Drilling out self tapping screws

[boxofbits]

Morning all

One of the annoying qualities of a self tapper is that it will quite easily snap when torqued, but when it comes to drilling the remains of it out it's as hard as granite!

I am trying to remove a 1/8" self tapper in an alluminium thread, and can't get any purchase on the screw itself.

I've tried a cobalt drill bit, but with limited success. Is it going to be a machine shop process or is there a certain type of drill bit for this?

Thanks for help

Kevin

[AndrewMAshton]

Hello, as its a self tapper it should have a very course thread, could you not 'tap' it to break it's hold, an old boy once showed me a way of dealing with rusted self tappers, tap the end of a screwdriver at the same time as you apply some turning force, seems to work, cheers, Andrew

[Motorsport Mickey]

If you can hold the part vertical and get a pillar drill or drilling machine on it you can feed the drill in very slowly waiting for the spinning bit to wear away the upstanding or uneven self tapper piece which is causing deflections.
A stellite or cobalt bit will get past most hard objects as long as you can avoid the drill wandering, very slow advancement here is the trick and lots of revs.

Mick Richards

[RobH]

You can't really drill it without the drill wandering but if not damaging the component is a priority you can dissolve it out using a solution of ALUM (Aluminium Ammonium Sulphate) which won't harm the aluminium.  The only other reasonable way is spark-erosion.

 

Its important that the alum solution is kept hot. Mixing proportions are approx four tablespoons of alum to a cup of water.  For a large component you can build a plasticine dam around the hole and heat the component. Takes a few hours to work.

Works on HSS drills and taps too.

Edited September 10, 2016 by RobH

[boxofbits]

Thanks all for your helpful answers. Fortunately I am able to get a drill bit on it without slipping off, and there is only around 3/16" left in there. Mick - my drill is only 660 rpm max so I guess this is not fast enough?


An engineer friend has suggested a carbide drill bit and Dremel drill for speed. Rob - As regards dissolving it with ALUM the hole is very small so I it would probably not be possible to keep the solution hot, other than that it would probably work.

Regards

Kevin

[RogerH]

Hi Kevin,

the problem with small carbide drills is that they usually are not tipped but solid - and they snap if you blink.

 

If the hole around the PK can be opened up then drill down the sides.

 

Spark erosion is very good but can also damage the threads in the body but worth a try.

 

Roger

[boxofbits]

Hi Roger

 

Yes breaking drills is a problem, and carbide at 2.5 mm is going to break.

 

I might splash out and get a Tungsten Carbide tipped drill but that's going to be about £16 a snap! Don't mind too much if it does the job though!

 

Bit worried about spark erosion, but will have to keep it as an option. I am currently using a 2.5 mm left hand cobalt Snap On drill bit and it's taking some punishment and hasn't broken yet, but I can see it becoming Snap Off if I'm not careful.

 

Kevin

Edited September 10, 2016 by boxofbits

[Steve T]

I've used a small masonry bit before. Re-sharpened it so it works backwards and reversed the drill. As it cut in it caught and unscrewed the self tapper. I used a diamond tile cutter disc to re-shape the drill bit.

[boxofbits]

Hi all

 

Just thought I'd update my progress on removal of a broken self tapping screw. As mentioned previously they are extremely hard meaning HSS drill bits won't touch it. I did manage to centre pop it which is essential, and then used a Snap On left helix cobalt 2.5 mm drill bit. I have to say this drill bit did not snap or wear out despite a lot of pressure being applied, so they do appear to be an extremely good make. It did make an impression but not sufficient. In the end I bought a 2.5mm TCT drill bit (Tungsten Carbide Tipped) and no sooner did I apply pressure to it, it went completely through. It did snap, but only due to snagging in the remnants of the screw.

 

I'd recommend going for this option straight away if anybody comes across a similar problem.

 

Kevin

[Tonyloz]

I've used a small masonry bit before. Re-sharpened it so it works backwards and reversed the drill. As it cut in it caught and unscrewed the self tapper. I used a diamond tile cutter disc to re-shape the drill bit.

[AlanT]

You are right on it SteveT. Masonry drills.

 

Wont stay sharp for very long so keep sharpening. Quite cheap and wont break easily.

 

Run slow with a fair amount of pressure. NO LUBRICANT. This will be a slow job.