Drilling Hard Bolts

[electrosteam]

I am fitting grease nipples and drillings in two 3/4 in W black bolts for a friend's 6 ft guillotine.

The first bolt was easy, 5 mm along the shaft in the lathe and 3 mm across in the drill press.

The second bolt complained with the 5 mm, and the drill was marked as needing sharpening.
The 3 mm across is a no-go - a new Sutton drill screaming at 2400 RPM, and blunt.
Same result with a near-new Dormer.
Tried a Chinese at 1400 RPM - same result.

I assume I have hit a hard spot in the bolt.
Would a carbide drill be any better ( drill press max is 3600 RPM ) ?
Any point in trying a different spot - 5 mm along shaft would be OK (perhaps 10 mm) ?
Different radial position ?

Any suggestions for a good source of carbide drills (near Penrith preferably) ?
Nepean Boltmaster is my local source for most things, and McJing is about 40 minutes drive.

Best wishes to all for the New Year,
John.

[Anorak Bob]

Hey John,

I'm no expert when it comes to drilling (or anything else) but I'd only be using those speeds for small ( tiny) drills. Have you tried drilling at a slower speed?

Bob.

[Stustoys]

Hi John,
Could you grind a flat on the bolt and try again?
I'd also think about drilling slower, I've found sometimes when drilling holes in unknown bolts some seem to harden like SS. As long as you keep the drill cutting its fine but if you ease off and it rubs for just a second its very hard to get started again.


Stuart

[electrosteam]

Thanks for the comments.
My training manuals suggest ordinary mild steel has a cutting speed of 30 m/min, giving a speed for a 3 mm drill at about 3000 RPM, hence my first go at 2400 RPM.
The 1400 RPM speed is already half of that.

I will try at half again.

John.

[Bryan]

Are there any markings on the bolt head? I wouldn't know what they mean, but someone will. Maybe it can be annealed?

[electrosteam]

Well, 740 RPM doesn't work either.

Graham McDonald at Girraween has 3 mm Carbide jobber drills at $29 each.
I will contact the bolt's owner to see if he can anneal it.
Would a propane torch do anything beneficial ?

John

[Michael G]

For a cheap source of carbide drills, buy some masonry drills. You just need to sharpen them so that the tips have an edge on them*. The guys in a tool room where I used to be used them all the time to good effect.

Michael

*Only take material from what would be considered the clearance side on a lathe tool. Use them as zero rake drill bits.

[BobL]

For a cheap source of carbide drills, buy some masonry drills. You just need to sharpen them so that the tips have an edge on them*. The guys in a tool room where I used to be used them all the time to good effect.

Michael

*Only take material from what would be considered the clearance side on a lathe tool. Use them as zero rake drill bits.

I use touched up masonry bits as you describe to drill up to 5/16" diam holes in the middle of chainsaw bar nose sprocket bearing supports. The bearing supports are incredibly hard and will blunt a HSS bit in a few seconds. I use a high steady pressure a slow speed and lots of cooling/lube.Going too fast and the carbide will disintegrate. Going a bit slower at break thru otherwise the bit can grab and shatter. A piece of hardwood backing helps at this point.

[electrosteam]

Success.

New Sutton 2.5 mm drill at 740 RPM, 5 mm offset towards bolt head and 180 degrees around.

Lots of lube, firm pressure.
It was tougher towards the centre.
Drill is now blunt and reversed in its holder to denote 'please sharpen'.
I doubt it would do a second hole.

I will put the advice on masonry drills into my file - could be useful in future.
Makes me think an EDM facility is not such a bad idea.

Thanks for all the help,
John

[Grahame Collins]

I am fitting grease nipples and drillings in two 3/4 in W black bolts for a friend's 6 ft guillotine.


3/4" W black bolts come cheaper than carbide drills. Even Grd. 8 bolts I woulld dare say.
Grahame