Center Drill Tip Broken Off In Hole

Topics include, Machine Tools & Tooling, Precision Measuring, Materials and their Properties, Electrical discussions related to machine tools, setups, fixtures and jigs and other general discussion related to amateur machining.

Moderators: Harold_V, websterz, GlennW

User avatar
BadDog
Posts: 4874
Joined: Wed May 17, 2006 8:21 pm
Location: Phoenix, AZ

Re: Center Drill Tip Broken Off In Hole

Post by BadDog » Mon Feb 03, 2020 11:49 am

Wow, that's a cool idea I never considered. His center drill is/was carbide, but could be slotted with a diamond dremel cutting wheel. Wouldn't cut happily being effectively negative rake, and cutting (presumably) PH 4140 likely will make it less happy, but doesn't have to cut easily or last long.
Russ
Master Floor Sweeper

JackF
Posts: 1565
Joined: Mon Jan 26, 2009 3:56 pm
Location: Caldwell, Idaho

Re: Center Drill Tip Broken Off In Hole

Post by JackF » Mon Feb 03, 2020 1:44 pm

When that happened to me I had enough room around the broken tip to use a small grooving tool to remove material around the tip and pick it out.

Jack. :D

btvarner
Posts: 11
Joined: Fri Nov 08, 2019 3:57 pm

Re: Center Drill Tip Broken Off In Hole *SOLVED*

Post by btvarner » Mon Feb 03, 2020 2:55 pm

**SOLVED** I did several things wrong. In the process I learned things I did not know. Plus I was able to save the part, and in the end was able to have a nice centered / reamed 3/8” hole.

Ok, I like to ensure that my questions have complete answers if/when I obtain a successful result. Especially since I am a novice machinist. I try to answer not only the specific question, but for the benefit of other newbie’s, some surrounding questions that the reader might not know either. If anything I say here is not right, be sure and correct my input.

The specific problem was that I broke off the tip of a center drill creating a hole I was to drill into an otherwise finished part. I wished to try and save the part. Not because it was a high dollar piece. Or even to avoid making another. Rather being new to this hobby, I wanted to try and solve the problem in this part so I would know how to do it. Then, next time when it might be a more important part, I would know how to solve that problem. Turned out the solution of drilling from the opposite side was a simple solution for several reasons.

My part (1 of 2) was machined from 4140 steel: .550” x .500” x 1.625”. The part required two holes through & through the .500” side. One of the holes would be reamed 3/8” in size. The other drilled and tapped & threaded 3/8” x 24. On my attempt at the first hole, I broke off the center drill tip.

My intent was to use the center drill first, followed by a 5/32” drill, followed by a 17/64” drill, followed by a 23/64” drill, followed by the 3’8” reamer. All this is done with the part centered in a 4-jaw chuck and the drills in the tail stock of my lathe.

Mistake #1 = I did not study enough about use and practices of center drills. I also did not even really know about spot drills.
A: Had I done due diligence in studying both, the problem would not have occurred in the first place.

Mistake #2 = I had a set of center drills. Numbers 1 to 5. I mistakenly used the smallest (a No. 1) rather than one of the larger center drills for a 3/8” final diameter hole. Assuming I would still use a center drill for starting a drill hole.
A: A larger center drill can be used to start a hole to be drilled if, one only center drills a dimple that is the tip of the tip in depth. That point of a center drill is 60° & can work. For full explanation of both center & spot drill, you can view this video: https://www.youtube.com/watch?v=Ot8wPGQW3JI

In my case I had used the No. 1 center drill to start the hole. The tip of a center drill that size is only about .040” in diameter. After the tip broke, I tried to push through the broken tip with the 5/32” drill. It soon became apparent that the drill would just wonder around the broken carbide tip in the hole.

So, I measured the other side of the part and used the very point of a No. 4 center drill (I had no spot drills) to dimple the opposite side. Then used the 5/32” drill from this other side to drill right up to where I measured the broken tip of be.

At that point I continued very-very slowly. Attempting to feel when the point of the drill contacted the broken carbide tip. I continued, and never felt any hard spots. Apparently the tip was small enough in diameter, while the drill was several times larger, so the tip must have just been pushed aside as the drill continued through the hole & exited the other side. Then following up as stated above, I completed that hole. A perfect slip fit for the designed part.

Amazon is amazing. I had ordered 3 spot drills last night. They arrived about noon today (Amazing). So I proceeded to drill and tap the second hole. Starting the hole with the correct spot drill to begin with. Suffice it to say, I will from now on know which drill (center or spot) and of what size to use in which situation.

Thanks for the help I received here………………

User avatar
BadDog
Posts: 4874
Joined: Wed May 17, 2006 8:21 pm
Location: Phoenix, AZ

Re: Center Drill Tip Broken Off In Hole

Post by BadDog » Mon Feb 03, 2020 4:32 pm

Look at the tip of your cross drill. I'm guessing this fits the case I mentioned where the web (chisel point) of the drill pushes out the broken tip (particularly in consideration of how small it was). It's possible that it was free enough to just fall out (vibration + heat). But look carefully at the drill you used under magnification, and you may find the chisel point damaged and in need of touch up.

Also, your steps for drilling are more difficult that is generally productive. If you are not dealing with machine power or rigidity limitations, for most "common" sized fractional drills (as found in a typical fractional drill index to 1/2"), on all but the lightest of machines, you really only need to (at most) make a pilot hole a bit larger than the web of the final drill. In fact, doing as you did may lead you to self-feed problems on aluminum (depending on alloy and state), and definitely will on brass/bronze.

You may also wish to investigate calculated speeds/feeds appropriate to cutter and part materials, proper application of *cutting* lubricants, relevant thermal shock issues (particularly with carbide combined with dabbed cutting lubricants), and "screw length" drill bits. If considering brass/bronze material, look into problematic self-feed issues along with techniques for dealing with that (particularly "drubbing" the cutting lips. The latter can also be useful when opening up a hole by a small amount in cast iron and other materials. There are also issues to be aware of with respect to drill bits wandering in the cut, making off-size and non-round holes, and related issues that also plague reamed holes, particularly when the drill has already introduced the problem characteristics. Your needs may be above the threshold for which such things matter, but it's good to be aware they exist. I'll stop there, but the list of such "need to know" items would go on for dozens of pages. These are just the ones that popped into my head related to your post...
Russ
Master Floor Sweeper

Harold_V
Posts: 18041
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Center Drill Tip Broken Off In Hole *SOLVED*

Post by Harold_V » Mon Feb 03, 2020 5:25 pm

btvarner wrote:
Mon Feb 03, 2020 2:55 pm
All this is done with the part centered in a 4-jaw chuck and the drills in the tail stock of my lathe.
Ahhh!
You just (unknowingly) revealed why the tip broke.

While some may argue the point (no pun intended), using a center drill in a lathe requires a light touch until the center drill is very well seated. While I won't discount the possibility that you simply failed to clear the chips, and the tip was broken for that reason, I'd like to make mention of what happens when a center drill is used in a lathe, and the tailstock isn't in good alignment (which is very common). What I'll describe is less likely to happen when using a center drill in a mill, although a poorly running chuck can yield the same results, as the center drill is called upon to behave like a boring bar. That's not good news for the tip if it is fed too rapidly.

Here's the deal. Because the tailstock on a lathe isn't always in good alignment with the headstock, if the center drill is held rigidly and forced to start drilling instead of giving it time to seek center, the hole gets started off center, causing the center drill to oscillate. If the tailstock doesn't allow it to do so, the tip breaks. That's a very common occurrence when using small center drills, and would be exacerbated in your case due to the brittleness of carbide. That your choice was a #1 made the matter all the worse. A larger one would have been more forgiving.

You resolve this issue by holding the center drill by a small portion of its length in the drill chuck, so it is free to seek center. Once it has done so, it can be fed with moderate pressure. Keep it lubed, and clear the chips often to avoid packing, which results in a broken tip.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

jcfx
Posts: 490
Joined: Sun Aug 09, 2009 1:24 pm
Location: NY

Re: Center Drill Tip Broken Off In Hole

Post by jcfx » Mon Feb 03, 2020 7:02 pm

mcostello wrote:
Sun Feb 02, 2020 8:05 pm
If You could slot the tip of the broken off center drill and make the slot a little bigger than the broken off piece it will core the broken piece out.
I'm having a hard time visualizing how this works .
By slot do you mean like a slotted screw slot across the diameter of the broken center drill ?

User avatar
BadDog
Posts: 4874
Joined: Wed May 17, 2006 8:21 pm
Location: Phoenix, AZ

Re: Center Drill Tip Broken Off In Hole

Post by BadDog » Mon Feb 03, 2020 8:44 pm

With the slot properly oriented and sized, it makes a sort of very primitive coring drill.

The slot would need to be just wide enough to straddle the broken embedded tip, and thus not stopped by the tip.

The slot would be oriented to align the trailing edge of the slot with the beginning of the cutting lip on the narrowest part of the "center drill" (the portion that makes the center mating pocket).

That would leave a small area of the slot cutting as it spins around the broken tip, which moves up into the slot. Really no worse (better even) than what happens with the chisel point (web) of a standard twist drill.

I haven't done it, but in my mind it appears to work well and provide a pretty slick solution.
Russ
Master Floor Sweeper

whateg0
Posts: 797
Joined: Sat Mar 28, 2009 3:54 pm
Location: Wichita, KS

Re: Center Drill Tip Broken Off In Hole

Post by whateg0 » Tue Feb 04, 2020 8:21 am

jcfx wrote:
Mon Feb 03, 2020 7:02 pm
mcostello wrote:
Sun Feb 02, 2020 8:05 pm
If You could slot the tip of the broken off center drill and make the slot a little bigger than the broken off piece it will core the broken piece out.
I'm having a hard time visualizing how this works .
By slot do you mean like a slotted screw slot across the diameter of the broken center drill ?

I'm right there with ya. Maybe slotting is really trepanning?

Dave

whateg0
Posts: 797
Joined: Sat Mar 28, 2009 3:54 pm
Location: Wichita, KS

Re: Center Drill Tip Broken Off In Hole

Post by whateg0 » Tue Feb 04, 2020 8:25 am

I can see where this could be a situation where the intent of ending up with a perfectly sized hole could backfire, especially if, as Harold suggested, the center drill didn't start on center. That would cause the drill to wander and then as the reamer followed the hole, it would waller it out at the mouth of the hole.

I also would never fault a novice for not knowing, in this case, the pitfalls of using a center drill to start a hole. Even experienced users sometimes make mistakes and when you are starting out, you don't know what you don't know.

I think it was Mark Twain who said, "Good decisions come from experience. Experience comes from making bad decisions.”

Harold_V
Posts: 18041
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: Center Drill Tip Broken Off In Hole

Post by Harold_V » Tue Feb 04, 2020 4:12 pm

You are spot on. I have memories of drilling deep holes, drilling from both sides. I've experienced holes meeting by mislocation of half the hole diameter, all due to drill wander related to misalignment. That, by the way, is the chief reason why one who uses reamers is well advised to take a boring pass before reaming. If nothing else, it will assure the hole is on location when finished. There's no guarantee of size, however, as the slightest variation when using a reamer can yield an unexpected diameter.

Your comments about a novice and not knowing. That's precisely why I post things like this. As absurd as some things may sound to those who haven't "been there, done that", I have, and have often learned the hard way. Sharing those experiences may help the guy who's struggling.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

User avatar
Bill Shields
Posts: 5720
Joined: Fri Dec 21, 2007 4:57 am
Location: Somewhere in the World
Contact:

Re: Center Drill Tip Broken Off In Hole

Post by Bill Shields » Tue Feb 04, 2020 5:34 pm

If i may suggest:

Mistake #0: Using the center drill in the first place.

Use a short SPOT DRILL whenever possible and leave the center drills to instances when that specific shape is called for.
Too many things going on to bother listing them.

jcfx
Posts: 490
Joined: Sun Aug 09, 2009 1:24 pm
Location: NY

Re: Center Drill Tip Broken Off In Hole

Post by jcfx » Wed Feb 05, 2020 1:51 am

BadDog wrote:
Mon Feb 03, 2020 8:44 pm
With the slot properly oriented and sized, it makes a sort of very primitive coring drill.

The slot would need to be just wide enough to straddle the broken embedded tip, and thus not stopped by the tip.
Ah-HA ! Now it makes sense.

Post Reply