Twist Drill Angle with Center Drills vs Spotting Drills

[markso125]

As for both as someone who has machined in shops for years, and also taught for years i can say from my experience centerdrills are used almost exclusively.
The reasons for this is accuracy and cost. The design of the tool keeps it very rigid thus making it very accurate, usually we find play in the machine causing holes to be off before we find the center drill walking.
The next is cost, a center drill is used for all materials, for all different machines. They are cheap, they all act the same and you don't have to waste time deciding which angle you need to use. We run them in everything from 15-5PH to 6061t6 and always have predictable results.
As for common drill angle, the vast majority of jobber drills are sold are at 118 degrees again this is the same as the center drills it is cost and predictability. Time spent checking angles for proper material in accordance with the cost of having a whole bunch of extra drills sounds counterproductive and more expensive to me.

 

[JR49]

the vast majority of jobber drills are sold are at 118 degrees again this is the same as the center drills

Could someone please explain this quote ? I'm pretty sure all center drills are 60 deg. combined angle, while the drills markso125 mentioned is 118 deg. combined. Unless he is referring to the tiny tip of the center drill, which (don't know for sure) might be 118 deg. or more but, if that is the case, wouldn't you need a HUGE center drill for the tip to be big enough to spot for even a fairly small dia. (say 3/8") final size drill. Thanks, JR49

EDIT: OK, I don't usually post things unless I'm 100% sure that what I'm saying is correct (which is why I don't post a lot, still a beginner). So, after submitting this reply, I went out and checked my center drills with some 118 deg. drills. It turns out that the tip of a #6 center drill will spot a hole big enough for a 3/4" main drill, and , as close as I could check, the tip is ground to 120 deg. I don't know how big center drills get, but the #6 was the biggest I had. So, like someone said up above, I guess yesterday I spent approx. 40 bucks on 2 spotting drills for nothing, except that:: "He who dies with the most tools WINS !! JR49

 

[David S]

I am in the process of modifying a jig that I made and have to relocate some holes accurately. So decided to try Bob's suggestion. I have a #5 centre drill. The main body is 7/16" Ø, and the "small" tip is 0.187Ø. I am spotting for a # 27 drill. And of course it works great.

And yes you only use the tip. The #5 is very stiff.

It is a slow week if I can't learn something.

David

 

[JR49]

I am in the process of modifying a jig that I made and have to relocate some holes accurately. So decided to try Bob's suggestion. I have a #5 centre drill. The main body is 7/16" Ø, and the "small" tip is 0.187Ø. I am spotting for a # 27 drill. And of course it works great.

And yes you only use the tip. The #5 is very stiff.

It is a slow week if I can't learn something.

David

I was typing when you posted this, thanks, and please see the edit to my post. JR49

 

[David S]

Ah JR we both got sucked into buying expensive spotting drills I see. And of course Bob waited 'till we had parted with our money to suggest a low cost....already have...alternative.

All in good fun my friends... and again really glad that the OP started this thread...who would have thunk?

David

 

[JR49]

And of course Bob waited 'till we had parted with our money to suggest a low cost....already have...alternative.

I know, how could Bob do that to us!! But actually, the link that Bob provided in post #5, sites about 4 other good uses for spotting drills, so all is good. Thanks Bob. Oh, and I forgot to say before that this has been an extremely informative thread, so thanks to the OP, for asking the question. These are the BEST threads, in my opinion. Happy Machining, JR49

 

[JPMacG]

Thank you everyone! This thread has been very helpful and very timely for me.

 

[mikey]

The best fun we have on HM is to get someone else to spend money on a tool. I tried it with @higgite but he wouldn't bite until I bought the tool myself and showed him the results. Sometimes we go to extreme measures to get the job done but we do get it done!

 

[LazyboyNZ]

Really good thread, I have learnt a lot.
Thanks

 

[Grandpop]

I was a tool and die maker for 25 years before I left the field for a desk job. In that time I must have drilled about a million holes, and always in harder tool steels. We made high production dies that could have the inserts replaced when trey were dull, so the hole locations had to match the drawing.

Most were located with a center drill, some were center punched from the described line intersections. Rarely did anything not line up, and typically could expect the final hole after center drill start to be in location within .005 . Not saying the spotting drills would not have been better, just saying we didn't have them and didn't see the need for buying them.