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Bill Shields wrote: ↑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.
Hard to argue with that logic, although I can honestly say that the 7½ years I spent at Sperry Utah in the machine shop I NEVER saw a spot drill used, and hundreds of thousands of holes were mill drilled in the R&D stage of the missile.
There's absolutely nothing wrong with using a center drill to locate holes. They've been used in that capacity as long as there's been center drills, and they cost one hell of a lot less money than spot drills, at least in my limited experience. The added bonus is one need not own multiple types of cutting tools---a real plus for the guy with shallow pockets.
CNC operations? I most likely would agree with you.
Said another way, there is no right or wrong in this instance. Just use either of them in such a way that they start on location. Spot drills suffer from not being on center, too. They're just harder to break.
H
Wise people talk because they have something to say. Fools talk because they have to say something.
I don't have an ounce of experience compared to Harrolds ocean, and I doubt many here are more "cheap" than me, but I didn't find spot drills to be overly expensive for their utility, particularly relative to other things like end mills. And the spot drills are practically not what I would consider "consumables". They have the added benefit of providing a better socket for starting the drill due to matching the drill point angle (assuming you have the matching spot drill). Having the angle match the drill bit helps keep it from starting chatter/lobing when entering the hole.
In my experience, most will really only need 4. One "small" and one "largish" in 118* and 135* should do it. The only reason I say "small" is mainly just for tight spaces and obstructions, and to provide suitably sized starting dimples for tiny drills. And of course the larger for starting larger drills where the smaller wouldn't even clear the chisel point. For example, typical home shop size stuff (where 1/16-1/2 drill index covers almost all), the sizes could be 1/8" and 3/8", adjusted/expanded as reality requires.
The one advantage a center drill has over a spot drill is that the web of the drill doesn't have to do any work until the drill is well seated in the hole. That, alone, will help prevent drill wander. As I said, they work just fine. Don't be dragged down the rabbit hole of logic that says they don't. Hundreds of thousands of machinists who have used them can't all be wrong.
H
Wise people talk because they have something to say. Fools talk because they have to say something.
I'm by no means saying they don't work, or that spot drills are a must have. I don't have the training to support, and haven't spent the time (lacking incentive) to gather anecdotal evidence of spot drill superiority, but as I recall, matching the angle of the drill point is noted in a number of references as a benefit when starting a drill.
Spot drills also have a tapered web and thus almost no web at the point, and so by matching the drill bit angle, the cutting lips of the drill bit engage fully in the cut before the web/chisel makes contact. And at some point it's likely to transition to a pilot drill (at least for starting) before webs get too large.
And as with too large a pilot hole, if the cutting lips engate the outer edge of a too acute angle center hole first, I've personally seen chatter and wingging/lobing result. I can't say for sure by memory exactly what was the condition of the bits that I observed doing that, so can't say for sure there were other factors involved. But I don't recall ever seeing that behavior with spot drills, which I've used exclusively for that purpose for quite a few years now.
Other than avoiding use of too small a center drill to start holes (thus risk breakage), I would never put down their use for starting holes more precisely on location. I was mainly commenting on the lack of significant economic differences, which combined with the purported benefits (assuming true) make adoption of spot drills quite reasonable even in a home shop. At least that was my reasoning, and for quite some time I've not used a center drill for any purpose other than creating a center socket. Likewise I found it compelling to invest extra in screw length drills, which are often used without even the spotting drill, though a jobber will generally do just as well (assuming locating start and sufficient z-space). I'm well satisfied with that choice, others will make their own decisions.
giggle...cannot remember the last time one of my cnc (usually a Swiss type of machine) customers, used a center drill...for just this reason.
When making 1000's of parts per day unattended, we cannot run the risk of losing the tip..break one tip and you lose not only the current part but all the parts that follow...as well as any following tools...
everyone uses spot drills and nobody worries about them wandering..even in very small sizes...
No....most of the machines do not have 'broken tip sensors' on every tool...so checking for a broken center drill is typically not practical.
I won't disagree that many many many parts have been made with center drills over the last 150 years...but that does not mean that there isn't a better way today..
After all...I glue things together instead of using press fits...but have not figured out a way to un-glue my way around this predicament.
Note that in my post, top of this page, that I agreed with you in regards to CNC operations.
The problem I see these days is that folks who are CNC oriented don't seem to understand that procedures used for such operations aren't always necessary for manual operations. Spot drills are a reasonable requirement, and I, too, use one when operating my Haas. Manual operations? Hardly necessary, and that was my point. And as to cost, not all folks have deep pockets. A center drill is pretty much a requirement for a guy who owns a lathe. If his free money is limited, he can spend what little he has on necessities, using a center drill to start holes, the center drill he already has.
Anyone chooses to use spot drills, and not center drills for starting holes? No problem, not as far as I'm concerned. There is no right or wrong, and both do an adequate job, each having somewhat different characteristics. I simply want to eliminate any doubts for those who are trying to learn, that either option is acceptable. When someone keeps demanding that a center drill isn't the proper tool, they are doing a disservice to those who are trying to learn. Just as a promoter of insert carbide tooling is doing a disservice to the guy who really needs to learn how to grind HSS cutting tools for his fractional horsepower lathe.
You like spot drills? Fine with me. Just don't tell others that they are a necessity. They aren't.
H
Wise people talk because they have something to say. Fools talk because they have to say something.
Worked in a screw machine shop for 5 years. Never had a spot drill, never broke a center drill either and Had a Guy who hand sharpener Them also. Can't explain the non breakage.
Clever Idea. Works when there is not enough room to use a grooving tool around it. 
