Using center drills

[MattsAwesomeStuff]

I learned something today by coincidence that Damien may find interesting...

Damien mentioned he frequently breaks center drills, and indeed broke one in his last tutorial.

Using center drills to start a drill operation is also how I was trained on a lathe, and is popularly done so, but, that is apparently the wrong tool for the job. The purpose of a center drill is to create the impression in the end of a piece of stock that you're going to be supporting between centers (one in the chuck, one in the tailstock).

The correct tool for simply starting a hole is instead a "spot drill". A spotting drill to me looks like a normal drillbit but with very short flutes to be more rigid.

Here's an excerpt:

"Despite their popularity, center drills are not designed for starting holes. Their purpose is to drill holes with 60° angles in the ends of parts. These holes are used to secure parts in the tailstock of a lathe or to mount a workpiece between centers for a grinding operation.

The center drill is inefficient. Its thick web puts a great deal of pressure on the tool. Pressure leads to heat, and heat leads to premature drill failure. The tip of the center drill is also prone to breaking, as anybody who has ever used one to drill stainless steel can attest.

The best tool for starting a hole is an NC spot drill, which is also referred to as a spotting/centering drill. NC spot drills come in several styles, but they all share similar characteristics. All are short and rigid and can handle much higher feed rates than comparably sized center drills."

Center Drill:



Spotting Drill:



I came across this as I was wondering how to build my motor-transmission coupler, and if I could do it without a lathe.

[konstantin8818]

From my expertise, center drills are constantly snapping because tailstock is off center.

[arber333]

I use center drills only to predrill and center my holes. Usually you tap your stock to mark there the drill will have center, but your ordinary drill has a problem grabbing small dent in material and can wader off. I use center drill to make this tap deeper. But not more than a mm or so. More than that and you really can break the carbide tip.
I am discussing hand drilling in fact. With lathe and mill i never broke a center drill. I simply dont push it.
Also when drilling, even centering i always spray small ammount of WD40 onto the tap. It helps lubricate the drilling process and lets the drill bite better, which saves you from overexerting force...

Large enough centering drills can be used for deburring too, but i dont recommend it. There are tools for that.

[Eflyers]

I use center drills only to predrill and center my holes. Usually you tap your stock to mark there the drill will have center, but your ordinary drill has a problem grabbing small dent in material and can wader off. I use center drill to make this tap deeper. But not more than a mm or so. More than that and you really can break the carbide tip.
I am discussing hand drilling in fact. With lathe and mill i never broke a center drill. I simply dont push it.
Also when drilling, even centering i always spray small ammount of WD40 onto the tap. It helps lubricate the drilling process and lets the drill bite better, which saves you from overexerting force...

Large enough centering drills can be used for deburring too, but i dont recommend it. There are tools for that.

This has been my experience with CDs. Keep the speed up and the feed light. Just as you would a drill bit the same diameter as the tip of the CD. Sometimes, in the lathe, the CD is the only tool you have to clean-up and correct the condition and location of the center hole in the end of a shaft. Speed up, feed light.

[SciroccoEV]

Centre drills are frequently used just because they're a short stiff drill with a minimal chisel edge, which all means they don't wander when starting a hole. Historically they were pretty much all that was readily available with these characteristics.

I've been using spotting drills for many years, they're a better tool for the job.

I also have a set of left hand drills.