Preferred spotting for drilled holes?

macardoso

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I run a smaller CNC and would like to preset a single tool to handle most or all of my spotting work. When doing general drilling I have always used a small center drill, but need to run high RPM's and find it doesn't clear chips well. I see NC spotting drills for sale in angles ranging from 90 degrees to 150 degrees. What would you recommend for a general purpose spot drill?
 

rgray

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I assume your using screw machine drills.
They usually come in 118 deg and 135 deg.
So if using the 118's get a 120 spotting drill and if using the 135's get a 140.
 

rgray

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If you use a larger spot than the drill being used, and get the depth right your hole will be chamfered before it's drilled.
Great benefit.
 

macardoso

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I haven't acquired many screw machine drills. Working off of a mismatched set of 115pc jobber drills. Bought cheap and replaced with cobalt and a few carbide over time. I should get a second index of screw machine drills.

That's a solid reason to use a 90 degree spot drill
 

mikey

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I assume your using screw machine drills.
They usually come in 118 deg and 135 deg.
So if using the 118's get a 120 spotting drill and if using the 135's get a 140.
I haven't acquired many screw machine drills. Working off of a mismatched set of 115pc jobber drills. Bought cheap and replaced with cobalt and a few carbide over time. I should get a second index of screw machine drills.

That's a solid reason to use a 90 degree spot drill
Not sure I follow the 90 degree spot drill thing. If you plan to use a spotter, the included angle of the spotter is typically larger than the angle of the drill in use as @rgray said. I do agree with the use of screw machine drills; they are much stiffer than jobber drills. Once you have a set then choose your spotting drill accordingly.

The chamfer thing has to do with using a spotter that is larger in diameter than the drill in use. For example, if you used a 5/16" spotter to near full depth with a 1/4" drill, you would have a chamfer left over. It will NOT be the correct chamfer angle for a screw but it will be nice for appearances.
 

MarkM

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If your not worried about the chamfer and want an accurate hole without the need to spot you could use a four facet drill and save that operation for most work. They tend to drill a more accurate hole closer to nominal size as well. I really like my Tormek wet grinder and the dbs-22 drill sharpening drill jig. Does a fantastic job with the four facet and can doctor a drill up for whatever material I come across. Not cheap but well worth it!

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Tornado220M

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I just use solid carbide which I can run so much faster on speed & feed.. also lasts longer and maintains a surface finish much smoother than a HSS or cobalt mix can.
 
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