Drilling rocker shafts - Advice please

The head is off to fix a leak (3/8" dia. holes for 5/16" studs = PO and Maney barrels). While its off I decided to do the single rocker feed mod. Now I've broken thru the hardening on the undrilled end with a quality 1/8" carbide centre drill but cobalt drills, as recommended, wont touch the core at all, not even marking the material. Maybe they're not cobalt but were sold as such. What do the engineers out there think of solid carbide bits? Depends on their quality as well I guess. I'm in at least 1/8" so I'm thru the hardened skin. Thanks.

Keith1069 said:

The head is off to fix a leak (3/8" dia. holes for 5/16" studs = PO and Maney barrels). While its off I decided to do the single rocker feed mod. Now I've broken thru the hardening on the undrilled end with a quality 1/8" carbide centre drill but cobalt drills, as recommended, wont touch the core at all, not even marking the material. Maybe they're not cobalt but were sold as such. What do the engineers out there think of solid carbide bits? Depends on their quality as well I guess. I'm in at least 1/8" so I'm thru the hardened skin. Thanks.

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I break through the case hardening from the outer end using a 1/8th inch ball end carbide burr. Then I drill from the other end using a cobalt bit. That way you start out in the softer center area and drilling is easy until you get close to the end.
Don't try to turn the bit very fast or it will burn it. Jim

Carbide tools work great in CNC machines because the machine is heavy and rigid, feeding the tool in and out with servo motors in a positive controlled manner keeps the brittle cutting edges from chipping. I drill out broken high speed steel tools like taps, endmills, ez outs on my manual milling machine with worn out carbide endmills all the time. The trick is to hold the quill down to the bottom against the stop so it can move no further. Feed the part up into the cutting tool cranking the table up slow and smooth. It makes for a more controlled cut because there no in and out slack to let the tool dig in and break. When the cutting tool grabs or digs into the broken tool I quickly move the quill up out of the cut, Flick out the debris, pull quill back down to stop and feed in cranking table up. If doing this on a manual lathe I hold the chuck in and twisted with my left hand while feeding in with right hand so the cutting tool wont chatter or suck into the part.

OK longwinded description of worst case but the point is carbide tools work best in machine tools but there are still tricks involved. I don't doubt a crafty guy with a hand drill can do it just think slow and steady keep it centered don't let the tool dig in.

I've done that mod several times on different motors. I simply ground though the hardened surface on the end with a stone and drilled from there. I never had a problem but don't remember exactly how I did it. I probably used cobalt bits but don't remember having to use carbide. Carbide bits for concrete are cheap and easy to re-sharpen if you can find one small enough. I've used them on Mallory metal when balancing cranks and they worked just fine.

Thanks gents. Unfortunately the so called cobalt bits I bought wont even touch the material inside. They were the UK equivalent of $3.20 each which seems to be about the right price for small qtys. At the outer end even 1/8" in they won't mark the material. Someone suggested masonry bits but going thru another 1.25"? I'll have to rethink this. Thanks for the input.

Masonry bits can be re-ground to cut through very hard steel however you'll need a special grinding wheel, sorry I cannot remember the type other than they are usually fine gain and coloured green.

Cash

I use a diamond lapidary wheel to sharpen my carbide bits. Cuts like butter.
Jaydee

cash said:

Masonry bits can be re-ground to cut through very hard steel however you'll need a special grinding wheel, sorry I cannot remember the type other than they are usually fine gain and coloured green.

Cash

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Silicon carbide wheels are the green wheels used for grinding tungsten carbide though a diamond wheel is preferred for finishing. Cheap carbide masonry drills work well even on very hard steels.