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Twist Drill Angle with Center Drills vs Spotting Drills

markso125

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#31
As for both as someone who has machined in shops for years, and also taught for years i can say from my experience centerdrills are used almost exclusively.
The reasons for this is accuracy and cost. The design of the tool keeps it very rigid thus making it very accurate, usually we find play in the machine causing holes to be off before we find the center drill walking.
The next is cost, a center drill is used for all materials, for all different machines. They are cheap, they all act the same and you don't have to waste time deciding which angle you need to use. We run them in everything from 15-5PH to 6061t6 and always have predictable results.
As for common drill angle, the vast majority of jobber drills are sold are at 118 degrees again this is the same as the center drills it is cost and predictability. Time spent checking angles for proper material in accordance with the cost of having a whole bunch of extra drills sounds counterproductive and more expensive to me.
 

JR49

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#32
the vast majority of jobber drills are sold are at 118 degrees again this is the same as the center drills
Could someone please explain this quote ? I'm pretty sure all center drills are 60 deg. combined angle, while the drills markso125 mentioned is 118 deg. combined. Unless he is referring to the tiny tip of the center drill, which (don't know for sure) might be 118 deg. or more but, if that is the case, wouldn't you need a HUGE center drill for the tip to be big enough to spot for even a fairly small dia. (say 3/8") final size drill. Thanks, JR49

EDIT: OK, I don't usually post things unless I'm 100% sure that what I'm saying is correct (which is why I don't post a lot, still a beginner). So, after submitting this reply, I went out and checked my center drills with some 118 deg. drills. It turns out that the tip of a #6 center drill will spot a hole big enough for a 3/4" main drill, and , as close as I could check, the tip is ground to 120 deg. I don't know how big center drills get, but the #6 was the biggest I had. So, like someone said up above, I guess yesterday I spent approx. 40 bucks on 2 spotting drills for nothing, except that:: "He who dies with the most tools WINS !! JR49
 
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David S

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#33
I am in the process of modifying a jig that I made and have to relocate some holes accurately. So decided to try Bob's suggestion. I have a #5 centre drill. The main body is 7/16" Ø, and the "small" tip is 0.187Ø. I am spotting for a # 27 drill. And of course it works great.

And yes you only use the tip. The #5 is very stiff.

It is a slow week if I can't learn something.

David
 

JR49

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#34
I am in the process of modifying a jig that I made and have to relocate some holes accurately. So decided to try Bob's suggestion. I have a #5 centre drill. The main body is 7/16" Ø, and the "small" tip is 0.187Ø. I am spotting for a # 27 drill. And of course it works great.

And yes you only use the tip. The #5 is very stiff.

It is a slow week if I can't learn something.

David
I was typing when you posted this, thanks, and please see the edit to my post. JR49
 

David S

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#35
Ah JR we both got sucked into buying expensive spotting drills I see. And of course Bob waited 'till we had parted with our money to suggest a low cost....already have...alternative.

All in good fun my friends... and again really glad that the OP started this thread...who would have thunk?

David
 

JR49

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#36
And of course Bob waited 'till we had parted with our money to suggest a low cost....already have...alternative.
I know, how could Bob do that to us!! But actually, the link that Bob provided in post #5, sites about 4 other good uses for spotting drills, so all is good. Thanks Bob. Oh, and I forgot to say before that this has been an extremely informative thread, so thanks to the OP, for asking the question. These are the BEST threads, in my opinion. Happy Machining, JR49
 

JPMacG

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#37
Thank you everyone! This thread has been very helpful and very timely for me.
 

mikey

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#38
The best fun we have on HM is to get someone else to spend money on a tool. I tried it with @higgite but he wouldn't bite until I bought the tool myself and showed him the results. Sometimes we go to extreme measures to get the job done but we do get it done!
 

Grandpop

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#40
I was a tool and die maker for 25 years before I left the field for a desk job. In that time I must have drilled about a million holes, and always in harder tool steels. We made high production dies that could have the inserts replaced when trey were dull, so the hole locations had to match the drawing.

Most were located with a center drill, some were center punched from the described line intersections. Rarely did anything not line up, and typically could expect the final hole after center drill start to be in location within .005 . Not saying the spotting drills would not have been better, just saying we didn't have them and didn't see the need for buying them.
 

Highsider

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#41
All good advice until the finished drilled hole
is a large enough diameter that you just can't push it through the material you're working with without a pilot hole.
 

bobshobby

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#43
These spotting drills that you speak of are a mystery to me, I don't believe I've ever heard of them before, never seen one, and certainly never used one. When I did my apprenticeship in the early 60's we were taught to pick up a center spot with a center drill, but only drill the point in, do not drill deep enough to start the 60deg taper for running a center in. the drill with a number of drills getting larger each time up to 1/64th smaller than the required hole, then finish the hole with an on size drill with the corners slightly knocked off. only using a reamer if super accuracy was required. Larger holes requiring high accuracy were normally bored anyway. Thus pilot holes were all the go. most of our work was in annealed tool steel, maybe something to do with it. I don't remember any problems with chatter or out of round holes.
 

Bob Korves

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#44
These spotting drills that you speak of are a mystery to me, I don't believe I've ever heard of them before, never seen one, and certainly never used one. When I did my apprenticeship in the early 60's we were taught to pick up a center spot with a center drill, but only drill the point in, do not drill deep enough to start the 60deg taper for running a center in. the drill with a number of drills getting larger each time up to 1/64th smaller than the required hole, then finish the hole with an on size drill with the corners slightly knocked off. only using a reamer if super accuracy was required. Larger holes requiring high accuracy were normally bored anyway. Thus pilot holes were all the go. most of our work was in annealed tool steel, maybe something to do with it. I don't remember any problems with chatter or out of round holes.
I very rarely have any trouble with drilling holes when using a mill or a lathe, and I imagine that other rigid machines would be about the same. Almost all the troubles I have ever had in drilling have been hand held or on my 17" floor model Enco drill press. If I do things the correct way, without cheating to save time, it works fine. If I get lazy, then occasionally I have real trouble, sometimes making nasty holes and occasionally ruining an otherwise nice part. I think the fact that this thread is now up to 44 posts says that others might have also had issues with drilling "simple" round holes to the correct size and in the correct location. It is great that we can all learn and share on these forums...
 

grzdomagala

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#45
Spotting drills may by useful on small mills with round column. Spotting drill is shorter than standard drill but longer than center drill - maybe not much but sometimes enough to let you use standard drill or reamer without moving the table and loosing position after spotting.
 

David S

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#46
Well I have been doing quite a bit of drilling, tapping and relocating holes accurately. Sizes are for 4-40 and 6-32 cap screws.
I am drilling and milling on my tricked out JET drill press. Using the #5 centre drill tip only has worked extremely well. The bit seems to snug right in without any wobble or digging in.

As a hobbyist I have been using centre drills for spotting for over 40 years. So far my take is that with my equipment the spotting drill or in this case the tip of the large centre drill is a lot better...so far.

And this is a bit of an aside, but I also learned on this great forum the difference between spiral point and spiral flute taps and have been using the spiral point on this project in aluminum. What a huge difference compared with straight flute. Just cuts right through with no having to back up and break the chips.

David
 

mikey

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#47
Well I have been doing quite a bit of drilling, tapping and relocating holes accurately. Sizes are for 4-40 and 6-32 cap screws.
I am drilling and milling on my tricked out JET drill press. Using the #5 centre drill tip only has worked extremely well. The bit seems to snug right in without any wobble or digging in.

As a hobbyist I have been using centre drills for spotting for over 40 years. So far my take is that with my equipment the spotting drill or in this case the tip of the large centre drill is a lot better...so far.

And this is a bit of an aside, but I also learned on this great forum the difference between spiral point and spiral flute taps and have been using the spiral point on this project in aluminum. What a huge difference compared with straight flute. Just cuts right through with no having to back up and break the chips.

David
I like form taps in aluminum - stronger, cleaner threads and NO chips.
 

mikey

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#48
I've been thinking about this subject and I think it comes down to this: use what works for you. You can use a center drill, spotting drill or just the drill. I don't think anyone is saying you have to use only a spotting drill to drill a hole, or that using a center drill is wrong. I suspect all of us are capable of testing these drills for ourselves and coming to our own conclusions.

The value of this thread is that we have been able to share what a spotting drill is, what it is used for and how to choose one.
 

EmilioG

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#49
My collection of center and spot drills ranging in size and point angles. 60 to 150°.
I will keep testing different methods with various metals. The HiRoc is 135° for hard steels.
IMG_20170319_142451104.jpg
 

Bob Korves

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#50
I recently bought some USA made cobalt, 135 degree, split point, screw machine (stub) length drills. Got a dozen each of 1/4 and 3/8" diameters, new and in the original sealed packaging, for less than $1 each, free shipping on eBay. I have given some away to my buddies, but I probably still have a lifetime supply of them on hand. I can use these as spotting drills, and they are split point so they do not need a center punch mark when using them on the mill with 90 degree smooth surfaces. I can also use them to start holes accurately in work on the lathe. The split points are also very useful on small center punch marks, even for hand held drill motors.
 

brino

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#51
I recently bought some USA made cobalt, 135 degree, split point, screw machine (stub) length drills. Got a dozen each of 1/4 and 3/8" diameters, new and in the original sealed packaging, for less than $1 each, free shipping on eBay.
Bob, would you have a link to that supplier?
Thanks,
-brino
Of course, not likely free shipping to Canada.....
 

Bob Korves

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#52

Bob Korves

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#55
I guess I shoulda asked in a PM! :cautious:

Nah, I hope you fellow H-M'ers enjoy them in good health! :encourage:

-brino
I probably should have PM'd you, Brino. :( PM me your address and I will send you a few...
 

tcarrington

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#56
been using stub drills in medium and small mills for a while now and it works amazingly well. Saves time and makes a better hole. Good advice all around.
 

Nogoingback

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#58
Very interesting thread. Like darkzero, I found some conflicting information when I looked into this as well. I just remembered where: www.guhring.com/documents/catalog/drills/ncspotdrills.pdf
Guhring specifically states that with HSS the spotting point angle should be less that the drill angle. But, as Bob suggests I'm going to buy a 120 degree spotter and compare with the 90 degree that I bought
before. I wonder if Guhring's advice assumes a CNC environment, where rigidity and accuracy is a given. Probably not aimed at folks with floppy old drill presses. :)
 
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brino

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#59
I probably should have PM'd you, Brino. :( PM me your address and I will send you a few...
No worries Bob, I'll keep an eye out at my local used tool shop and I'm sure I can find some.
Thanks for the considerate offer!

-brino
 

mikey

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#60
Very interesting thread. Like darkzero, I found some conflicting information when I looked into this as well. I just remembered where: www.guhring.com/documents/catalog/drills/ncspotdrills.pdf
Guhring specifically states that with HSS the spotting point angle should be less that the drill angle. But, as Bob suggests I'm going to buy a 120 degree spotter and compare with the 90 degree that I bought
before. I wonder if Guhring's advice assumes a CNC environment, where rigidity and accuracy is a given. Probably not aimed at folks with floppy old drill presses. :)
Back when I tested these drills (can't even remember how long ago that was), I used a 1/8" drill bit. I tested them because I noticed that my smaller drills were not drilling where I wanted them to when I spotted with the 90 degree spotter. I also noted that a freshly sharpened drill was not outputting equal-sized chips, which suggested to me that the edges were being dulled or damaged somehow. At the time, I didn't even know that there were other angles available in spotting drills but when I looked, I saw that a 120 degree drill was available so I ordered one and all the issues went away.

Were it not for the fact that I was trying to fit a part to another part that required the holes to line up precisely (I was building a precision square) I may never have stumbled upon the difference between these spotting drills.

My point is that when using larger non-carbide drills with greater inherent stiffness and less fragile edges, a 90 degree spotting drill may be fine. For myself, I prefer a 120 but admit that I are a bit anal about it.
 
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