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Help and advice on a drill bit for hardened steel

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Ian m 114/10/2019 11:59:02
9 forum posts

I am confused as to what type of drill bit I need to drill a hardened steel stud that has snapped due to someone's ham fistedness.

I think I need cobolt, is this correct 5% or 8%?

what tip angel and helix angle do I need? Seeing references to these on the internet has blown my mind.

I'd appreciate what good brands to look at.

I want to get 4mm, 6mm and 7.5mm drill bits. These will be used in an electric hand drill not a pillar drill or lathe.

thank you in advance

Alistair Robertson 114/10/2019 12:25:25
88 forum posts
6 photos

I don't know what your broken stud is screwed in to, aluminium,steel or cast iron.

Personally I wouldn't attempt to drill a hardened steel stud unless it was with a drill press or mill and with the component clamped in place.

A far better way is to build up a pillar of weld on the exposed end of the stud using a mig welder, then when the pillar is high enough to then weld a suitable nut on top of the whole lot. The heat of the weld is almost always enough to free off the stud and is a simple twist-out job. Use an aluminium bush to prevent the weld and spatter sticking to the surrounding metal.

I have never failed to extract a broken stud in this manner in forty years since I got a mig welder!

We had one welder who was so good he could build up a pillar on a 3mm stud and successfully get the offending stud out.

SillyOldDuffer14/10/2019 12:55:59
5607 forum posts
1153 photos

To get an idea of what's available, prices and 'good' brands have a look at RS Components.

The nature of the steel stud may matter. Do you know what it is? Stainless steel work hardens but can be drilled with ordinary HSS provided the right technique is used. (Basically pressing hard enough to cut, but not overdoing it. Needs practice.)

Other hardened metals are too hard for HSS, or at least blunt it rapidly. Various choices for these, in rough order of rising hardness and cost. HSS coated with yellow TiN, Cobalt (5 or 8%), Coated Cobalt, Carbide Tipped (TCT), solid Carbide, and exotics like Diamond or Boron Nitride. DIY store drills are best avoided, buy from someone selling to metalworkers.

Assuming your stud is hardened within ordinary limits, I'd try a TCT drill as offered for drilling stainless steel. If that fails, solid carbide, but they're expensive and brittle. Attempting the job with a hand drill is disadvantageous because they move about and magnify operator mistakes. Getting the cut to start can be tricky because the point tends to skate about. Good idea to centre-pop the stud (if you can) to steady the cutting point and to use split-point drills. Easier to drill the stud if the drill's held firmly in a stand, even a home made one.

I don't have a good track record drilling out hardened studs - broken drills, blunted drills, and scars around the work. No help at all, but the professionals use spark-erosion or weld a temporary new head on the stud to get it out. I expect someone else will offer better advice!


PS Must type faster, while I dithered Alastair got in first!


Edited By SillyOldDuffer on 14/10/2019 12:58:54

Dave Halford14/10/2019 14:12:39
694 forum posts
6 photos

Carbide tipped will drill it, allegedly possible with a reground masonry bit.

If this is to drill out an exhaust manifold stud or thermostat housing it is probably rusted in and the welding method is best

J Hancock14/10/2019 14:34:07
388 forum posts

If you are using a hand drill you will really need to somehow fix a guide hole over your stud.

Cobalt may do it but if it is as hard as you say then carbide is what you need.

Using the right speed will also be important.

HOWARDT14/10/2019 14:40:04
535 forum posts
15 photos

You say a hardened stud, how hard. I would think the stud would be tough not hard, as in En8 or 16. Hardening would make the thread brittle and likely to break. Usually studs can be drilled out and removed with normal HSS drills. I have never come across a hard stud since forces are in tension with no abrasive action requiring hardness.


old mart14/10/2019 14:44:13
1516 forum posts
136 photos

Solid carbide would be the answer if you could use a drill press, but with a hand held drill you would just break the bit. However, there are Bosch Multiconstruction drills in sets which have carbide tips and steel shanks like masonry drills. It is certainly possible to sharpen the tip of a regular masonry drill, hard going with a standard grinding wheel, but easy with a green wheel, or diamond wheel.

Some sort of drill bush to keep the drill on line would help if it is possible, and maybe using a Dremel to create a start point for the drilling especially if the fractured end is not flat.

not done it yet14/10/2019 15:30:04
4482 forum posts
16 photos

I’m in the Howard camp on this one. Just don’t break anything carbide in the hole or the problem may well multiply.

I tend to use left handed drills, but one must drill right down the centre if expecting the stud to turn out easily.

Welding, drilling (even dissolving in alum might be appropriate on the odd occasion in the right circumstances - I just removed a broken tap from a brass item that way - although this is presumably into steel as even really tough bolts are not usually needed into aluminium).

Guessing it is a 3/8” stud? Good luck without any effective alignment aids.

Ian m 114/10/2019 16:41:15
9 forum posts

gents thank you for the replies. An update.

I found a clip on Youtube on how to drill a stud that had snapped below the surface. Mine is above the surface but it gave me an idea on how to make a guide.

I used a 10mm joining nut on a 10mm bolt, I added a single 10mm nut and locked the joining nut and ordinary nut together. I cut the bolt to the length of the two nuts. placed the lot into the chuck of the lathe holding by the joining nut and pushed back in the chuck so the single nut was clear of the jaws. I then centre drilled and followed this with a 4mm hole all the way along the length of the bolt.

I made three using the process above, the other 2 had a 6 and 7.5 mm hole. The two nuts were then separated and the bolt removed. The joining nut then screwed onto the snapped stud that was on the engine. The drilled bolt with the 4mm hole then screwed into the joining nut. I had a guide.

Things had gone well to this point but my 4mm drill bit would not touch the broken stud and so I turned to the forum.

Whilst getting replies I changed the 4mm guide for the 6mm guide, not sure why. Anyway the 6mm drill bit worked. My cordless drill went flat so had to recharge it. Then progressed to the 7.5mm guide. I was not gentle enough, the bit bit got stuck and sheered the rest of the stud as the walls were now thin.

Anyway I progressed all by hand and no guide, 7.5, 8.5, 9, 9.5 and then finally 10mm.

OMG the guides were brilliant, if it were not for Youtube I would never have gotten the idea on how to create a guide.

In the end I think the problem was the 4mm bit not being very good. I apologise if I have wasted your time but when I posted I was desperate and you guys I thought would know.

thank you for the advice. now to order a nut and bolt

all the best

old mart14/10/2019 22:03:28
1516 forum posts
136 photos

Not a waste of time at all, helping others and knowing others can help you is a very important function of any forum.

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