Spotting drill angles and sizes

[grateexpectations]

I just got a small lathe and am eager to start on a project to install a QCTP. Quick question, what spotting drill would you use to start an M10 tapped hole?

If I'm not mistaken the 'proper' method would be either
- 90 degree spotting drill, larger diameter than the tap drill, or
- 120 degree spotting drill, smaller diameter than the tap drill

I see lots of youtubers using centre drills for starting holes, but is that really a cromulent method?

 

[benmychree]

Center drills are commonly used to start drilled holes, and have been for a long time, but spotting drills do not break the tip off as center drills often do, the angle of a spotting drill should be such that the actual drill will make first contact at the large diameter of the spotting drill, no the bottom of the spot hole.

 

[JimDawson]

Center drills are commonly used to start drilled holes, and have been for a long time, but spotting drills do not break the tip off as center drills often do, the angle of a spotting drill should be such that the actual drill will make first contact at the large diameter of the spotting drill, no the bottom of the spot hole.

I normally use center drills for spotting holes, but this is not the really the proper way. But they work fine for most applications, for production applications I do use spotting drills when needed.

I have to disagree with John. A real spotting drill angle should be greater than the tap drill so the tap drill is forced to the center of the spot, and not touch the outside edges of the spot. So if you are using a 135° tap drill, the spotting drill should be 140° or 150°

 

[Bob Korves]

Center drills are commonly used to start drilled holes, and have been for a long time, but spotting drills do not break the tip off as center drills often do, the angle of a spotting drill should be such that the actual drill will make first contact at the large diameter of the spotting drill, no the bottom of the spot hole.

I have learned to do it the opposite way, John. Included angle of spotting drill larger than the following drill, and the cut need not be larger than the chisel point of the following drill. It keeps the following drill centered in the starting hole. I learned to drill holes the way you prefer, John, and it often works well, but sometimes one cutting edge catches the outside diameter of the starting hole first, and then an ugly lobed shape hole is generated with heavy vibration, ugly and difficult to make better for getting the rest of the hole finished to size and in the proper location. I have never had that problem again using my current method, which also keeps the following drill well centered with the pilot hole. You have WAY more experience than I do, John, please explain how your method works better than mine does, in detail. BTW, I never use lathe center drills as spotting drills any more, makes no sense to me at all...

 

[benmychree]

The problem is that the spotting drill hold is not perfectly cone shaped, and the tap drill skates around in the bottom of the hole and does not always start concentrically. I do all this sort of drilling in the lathe from the QCTP, and it is easy to center the drill with the cross slide. What I stated in regard to the angles is what I read in various publications. I have only one spotting drill and rarely use it, at my old shop downtown I used them only on the turret lathes. For spotting on the drill press or mill, I use only center drills and do the chamfering for the tap with 82 deg. countersinks. When I worked at Kaiser Steel machine shop, there was not a spotting drill in the whole shop.

 

[Bob Korves]

I also do not use "spotting drills," instead I use a stubby 135 degree drill to start holes when followed by a 118 degree finishing drill long enough to do the hole, minimum excess length when and where possible. I want first contact as close to the center of the small pilot hole as possible. Works for me...

 

[projectnut]

The problem is that the spotting drill hold is not perfectly cone shaped, and the tap drill skates around in the bottom of the hole and does not always start concentrically. I do all this sort of drilling in the lathe from the QCTP, and it is easy to center the drill with the cross slide. What I stated in regard to the angles is what I read in various publications. I have only one spotting drill and rarely use it, at my old shop downtown I used them only on the turret lathes. For spotting on the drill press or mill, I use only center drills and do the chamfering for the tap with 82 deg. countersinks. When I worked at Kaiser Steel machine shop, there was not a spotting drill in the whole shop.

I'm in the same camp. In the 20+ years I spent in the company machine shop I never saw a spotting drill in the cabinets. We had center drills of all sizes and configurations. They got used on the mills and lathes for both piloting standard drills, and to make pockets for centers. To this day I use center drills almost exclusively. I did buy a few spotting drills some years ago, but I can count on one hand the number of times they've been used.

To me there's no right or wrong answer. It's a matter of comfort and preference.

 

[mikey]

I just got a small lathe and am eager to start on a project to install a QCTP. Quick question, what spotting drill would you use to start an M10 tapped hole?

I prefer to use a spotting drill myself. The correct spotter depends on the drill geometry of the drills you use. If you use 118 degree drills, use a 120 degree spotter; for 135 degree split points, use a 140 degree spotter.

While you can use center drills for starting holes, spotters are more accurate in my experience. We had a great discussion on spotting drills in this thread.