Hard spot?

[hitandmissman]

Tried to drill a #50 hole in a casting. First hole went thru great, had three more to do. The next after putting in a punch mark, started to cut than just stopped and got hot. Tried a new bit and it would not even start. So went to the next hole, with a new bit again, no go. Turned the casting over but could not reach the spot to drill with a #50 so went larger, really larger, 5-40 drill went thru just like butter from the back side. I had spot faced these places before I drilled with the #50. Did I harden the casting by doing that or was it just a bad casting?? Should you spot face before or after drilling a hole?? Thanks in advance for any advise.

 

[Rocket Man]

The first drill bit probably was a little bit dull after drilling the first hole. It drilled a little then stopped drilling and produced a work harden spot in the bottom of the hole.

The replacement drill bit probably got dull trying to drill trying to drill the work harden spot that is why it would not drill the third hole.

The only way to drill through that work harden spot is, with a carbide tip drill or use a new drill or sharpened drill poke the #50 drill pretty hard in to the bottom of the hole if it does not break off it might drill through. You can also drill through from the opposite side. Sometimes a pilot hole will drill through then use the larger #50 drill and run the drill much slower. Use oil to reduce heat to prevent work hardening.

Sharpen the drill it should drill the third hole

 

[Lew Hartswick]

What sort of material? "Casting" just doesn't say.
If cast iron, then it doesn't work harden.
Could be an inclusion you hit on the second hole and that did the bit in.
...lew...

 

[MachineTom]

I'm with Lew, cast iron can have hard spots but it does not seem to work harden as do other materials, SS and Ti others. You said you first spot Faced the area, or did you spot Drill the hole?

 

[Jasonb]

If its CI then a said it could well have been chilled which makes it very hard, best thing it to put it in a fire or stove for a few hours and then let it cool in the ashes over night or if its a small casting a 10-20min blast with a big propane torch and allow to cool slowly may well get it soft enough.

I would spot face first as it stops the drill going off line if the surface is uneven.

J

 

[hitandmissman]

Opps should of said it was cast iron. I used an end mill to spot face first than just a center punch to mark the drill hole. Thanks for all the replies.

 

[rkepler]

Cast iron can 'chill' - cool fast enough that it gets hard. Thin sections are particularly prone to this but any relatively thin areas can chill if they're broken out of the mold too soon..

If you have a heat treat oven available you can heat treat them, but it's at a cost of a grade in the cast iron (Grade 40 goes to Grade 30, etc.) so you lose strength and maybe a little skin if it's not an atmosphere controlled oven. If it's a small enough area you can sometimes just bull on through with carbide tooling or a lot of resharpening of tooling. But carbide will deflect on white iron - it cuts with enough pressure but then you gouge in a bit on the soft area before and after it.

If it's a casting that you bought recently you might check with the seller and see if they offer a warranty of any kind, and you might be doing them a favor letting them know that the foundry is messing up their castings. But sometimes they don't really seem appreciative of the notification...

BTW: 'chilling' is sometimes used to generate hard area on cast iron - old time rolls were often made of chilled cast iron and were ground to finish. A 63-65RC roll lasts a long while.

 

[hitandmissman]

Thanks for the replies. The casting is a PMResearch 5BI that I had bought a couple years ago and am just now getting around to. I am a little slow getting my stuff done. Have done a couple others from PM with no problems so thought it might be something I was doing wrong. Anyway got them thru by drilling from the back side however bigger but shouldn't look too bad when all is done. Thanks again.