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How do you know when to use a certain type of screw

Update:01-06-2020
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Really, you can use whatever drive style you like as lo […]

Really, you can use whatever drive style you like as long as you can find it in the type of screw you need. The only times, as far as I can think of, that you would need a specific drive style comes down to three things:

Point 1: if you need a specific (particularly high) torque, you will be far better off using a star or square drive screw rather than a Phillips or flat head screw. The Phillips drive style was a great choice but once you exceed a certain torque the drive will slip causing cam-out.

Point 2: take a look at your nearest outlet. You’ll notice the cover most likely has slotted head screws in it. This is because slotted head screws are considered to be less obtrusive on the eye. Making the product look more finished. In fact, almost all electricians use slotted head screws for this type of application even though it is not truly necessary.

Point 3: Back before manufacturing was as advanced as it is today, slotted screws were easier to make which made them the only choice for a lot of installations. Now when someone is doing a restoration of something made during that time frame, they want to make sure they get the right screws for the era it was made it to not cheapen its value. A lot of fakes can be easily spotted this way.

Material: Each has a different corrosion resistance, hardness and brittleness. Drywall screws are cheap and brittle. They will snap off quite easily, but withstand drywall. Brass screws are typically soft, and unless used very carefully (pre-drilling), Robertson or Phillips heads will deform and slip if soft brass. Slotted heads are still common for brass. If screws are to be countersunk and filled, if removal is needed at some point, old slotted screws can be cleaned out easier.

Sometimes a specific type of head is the only kind for a particular material you can get. Stainless steel or bronze screws of a desired size may only be available with oval Phillips heads.

Robertson (square head) screws will hold on a driving bit quite reliably for placement. For others, not so much.

If you are drawing two pieces together, and most especially wood, you will want screws with a section of unthreaded shaft between threads and head. This allows the threads to draw the head down through the top piece. Threads stop such compression.

Sometimes you want the screw to not draw down into a soft material (wood) or to allow a washer to sit flat between the head and the workpiece, so then you want a round-head type of screw with a flat face beneath the head. Other times you specifically want the screw to be drawn down into the material, in which case the typical flat-head screws with a tapered lower face is recommended. You can even get screws with different tapers!

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