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  #1  
Old 02-07-2006, 05:21 AM
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Cobalt bits to drill in sprocket ??

I have to remove a crank sprocket (other car) that's stuck solid, in order to replace the oil seal that's behind it. My plan is to drill some 5mm holes in the sprocket, so that it gives way (don't like grinding: too brutal )
This sprocket looks like it's made of hardened steel, so the regular HSS bits won't work I guess.
I've heard of these cobalt drill bits, can someone recommend these, or even better ones for this job ??

Thanks,
V.
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  #2  
Old 02-07-2006, 11:12 AM
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Can you take a snap shot of the crank gear and paste the image to a response? If we can see what you are up against we might be able to direct you more effectively with some real world experiences.

IMHO-cobalt bits are one of the best bits to use; however you do need to keep in mind that when a drill bit spins too fast all it will do is burn the cutting edge. When drilling you want to see curly cues of steel being removed, if you do not see them than you are spinning the bit too fast..........
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  #3  
Old 02-07-2006, 11:20 AM
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cobalt bits

IMHO-cobalt bits are one of the best bits to use; however you do need to keep in mind that when a drill bit spins too fast all it will do is burn the cutting edge. When drilling you want to see curly cues of steel being removed, if you do not see them than you are spinning the bit too fast..........

He is right on about the danger of insufficient pressure on the bit resulting in overheating and ruining the bit. You're best off with a half-inch drill and positioning yourself to exert enough force so that you see thin strips of the sprocket as you drill. It is the thin strips which removes heat from the drill and sprocket as you drill.
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Old 02-08-2006, 02:30 AM
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Thanks Fella's,
Here's a pic of the wheel with the little woodruff key in the centre positioning the sprocket on the crankshaft:


It looks like as if the circle in de middle is composed of different metal than both the inner circle and the cogs on the outside. The sprocket measures 2.5 inches diameter, and is less than 1 inch deep. The diameter of the crank is a little over 1 inch. My plan is to drill two holes of 5mm above the woodruff key, to create a wedge running from the centre to the cogs, to release the grip of the sprocket on the crank.

At the back of the sprocket there's one (now damaged ) of the two conical disks that guide the belt. Behind the disk is the oilpump, and the black object beneath is the oil sump
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  #5  
Old 02-08-2006, 06:57 AM
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Since you are planning to replace the gear, you might consider an air chisel straight down into the root of the gear teeth like you were trying to split the gear. Spray with penetrating oil first then try to spit the gear. You shouldent need to split it all the way. Once you get a wedge about 3 mm down with the chisel the gear will probably slid right off.

John Roncallo
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  #6  
Old 02-08-2006, 07:35 AM
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i would be

leery of trying the chisel behind the sprocket. the block appears to be aluminum. you could damage it i think. the sprocket is too tight to get the fingers of a puller in there?

the center of the sprocket seems to be a softer material to alleviate vibrations. so it sounds like you are will ing to destroy the sprocket. if you do that maybe there will be room at the back of the center piece of steel at the base of the sprocket. can you find a cross section of the sprocket?? to see if this might be true? i think it prob is not true. but might be worth checking into. what does the repair manual say?

tom w
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  #7  
Old 02-08-2006, 08:15 AM
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Tried several pullers (and loads of WD40), but there's not enough space beside & behind the sprocket to position the fingers properly. And the pullers I used with thin fingers turned out too weak. Cover behind the sprocket is the aluminium oil pump. Applying some pry bar will certainly give damage. Yes, ...I've given up hope that this sprocket can be re-used, have ordered a new one already.
Using a chisel on the crankshaft makes me a bit nervous

You reckon the drilling option might work??

V.
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  #8  
Old 02-08-2006, 09:00 AM
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I would try to heat it up, if its sufficiently warm it may expand enough to slide off.
One of the methods is to use a electric welder and just weld in a circle on the wheel. Donít touch the aluminium with the welder, it will just burn away.
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  #9  
Old 02-08-2006, 11:25 PM
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the drilling

will work if you can get close enough to the crank to weaken the metal that contacts it. (without getting into the crank).

heat might work.

the welding thing would require care not to damage the crank i think.

tom w
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..I also have a 427 Cobra replica with an aluminum chassis.
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  #10  
Old 02-09-2006, 01:01 PM
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Quote:
Originally Posted by Rob Pruijt
I would try to heat it up, if its sufficiently warm it may expand enough to slide off.
One of the methods is to use a electric welder and just weld in a circle on the wheel. Don’t touch the aluminium with the welder, it will just burn away.
I agree, I would fit the Gas-Wrench (a 2-gas that is) with a fine tip so I could direct the majority of heat to the gear itself. I would heat the circumference of the gear approx 1/2" from the crank snout and then try a little prying. You may need to heat the gear a couple of different times in order to get it to release. If it is really stubborn after heating it up a couple of different times, I think I would use an air hammer to see if I could get it to break the static resistance between the 2 pieces. The air hammer must be used with thoughtfulness because you want to avoid cremating the gear as that could lead to more unwanted tension on the crankshaft. Subtle, controlled hammering with a pin bit traveling the circumference of the gear would finish releasing the stubborn little sh*%

.

Last edited by MrCjames; 02-09-2006 at 01:28 PM.
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  #11  
Old 02-09-2006, 01:09 PM
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I'd be suprised if it was that hard!

You can probably cut it with HSS. The pull it method will work I bet- like you are planning. I'd probably get it hot with the blue wrench and get behind it with my trusty Mac tool diesel injector pullers- little prybars. That or even a pair of large channel locks.
I'd avoid the welder method b/c if it doesn't move, as the weld cools- it will be make the whole thing shrink requiring more force!

Do you have the ability to sharpen drill bits? I can do some sizes well on my small lathe with a grinder. Done big non-split points by hand on a bench grinder... That would be the down side of cobalt.

Excellent bit though!

Michael
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  #12  
Old 02-09-2006, 05:31 PM
88Black560SL
 
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Quote:
Originally Posted by t walgamuth
leery of trying the chisel behind the sprocket. the block appears to be aluminum. you could damage it i think. the sprocket is too tight to get the fingers of a puller in there?


tom w
Dont Chisel behind the sprocket. Chisel straight into root of the sprocket teeth. The chisel blade should fit in between the teeth. Chisel down toward the crank shaft about 1/8". At that point you can probably remove the sproket by hand. This is a very common technique to remove the press fit axel bearing reatiner on American car rear axels.

John Roncallo
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  #13  
Old 02-13-2006, 04:13 AM
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Update: drilled two holes next to eachother with a 5mm cobalt bit, pried the hole with a large screwdriver and 'crack' said the wheel, after which it came off like a good boy.

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  #14  
Old 02-13-2006, 05:15 AM
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Good job. Put some antisieze inside the new one..

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  #15  
Old 02-13-2006, 07:43 AM
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woohoo

that was fun.

perserverence wins out in the end.

good work

tom w
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..I also have a 427 Cobra replica with an aluminum chassis.
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