Drilling: very basic question

[cazclocker]

I've been tinkering with a very small machine shop for several years, but I'm a perpetual newbie. My question is about what point angle I should use for my everyday drills.

OK, I've been in the habit of buying 135-degree split point for all my drills, mainly because they seem to be sturdy and resist walking during the first moment of entry. Most of the time they perform beautifully. Actually, all the time, really.

My question is about those times when I need to drill a small hole in a very controlled, precise location. I have a set of combined drill/countersink to locate a tiny hole into the material, and then follow with the actual drill bit. I should have thought of this a LONG time ago, but I just realized that the cutting angle is LESS than my 135-degree drill bits... I'm not sure of the angle, but they look like about 60 degrees. So when I press the drill bit into the hole made by my combined drill/countersink, my drill bit is not going to immediately hit bottom, right?

If I'm going to use 135-degree split point drills, what should I use to make my initial hole? Whatever I use needs to make a larger degree-angle than 135 degrees, right? I know there is such a thing as spotting drills, but they look just like normal drills... if I'm going to use those, what's the difference between (1) making a pilot hole with a spotting drill and following up with a normal drill, and (2) just making the entire drill hole with the spotting drill? I mean, if the spotting drill is so superior with making the original hole, then why not just finish the hole with it?

What am I missing? You can tell I'm confused....

 

[markba633csi]

I think the idea is just to make a dimple in a precise location for the real drill to follow. For example, using a center punch will
do pretty much the same thing- as long as it's deep enough to give the drill a good start, and wide enough to catch the full web tip of the drill. Personally I use center drills; I don't even own any
spotting drills, and I know the angles are different on those.
Mark S.
ps great avatar

 

[Glenn Brooks]

I do know you can also buy 125* drill bits. Maybe they are 120*’ don’t remember...

How small of diameter holes are you thinking of? And what level,of precision are you trying to achieve? I’ve found a big deterrent to drilling accurate placed holes is that the center punch mark sometimes is not at nadir, e.g. not made at 90* to the work. So when I put the drill bit into the center punch, the bit wanders a few thou off center until it bites into the work. I’ve taken to using a different center punch just to ensure Iam drilling where I want the hole to be drilled.

Mr Pete has a YouTube video out on locating and drilling accurate holes. He uses a small precision drill press to locate and start the hole, then switches over to his floor model to hog put the actual diameter.

Glenn

 

[Bob Korves]

You can grind one of your center drills to a 140 degree angle point, and then just use the tip of the center drill, and not the cylindrical portion enter the work. That is an all around win for spotting, but maybe not so much for other center drilling jobs. It would work, but would be slower cutting. It would also be better for harder materials, so several trade offs there. You could also just purchase a 140 degree spotting drill and be done with it...

 

[cazclocker]

Thanks guys. Glen Brooks, Tubalcain has so many videos, I can't find the one you're referring to. Any idea which one it is? They're numbered... hopefully you know which number it is.
Bob Korves.... so is that what spotting drills are for? What about the last question of my original post?

 

[mikey]

Small drills tend to bend so spotting is actually more important for small drills than for larger ones. For small drills, I use a spotter or a relatively large center drill to create the initial divot. Sometimes I use both if I need to get the hole precisely centered on a mark. Let me explain.

A spotting drill that has an included angle wider than the drill tip is the way to go. For 118 degree drills, a 120 degree spotter works; for 135 degree drills, a 140 degree spotter is good. The problem with spotting drills is that their nose is broad and it can be difficult to center it on a cross mark. A center drill is actually easier to use for precision positioning so I use it as a spotter (it has a 120 included angle at the tip) for small drills. I normally use 118 degree drills so this works for me. If I use my 135 degree drills and I find the small drill tends to bounce then I use a 140 degree spotter to kiss the hole and widen it, then use the small drill. Does that make sense?

For larger drills that need to be accurately positioned, I use the center drill to get me on the spot and a spotter to open up the hole a bit, then the main drill. This is a lot of hassle to work this way but if the hole needs to be accurately positioned and I think I have one shot at it, this is what I feel I have to do.

Punch marks are not an accurate positioning aid. It is more accurate to use layout dye or a Sharpie and mark the hole position with a cross, then come in with a magnifier and center drill and use it to hit the mark. Use a relatively high rpm so the center drill cuts instead of skating and it will create a nice divot for a small drill or a spotting drill. You won't do this all the time but if the hole has to be in the right spot then its worth the hassle.

As for using a spotting drill to drill the entire hole, remember that a spotting drill has no flutes so chip clearance is just not going to happen and the spotter will bind up. Better to use it as intended.

 

[RandyM]

Thanks guys. Glen Brooks, Tubalcain has so many videos, I can't find the one you're referring to. Any idea which one it is? They're numbered... hopefully you know which number it is.
Bob Korves.... so is that what spotting drills are for? What about the last question of my original post?

Here is one on accurately locating holes. It is a two part video. Maybe it is this one.

 

[benmychree]

I do know you can also buy 125* drill bits. Maybe they are 120*’ don’t remember...

How small of diameter holes are you thinking of? And what level,of precision are you trying to achieve? I’ve found a big deterrent to drilling accurate placed holes is that the center punch mark sometimes is not at nadir, e.g. not made at 90* to the work. So when I put the drill bit into the center punch, the bit wanders a few thou off center until it bites into the work. I’ve taken to using a different center punch just to ensure Iam drilling where I want the hole to be drilled.

Mr Pete has a YouTube video out on locating and drilling accurate holes. He uses a small precision drill press to locate and start the hole, then switches over to his floor model to hog put the actual diameter.

Glenn

"Normal" drills have a 118 degree point included angle. The way to locate holes precisely by layout is to first (lightly) use a prick punch, use a magnifier to see that it is precisely on center, if not, use the prick punch at an angle to chase the mark onto center, then still using the prick punch, hit a bit harder to make the impression at nadir as you suggest, then follow with a normal center punch to make the impression angle mor close to the drill point angle. I think all too many people do not pay enough attention to the sharpness of their punches; if the prick punch is sharp, you can literally slide it along one line and feel when it drops into the cross layout line (this is done with the punch at an angle while dragging it, then erect the punch and hit it lightly with a hammer. This technique can have results that are literally within a few thousandths. I have no machines with DRO, so this is my go to technique for close work; even if I had a DRO, I would still do an accurate layout with a vernier height gage to avoid mistakes in computation or whatever.

 

[Dan_S]

My question is about those times when I need to drill a small hole in a very controlled, precise location.

To me the first question would be what do you consider a precise location +- 0.01",0.005",0.001"? The other factor is what sized drill are we talking about, as that also plays a part?

 

[cazclocker]

Small drills tend to bend so spotting is actually more important for small drills than for larger ones. For small drills, I use a spotter or a relatively large center drill to create the initial divot. Sometimes I use both if I need to get the hole precisely centered on a mark. Let me explain.

A spotting drill that has an included angle wider than the drill tip is the way to go. For 118 degree drills, a 120 degree spotter works; for 135 degree drills, a 140 degree spotter is good. The problem with spotting drills is that their nose is broad and it can be difficult to center it on a cross mark. A center drill is actually easier to use for precision positioning so I use it as a spotter (it has a 120 included angle at the tip) for small drills. I normally use 118 degree drills so this works for me. If I use my 135 degree drills and I find the small drill tends to bounce then I use a 140 degree spotter to kiss the hole and widen it, then use the small drill. Does that make sense?

For larger drills that need to be accurately positioned, I use the center drill to get me on the spot and a spotter to open up the hole a bit, then the main drill. This is a lot of hassle to work this way but if the hole needs to be accurately positioned and I think I have one shot at it, this is what I feel I have to do.

Punch marks are not an accurate positioning aid. It is more accurate to use layout dye or a Sharpie and mark the hole position with a cross, then come in with a magnifier and center drill and use it to hit the mark. Use a relatively high rpm so the center drill cuts instead of skating and it will create a nice divot for a small drill or a spotting drill. You won't do this all the time but if the hole has to be in the right spot then its worth the hassle.

As for using a spotting drill to drill the entire hole, remember that a spotting drill has no flutes so chip clearance is just not going to happen and the spotter will bind up. Better to use it as intended.

Mikey, thanks for the clear explanation of a spotting drill’s inability to clear chips. I’m not really willing to give up my 135-degree drills for 118’s, so it sounds like I’d better invest in some spotting drills.
I’m still not clear on the difference between spotting drills and center drills, though. Can you explain please?