How big is the part in which they are stuck?

Clamp it to a milling machine table and have at them with a carbide end mill and high speed.

If you have removed some of them, you will know the thread size and hence the tapping drill size.

Mill/drill them to no more than that size. The two axes of movement the mill offers will allow you to centralise the hole fairly well. If the mill you are using is smaller than the tapping drill size, once the central portion of the screw is removed, you can move a little back and forth and left and right until only a thin shell of screw remains, which can be picked out with a pointy object.

If you cannot bring the part to the machine, make up a drill bushing that can be fixed centrally over the hole and use a carbide mill in an electric drill.

Larry sounds like you have exhausted all the options, however drilling out should work OK but you will have to remove the rotor block and mount in a vice on your mill or drill. Then you can be sure of drilling on centre and true to to the original hole.

Just my thoughts John

I would carefully drill it and have used LH drills with good results when I did this sort of thing for a living.

Tony

If these tapped holes are on a tangent then drilling the grubscrews with a hand held pistol drill is not likely to go well as you have discovered.

If the threads are M6 and you have a lathe I suggest making drilling jigs from some M6 bolts by drilling them concentrically (say 4mm) and using them to guide a good quality sharp drill (ideally following the use of a spotting drill) to ensure the best chance of staying on axis. The donor bolts need to be long enough for their hex heads not to collide with the surface of the planer drum

Make several drilling jigs as its likely the bore of the jig will get ovalised.

I have recovered quite a few butchered Allen grub screws using the above method

Ian P

IF the grub screws are good ones (unbrako) then they WILL be very hard ! Any talk of drilling or milling will HAVE to be with carbide or tipped drills. Fine in a pillar drill but hand held, the brittleness of the drill or tip is all to likely to brake the bit or it's cutting edge. Fill hole full of wd40 or similar, hammer on it persistently with an 8Oz hammer using a core size punch. Then hammer in the next size up allen key, pray, then try to undo! Good Luck, you'll need it . Noel.

+1 on Neil's suggestion above. And be sure to use good quality allen keys such as Unbrako. Junk ones round off easily.

Another dodge if the hex hole is still reasonably intact is some coarse valve grinding paste in the hole and on the allen key to give it some grip.

Or sometimes you can hammer an oversized good quality Torx driver into the remains of the hex hole and get a grip that way via its sharp splines.

If you do resort to drilling — which as said can be tough on high tensile screws — left hand drill bits that spin in the reverse direction are the best bet. Run very slow in intermittent bursts in a slow but torquey battery drill seems to quite often grab the broken screw and wind it out before needing to fully drill through.

I'm not that familiar with how planer blades are held within the rotating drum but if they are just in a slot and its only the pressure from the screw that prevents them being flung out by centrifugal force, then your idea of somehow removing the tip of the screw sounds good.

If there is nothing holding the blade besides the screw pressure (only two remaining now) can you get a wedge or use a screwdriver tip to lever up the the blade in its slot and gradually extract it. The exposed tip of the grubscrews might then be long enough to grip with narrow (or ground down) jaws of Mole type grips. You could then rotate then to screw the grubscrew further into its hole to loosen it and then remove the grubscrew by cutting it off in stages.

The drilling guides I suggested earlier do rely on there being a sufficient length of exposed thread above the butchered screw.

Ian P

Screw in for preference if the chopped end works, far less to screw in than out.

Regards Ian.

Hey Larry where abouts are you ? If your not to far off I will dig out the spark eroder ! It will make holes in any conductive material, the party piece is to put a hole in a carbite tip so it can be screwed to a holder – with a bit of copper. A hex electrode would even make a new key hole. Noel.