Twist Drill Angle with Center Drills vs Spotting Drills

[Bob Korves]

This disagrees with some of the info shared here. Source: http://www.guhring.com/Documents/Catalog/Drills/NCSpotDrills.pdf

Jeff, you will find many opinions and directions for drilling holes, in this thread and elsewhere. Many will contradict 180 degrees from each other, and all will be touted as gospel by someone on the worksite or posting in the thread. Ultimately, you will make the call on how to proceed in your home shop. Consider the options, and visualize the mechanics of how each of them works, and how they might or might not improve workflow and accuracy in your shop. Try a few ideas and see what seems best for you. Go with it, but remember that there are lot of other ideas that might work best for drilling your next hole.

 

[petertha]

Bob, that's the conclusion I'm arriving at in my own amateurish way. I have conventional 60-deg center drills plus 90, 118 & 120 deg spotting drills in my collection. I've been randomly trying different ones in different materials, just as I'm going about machining my parts. My drills are typical 118 point HSS. If I watch the drill very carefully just as its entering the pilot, a 'general' observation is the shallower angle centers will on average have a higher chance of grabbing & displacing the bit over a smidge. Not always, not necessarily the same amount, but on average. What it does for the next 1mm into the hole seems to be a function of drill size, material, speed... all that stuff. Sometimes it will straighten itself out, sometimes I just have to assume its a slightly deviated hole.

The higher angled spotters generally seem to be less of this, all things equal. Again very unscientific. So theory says 118 spotter should be better than 120 but I'm having trouble seeing anything significantly different between them. In fact, almost tending towards 120. Which is maybe good news because 118's seem to be harder to source, so I probably wouldn't re-buy them again.

In terms of the question about power demands going to the big drill right up front vs. progression drilling, totally point valid IMO on many hobby sized machines. Possibly an annular cutter but that doesn't work for blind holes & has depth limits under say 2". I have a 14x40 lathe which has decent power, but I've ran into other issues just gripping the part in the chuck with the axial load without excessive gronking. And teh tailstock sliding back which I think I've solved. So progression drilling is about the only compromise I can see. I treat drilling as a roughing operation anyway, basically the unnecessary evil to make room for a boring bar - LOL

On another note, I had to make some deeper counter-bores for M10 cap screws. I don't have a proper tool that does this, but I sunk in a fine tooth roughing end mill & was amazed at how effortless it was to open up the hole. It had a pilot hole for the bolt shank of course & that may have helped things. the end mil was short & rigid. I'm not sure if this is 'proper' machining but it worked well in this application.

 

[woodchucker]

Jeff, you will find many opinions and directions for drilling holes, in this thread and elsewhere. Many will contradict 180 degrees from each other, and all will be touted as gospel by someone on the worksite or posting in the thread. Ultimately, you will make the call on how to proceed in your home shop. Consider the options, and visualize the mechanics of how each of them works, and how they might or might not improve workflow and accuracy in your shop. Try a few ideas and see what seems best for you. Go with it, but remember that there are lot of other ideas that might work best for drilling your next hole.

I agree, I just thought I would present their knowledge base on the subject. A little stirring the pot, seems like we are low on the discussions these days.

 

[Bob Korves]

I agree, I just thought I would present their knowledge base on the subject. A little stirring the pot, seems like we are low on the discussions these days.

It would have been more interesting if Guhring talked about the "why" of their recommendations.

 

[Bob Korves]

So, this thread advocates going directly to the size of hole you want without drilling smaller sizes out first.
What is the backup plan if your lathe/drill press/mill does not have enough power to drill the hole in one step?

Finishing a hole with a boring bar is the best idea for making a hole that ends up placed where you want it, and is also a way to open up smaller drilled holes while gaining accuracy, not losing accuracy. It does not save time, however. I have never done boring on a drill press, don't really have a setup put together that would do that, but boring holes sure works great on the lathe and on the mill. Taking the last few thousandths with a reamer of the correct size is a good way to get an accurately finished diameter hole.

 

[woodchucker]

Finishing a hole with a boring bar is the best idea for making a hole that ends up placed where you want it, and is also a way to open up smaller drilled holes while gaining accuracy, not losing accuracy. It does not save time, however. I have never done boring on a drill press, don't really have a setup put together that would do that, but boring holes sure works great on the lathe and on the mill. Taking the last few thousandths with a reamer of the correct size is a good way to get an accurately finished diameter hole.

I have had mixed results with reaming. And recently when I saw the gunsmiths use a floating reamer, I realized my tailstock is the reason reaming doesn't work all the time. I think it's the wear in my bed, depending on where the tailstock is setup and where the wear is. I want to take a look at that, or make a floating holder. Seems like it would be like a nice project.

 

[mikey]

So, this thread advocates going directly to the size of hole you want without drilling smaller sizes out first.
What is the backup plan if your lathe/drill press/mill does not have enough power to drill the hole in one step?

Mitch, you make a valid point. Like @Highsider said in post 41 of this thread, it can be tough to drill a big hole without first using a pilot drill and he is right.

So, how big is big? I find that once a twist drill gets much above 5/8" I need either a pilot hole or a really rigid set up. The reason, of course, is that the tip of the drill in the web area is not cutting; the large surface area of the tip on a big drill is too large for us to literally push through the material without a lot of power and leverage so it spins and the cutting area of the flutes cannot engage. A pilot hole overcomes this scenario by eliminating this point contact, which is why we only need a pilot hole large enough to span the web of the main drill. Sort of makes sense, right?

I can't speak for others but I spot drill and go to the main drill for everything up to about 1/2" in most cases. On my lathe, I can go up to about 3/4" (using a morse taper shanked drill). Once I reach these limits, I use a pilot drill large enough to just span the web of the big drill I'm using and accept that my accuracy will be a tiny bit off. That's okay since big holes for me are usually either through holes for a bolt or are roughing holes in preparation for a boring tool.

Even with a properly sized pilot drill there will be some bouncing of the main drill but I find that if I gently engage the drill and allow it to shave a small bevel before applying pressure I have a lot less chatter. Once the tips of the flutes are buried, I pour on the leverage to cut the hole.

Hope this helps.

 

[cg285]

a good friend who is the milling foreman for a fairly large manufacturing co uses center drills, so he says and recommends. maybe some day if i no longer have a dozen projects facing me i may do a comparison.

 

[Bob Korves]

a good friend who is the milling foreman for a fairly large manufacturing co uses center drills, so he says and recommends. maybe some day if i no longer have a dozen projects facing me i may do a comparison.

Lots of people use center drills as pilot drills, especially those who own lathes. They are rigid, probably the most rigid drilling tool, and they are already in the lathe tooling, nothing needed to purchase. They are designed for placing center holes for work to be held between centers, not for spotting holes for drilling. However, the cutting point of a (larger) center drill, not including the cylindrical portion beyond it and before the wider cutting section, just the tip, makes a nicely formed divot for a larger drill to follow. The massive rigidity keeps the tool from wandering. If you use it like how it would be used for placing a lathe center, then I think it loses some ability to help center the following drill, unless the following drill has a smaller angle than 60 degrees (unlikely.)

 

[Bob Korves]

So theory says 118 spotter should be better than 120 but I'm having trouble seeing anything significantly different between them.

Actually, I think the spotting drill should have a slightly larger included angle than the following drill for most scenarios. If we are using the first bit to locate the hole, then we want the second drill to follow the first drill as accurately as possible. If the spotting drill is the same angle or narrower than the following drill things can get interesting at the moment of first contact of the following drill. Any asymmetry or runout or location error will cause one flute of the drill to contact the work before the other. That starts a dance called "walking." The farther out from the center of the hole that contact is made, the more leverage that is snagging on the drill until it starts to cut. Something called "lobing" can happen where the grabbing escalates, causing big vibration and making three or more lobed divots in the hole, considerably oversize. I have had that happen often on not so rigid drill presses and hand held drills. It also happens on a smaller scale, and still ruins holes on heavier setups. If the angle of the cutting edges on the primary bit is larger than on the following bit, then the first thing to make contact is the chisel edge of the following drill. If one hits before the other, the leverage on the drill is much diminished, causing much less leverage induced walking, and also rapidly making a seat to support the tip of the drill, the support increasing as the diameter of the drill in contact with the hole is also increasing. No drama, the drill stays in the middle of the divot from the spotting drill. More accurate as well. That is how I see it, and using that technique has helped my drilling immensely. I do not use it all the time. If I am using my rigid mill and drilling into rigidly held work with a smooth surface 90 degrees to the drill, then why waste time with a starter drill? Use a screw machine (stub) length cobalt drill with a 135 degree split point and just peck it a couple times and then get on with drilling, much time and effort saved.