Center "punch" for CNC mill?

[kerryveenstra]

I noticed while drilling a 7/64" hole that without a centerpunch's dimple, the drill bit wanders slightly, offsetting the hole. What is the trick here? Is there a kind of "CNC centerpunch"?

Thanks for any ideas,

Kerry

[philbur]

How about starting the hole with a center drill, or a shortened drill bit or even an endmill.

Phil

Originally Posted by kerryveenstra I noticed while drilling a 7/64" hole that without a centerpunch's dimple, the drill bit wanders slightly, offsetting the hole. What is the trick here? Is there a kind of "CNC centerpunch"? Thanks for any ideas, Kerry

[Hursty]

11.9.1 Centre Drill
Only use on turning centres to drill for tailstock centre location or for the specific chamfer angle.
11.9.2 Spot Drill
Before drilling with a twist drill, first produce a centering hole. This supports the drill body before the point starts to cut and act as a lead for a tap; an unsupported drill may not start concentric resulting in a hole out of position or a broken drill. The point angle of the centring drill is traditionally between 90o – 100o; a 90o angle presents the simplest solution producing a 45o chamfer with ease of programming.
11.9.3 Drilling (Twist Drill)
To maintain the accuracy of a drilled hole, after centering, use light feeds to produce a straighter hole than that when forcing the drill using higher rates. When drilling a deep hole (>3×D), for materials that produce continuous chips, the method used is a peck drilling or chip removal operation. Withdraw a standard drill after a maximum depth of 5×D; gradually reduce this depth to 1×D for deep holes. For extra deep holes reduce the speed and feed progressively to half speed and minimum of 0.025 mm/rev (0.001”/rev), the drill supplier normally provides formulae for these and other calculations. The following graph is a Guide to the expected diameter oversize in normal drilling operation.

[soszi1950]

hi to position a hole accuratly it is advisable to use a centre drill or spotting drill first to a depth of say 3 mm.

The spotting drill usually costs a bit more but the life is much greater. (spotting drills are used to mark new holes from old parts or PCDS) hope this helps

Don

Quote

If the finest genius studies at one of our colleges, and is not installed in an office within one year afterwards in the cities or suburbs of Boston or New York, it seems to his friends and to himself that he is right in being disheartened, and in complaining the rest of his life. A sturdy lad from New Hampshire or Vermont, who in turn tries all the professions, who _teams it_, _farms it_, _peddles_, keeps a school, preaches, edits a newspaper, goes to Congress, buys a township, and so forth, in successive years, and always, like a cat, falls on his feet, is worth a hundred of these city dolls. He walks abreast with his days, and feels no shame in not `studying a profession,' for he does not postpone his life, but lives already. He has not one chance, but a hundred chances.

Self-Reliance
from Essays: First Series (1841)
by
Ralph Waldo Emerson

[kerryveenstra]

Thanks everyone. "Spotting drill" is what I'm looking for.

[Geof]

Originally Posted by kerryveenstra Thanks everyone. "Spotting drill" is what I'm looking for.

And in some tool catalogues you will never find "spotting drill", you will find "CNC spotting drill".

Just a hint that may save you some searching time.

[SMW Precision]

Originally Posted by kerryveenstra I noticed while drilling a 7/64" hole that without a centerpunch's dimple, the drill bit wanders slightly, offsetting the hole. What is the trick here? Is there a kind of "CNC centerpunch"? Thanks for any ideas, Kerry

Kerry, try screw machine drill bits. Shorter and much more rigid.

[Don Clement]

In lieu of a spotting drill or to use fewer operations, I use screw machine length drills with a 135* split point. The 135* split point drill does not wander as much as 118* drill without split points. Also solid carbide drill is more rigid than HSS.

Don Clement
Running Springs, California

[holbieone]

90 degree spot drills work nice


drill radius in depth + .005 will leave a nice chamfer on the finished hole

[kerryveenstra]

> screw-machine length drills with a 135* split point
...
> Also solid carbide drill is more rigid than HSS.

As usual, a quality suggestion from Mr. Clement! Thanks!

When I first started looking through the Travers master catalog a few months ago, I was thinking, "why are there so many different kinds of X" where X was nearly anything that I was looking for.

I'm starting to understand why: there's a *reason*.

Yep, there they are, on p. 4. But no solid carbide versions. use-enco.com has carbide ones, as individual bits. (Hmmm... A 10-to-1 difference in price for different USA sources of the same size. More homework for me.)

> 90 degree spot drills work nice
> drill radius in depth + .005 will leave a nice chamfer on the finished hole

Yes, those 90s do seem to be great for chamfers. That's just too tempting to ignore.

My research suggests that the angle of the spotting drill should be no less than that of the drill (although I've seen opinions both ways, I'm preferring the recommendation from a spotting-drill manufacturer.)

But since 135* split-point drill bits are available individually, I can try a few the next time I place an Enco order.

Kerry

[Don Clement]

I converted a S&D HSS 5/8” diameter 118* drill to 135* split point using a model 750 drill doctor. I use the converted split point drill without a spotting drill. The drill doctor has a diamond wheel and also sharpens my solid carbide split point drills.

Don Clement
Running Springs, California

[wildcat]

When buying your spotting drills pay attention to the overall length. Most of the ones that I have got have been really short and work great in chucks or collets. However, I needed some 82 degree spotters, had to get them from a different manufactor, and they are REALLY long. They chatter like nothing else in a chuck cause the stick way out. In a collet pushed all the way up they are fine. May end up cutting the excess off.