Spotting drills- Function?

[HMF]

Another forum had a question:

"Can I drill with a 90 1/8 spotting drill right through 1/8 6061. I'm drilling about 48 holes every 1/4" for an indexing bar? I'll be chucking the spotting drill right in an 1/8 R8 collet."

What is the drill (spotting drill) that they are talking about? Why would you use a drill just to spot a hole?

Thanks,


Nelson

 

[HMF]

Dave,

Thanks- that is exactly what I was wondering, why one wouldn't use a center drill instead.

I have used center drills on my drill press and never broke one- of course, I feed it very slow with some cuttng oil. Pushing hard is what seems to break the points.

Thanks,

Nelson

 

[RandyM]

Thanks guys, good info here. I did not know this. In fact, I was uncertain what a spotting drill is, are they something relatively new?

 

[HMF]

Thanks guys, good info here. I did not know this. In fact, I was uncertain what a spotting drill is, are they something relatively new?

Which is precisely why I ask "Nelson's stupid questions." I learn a lot and I may hit on a topic that can help someone else too.

The great thing about you guys is, now that you answered my question, I actually understand what that person on another board was asking.
You've removed a little bit of my machining ignorance, and for that, I am thankful.

Best,


Nelson

 

[Oldnbroek]

Using a 90 degree spot drill, dive in until the spot is a little larger in diameter than the hole you will be drilling. Then when you put in your standard 118 degree drill, both lips will start cutting from the outside instead of somewhere in the middle. They'll be forced onto center by the cone that your spot drill put there, with the chips gradually gaining in width until the whole drill point is engaged. That way, the chisel point on your 118 can't walk at all, plus the hole will be chamfered when you're finished, with no (or minimal) deburring.
A center drill will do the same thing, but since they're 60 degree (think lathe center) they'll have wider chips than a 90 degree spot drill for the same diameter hole, which may cause them to chatter unless you slow down (RPM) or feed faster. Either is vastly better than no spot.