Soft blank end arbour rather harder than expected?

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sploo

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I have a couple of blank end MT3 arbours from rdgtools, and I've drilled and reamed a 19mm hole out of the 28mm diameter blank end with no problems, but next I needed to drill and tap a hole into the side for a grub screw... and that's going nowhere. It appears that the outer shell of this softened blank end is not soft. I've not seen this before with other blank end arbors I've machined. Is this normal?
 
I have never had that experience personally. The arbors I have machined have been pretty tough but not hardened, not even a hard skin. Maybe the heat treat left a bit of a skin, do you need it to be 28mm diameter? If not, turn away a mm or two and see it goes then, the lathe tool should be able to dig under the hard skin and remove it.

If you do need it to stay at 28mm, grind away a little flat where you want to put your grub screw, that might be enough to get under the skin. Last choice, buy a carbide drill bit, that will go through no trouble.
 
I thought about removing the outer skin, but a carbide insert lathe tool just skates over the outer surface. Though I do need too much of the tool sticking out of the toolpost, as I can't get the toolpost close enough with the arbour mounted in the morse taper of the lathe - so it's not ideal.

I was using some decent cobalt drill bits but they've gone nowhere (first time I've ever seen them "fail"). However, I do have some carbide endmills so I suppose that might be an idea to get through it.
 
That definitely sounds like a manufacturing mistake to me. Hope you manage to work around it.
 
Good call on the carbide; I managed to get through the "diamond crust" with a 2.5mm solid carbide end mill, then took it to 4mm. I didn't have a 5mm end mill (M6 tapping size) so used a 6mm carbide ball end mill to try to get through a bit of the "shell" before drilling to 5mm. That (just about) worked. As you can guess, tapping the holes was "fun". Got there in the end though, and it seems to work fine.
 
I remember those types of cutters from work and the other type with a tapered flat. The slang for them was whistle notches. I have no idea what the proper name was. They were common on the older tool holders. The newer high speed ones were induction heated and shrunk around the tool. Had to use heat again to get them out.

Pete
 
The induction heated holders fascinate me. Not something that's probably common in the home shop, but I guess (with high tolerance holders and tool shanks) a really accurate way of gripping the tool.
 
The induction heated holders fascinate me. Not something that's probably common in the home shop, but I guess (with high tolerance holders and tool shanks) a really accurate way of gripping the tool.

Yes very interesting. I suppose it's really just a form of collet as it is using the natural physical properties of the metal to hold the tool.
 
Basically a shrink/interference fit. The tricky part is to account for the growth and subsequent shrinkage for the heated tool holder so the bit was the correct gauge length for the cutting operation. Because the bits were turning up to 25,000+ RPM they had a timed life before being changed out automatically with another in the magazine. The length had a very close tolerance to maintain finished part sizes.

Pete
 

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