# In the "you don't know what you think you know" department....



## dbb-the-bruce (Jul 16, 2020)

Perhaps this should be in the beginners forum, but it struck me as yet another example of no matter how much you know, you have assumptions that are over-simplifications. Sure they probably work for you most of the time, but there is more going on than you realize.

For a completely unrelated reason, I went looking for the angle is on my center drills. I've done quite a bit of wood lathe work before getting into metal lathe work where I first encountered center drills. So it was easy to grasp that the best way to get true work is supporting it between two centers (wood spindle turners know or lean this to be successful).

The job of a center drill in metal lathe works is to make a matching hole for the centers that includes relief at the bottom for the tip of the center. Got it/knew that. And the tip angle is standardized at 60Deg. Then there is the additional use of a center drill to make a conical starting point for drilling, keeps the drill from wandering around when you are starting.

After getting my benchtop mill, it just seems natural to always start drill holes with a center drill, in fact the "starter set" of tooling for the miil includes a set of center drills.

So back to searching for info on center drill angles, and I find this: CENTERDRILLS DON'T CENTER DRILLS
Of Course!

I think I'll be looking for some spotting drills of various angles.
I've got a menage of old drills sharpened to various angles - I'll be paying a little more attention to the tip angles.
I don't think I've got any carbide drills, but will change my strategy for starting holes if I end up using one.
For more critical holes in general, I'm going to be paying attention to tip angles when spotting and enlarging.

-Dave B


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## Dabbler (Jul 16, 2020)

Thanks for bringing this up.Ghuring has a similar thing in their training.

I've been teaching for many years that people should NEVER use a centre drill to use as a first operation for drilling - only use spotting drills.  I can't believe the push-back I get from people who've done it wrong for many years, so it must be the right way....  My mentor, a tool and die maker for over 60 years never converted to the better way to do it.  Every one of his cleveland HSS drills had weakensses and dull spots in about the same place - where the 60 degree point most often hit the 118 degree angle...


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## DavidR8 (Jul 16, 2020)

So something like this should be used instead?




__





						KEO,3/8 KEO 118 DEGREE SPOTTING DRILL,1-089A-024,KBC Tools & Machinery
					

KEO,3/8 KEO 118 DEGREE SPOTTING DRILL,1-089A-024,KBC Tools & Machinery




					www.kbctools.ca


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## Dabbler (Jul 16, 2020)

Mine are 120 degree, but 118 will do just fine as well!  THe KEO are shorter than mine, but may be better for it...


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## Mitch Alsup (Jul 16, 2020)

A reason some recommend starting a hole with a center drill is the stiffness of the short non-fluted flank enables the first feature drilled into the part under machining to be accurately positioned.

A spotting drill has all of these advantages and more! 

Now if they just sold an assortment of spotting drills for the price of an assortment of center drills.


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## Tozguy (Jul 16, 2020)

What happened to centre punching? Many drilling jobs do not need a pilot hole. Just a well placed punch mark.
A poorly sharpened drill will wander whether there is a pilot hole or not.
Get a set of quality stub drills or screw drills (or make your own from jobber drills) and see how big a hole they can drill with no starter hole at all. When used the way they are intended, they stay sharper longer. You will see the difference right away.


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## middle.road (Jul 16, 2020)

Cool, learn something new everyday.


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## mmcmdl (Jul 16, 2020)

If you ever ran production on cnc machines you would know all about spotting drills .


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## bakrch (Jul 16, 2020)

I drill tens of thousands of holes every week, for close to 15 years now.  Take that FWIW.

142 degree spot drills are made specifically for starting a 140 degree carbide drill. We use coolant thru OSG. I am still amazed how far a single drill can go.  

If one doesn't have that, the tip of a center drill is next best to match typical drill angles. If you can get away with just making a .02/.03 depth dimple with the center drill you will get great tool life.  However,  the web is rather thick and cannot be used for very small diameter holes.  This is when a spot drill comes back into play. 

It's quite a rabbit hole, context surely matters and there is really no one-size-fits-all answer.


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## projectnut (Jul 16, 2020)

I guess I'm one of those fossils that can go either way.  In reference to using either a spot drill or a center drill that is.  In the last 40 years I've drilled more holes than I can count.  The vast majority were started with center drills since that's what the shop supplied.  To put things into better context 90+% of the holes we drilled were in 316 stainless and done with HSS drills.  

I don't ever recall chipping a drill or having one wander because the pilot was the wrong profile.  Drilled holes were made where precision fits weren't required.  Most often for bolt hole clearances.  The standard was 1/32" over bolt diameter.  Some where closer fits were required were .015" over bolt diameter.  Anything needing closer tolerance was either bored or milled.

To this day I only use carbide drills and mills when absolutely required.  They may be faster, (if you have the equipment to handle the speeds and feeds) and last longer in some materials, but I don't believe they are any more accurate.  Chipping has always been a concern when using carbide.  It doesn't like interrupted cuts, welds, or inclusions.

I do have multiples of several sizes of spot drills, and carbide drills, but in the scheme of things they get used far less then the center drills and HSS drills of the same sizes.  If I was in a production situation and time was money, I would definitely be more interested in carbide and in the spot drill camp.  As it is the older time tested methods work just fine, and cost considerably less.


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## mmcmdl (Jul 16, 2020)

^^^ +1 on what PN said . If not in a hurry , I can't see a difference in either . The spot drills were much easier to program for when the print called out a certain chamfer with the drilled hole . Less time with the z axis when doing multiple holes in a production environment . CNC work is quite competitive and every minute counted . As for a home shop  , I use both depending on size .


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## tcarrington (Jul 16, 2020)

It seems to me the assumption through all this is the twist drills are ground with the correct geometry. The off-shore drills, IMHO, FWIW, based on my experience are not necessarily so ground. 
Those doing this for a business those who have bought better sets are probably getting "to specification" tooling because you can't afford not to do so. 
A small dimple that will force or allow a correctly ground twist drill start with even curling chips on both sides will likely give a good result. In the same way a machine screw or stub length drill is a fairly good substitute for a carbide spotting drill.
I also suspect the center portion of the drill is critical for starting a hole, spotting drill or not. Would a split point be better? I think so.

In my reality, I have learned how to grind them correctly since I have 114 or so that need it. I don't have the split point worked out on the smaller sizes. Oddly, the 0.125 (1/8) inch in my off short set was spot on.  I also have a good set of number drills and have made some other loose bits stub length to add to my arsenal.


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## bakrch (Jul 16, 2020)

Production is one thing, as far as CNC is concerned.

It is highly situational. Personally, I would rather spend 6-8 hours drilling a part with 11,000 holes on our coolant-thru Doosan VMC, than a week and a half on the HAAS using cobalt(only flood coolant there).

We have CNC, but are mainly a repair shop and there is no need for production mentality in most cases. Sometimes we need to meet a deadline, so there is an obvious need to go fast there, but more often than not we have plenty of time. If one of the guys wants to drill with HSS on the HAAS, we aren't going to tell him otherwise as he is a respected machinist and probably has the free time to do so. We have proven time and time again that it is cheaper in the long run to use carbide, but we have a lot of HSS and cobalt tooling laying around from doing things the old way. It doesn't get cheaper than using what has already been purchased.

Another thing I like to consider, is you are going to change that HSS drill quite a few times for some jobs, yet the carbide drill is likely going to last the job, and then another, and another ... with no tool change or potential for a crash due to human error setting the tool. On a manual machine, nothing to worry about here, really.  There are also no pecks for a CNC fed thru-coolant drill, so fewer problems on that front as well.

Also, say the drill breaks ... it is much easier to break the carbide and blow it out with an air hose provided the hole diameter is large enough to get a decent punch in there. Soooo many variables.


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## mmcmdl (Jul 16, 2020)

I still have plenty of my old Valenite insert drills used on the Makino VMCs and HMCs . Some of these I've turned the shanks down to fit my end mill holders for the BPs . While I don't/can't take advantage of the thru spindle coolant , these things can blow material out like crazy . One problem with them is they throw a slug out the bottom when making a thru hole , otherwise they are great . I think they took the wcem inserts if I remember correctly , it's been awhile .


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## dbb-the-bruce (Jul 16, 2020)

A lot of good comments, I do think that some people misunderstood the meaning of my post.

It's not that using a "center drill" is wrong, it that the true meaning of "center drill" is a drill that makes holes for you to hold a cylinder between centers on a lathe. 
It's short and stubby so it won't wander around, angled to match your lathe centers with a relief for the tip of the center. 
So it also happens to be a pretty good choice for starting a hole when you don't want the drill to wander on entry.

However, there is more to it than just making a little funnel for the drill bit to start in. I kind of always assumed that the idea was to catch the outside of the bit with the funnel walls and that helps direct it do the center. And it probably does in a lot of cases. But there are also downsides to using a center drill this way. etc. etc. etc.

I never thought of using just the tiny tip of the center drill, I'm sure it works but it can/will cause divots because the cut is not starting at the point. No big deal if you aren't drilling much or like to sharpen your bits. 

A punch is also a good locater, if you can get the punch on the right spot by hand and eye - I suck at it, I'm not good at all with manual layout. I get better with practice but....

So, if it gets the job done with what you have and with the quality you need I'm all for it, go for it!
There is always more than one way - and always different priorities (speed, tool life, what do I have to use right now etc).

There is always more to learn,  and the corollary - you don't know as much as you think you know.

Have fun, make chips.


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## mmcmdl (Jul 16, 2020)




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## bollie7 (Jul 16, 2020)

A lot of us don't know what we don't know either sometimes. I include myself in that group too.
Peter


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## Dabbler (Jul 16, 2020)

for many years, before I could afford a spotting drill in carbide I used #5 or #6 centre drills to drill a little divot for my 120 degree HSS drills.  It's a lot cheaper to do it that way!


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## mikey (Jul 17, 2020)

dbb-the-bruce said:


> I never thought of using just the tiny tip of the center drill, I'm sure it works but it can/will cause divots because the cut is not starting at the point. No big deal if you aren't drilling much or like to sharpen your bits.



The reason the very tip of a center drill works well as a spotter is because the tip angle is 120 degrees, the same angle as a spotter for use with 118 degree drills. We had a good discussion about spotters here.


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## dbb-the-bruce (Jul 18, 2020)

mikey said:


> The reason the very tip of a center drill works well as a spotter is because the tip angle is 120 degrees, the same angle as a spotter for use with 118 degree drills. We had a good discussion about spotters here.


So only use the very tip, choose a large center drill and don't go so deep that you have sides.
I've been doing the funnel thing - Wrong!


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## ericc (Jul 19, 2020)

Berry interesting discussion, thanks. Now I know what those large center drills are good for.


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