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What size of drilling screw diameter do I need for a worn bore thread? (In my case, it is related to eyeglasses.)

The original screw thread diameter is 1.4mm, but since the bore thread is worn, I need to use a drilling screw that, I assume, should be larger (but not equal) than the original one. If it should be larger indeed, how much larger should it be? +0.1 mm or less?

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I am fairly certain that neither self-tapping or self-drilling (not the same thing) screws exist for diameters that small. Furthermore, self-tapping threads that are that fine which are required for lengths that short do not seem workable.

You are also probably not equipped to drill it and tap it even if you had the drill and tap. It's just so small and short your that the alignment is going to be off or something is going to cross thread or break.

If you did drill out and re-tap the hole, you would use a 1.25mm drill (which is the tap drill size for a M1.6 screw) and then follow that with a M1.6 tap. And you would not use a power drill, because if you had an appropriate drill and setup you wouldn't have asked your post. You would use a pin vise and do it manually by hand but that makes alignment much more difficult.

Might I suggest pinning it? You would not necessarily need to drill out the hole which would giving you more attempts at it if you mess up. If you can't get a press fit to work, then you cal also flare the pin on both ends of the pin with a punch to get it to stay in.

Also, how did you get that 1.4mm measurement? For the precision you require and the diameters you are working with, caliper measurement should not be trusted.You would need a micrometer for that.

UPDATE:

I did look at on McMaster-Carr and there are M1.4 "self-forming screws for soft metal". Forming a thread does seem like it would work better than cutting a thread for something this small.

So if you trust that M1.4 measurement, then get M1.4, M1.6 and M2.0. I would try the M1.4 first to see if you can't just reform the thread. But it will probably not be removable safely after that. If that fails, then eyeball the M1.6 and M2.0 for which one you think will work. If that fails then you can go with the pinning approach previously mentioned. If you go to M1.6 or M2.0, you would not be drilling it out because you're probably going to mess that up, even if you use a pin vise and to it manually. You're going to be trying to force it in so it will take more effort than normal, but it might also not work and break or strip so you need to judge when it is not working and back off to go with a different method.

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  • $\begingroup$ I got 1.4mm with caliper(0.05mm),i guess it is not 100% accurate,but i assume that this is the size.In regards to the M1.4 self tapping screw,Is there a chance that it will reform the worn thread(from your experience)?In theory(in that case),what should be the ideal self tapping screw diameter to reform the worn thread?M1.41?M1.42?... $\endgroup$
    – Dan
    Commented Jan 21 at 14:10
  • $\begingroup$ @Dan Screws are only produced in limited diamaters and pitches so you don't really have a choice. Reforming a worn thread will only really work in really soft material and the chances are less likely in a really small screw that where the head strips really easily. What you technically want to use is a M1.4 forming tap, but forming taps are difficult to get ahold of and costs more than a screw. You could get an M1.4 cutting tap and run that through by hand but you need to make sure your alignment is dead on and get it to start following the existing thread. $\endgroup$
    – DKNguyen
    Commented Jan 21 at 18:25
  • $\begingroup$ @Dan Assuming you could properly align a drill and tap to drill this (with a pin vise, by hand, not a power drill) you would use the recommended tap drill size for a M1.6 screw and then follow that with an M1.6 tap. Then use an M1.6 screw. That's the proper way to fix it but I feel you're probably going to mess the alignment up which is why I suggest just pinning. $\endgroup$
    – DKNguyen
    Commented Jan 21 at 18:26
  • $\begingroup$ By, the way, the recommended tap drill size for a M1.6 screw is 1.25 mm for a cutting tap. It is different for forming taps (and needs to be much more precise too). You can find all this by searching around on McMaster-Carr for taps, and screws, and all the taps have the tap drill size listed next to the tap. $\endgroup$
    – DKNguyen
    Commented Jan 21 at 18:29
  • $\begingroup$ In regards to the M1.4 forming tap,Are there different pitch distance of M1.4 forming taps?As i need to screw back the original screw and i wonder if there might be a problem due to a possible difference between the pitches of the original screw and the M1.4 forming tap. $\endgroup$
    – Dan
    Commented Jan 23 at 11:31
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You're not looking for a "self-drilling" screw. The hole is already there. You're looking for a self-tapping screw - one which will cut its own threads.

This isn't really likely to work well.

  • You'll need to enlarge the clearance hole in the top of the frame hinge screw entry point.
  • You'll need to enlarge the clearance hole in the arm.
  • You won't have the benefit of the shoulder on the screw to allow you to lock it down without pinching the hinge tight and running high friction on the hinge. (A quick image search shows that not all screws are shoulder type.)

If you insist, then look at the self-tapper screw charts and pick one with a thread minor-diameter < 1.4 mm. Then the threads will have maximum grip. Take note of the major thread diameter and buy a drill bit to suit for the clearance holes.

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  • $\begingroup$ When you adviced:"...look at the self-tapper screw charts and pick one with a thread minor-diameter < 1.4 mm."is there a typo with the"<"mark?didn't you mean to pick self tapping screw with a thread that is larger than 1.4mm?since a smaller than 1.4mm will be smaller than the bore,am i wrong? $\endgroup$
    – Dan
    Commented Jan 21 at 14:19
  • $\begingroup$ @Dan, the minor-diameter is the diameter of the actual shank of the screw. The major-diameter is that + 2 x thread depth. Since you're screwing into a stripped 1.4 thread then the shank has to fit through that. The threads can't enlarge the hole for you - only cut threads. e.g., An M6 screw needs a 5 mm hole drilled for the tap. You can't get the tap into a 4 mm hole. (Well, you probably can force it but it will not be working properly and may break.) $\endgroup$
    – Transistor
    Commented Jan 21 at 15:08
  • $\begingroup$ If M6 screw needs a 5mm hole for cut threads,it means that the hole is actually enlarge by the threads to 6mm,right?So,if my eyeglasses screw is 1.4mm(major diameter),Does it mean that the eyeglasses bore(before tapping)was actually 1.3mm?Does it mean that 1.4mm(major diameter)self tapping screw is the right size to fix the stripped thread,as the original screw size of this eyeglasses is 1.4mm(major-diameter)? $\endgroup$
    – Dan
    Commented Jan 21 at 17:05
  • $\begingroup$ @Dan, no the hole is not enlarged to 6 mm. The hole is still 5 mm but threads are cut in its surface. That doesn't make it a 6 mm hole. $\endgroup$
    – Transistor
    Commented Jan 21 at 17:08

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