What drill bit for drilling 1 inch long 5mm hole through 10mm grub screw?

Having problems drilling a short (approx. 1 inch) 5mm hole through the centre of a 10mm grub screw. This is part of a long-winded, and possibly doomed, attempt to fix an injector problem on my van, but since I've no money really, needs must, and I'd rather fail by destroying the thing than by giving up, so any advice would be appreciated!

HSS bits got me nowhere, so then went shopping and everybody in toolshops advised me to use cobalt drill bits but these only created a dimple and a red hot tip of the drill bit. Then bought some tungsten carbide bits off ebay - think the 5mm one was about £3 which was fairly reasonable I thought. However, I did a practise an a bolt and it got through about 20mm, then stopped. I'm sure I kept going for too long after it stopped going any further as now there's not even a point on the bit, it's just flat with a tiny nipple.

Obviously I can order another couple of these from the internet, but since it's now Saturday evening, nothing would arrive until Tuesday at the very earliest, so wondering if there's a way of using something that's available over the counter anywhere in order to speed things up here!

The hole has to be 5mm, by the way, so that I can tap a thread for a 6mm bolt that will ultimately screw into the grub screw. I don't imagine it will matter too much if the hole isn't absolutely straight, so long as it comes out fairly near the middle of the bottom! And if the top is a tiny bit mangled before it goes into a 5mm hole doubt this will matter too much either. Plus I've stupidly left the grub screw at my unit, so no information available on how hard it is, but don't recall seeing any numbers on it like you get on top of bolts. It's black, M10, regular pitch, and has 5mm allen key slot at top for turning.

Cheers.

If you are having that much of a job drilling it then a HSS tap won't stand much chance even if you get a hole through it. Only thing that may work is annealing the grub screw to get rid of any heat treatment and then have a go.

Have you tried sharpening the carbide tipped drill you already bought, would need a green grit or diamond wheel

J

Edited By JasonB on 02/08/2014 20:55:57

The grub screw in question is properly a 12.9 grade , if a cobolt drill bit won't drill it ,try a multi construction drill bit , flood the grub screw with coolant while drilling is your best option .

It would help me greatly if you could give me a brief outline if what you are trying to do, and why. The 5mm grub screw is not going to work unless, as Jason suggests, you anneal it first(not difficult - just heat to cherry red and allow to cool slowly).

graham.

Use a mild steel/stainless steel M10 bolt.

It might be somewhat softer but it would get you 'out the poo' whilst you're stint.

Cheers for all the replies. Reason it has to be a grub screw is that there was originally a smaller bolt in the hole and this bolt secured a clamp which holds down a diesel injector - there's a hole in the clamp, which if I make it any bigger will probably break the clamp. This bolt had worked its way loose and stripped all the threads, so have tapped bigger thread in order to get bigger bolt in it, but obviously a new bolt wouldn't fit through the hole in the clamp, so if I put a grub screw in and then screw original sized bolt into middle of grub screw it might work. (Have actually enlarged hole in clamp to 9mm but looks dodgy to take it up to a 10!)

It probably is 12.9 as I'm sure that's what's written on the black bolts of same type. Is 8.8 softer? I could try and find a softer grub screw I suppose, that's definitely one option. Or chopping the head off a softer bolt, drilling and tapping that, then make it take a flat screwdriver. Will also try a multipurpose drill bit to see what they're like as well.

Being a proper amateur with not much knowledge of these things other than when necessity dictates (though I do quite enjoy it in a strange way!), I have no real access to heat - does this annealing process last? Now this may be a stupid question, but could I just chuck it in a fire and then fish it out once it's cherry red, as one of you described, and if so would this last for good, or does the drilling have to be done straight after it's had its initial cool down? Plus, if I was to do the same thing once the hole was drilled out would this make the tapping with an hss tap any more likely to be a success?

PS carbide tipped bit definitely unsharpenable - think I've burnt all the carbide off with my enthusiasm on trial bolt!

Thanks again, and sorry about all the questions!

PPS What's best to use as coolant substitute? Grease or engine oil, or neither?!

Hi Ian

Now that we have more info on the job in hand.

The object is to secure an injector into the cylinder head on a diesel engine of cast iron or aluminum alloy.

Drill out the 10 mm hole and tap out to 14 mm and fabricate a softer bolt 8.8 as max strength 5.5 will be more than adequate with a slot to drive it in with a screw driver.

Loctite the 14 mm screw in the head and allow it to harden and dry.

Make sure that the internal thread is absolutely clean and no loctite liquid still in the thread to avoid making the injector hold down bolt a permanant fixture.

The insert screw does not need to be super hard as the cylinder head is relatively soft!

To finish with a question what is the diameter of the original hold down bolts for the injector as i have serious doubts that it is only 5 mm @ approximately 25 mm long

If as I suspect that is 8 mm then use a 14 mm for the plug.

Good luck with the repair and be positive this can be fixed.

Eric

Edited By oldvelo on 03/08/2014 00:51:07

Original bolt was about 3 inches long and M6, but threads didn't start until 2 inches down the hole meaning couldn't get tap down there. So decided to tap accessible top of the hole to M8 but this didn't grip enough ( probably hole was about 7.5mm wide I'm thinking), so have now tapped it to M10, hence the grub screw idea. (Had to leave some of the details out of messages as if I mentioned absolutely everything that had happened nobody would have energy to read it!) Will make a softer grub screw at M10 and follow yours and others instructions! Could go to M12 at a push I think, but any more it will probably be too close to hole for actual injector.

Need to drill hole at 5mm in grub screw so can tap threads for an M6.

Cheers, Ian.

The annelaing will last until you heat treat the screw again. If you have a coal fire or log burner then chuck it in there so it sits at red heat for some time. Then let it cool slowly, best way is to leave it in the fire overnight so it sits burried in the ash.

Engine oil will do if thats all you have. Are you drilling with a handheld electric drill or in a lathe/bench drill?

J

It sounds a bit too late now, but perhaps you should have used a thread insert which would have brought the tapped hole back to its original size. I think that what you are in fact making is a stepped stud which is 10mm on one end and 6mm on the other. May be a lot easier to get a 10mm bolt, turn one end down to 6mm, then run a die down the 6mm bit.

Did the OP forget to tell us the RPM he used to drill that 5 mm hole? In a drill press I suspect?

No one asked the important questions...

Regards, Frank

Can you use a stud and nut instead of a bolt? If so you could make a stepped stud.

I have always solved this type of problem with "Helicoil' inserts but if you have already drilled the hole out to tap 10 mm it's too late for that.

Russell.

Just an update to say that me and the van have made it back home in one piece at last today. Have been walking 7 miles each way to my unit for last month or so to try and sort this headache so in a fine mood here! Thank you very much to everybody for their very helpful advice - basically pinched bits from each one and it came good in the end - was slowed down by my cheap (but pretty useless!) bench drill holder from Machinemart - wouldn't have managed without it, just a £20 thing you put your power drill into and lower it down, but the vice that came with it broke, meaning that I had to G-clamp the flimsy thing shut all the time.

After further slight complications, ended up tapping the top of the hole to an M12, which is the only time it cut first time nice and easily, maybe it's easier to do big jumps in tap sizes in aluminium, when tried to go from M8 to M9 and then M9 to M10 was really hard to get a thread started. Made a grub screw from softer steel threaded rod.(Now an expert on my 8.8s and 12.9s, handy pdf document by Thomson rail, quite informative for the beginner I thought - just type in 'A Short Guide To Metric Nuts and Bolts Thomson Rail' and it comes up on Google). Drilled a 5mm hole with a cobalt bit into the softer steel, then tapped that with an HSS tap to M6, and hacksawed a slot for a screwdriver. This caused a problem as took me ages to find an 8mm slotted screwdriver bit, which I needed to use to properly tighten it down, the 7mm just kept falling down M6 hole, and couldn't use a normal 8mm screwdriver as bonnet was in the way! Didn't look particularly professional, but threadlocked it down and no sign of anything getting worse after doing 30 laps of my industrial estate, so took the gamble and drove home tonight.

If I'd known it would have ended up with an M12 would definitely have used Hellicoils, and if I had access to a lathe would then have made a stepped stud. Have read up on drilling speeds, and now a bit wiser, but definitely of the opinion that cobalt not much better than HSS and should get solid tungsten carbide drill bits in for all tricky jobs in the future. Plus why do they produce M7, M9 and M11 taps when it's virtually impossible to buy any corresponding bolts?! Not everyone has their own boltmaking equipment, although I am tempted to one day buy a lathe and start selling custom made bolts of strange sizes through eBay - the only M9 bolt of any length available in the whole world it seems is something from BMW, which has one of those male torx heads on it, and if someone doesn't have those sockets to hand, that's another £20 away! I think a fiver a custom made bolt might be a winner.

Anyway, waffling now, but wanted to say thank you for everyone's help, and will drop in from time to time, but unlikely I'll be able to assist with anybody's problem with my sparse knowledge, but you never know!

Cheers, Ian Dodds

I quite often need to drill unbrako bolts and grub screws or bolts of a similar hardness, I would drill first with a centre drill then a cobalt drill but would use a cutting oil like,Molyslip extrme pressure cutting oil,I might smoke a little but the drill would not have a problem cutting and staying share.

MWF (Metalworking Fluid)

MOLYSLIP MWF is a concentrated neat cutting fluid containing a unique oil-soluble molybdenum compound together with extreme pressure anti-weld additives. Helps give greater cutting accuracy and prolonged tool life.

There are many other cutting oils but here is one that I use mostly.

Regards

John

I just used engine oil cos was skint, so no idea whether that helped that much! Being very much an amateur didn't know if they were actually going to be much better (the marketed cutting oils), than normal oil, but I take it you've seen this yourself. I've had to do quite a bit of chainsawing of logs last couple of months, and switched from chainsaw oil at £5 a litre to sunflower oil at £1 a litre with no noticeable change, so did wonder whether it was a lot of extra money for like a 2% difference in performance!

Cheers

Hi Jan,

I had a bit mixed feelings about what you described here, to be honest I can't imagine how the part you made looks in the end.

But one thing (well rather two) things I can tell you: You can't find M7, M9 and M11 screws/bolts because these are not used as fasteners! They are 'non-preferred' diameters, but that doesn't mean that taps and dies don't exist! There are even more such non-existing fasteners (every odd dia. from 11 upward); it would be crazy having screws every mm.

The other thing: keep off thinking that carbide drills would solve your problems! I bet that such a drill in your machine (whatever it is) would not last more than 30 seconds. It IS possible to drill all usual (for a mod,eng.)materials with HSS drills. Just today I did quite a lot of holes in 5 mm stainless, all with HSS.

Regards, HansR.

But why sell the taps if you cant buy/sell a fastener is my query! All I made in end was an M12 grub screw with a threaded hole for an M6 bolt to go through in the middle of it in the end. It's my lack of knowledge and experience that makes it sound so complicated - but had to drill through 30mm bolt/grub screw, which proved too much for cobalt bit in 12.9 bolt, but the tungsten carbide bit went through it easily. Ended up knackering the carbide tipped bit in the end (by, emabarrasingly flicking the switch to anticlockwise inadvertently!), so had to use milder steel for bolt / grubscrew used in the end, which was 8.8.

Cheers, Ian

Ian

I was very glad that my cheap tap and die set included 7mm when I ran Peugeot 504s on the road and used them in competition. It seemed to be a favourite bolt size on those cars, Various engine bits were attached using them and I think even the front suspension top mounts used them. Don't assume that just because you can't get them over the counter in a hardware supplier that they aren't used at all.

Keith