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Hardware without an undo function?


(Julian Warren) #1

Has anyone met a case on an electrical appliance that seems to have screws at the bottom of wells (about 14mm down so tricky to reach), but where the rounded top of the screw is perfectly smooth (apart from a tiny >1mm flat plateau on the top)?

My guess is that either:

  1. They’re special screws that had a small top on them that was then milled off.
  2. They’re some sort of heat rivet set into the plastic.

The question is how do I get the blighters out?


(Chris) #2

Sounds like fun! What item are they on? Might lead to some more clues. If it’s something relatively common I would suspect it’s number 2, number 1 sounds fairly complicated for any kind of mass production.

You might find a set of cheap “screw extractors” might help, there’s a few kinds if I remember correctly, one that requires a pilot hole drilled and some that don’t.

Failing that, drilling it out is my usual approach to getting at electrical goodies, especially for rivets (in fact I think I did this recently for my ham radio mast thing at the space).Your goal really is to separate the head off the screw, and that usually works quite quickly with the judicious application of the correct drill bit size! :slight_smile:


(Julian Warren) #3

It’s on the (I’d be) dead (if I relied on this) man’s handle of an electric lawn mower.

Out of interest, can you remember if the pillar drill can run in both directions or not?


(Chris) #4

Unfortunately I don’t believe it does. The motor is definitely mains AC and I don’t think it has a reverse pulley wheel of any sort.

If it’s a universal motor we could try to be clever and stick DC over it, and see if that let’s us reverse the direction somehow. :slight_smile:


(Julian Warren) #5

If it’s a single phase induction motor instead there’s a decent chance that we might be able to add a reverse option too…


(Chris) #6

Excellent. Let’s have a look tonight, would be a good hack :slight_smile:

And it should be mandatory now for me to mutter something about ‘if we had a mill, that would have a reverse gear…’


(Mike Bookham) #7

How did you get on with this?

It’s a bit of a coincidence, as today I’ve just fixed the mother in law’s dead man’s handle on her mower. One of her friends had replaced the mains cable but ended up with a left over spring which he couldn’t work out where it fitted. It turned out that the spring returned the switch handle to the off position when you let go of it.
At least this mower’s switch box was held together with Philips screws, so getting it apart was easy, though working out where the spring went was a bit of a mission, but I got there in the end.


(Malcolm Napier) #8

I think that what you have encountered is a shear head security bolt:

(from your description, probably the left hand one).

If it is a really nasty one, then it will be hardened steel as well but I suspect not in this case.

Drillign the head off is the only option. Good luck!


(Julian Warren) #9

I guess it must be the lawn mower peak malfunctioning season @MikeBookham! I managed to drill one of them out (but probably a bit over enthusiastically).

That could well be it @Malcolm_Napier - I wondered if the top was too neat to have been sheered off, but it seems the simplest design.

I do have an alternative idea for a way to shift the blighters, other than straight out drilling. It involves slightly more work, but hopefully less destruction!