for a particular screw. Mostly interested for the use of small electronics because stripping the head seems to be like the worst thing that can happen.

Not sure if I'm using the correct terminology

Answer 1

Michael said many good things in that response. Read it. As an addendum....

In most cases, if it feels loose, then use a larger driver. A screwdriver should fit snuggly in the recess of the screw you are driving, without taking force to inset the bit into the screw. If it does not, then you are using the wrong one. It should have a shape that matches. Do not use something with only approximately the right shape, and jam it in. Thus, do not jam an Allen head bit into a Torx screw. This means you must know what kind of screw it is. If you don't/can't recognize the type of screw, then don't try to turn it.

There are some screws that are a combination head. For example, some have a doubly slotted head that will take a Phillips driver, but also a square recess for a Robertson driver. In that case, use a square (Robertson) drive. It will be a better fit, and is far less likely to slip and cam out. Even better is to use a driver that is designed to fit into that style of screw, with a shape that completely fills the recess to yield the highest torque.

For a flat head screw, again just use the largest driver that will fit snuggly into the slot. This will offer the greatest amount of leverage and thus drive the screw with the greatest torque for the largest turning power. Since large screws with large slots may require a large amount of force to drive and remove, this just seems like common sense.

Answer 2

Stripping a screw is indeed a terrible thing to do. It is best to avoid the problem of stripped screws by not stripping them in the first place, but you should also be prepared to deal with a stripped or rusted screw in the event that you have to deal with one.

Your first step should be to look at the screw's head and find several screws that you might think fit well in it. Don't bring one and try to force it to work. Bring several and test as best you can to find the one with the best fit.

Take your time. When driving a machine screw into a tapped hole, line it up and start driving it, being gentle and operating on the assumption that the screw will go in without much effort. Again, do not "force it" to work. If effort is required, assume that you have crossthreaded the screw and take it back out. If, after several attempts, you have determined that you're not crossthreading the screw, try to figure out why so much force is being required. If the tapped hole is large enough, you may want to try spraying it out with compressed air. You can try putting a drop of lubricant, a tiny spray of WD40 and letting it sit for a few minutes, or using anti-seize (lubricating compounds that prevent galling.) If all else fails, you can try driving it home with force (the ability of the driving bit to stay in the screw is dependent on its fit, forward pressure, and torque; in order to apply a lot of torque, you must first firmly press the driving bit forward, into the screw)... or you can drill the hole out a tiny bit and retap it, using machine screws from a local hardware store that you know will fit in your newly tapped hole.

If you are stripping larger (1.5"-3.5") screws that are being driven into wood, you can use an impact driver if you think the screw can withstand the force. (18V right-angle impact drivers have around 500 in-lbs of force for screws that can't take the 1300-1400 in-lbs of force that a standard impact driver puts out.) If you know the driving bit is about to disengage and chew up the screw, back the screw out an inch and try again. The first pass serves as a sort of "ghetto" pilot hole. If all else fails, drill a proper pilot hole. You may need to secure the two pieces of wood together somehow to keep them stationary during the process of drilling the pilot hole and then following it up with the screw. Possible options to secure the work pieces include C-clamps, temporary screws, temporary nails, vise grips, temporary supports, etc.

If you strip a screw, take it out and replace it. (Hopefully it isn't hard to find a replacement.) Removal options include (1) gouging a slot with a dremel tool and using a slotted screw driver to extract the screw, (2) using Eazy-Outs / Alden's "GrabIt!" / other screw extraction kits, and (3) cutting/grinding the head off the screw and then drilling the hole out and retapping it, regardless of whether or not there's a screw still left in the hole. Let the drill bit chew it up.

Answer 3

Currently the most common type of screw used on small electronic and household equipment seems to be Phillips head (named for the inventor, Henry F. Phillips). These come in a variety of sizes. The most common size is a #2 head, but many items use the smaller #1, and even the #0 size. There are even smaller sizes available in specialty sets for very small screws, but these are usually found on internal parts of electronics equipment, not usually on consumer accessible areas (one notable exception is the battery compartment of small electronic gadgets). There are even larger #3 and #4 size heads, but those are usually used only on the largest screws, and much less frequently in household items.

As noted by woodchips, you should always use the largest blade that fits the head. Using either a too small or a too large driver can chew up the driving recess. It is a good idea to have one of each of the standard sizes of drivers available, and to test which one fully goes in the recess fairly snugly.

With regard to slotted screw heads, there are flat bladed screwdrivers on which the blade tip flares out higher on the shaft. Again, the tightest fit available should be used, but the flare on that type of blade often gives a tighter fit if the slot is deep enough.

Answer 4

There are several types of screw heads that look like Phillips screws but aren't quite the same--Reed & Prince, JIS, Pozidrive, etc. The JIS screw head is likely to be found in electronic equipment.

List of Screw Drives