Let's talk drilling...

[HennieL]

What is your preferred method of drilling deep holes as accurately as possible using a lathe? I have been thinking about this for many years, trying to decide the "best" way to drill a reasonably large hole (say 16mm diameter...) for a depth of (say) 10 x diameter into steel. In your opinion (for hobbyists, and not using specialised through lubricated gun drills, etc.):

• would it be better to drill a small diameter pilot hole (say 4mm dia.) as deep as it can go, and then only drilling once using the final size drill, or
• would it be better to use a succession of increasing sized drills (say 4mm, then 8mm, then 10mm, 12mm, 14mm and finally 16mm

Using the second method one would make chip evacuation easier, and put less strain on the drill (both helping to increase the accuracy of the hole), but each successive drill bit would be "guided" only on the outer rim of it's cutting edges, which would make it easier to go off track. The first method would guide over the drill bit's full cutting edges, thus keeping the drill centered, but would create a much larger amount of chips to be evacuated, which would tend to push the drill off center, thus requiring more "peck drilling"...

I normally use the first method, but would appreciate your comments on this. Also, please feel free to comment on any other method you use, and also on drill bit types, twist rates, using a drill chuck vs using Morse tapered drills mounted directly in the tailstock, etc. etc.

Thanks for your comments
Hennie

 

[kwoodhands]

I use a spotting drill first. More accurate then a center drill. If the part is thin I drill with the largest drill after spotting drill. Thicker parts are bored with 1/4" drill and then the largest drill.
Too many variables to help with best results, depends on the material, hole size, motor power, lubrication etc.
I only have three MT shank drills but would use them exclusively if I owned more. I had problems with drills turning in a chuck, marring the shank and the drill sliding into the chuck. I bought a better chuck and this problem went away. I use mostly wire gauge drills and buy them from McMaster Carr. I think Greenlee is the brand. Fractional drills are from Harbor Freight. I have 4 sets of the 1/16"/1/2" drills.
One set I modified for brass, the others I bought because for $10.00 a set it's hard to go wrong.
I found that these sets are more than adequate for general drilling in most metals.
mike

 

[HennieL]

Thanks for your reply, Mike.

I use a spotting drill first.

Yes. I omitted to state this, but I always start my drilling in the lathe with a center drill, as we call them, and then move to the small diameter drill (usually a 4mm short spiral "deep hole" drill), before drilling the large diameter hole. I also prefer to use my More taper shank drills, but only have a set ranging from 5mm to 16mm, in 1mm increments - would love to get some 0.5mm increments as well, but that's quite expensive to source in South Africa.

I would still love to hear more opinions on my original question - is it better (i.e. more accurate) to drill only a small pilot hole and then go directly to the final size drill, or is it more accurate to increase the hole size in small increments (of say 2mm per increment)?

Regards
Hennie

 

[Vietti]

I like screw machine drills, have a set that goes 1/16 to 1/2. They are stiffer than a jobber drill and depending on precision desired can be used without a center drill start. Irritating that they cost more than jobber length and haven't found a source for good relatively inexpensive ones.

Sometimes I use a center cutting end mill to at least start or finish a hole where any wandering cannot be tolerated, beware they don't cut an exact size hole either.

John

 

[kwoodhands]

Thanks for your reply, Mike.

Yes. I omitted to state this, but I always start my drilling in the lathe with a center drill, as we call them, and then move to the small diameter drill (usually a 4mm short spiral "deep hole" drill), before drilling the large diameter hole. I also prefer to use my More taper shank drills, but only have a set ranging from 5mm to 16mm, in 1mm increments - would love to get some 0.5mm increments as well, but that's quite expensive to source in South Africa.

I would still love to hear more opinions on my original question - is it better (i.e. more accurate) to drill only a small pilot hole and then go directly to the final size drill, or is it more accurate to increase the hole size in small increments (of say 2mm per increment)?

Regards
Hennie

I happened to read an article this morning in one of my machining books that addresses this question in a way I have never heard of. The author said to start with a large drill, then progressively smaller drills til the depth is reached.
Then install the size drill you want to finish with and bore the hole.
He said that this method leaves clearance for each successive drill and is best way to drill deep holes.
I will try this myself when I need to bore a deep hole.
Even though it may seem odd , I think it may be the way to bore deep holes.
mike

 

[clockworkcheval]

Kwoodhands is right in my opinion to start with a spotting drill rather than a center drill. A center drill prepares a hole for a center, thus has a top angle of 60 degrees and also makes a little top-pocket to protect the sharp point of the center. When you enter such a hole with a 118 degrees top angle drill this drill will hit the hole at the rim rather than in the center. And shapes never being ideal one cutting edge of the drill will hit the rim first and thus act as a temporary center of rotation till the other edge hits and takes over the role of center of rotation. Doing this in thin material nicely shows the resulting typical triangular hole. The small holes started with center drills are also triangular (or multi-angled), only so slight that it is not all that obvious. The spotting drill with a top angle of 120 degrees on the other hand gives a drill with a top angle of 118 degrees a well-centered start.
As you drill the hole the hole will deviate from the centerline by a-symmetric forces. The amount of deviation caused by an a-symmetric force increases with the length of the drill to the third power and decreases with the diameter of the drill to the fourth power. Thus if you compare a drill of diameter 16 mm sticking out 50 mm with a drill of 4 mm sticking out 100 mm the 4 mm drill will deviate (16/4) to the power 4 times (100/50) to the power 3 equalling a whopping 2048 times more.
For a long hole I suggest you start with a 120 degree spotting drill, then a 4 mm diameter drill to a depth of say 15-20 mm. If you now follow with a 16 mm diameter drill it will nicely start at the center, follow the 4 mm diameter hole for the 15-20 mm pre-bore and further on be guided by its own 16 mm diameter hole. Starting with a short drill and exchanging it with longer drills as you get deeper will give you the best shot at drilling a long concentric hole. You need to retract the drill often to get rid of the chips. In short, a well-centered short pilot hole followed by a full size drill. The variation mentioned by Kwoodhands of using drills of decreasing diameters seems to me to lower the stiffness of the ever longer drills whilst not benefitting from the guidance of its own full size hole. For long holes the way you hold the drill whether by Morse cone or chuck is not relevant for the precision of the position of the tip of the drill - the tip will follow the centerhole first and after that the drill will be guided by its own bore.
Boring of course is the more precise option.

 

[Peter Murphy]

Yes. I omitted to state this, but I always start my drilling in the lathe with a center drill, as we call them, and then move to the small diameter drill (usually a 4mm short spiral "deep hole" drill), before drilling the large diameter hole. I also prefer to use my More taper shank drills, but only have a set ranging from 5mm to 16mm, in 1mm increments - would love to get some 0.5mm increments as well, but that's quite expensive to source in South Africa.

I would still love to hear more opinions on my original question - is it better (i.e. more accurate) to drill only a small pilot hole and then go directly to the final size drill, or is it more accurate to increase the hole size in small increments (of say 2mm per increment)?

Regards
Hennie

Hi Hennie, a spotting drill is different to a centre drill. A centre drills cutting angle is ground to the correct angle to support a tailstock centre. A spotting drills cutting angle is ground to the correct angle to align a drill bit & will give you greater accuracy. I personally don't interchange them. When to Use a Spot Drill [ 7 Useful Tips and Techniques ] - CNCCookbook: Be A Better CNC'er
I do like that idea of starting with a larger bit (preferably an MT bit) & progressally stepping down in bit diameter as you go deeper, then go up in size & finally bore to finish. This would give you the most accuracy, so long as you have some small diameter long drill bits

 

[Peter Murphy]

Hi Clockworkcheval, Fully agree with you & sorry for sort of repeating your post, we had a blackout here due to high winds & I was on my laptop with no internet. Your post appeared as I was posting when the power & internet came back on. Cheers Peter from OZ

 

[ajoeiam]

Monsieur Clockworkcheval describes a technique that works reasonably and here is another - - - - especially useful when the resulting hole will be tapped.

I never have had the fortune of using a spotting drill - - - was just one more tool and, well, that would have meant more cost and time for the boss so that didn't just fly very well. So - - - albeit I think using a spotting drill is likely more accurate this technique works well especially for tapped holes.

1. hit the shaft end with a center drill
2. use a drill that is large enough to provide a divot that is large enough to be say 0.5mm or 1/64" larger in dia than either the thread OD or your finished
hole OD
3. using an accurately sharpened drill start your hole - - - - starting hard and pushing strongly for at least 3 if not 5x D (diameter)
4. spin that tailstock back and now start using a peck cycle
5. if you're only drilling to 7xD or you have a drill where the flutes are passing chips very well you might get to 10xD then #4 suffices
6. if, on the other hand you're drilling something like a grease hole that might be some 12 or even 20" long (its a bloody long bit!!!) I found using a shuttle kind
of drilling worked (with care).
This was peck once and maybe twice with a small amount of hand wheel back motion on the tailstock. Then after this very small number of 'pecks' unlock the
tailstock and drag the setup away from the piece. Found that moving the setup some 4 to 6 inches would shift the material down the flutes of the drill. Care
needs to be taken so that you don't really jam up the chips in the leading part of the drill. You might then need some serious pull to remove the setup. I would
think that balled tight enough and you just might have a drill bit welded into the material which will have a tendency to cause the surrounding air to change
hue - - - remember it seeming to take about half of forever but it does work.
To heavy a push on the drill cycle can cause the drill to wander and that makes hitting the center hole with the cross hole for the grease a bunch trickier.

HTH

 

[HennieL]

Hi Hennie, a spotting drill is different to a centre drill. A centre drills cutting angle is ground to the correct angle to support a tailstock centre. A spotting drills cutting angle is ground to the correct angle to align a drill bit & will give you greater accuracy. I personally don't interchange them. When to Use a Spot Drill [ 7 Useful Tips and Techniques ] - CNCCookbook: Be A Better CNC'er

Thanks Peter - just goes to show again... one is never too old to learn

Thanks for all the other replies, especially the very informative ones by clockworkcheval and ajoeiam - much appreciated.

I sharpen all my drills (well, all those larger than 4mm...) to a 130° point angle, as I mostly work in harder steel - am I correct in deducing that this would then have a larger influence in forcing the drill to go off-centre, even when starting with a centre drill?
It should be fairly easy (I think, having never tried it...) to re-profile the points of my larger centre drills to 130° and then only drilling the first stage (i.e. up to where the larger angle for cutting the tapered hole starts) - would that make a positive difference?

 

[Ken I]

I once drilled a Ø20 hole through a 600mm length of stainless (on a Graziano SAG14) - The long series drill being made by turning down the drill shank and attaching it to a length of Ø12mm rod by silver soldering.
The idea was to use the drill as standard at the start, add the extension piece and "peck" my way deeper - as the flutes are going to get swarf bound quite rapidly.
On my first attempt, I was a bit slap dash and the drill bit was clearly wandering at the start (not really well centered) - the damn thing came out the side of the Ø40mm bar shortly before it reach full length.
I was shocked - I couldn't believe you could drill "around a corner" that badly.
On my second attempt, I was far more careful about getting the drilling started correctly - I also bored the start up to drill size as accurately as I could - so as to provide initial "guidance" to the drill.
This time it went through perfectly and was only ±0.1 TIR at the breakthrough end.

Lesson: Make sure that you start it as perfectly true as possible and bore the start if possible. Everything must be as perfect as you can do.

Here's a link to a Ø4 x 123.5mm long - that's also over 30 times the diameter.

Six Shooter Elbow Engine

Page down to my Jan 7 entry.

Get the start right and the rest follows.

Postscript: I have had the odd problem on deep drilling where the central grain structure is "corkscrewed" during spin casting or other processing - then the drill tends to follow the corkscrew - it can be a right PITB.

Regards, Ken

 

[aRM]

Kwoodhands is right in my opinion to start with a spotting drill rather than a center drill. ................

Hello there
This is one of the BEST Explanations I've read in a long while on WHY a CENTRE DRILL will not lead to a very precise and accurate Drilled Hole.
Much appreciate the Post
Thanks again for the input
aRM

 

[Richard Hed]

Thanks Peter - just goes to show again... one is never too old to learn

Thanks for all the other replies, especially the very informative ones by clockworkcheval and ajoeiam - much appreciated.

I sharpen all my drills (well, all those larger than 4mm...) to a 130° point angle, as I mostly work in harder steel - am I correct in deducing that this would then have a larger influence in forcing the drill to go off-centre, even when starting with a centre drill?
It should be fairly easy (I think, having never tried it...) to re-profile the points of my larger centre drills to 130° and then only drilling the first stage (i.e. up to where the larger angle for cutting the tapered hole starts) - would that make a positive difference?

Wait a miniute. I am too old to learn! But it is always helpful to know about these spotting drills. I never knew about them till I lookt them up because of all your posts. I thimpfk I have never owned one of these drill bits. Maybe I 'll get a couple to test out. Even so, I am still incapable of learning.

 

[L98fiero]

I sharpen all my drills (well, all those larger than 4mm...) to a 130° point angle, as I mostly work in harder steel - am I correct in deducing that this

There are 130° spotting drills as well.

 

[tornitore45]

Until the final size drill is deep enough to be past the conical tip it is free to wander, drill oversize and veer off.
Once the drill margins are into the hole, the freedom to wander is much limited.
A second consideration is that uneven lips can push the the bit on one side but this is greatly reduced with a pilot hole to clear the web.

I would drill a pilot with the normal size, that is, a bit larger than the web. Then if the pilot drill can not reach the final hole depth I would change to a larger pilot that can, obviously still smaller that the final size.
Then BORE the first 1/2" or so, to size or to the size for the reamer if one plans to ream, to guarantee that the final size bit is fully guided.
Now you can drill with the reamer size or the final size, knowing the bit is constrained by the short bored hole.
If you have to, ream to size.

It does not really matter if one starts the hole with a spot drill or a center drill since that operation is just to start the pilot hole.

 

[clockworkcheval]

Good to see from the many comments that drilling is still the Mother of Manufacturing, a firm descendant of Stone Age Technology.

 

[goldstar31]

I recall that in 1973Prof Dennis Chadock writing on his Mark1 Quormn T&C grinder advocated the use of 4 and 6 facet twist drills. More recently, our own BaronJ published is design which appeared in the GadgetBuilder.com site.


No mention- WHY?

 

[Peter Murphy]

Until the final size drill is deep enough to be past the conical tip it is free to wander, drill oversize and veer off.
Once the drill margins are into the hole, the freedom to wander is much limited.
A second consideration is that uneven lips can push the the bit on one side but this is greatly reduced with a pilot hole to clear the web.

I would drill a pilot with the normal size, that is, a bit larger than the web. Then if the pilot drill can not reach the final hole depth I would change to a larger pilot that can, obviously still smaller that the final size.
Then BORE the first 1/2" or so, to size or to the size for the reamer if one plans to ream, to guarantee that the final size bit is fully guided.
Now you can drill with the reamer size or the final size, knowing the bit is constrained by the short bored hole.
If you have to, ream to size.

It does not really matter if one starts the hole with a spot drill or a center drill since that operation is just to start the pilot hole.

But if you use a 60deg centre drill & your pilot hole is off because the pilot drill can't go in deep enough to align the pilot drills point with the centre of the divot then everything after that including the reaming will be off. If you use the correct size & point angle spotting drill your pilot drill will be guided to the centre of your spotting drill divot, everything after that has a better chance of being true.

 

[jack620]

It does not really matter if one starts the hole with a spot drill or a center drill since that operation is just to start the pilot hole.

I disagree. A spotting drill is designed for drilling starter holes for subsequent drilling. Centre drills are not.

 

[clockworkcheval]

It is my understanding that there are two main methods of grinding drills. The first method is forming a slanted cone for each cutting edge. This cone will have the right cutting angle at the cutting edge and then drop off leaving space for the chips. You can form the cone with a grinding tool or with some exercise holding the drill in your hand and making the cone with a movement of your wrist. The second method starts with forming a small straight flat cutting edge with the correct angle - 2 facets. Being flat this leaves not enough room for the chips to evade so you grind two more flats behind the cutting edge at a steeper slant - 2 more facets. If this is done right the four facets will meet at the point of the drill making the drill more self-centering. You can improve this by cutting away the web towards the point. Finally considering that most wear will occur where the cutting edge meets the outer diameter you can grind two more facets at the cutting corners - 2 more facets. The 6 facet drill with webpoint cutaway is superior in selfcentering and in tool lifetime. The setback is that you need a machine to do it correctly.