Twist Drill Angle with Center Drills vs Spotting Drills

[Bob Korves]

You're right Bob. Thus far the discussion has been about how best to center a drill relative to a preceding angled spotting pilot hole. But when opening up a hole with progressively larger drills, the 118-deg tip now sees a 90-deg corner from the prior hole as its first point of contact. Out of habit I just kiss that edge or peck a bit until it gets started. But I don't see any other way around this using drills.

The way around it is to never get into the situation that I described, getting into trouble with enlarging a hole without solid support for the drill. And enlarging a cone is enlarging a hole, same issue, maybe worse, because of the wider chunk a flute can grab on first contact and musch less initial support. The answer is to start in the center of the hole, forming a small cone, and then moving gradually deeper and outward to a bigger hole with enough central cone contact to support the cutting forces as the hole enlarges. That is how I see it, anyway, and it works for me.

 

[Downunder Bob]

I was taught as a tool making apprentice to start with a center drill selecting one where the chisel point of the drill was as close as possible to the dia of the starting point of the center drill and do not drill down to the 60 deg section, thus the following drll would not wander. This what we always did in the tool room where I was. I have always used this method ever since, and it has never let me down.

I don't recall hearing the term spotting drill in this sense until joining this forum. A spotting drill to us was also known as a step drill for drilling counter bores for SHCS using a pilot to set it true to the original hole, can also be used to spot a face around a hole in a casting or where the surface is not flat or square to the hole.

 

[EmilioG]

OK, let's look at it a bit differently. I drill a 7/16" hole in steel on the drill press. Then I decide to open it up to 1/2". The odds are very good that the drill will walk and make an ugly oversize lobed hole while everything shakes badly. There is no cure at that point beyond starting over. I have had this happen many times doing fabrication work. I think the drill is "walking" around the hole. If I make a center punch hole or a starter drill hole a little larger than the chisel point of the drill, I always get a clean hole, no exceptions. Slow or fast starting feeds make no differences. Tell me what you think is causing this scenario.

Would using a 1/2" end mill work? Center cutting of course.

 

[mikey]

I was taught as a tool making apprentice to start with a center drill selecting one where the chisel point of the drill was as close as possible to the dia of the starting point of the center drill and do not drill down to the 60 deg section, thus the following drll would not wander. This what we always did in the tool room where I was. I have always used this method ever since, and it has never let me down.

I believe this is what Bob K. suggested earlier but the guys lightheartedly gave him grief for not mentioning it until after they already ordered/bought their spotters but I agree with you - it works.

 

[Bob Korves]

I was taught as a tool making apprentice to start with a center drill selecting one where the chisel point of the drill was as close as possible to the dia of the starting point of the center drill and do not drill down to the 60 deg section, thus the following drll would not wander.

As Mikey says, that is an OK approach, the center drill is quite stiff, but the secret is more than not drilling down to the 60 degree section, it is in not drilling down to the parallel section, just use the 120 degree point of the center drill, and do not allow any of the parallel portion of the center drill to enter the work. Now we have a spot accurately located by a stiff starting drill, and with a small cone a 118 degree drill can follow faithfully.

 

[petertha]

The answer is to start in the center of the hole, forming a small cone, and then moving gradually deeper and outward to a bigger hole with enough central cone contact to support the cutting forces as the hole enlarges. That is how I see it, anyway, and it works for me.

If I understand, you mean do the progressive enlarged drill stepping just at the beginning of the hole & then plunge all the way with the biggest size? If so, that would require a lot of in-feed force for the final drill, at least for my equipment. For example my cylinder liners ~ 1" bore x 2" long. No way I could push say a 5/8" or 3/4" drill through steel in one go. Even with a 3/8" drill the material wanted to slide back in the chuck jaws, so I turned a small step that butts against the front of jaws & do the operation blind hole. Actually I consider 'drilling' as a roughing operation just to min ID for my boring bar. I'm less concerned by hole quality than the drill cone catching on the bore of the prior drill & walking off center. But aside from using an annular cutter from both sides & go straight to boring bar, I see no other option.

How the h*ll do they drill long gun barrels anyway? :/

 

[mikey]

If I understand, you mean do the progressive enlarged drill stepping just at the beginning of the hole & then plunge all the way with the biggest size? If so, that would require a lot of in-feed force for the final drill, at least for my equipment. For example my cylinder liners ~ 1" bore x 2" long. No way I could push say a 5/8" or 3/4" drill through steel in one go. Even with a 3/8" drill the material wanted to slide back in the chuck jaws, so I turned a small step that butts against the front of jaws & do the operation blind hole. Actually I consider 'drilling' as a roughing operation just to min ID for my boring bar. I'm less concerned by hole quality than the drill cone catching on the bore of the prior drill & walking off center. But aside from using an annular cutter from both sides & go straight to boring bar, I see no other option.

How the h*ll do they drill long gun barrels anyway? :/

I probably do it wrong but when I need to make a precision hole, like your cylinder liners, I spot and then drill with the biggest drill I can use without step drilling. This gives me zero chatter and the best chance at a fairly clean, straight-ish hole. Then I bore it from there. Sometimes this involves a fair amount of boring but at least I end up with a precisely sized concentric hole with straight sides and a good finish.

 

[Downunder Bob]

Let's not get carried away guys, if the hole is going to be bored the accuracy of the drilled hole is not important, it's just a starting point for the boring process.

However if the drilled hole is the final act, then get it right the first time. Locate the hole with a center drill then drill as close to the final size as possible, within 1/64 or 0.5 mm, in one act, finish with a reamer. if a reamer is not available, then modify the finish drill by rounding the corners of the drill bit This drill will follow the existing hole and finish on size. The poor mans reamer, if done correctly it works very well.

 

[Bill Gruby]

If you want the most accurate hole using only a drill I would use a spot drill and not a center drill. Center drills are for spotting a work piece for a lathe center. This is my thoughts here. Others may differ.

"Billy G"

 

[David S]

If you want the most accurate hole using only a drill I would use a spot drill and not a center drill. Center drills are for spotting a work piece for a lathe center. This is my thoughts here. Others may differ.

"Billy G"

For decades I used the centre / countersink drill for starting, but a while back with info from this great site switched to spotting drills and I got an immediate and noticeable improvement. Huge improvement.

So I fully agree Billy G

David