# Center drill question



## cazclocker (Jun 29, 2022)

I'm getting set to buy a set of 5 center drills (drill & countersink combination). All the drill bits I own are 135 degree split point. But the center drills I see are almost universally 60 degree included taper. That means I can start a hole with a center drill with no problem - but when I follow up with my drill bit, the first thing my drill bit will cut is going to be the outer circumference of the starter hole. To get the best concentricity, shouldn't the split point of my drill bit be the first thing to begin cutting the workpiece? What am I missing?


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## Liljoebrshooter (Jun 29, 2022)

Center drills are for using on a lathe to use a live or dead center on not for using as a guide for drilling a hole.
Spotting drills are for starting a precise hole for drilling .

Joe


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## Nutfarmer (Jun 29, 2022)

Use a spotting bit that matches the angle of the drill bit. A center drill is for drilling centers on the lathe for turning between centers. A lot of us will use a center drill for starting a drill just because that is what we have not because it’s the best way. Each their own. It all works.


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## mmcmdl (Jun 29, 2022)

Joe sums it up correctly above .   And nutfarmer is correct also . We all use center drills to spot holes because it's what we have . It doesn't cause issues when drilling .


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## cazclocker (Jun 29, 2022)

Liljoebrshooter said:


> Center drills are for using on a lathe to use a live or dead center on not for using as a guide for drilling a hole.
> Spotting drills are for starting a precise hole for drilling .
> 
> Joe


Ah, okay. My mistake - glad I asked. My intention is to drill a series of 12 precisely-placed holes, about 1/4" diameter, in 1/4" steel plate of unknown composition. Thank you!


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## mmcmdl (Jun 29, 2022)

cazclocker said:


> Ah, okay. My mistake - glad I asked.


Not a mistake cazclocker , either will work fine . One big difference is when running the CNCs with a 1000 holes that need to be chamfered for tapping or whatever . The spotting drills don't have the pilot of the center drill , thus the Z depth may be cut by 3/4s saving loads of time . Bottom line , use what you have .


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## jmkasunich (Jun 29, 2022)

My two cents:  if you care about hole location, get and use spotting drills.  You only need one or two (one for little holes, maybe 1/8" or so, and one for bigger holes, 1/4" or so).  Can be purpose made spotting drills (named and sold as such) or just screw-machine-length drill bits.  The key factor is that they are short and stiff so they don't go wandering off.  Once you've spotted the hole in the correct location then a longer more flexible drill can be used to finish it.


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## cazclocker (Jun 29, 2022)

mmcmdl said:


> Not a mistake cazclocker , either will work fine . One big difference is when running the CNCs with a 1000 holes that need to be chamfered for tapping or whatever . The spotting drills don't have the pilot of the center drill , thus the Z depth may be cut by 3/4s saving loads of time . Bottom line , use what you have .


Thank you for that. I should have mentioned that I'll be doing this project on a benchtop drill press, not a milling machine. Sadly, I sold my 5-piece set of center drills along with my milling machine when we moved to be close to family. Bottom line, at this point I don't have either center drills OR spotting drills!


jmkasunich said:


> My two cents:  if you care about hole location, get and use spotting drills.  You only need one or two (one for little holes, maybe 1/8" or so, and one for bigger holes, 1/4" or so).  Can be purpose made spotting drills (named and sold as such) or just screw-machine-length drill bits.  The key factor is that they are short and stiff so they don't go wandering off.  Once you've spotted the hole in the correct location then a longer more flexible drill can be used to finish it.


I just looked at MSC and McMaster-Carr, and I see that MSC has a good variety of drill point angles - I see 140 degrees, which seems appropriate for my collection of 135 degree cobalt & HSS drill bits (all screw machine length).
Given my drill bit point angle, does a spotting drill point angle of 140 degrees seem appropriate to you guys? MSC also carries 130 degree point angle spotting drills, but 140 seems more logical to me.


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## jmkasunich (Jun 29, 2022)

cazclocker said:


> Thank you for that. I should have mentioned that I'll be doing this project on a benchtop drill press, not a milling machine. Sadly, I sold my 5-piece set of center drills along with my milling machine when we moved to be close to family. Bottom line, at this point I don't have either center drills OR spotting drills!


If you are doing the work on a drill press, how are you doing the layout?  You don't have leadscrews and dials....

If you are scribing lines and centerpunching, you do NOT want spotting drills (or center drills).  

Think of it this way:  on a mill, you use the dials to accurately put your target spot directly under the spindle.  Then you use a spotting drill to make a dimple is accurately centered under the spindle before following it with a more flexible drill bit that hopefully follows the dimple.  But with a drill press, your layout and center punch mark is the target.  So a flexible drill that will find and follow the center punch mark is better than a stiff spotting drill that will follow the spindle centerline and ignore the center punch mark.


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## mattthemuppet2 (Jun 29, 2022)

one way that you can use center drills to spot a hole is to use a follow up drill that is the same diameter as the tip of the center drill. Essentially you're creating a piloted hole for a pilot, then you can follow up with your larger drill.

I also second using screw machine length drills - if you need any, I have lots  That's all I've used, though I do have spotting drills now. As Dave ^ wrote, they're handy for adding a chamfer to the hole first, so that you don't need to go back and do it after drilling the hole.


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## cazclocker (Jun 29, 2022)

jmkasunich said:


> If you are doing the work on a drill press, how are you doing the layout?  You don't have leadscrews and dials....
> 
> If you are scribing lines and centerpunching, you do NOT want spotting drills (or center drills).
> 
> Think of it this way:  on a mill, you use the dials to accurately put your target spot directly under the spindle.  Then you use a spotting drill to make a dimple is accurately centered under the spindle before following it with a more flexible drill bit that hopefully follows the dimple.  But with a drill press, your layout and center punch mark is the target.  So a flexible drill that will find and follow the center punch mark is better than a stiff spotting drill that will follow the spindle centerline and ignore the center punch mark.


I'll be marking the hole locations with transfer punches. The hole locations will be guided by other pre-drilled parts which need to be bolted to the steel plate that I'm drilling. I'll clamp the pre-drilled part in the desired spot on the steel plate and use my transfer punches to mark a small spot on the steel plate. Once I've got all 12 spots marked, I'll take the steel plate and clamp it in position under the drill press quill. Then, I have a homemade piece of drill rod sharpened to a fine pencil point that I'll chuck up into the drill press. I just lower the quill and wiggle the plate's position until the "steel pencil" is pointing exactly to the transfer punch mark. Once I've got it where I want, I'll replace the steel pencil with a spotting drill, and drill a shallow hole - and follow up with the final drill size. It's stupid and inefficient, but I'm retired and patient.
I've done similar projects before using the same methodology. It works great. Someday I'll buy another milling machine!


mattthemuppet2 said:


> one way that you can use center drills to spot a hole is to use a follow up drill that is the same diameter as the tip of the center drill. Essentially you're creating a piloted hole for a pilot, then you can follow up with your larger drill.
> 
> I also second using screw machine length drills - if you need any, I have lots  That's all I've used, though I do have spotting drills now. As Dave ^ wrote, they're handy for adding a chamfer to the hole first, so that you don't need to go back and do it after drilling the hole.


Some good ideas there! Thank you!


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## cazclocker (Jun 29, 2022)

My last question: all of my drill bits are 135 degree split point. Should I buy 130 degree or 140 degree spotting drills?


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## mattthemuppet2 (Jun 29, 2022)

140 degree


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## cazclocker (Jun 29, 2022)

mattthemuppet2 said:


> 140 degree


Thanks very much.


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## BGHansen (Jun 29, 2022)

As mentioned above, center drills are primarily for making a 60 deg. cone for a 60 deg. center on a lathe.  However, the tip of all of my center drills is at 120 deg.  If you only dip the tip, they are essentially 120 deg. spotting drills.  It wouldn't be best for your 135 deg. bits, but works fine for 118 deg. bits.

Bruce


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## Parlo (Jun 29, 2022)

jmkasunich said:


> If you are doing the work on a drill press, how are you doing the layout?  You don't have leadscrews and dials....
> 
> If you are scribing lines and centerpunching, you do NOT want spotting drills (or center drills).
> 
> Think of it this way:  on a mill, you use the dials to accurately put your target spot directly under the spindle.  Then you use a spotting drill to make a dimple is accurately centered under the spindle before following it with a more flexible drill bit that hopefully follows the dimple.  But with a drill press, your layout and center punch mark is the target.  So a flexible drill that will find and follow the center punch mark is better than a stiff spotting drill that will follow the spindle centerline and ignore the center punch mark.


If your workpiece is not clamped then the spotting drill will find the centrepunched mark nicely.


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## wachuko (Jun 29, 2022)

Oh great... now I need to order spotting drills... all along I was using the center drills for everything...


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## jmkasunich (Jun 29, 2022)

Parlo said:


> If your workpiece is not clamped then the spotting drill will find the centrepunched mark nicely.


Depends on the part.  If it weighs a few ounces, sure.  If it's 50 pounds the spotting drill & centerpunch mark aren't going to be able to drag it across the table to self-align.  Cazclocker's plan of using a sharp pointed rod in the chuck to align the part should work regardless of the size of the part.


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## IamNotImportant (Jun 29, 2022)

wachuko said:


> Oh great... now I need to order spotting drills... all along I was using the center drills for everything...


that's "spot on"!


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## Parlo (Jun 29, 2022)

jmkasunich said:


> Depends on the part.  If it weighs a few ounces, sure.  If it's 50 pounds the spotting drill & centerpunch mark aren't going to be able to drag it across the table to self-align.  Cazclocker's plan of using a sharp pointed rod in the chuck to align the part should work regardless of the size of the part.


I was thinking of the point on the spotting drill instead of the rod as it is only 1/4" plate.


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## pontiac428 (Jun 29, 2022)

You can make your own spotting drills for free.  You can re-point a broken drill to the angle you want for spotting regardless of helix angle.  You can thin the web and use a 2-angle grind or other little tricks to get the result you want.  Since chip removal isn't a problem, I just grind D-bits in a conical profile with a good clearance cut past the cutting edge.  They can also be ground to match fastener heads and run as a countersink.  If the subject interests you, read Harold Hall's tool grinding book that is included in our site's free library.  He does more to explain drills in four pages than you'll find in some of the big references.


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## Jimsehr (Jul 3, 2022)

wachuko said:


> Oh great... now I need to order spotting drills... all along I was using the center drills for everything...


No. What you need is to learn how to grind a short stubby spotting drill.


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## Toolmaker51 (Jul 3, 2022)

Be reminded drills do not commence with material removal, with common web variety as the most obvious, split points the quickest. They extrude metal for the leading edge of flute to engage. Being fluteless, spotters offer high rigidity, yet the points can be fragile. A center drill does a good start, when followed by a correctly ground bit, by initiating a cut immediately. 
I usually penetrate a bit deeper than one diameter of the center drill point, deeper yet to accommodate larger bits.  In very small drills, a center drill barely more than a punch mark is sufficient.


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## tq60 (Jul 3, 2022)

Get a good, sharp and hard center punch.

Your transfer punch will make a good starting point.

A good center punch with proper hold and good smack will make a good starting point for your drill.

Center drill work well as they are designed to hit a moving target and cut properly.

For using a drill press everything is going to move, your drill rod in chuck as a guide will help.

Sent from my SM-G781V using Tapatalk


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