Twist Drill Angle with Center Drills vs Spotting Drills

[David S]

Thanks Jim,

I came across that explanation after I had posted way back when. For the normal spotting I make sure the "dimple" is smaller than the diameter of the follow up bit. For chamfering it would be larger than the follow up drill bit. Saves an extra step. I like it.

David

 

[Scruffy]

1st time you break the tip Off a centering drill in a piece and spend hours trying to save the piece you will think ,theirs gott’a be a better way.
Only screw machine or spot drills for me.
Thanks Ron

 

[Bob Korves]

You use a 90 degree spot drill when you want to put a 45 degree chamfer on the part and also use drill as a starter.

That idea can save time. It can also cause an ugly hole. Think carefully about a 118 or 135 degree drill coming down to intersect the oversize 90 degree cone left by the chamfering tool. The first points of contact (impact!) will be made by the outside corners of the drill's cutting edges. The first corner that makes contact deflects the drill, which causes the second corner to hit even harder, and this increases to whatever flex the drill is capable of under the load. The drill walks around the hole, making lobes, often in a triangular pattern. The drilling machine shakes at high speed and amplitude. It ain't pretty. The drill is often damaged. And I have been there, more times than I would like to admit. It mostly happened on my 17" floor model drill press, which is much less rigid than my mill. It ruins the work or leaves it ugly for all to see. Having a flatter cone than the following drill makes the drill start at the center of the hole and then gradually open up the hole at a larger cone angle than the starting drill had. I might attempt the chamfer first idea when using a rigid mill and setup, while cutting something like soft aluminum, otherwise I would not try it at all. Too many personal experiences with failure...

 

[David S]

1st time you break the tip Off a centering drill in a piece and spend hours trying to save the piece you will think ,theirs gott’a be a better way.
Only screw machine or spot drills for me.
Thanks Ron

Indeed, I have a incomplete selection of stub bits and use them whenever I can. And consider that they are about 8 times stiffer than jobber length bits.

David

 

[petertha]

Interesting stuff here. I threw together some sketches of different combinations of spot/center hole angle & typical 118-deg twist drill angle, deep/shallow etc. I too have been guilty of the classic 60-deg center drill, having been instructed that way. After watching some of my drills drift away or worse yet grab in softer alloys, it was time to ponder. Hopefully pics help illustrate words of what might be going on. I figure the trouble-maker is the point contact (red arrow) with wide relief gap on either side. It doesn't seem to matter much (visually) whether spot drill is shallow or deep, its a similar issue on a different point of the drill. A 90-deg spot hole is a bit of improvement over 60-deg, but not hugely. I'm sure there is more to the story in reality - different hardness, chip forming etc. But this kind of makes sense to my amateur eye.

With a spot hole angle slightly larger than drill angle (120 vs 118 deg respectively), there is still theoretical 'point' contact, but now a very teeny, shallow relief angle between them which allows for a nice contact start & then true centering as the drill progresses. At least that's my own personal conclusion.

The only thing I wonder about is a combo 118 center & 118 drill. Yes the angles match so its like a cone within a cone, but now the entire cutting edge of the drill contacts hole simultaneously. To me this is like putting an extra wide cutter or form tool in the lathe. You need more power & rigidity, but also more friction, heat & chatter potential vs. a smaller cutting contact area. Maybe now we are splitting gnat hairs. Who knows, my 118 drills could be 117 or 119 or any run-out will start cutting on one face first anyway. I only have a mish-mash of 118 centers but generally & the improvement in hole making was noticeable. But I think I'm convincing myself to get a progressive series of 120-deg spot drills.

 

[Bob Korves]

It doesn't seem to matter much (visually) whether spot drill is shallow or deep, its a similar issue on a different point of the drill.

Look at where the red arrows intersect the drill in your illustrations. The farther out on the drill cutting edge where initial contact is made, the more leverage there is to start the drill walking around the cone.

The only thing I wonder about is a combo 118 center & 118 drill. Yes the angles match so its like a cone within a cone, but now the entire edge of the drill contacts hole simultaneously. To me this is like putting an extra wide cutter or form tool in the lathe. You need more power & rigidity, but also more friction & heat vs. a smaller contact area. Maybe now we are splitting gnat hairs. Who knows, my 118 drills could be 117 or 119 or any run-out will start cutting on one face first anyway. I only have a mish-mash of 118 centers but generally & the improvement in hole making was noticeable. But I think I'm convincing myself to get a progressive series of 120-deg.

Then you are in an unstable situation, where it might go well if the entire drill (or just the center) contacts first, but might also go very wrong if by chance one side of the drill first catches at the O.D. of the hole while the opposite side makes contact near the center of the hole or with no contact at all. The drill is most likely to have a successful start with a starting hole angle enough larger than the drill angle so that there would be no chance of the outer portions of the drill making first contact with the work. Using a starter hole just over the size of the drill chisel point would achieve the same thing, and would also reduce wear and time on the starter drill.

 

[brino]

Interesting stuff here. I threw together some sketches of different combinations of spot/center hole angle & typical 118-deg twist drill angle, deep/shallow etc.

@petertha, that is exactly the series of pictures I have in my head from my experiences at the drill press.....but I've never sat down and drawn them out.

he only thing I wonder about is a combo 118 center & 118 drill. Yes the angles match so its like a cone within a cone, but now the entire edge of the drill contacts hole simultaneously. To me this is like putting an extra wide cutter or form tool in the lathe. You need more power & rigidity, but also more friction & heat vs. a smaller contact area. Maybe now we are splitting gnat hairs.

I understand what you mean about presenting the wider tool face to the work. Typically this is worse for lathe work.
However, in this case I believe it is orders of magnitude better than the deflection that would be caused by using the "shallower" starter drill.

Thanks for sharing your thoughts and pictures, it is greatly appreciated!

-brino

 

[Bob Korves]

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.

 

[Scruffy]

I agree with wreck unles your doing a thousand it doesn’t matter. I don’t think anyone on this forum is working for nasa.
Ron

 

[petertha]

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.