Thread: Drilling Hard or Case Hardened teel

The photo attached shows a Dickson tool holder from a mate's Harrison M250 lathe in the process of being re-configured.
Originally it was a MT1 holder with the plan to make into a standard square shank holder.
You can see a faint trace of some of the taper on the left, and remnants of the the tang restraint dowels on the right.

The slot was done on a CNC mill of some sort, but not correctly, so the decision was made to reverse the holder top/bottom to get more meat for the shank clamping grubscrews.
This meant 5 holes to be drilled and threaded M8, the height adjuster plus 4 shank clamps.
The height adjuster has been done and is OK, but at some cost:
- two Chinese drills with obvious cutting edge damage in the queue to be re-sharpened,
- one new Sutton drill in the same queue with the same damage,
- one intermediate tap with a broken flute.

As the drill would break through partially into the slot, a new Sutton end mill was used to remove some metal from the slot at the end of of the drilling without any duress - you can see the circular recess at the bottom of the threaded hole at the top, and some cutting of the slot side wall.
The start of the threaded hole was made with a centre drill, tough but OK.
The Chinese drill would not look at the task, banish the drill.
A new Sutton 4 mm drill went through hard, high resistance then a run-away.
A new Sutton drill sized for the thread refused to start, and was added to the queue.
I counterbored about 0.5 mm through the suspected case hardening with a solid-carbide slot drill - high resistance then run-away again.
Another Chinese drill finally got the hole done.
I am using a small milling machine and speeds about 1100 and 680 RPM.
The hand threading seemed to go relatively easy, hard with frequent clearing, but no real problems.
On withdrawing the intermediate tap, one flute was found to be broken.

The steel is either tough with a work-hardening component, or case-hardened.
It is not that hard because marking-out with a height gauge on the table gave an adequate scribe mark, and the centre-pop was done with an ordinary cheap automatic one.

I now have 4 more threaded holes in this one, and then something like 5 more holders to do.
Suggestions ?

John.DSCN3688.jpg

Try Guhring and get drills and taps suited to a higher hardness material than plain everyday taps for normal steels.