How to melt a drill bit

[kvom]

First you try to center drill on a CNC mill with too fast a feed rate and snap off the tip of the center drill in the aluminum workpiece.

Second you don't remove the broken tip from the workpiece.

Third you try to drill the hole.

 

[doc1955]

A tip you may or may not know about. If you break a center drill just take the same broken c/drill and hand grind half of the drill away creating a treepan tool and go back to the hole it broke on and go a little deeper with the tool and it will remove the broken tip.
I have used this a few times and it works every time.

 

[Mo deller]

My method was to drill into a live cable buried in a wall. Melted the tip nicely. ;D

 

[doc1955]

Here is a model showing how to modify a c/drill to treepan a broken tip out.

 

[mklotz]

We all do it but it still isn't right. You shouldn't be using a center drill to spot holes. Center drills are for making CENTER holes on the lathe. Get some proper spotting drills and save the center drills for their intended purpose.

 

[Metal Butcher]

We all do it but it still isn't right. You shouldn't be using a center drill to spot holes. Center drills are for making CENTER holes on the lathe. Get some proper spotting drills and save the center drills for their intended purpose.

Hi Marv. I've heard this before on the forum. Why not use a center drill? ???
I raised this 'improper use' issue (off forum) several times, and some time ago. The experienced professional machinists, and a retired tool and die maker told me that they use(d) them, and also said that this is the way its been done in shops on manual machines all over the country for as long as they could remember. They brushed off my idea of buying some 'spotting drills' as simply a waste of money.

The reason I asked them was based on my noting the availability of 'spotting drills' in a tool catalog. After speaking with these experienced professionals I brushed off their availability as nothing more than gimmick tool, a better mouse trap, so to speak.

I always use a center drills to spot a starter dimple before drilling, I find it better to just use the tip.

-MB

 

[1hand]

First you try to center drill on a CNC mill with too fast a feed rate and snap off the tip of the center drill in the aluminum workpiece.

Second you don't remove the broken tip from the workpiece.

Third you try to drill the hole.

Been there!!! Same thing happened to me with a extra long 1/4" end mill. The broken piece was small enough that you really couldn't see it, but #2 end mill found it! So $24 worth of endmills later, we where back to business.

Live and learn, Live and learn.

Happy New Year,
Matt

 

[tel]

Well MB, the spotting drill is considerably more robust than the centre drill, with it's reduced nose section. Also, I believe they start much truer, even on curved surfaces. I changed to them a couple of years back, when they became reasonably available, and I'm glad I did, tho' I'm still guilty of using a centre drill as a matter of expediency from time to time.

 

[mklotz]

MB,

Hence the "we all do it" in my post. Center drills work for the intended purpose, no doubt.

However, a spotting drill lacks the delicate tip and cuts a recess that exactly matches the angle of the drill so the drill edges are starting on a flat surface rather than a ledge.

I only had to mess up one half-finished casting with a broken center drill before I switched permanently to spotting drills.

 

[Henk]

I realize it's off topic, but it may interest more people. The way I understand it from Marv's post, the main difference between a spotting drill and a center drill is the cone angle (which should be the same as that of the drill) ? Or are there other relevant differences?

I wouldn't know where to get spotting drills around here (BTW nor even the proper dutch word for it), but still I may be tempted when I do see an opportunity. At least recently, I just used a center drill (no punch marks), and I noticed a lot of wandering of the bit, before it started to dig in. This was on a my mill where I assumed machine rigidity + cross feed dials would get me a better looking row of holes than punching marks (which is a skill I have not really mastered). The result looked OK (I did not really take measurements), but the drill wander bothers me a bit. Would that likely be any different with a spot drill?

 

[doc1955]

I agree with Marv spot drills are also less $ than center drill but I too am guilty of using a center drill on the mill when picking up scribe lines.

 

[mklotz]

Henk,

Here's a picture of a spotting drill...

http://www.use-enco.com/CGI/INSRIT?PARTPG=INSRAR2&PMAKA=325-4066&PMPXNO=5809603

As you can see, it's short and stiff but lacks the delicate, reduced diameter tip typical of the center drill. Typically it's ground to match the drill angle (e.g., 118 deg) as you surmised.

 

[Henk]

Marv,

Thanks. Small shame on me, as indeed a google search on spot drill does show similar.

I guess then it is also advantageous to have a set with diameters of the drills one uses a lot, where you plunge in until the full diameter is say a few mm / .1'' into the material?

And for the less used drills sizes one does not proceed with the spotting drill until the full diameter is engaged? Or in the latter case, first use a spot drill / drill combination (largest size you have but still smaller the hole you want to make) and drill as deep as the hole should be and then open up the hole to get where you want to be?

 

[mklotz]

I manage with three sizes - tiny, small and medium.

You don't want to plunge in past the conical tip since that would defeat the conical depression to seat the drill. The drill lips would be riding in a line contact on the edge of the cylindrical hole formed by the spotting drill. You just want to create a conical depression that the drill, regardless of size, will naturally "walk" into rather than "skating" around and starting off location.

For very small drills, one must develop a fine touch with the quill lever so you don't create a spotting divot with a diameter larger than the drill diameter, thus yielding a countersunk hole when one doesn't want a countersunk hole.

 

[kvom]

I'm aware of spotting drills, but as I have a good supply of "free" center drills I've avoided buying any. The bad hole in question was on a small raw piece of stock, so the trepan technique wouldn't have been worth the trouble, but it's nice to know about it.

 

[miker]

Are these the type of Spotting Drills being reffered to?

http://www.sutton.com.au/Assets/4314/1/499980430_Drill_Spotting_Brochure.pdf

There appears to bo 90 degree and 120 degree point styles.

 

[shred]

Apparently the term "Center drill" has been replaced by "Combined Drill and Countersink", which makes a little bit of sense since they're available in angles that don't match available centers and in formats that would be nearly useless for traditional center work (say, the 6" long #1..)

I have a few of the Keo spotting drills in 3/8" size, but don't use them all that much-- in that size I've not found them any better than center drills or other spotting methods. Someday I may try the smaller sizes and compare. Also (as Kvom says), I have a raft of center drills and suspect they grow in toolboxes since the only one I can remember buying was to match an odd countersunk screw head angle.

 

[doc1955]

Are these the type of Spotting Drills being reffered to?

http://www.sutton.com.au/Assets/4314/1/499980430_Drill_Spotting_Brochure.pdf

There appears to bo 90 degree and 120 degree point styles.

Yes those are what I refer to and I believe Marv also.
They also work good for 45 degree or 60 degree chamfer cutters for milling chamfers and yes I know they are really not meant for that but they do work good in softer materials such as brass and aluminum.

 

[mklotz]

Are these the type of Spotting Drills being reffered to?

http://www.sutton.com.au/Assets/4314/1/499980430_Drill_Spotting_Brochure.pdf

There appears to bo 90 degree and 120 degree point styles.

Yes, but there is an even greater selection of head angles available. ENCO offers head angles of 60, 82, 90, 118, and 120 deg.

 

[jpeter]

I don't use spotting drills cuz I don't own any. I do know though one of the easist ways to get a hole off center is to start it with a center drill. I've got lots of work arounds if its important to get a hole dead nuts on. Maybe I'll buy some spotting drills. I often start holes for small drills using a ground point on a piece of drill rod to create the starting hole.