Why are different screw heads used in UK 3 pin plugs?

The manufacturer is at liberty to select the type of screw head used, so today it's a question for each manufacturer's design team; however, there are some trends ... see below.

AJJewsbury said:

I suspect it's just down to what's most efficient/convenient in the factory

To an extent, but there is be a bit more of a logical explanation.

Originally, all screws were flat head. Then, 'tap-tite' screws started to be used when the old cardboard cable grips were replaced by plastic. This is a manufacturing decision, because of production line power tools used because of the strain on hands with new plastic parts that have not had a screw inserted previously, and to avoid damage to screw heads and injury from flat screwdriver bits with power tools. Typically, the tap-tite screws were pozi, but later (as per the image) flat-pozi or flat-philips combination screws were used (like the example shown).

Next, combination screws (flat/pozi) have been used for, first, the top cover, then later the terminal screws.This enabled flat to continue to be used post-manufacture, but pozi/philips in the factory. Terminal screws were last for the treatment, because only more recently has it been possible to use torque-controlled power tools for the terminal screws in the factory.

Occasionally, we do now see pozi-only or philips-only options, but this is the decision of the manufacturer.

Lisa Miles said:

So, I went to unscrew the plug so I could pop the bung back in but then noticed that there are different types of screwheads used in the plug?

 Lisa Miles , if you look closely, the manufacturer of your plug has chosen an option of a combination flat-pozi, for the cable grip screws, so you can use flat only for all of the screws.

Lisa Miles said:

Why is that? Why are they different? Is that done deliberately?

Yes, see above, this is a deliberate decision for manufacturing power tool use.

The manufacturer is at liberty to select the type of screw head used, so today it's a question for each manufacturer's design team; however, there are some trends ... see below.

AJJewsbury said:

I suspect it's just down to what's most efficient/convenient in the factory

To an extent, but there is be a bit more of a logical explanation.

Originally, all screws were flat head. Then, 'tap-tite' screws started to be used when the old cardboard cable grips were replaced by plastic. This is a manufacturing decision, because of production line power tools used because of the strain on hands with new plastic parts that have not had a screw inserted previously, and to avoid damage to screw heads and injury from flat screwdriver bits with power tools. Typically, the tap-tite screws were pozi, but later (as per the image) flat-pozi or flat-philips combination screws were used (like the example shown).

Next, combination screws (flat/pozi) have been used for, first, the top cover, then later the terminal screws.This enabled flat to continue to be used post-manufacture, but pozi/philips in the factory. Terminal screws were last for the treatment, because only more recently has it been possible to use torque-controlled power tools for the terminal screws in the factory.

Occasionally, we do now see pozi-only or philips-only options, but this is the decision of the manufacturer.

Lisa Miles said:

So, I went to unscrew the plug so I could pop the bung back in but then noticed that there are different types of screwheads used in the plug?

 Lisa Miles , if you look closely, the manufacturer of your plug has chosen an option of a combination flat-pozi, for the cable grip screws, so you can use flat only for all of the screws.

Lisa Miles said:

Why is that? Why are they different? Is that done deliberately?

Yes, see above, this is a deliberate decision for manufacturing power tool use.