Spot drills

I can't recall when I last used a center drill on any of my mills, manual or CNC. That should cover the how and when, I think.

michael

Don't feel you're alone here. I started in the trade in '57 in a missile facility, then went on to work in other shops, including my own, spending a total of 26 years actively machining in the trade, mostly aero-space/defense industry, but totally on manual machines. I have no CNC background. In all those years I never encountered spotting drills. When I broke into the trade, center drills were used for drill spotting. Not even our well stocked tool crib had them. Makes me wonder when they were introduced to the trade.

I commented to Michael, on the side, that CNC likely was a large factor in the acceptance, perhaps even the development of, spotting drills, which are far less likely to lose their tips than are center drills. I believe they also have a much smaller chisel point, thus cut to center, or nearly so. They are capable of providing a larger starting point than center drills without drilling deep.

Perhaps someone that knows tool history will comment.

Harold

We use them when we want the drill to be exactly on the location. Some drills have a selfcentering point on them but will still walk a few thou, so the addition of the spot drill keeps it on track. On the cnc when you are drilling a deep hole the drill will tend to walk even more. One of the problems with use spot drills is you have to match the point angle or go greater so that the od of the drill doesn't wear out to fast making the regrind a pain. Some spot drills can do the hole complete because of the size/ thickness of the material this would allow you to feed faster through the material due to strength of the spot drill ie less flute length. Also you could chamfer the hole and spot it at the same time but the problem comes back to the od wear on the tool but makes for less tools to drill a hole ie spot and drill compared to spot drill chamfer. Spot drills can be had in both HSS and carbide.

I don't use commercially made spot drills, but we use screw machine length drills all the time when we suspect any trouble getting a drill to start properly. -- either cnc or manual use

Don't know exactly when spot drills came to be, only wish I had begun to make use of them sooner. ISTR seeing them first in SPI catalogues, if that means anything. I use them all the time in 90 degree configuration. I know what is said about the spot and twist drill point angles, but I have not noticed any big deal about tool wear or life. Someone likely has some stats to prove the point one way or the other. I may drill several thousand holes without a drill change, and often do just that, so then what? Do I bother with sharpening a $1 or $2 drill? If it still had been making good holes and looked decent, I use it another time, or throw it into the sacrificial box where I keep drills for hand drilling trailer frames or pilot holes in walls for screws, or whatever. It's easy to figure depth for leaving desired chamfers and they work nicely for cruising around part profiles to deburr or give a specific 45 degree bevel. They work nicely for engraving, but I often use a modified #2 center dill for that, depends what is being engraved. Center drills are for making centers, for drill locating they load up too easily and must be fed too slowly. Read that as PITA.

michael

Ive made it a practice to have just about only screw machine bits in my hobby shop. Its seldom that I will drill a hole deep enoug to need a "jobber" bit and they are a hassel once you get used to SM bits.

Now if I could only remember to check which twist they have..right or left hand..., before turning on the drill...

Gunner

Confronting Liberals with the facts of reality is very much akin to clubbing baby seals. It gets boring after a while, but because Liberals are so stupid it is easy work." Steven M. Barry

I've seen screw machine bits listed, individually, but I don't ever recall seeing them sold as sets. Does anyone know who might sell them in complete sets?

My mill has limited Z-axis space. If I could get a set of shorter drill bits it would certainly help me a great deal.

TIA

Orrin

KBC has an import set and a US made set in their catalog.

I dont think you will EVER find screw machine "sets" as the industry and common usage pretty much precludes this.

Gunner

Cum catapultae proscriptae erunt tum soli proscript catapultas habebunt.

Well Ill be damned. Mea Culpa. Wrong again. Hope it doesn't become a trend on my part

Gunner

Cum catapultae proscriptae erunt tum soli proscript catapultas habebunt.

I must be confused. I have a numbered set and a fractional set of chicago latrob screw machine drill bit sets in a nice metal index. Bought em from grainger or MSC. chuck

Wholesale Tool has 'em too.

Dan Mitchell ==========

Gunner wrote:

Following my previous post, I use these sets in the student toolboxes here at the university. Screw machine drills are a LOT less prone to breakage when used ham-handed ... as the student often do. Jobbers drills are available when needed (not often).

Dan Mitchell ==========

Gunner wrote:

Well, what about the screw machine industry? ;-)

Regards,

Robin

My Mea Culpa was presented to one and all, in another earlier post.

I spend a lot of time in screw machine shops, and have never seen a set. Screw machine shops purchase them by size, in each of the 3 standard gages as well as metric.

Gunner

Cum catapultae proscriptae erunt tum soli proscript catapultas habebunt.

As my post of a moment ago indicated..they buy them, a dozen or two at a time, by sizes, never sets.

Gunner

Cum catapultae proscriptae erunt tum soli proscript catapultas habebunt.

MSC sells them. I have a nice high-cobalt screw machine set in number sizes bought from them. The index is a standard HUOT index, except that it has baffles positioned to keep the shorter screw-machine length drills from shaking out of their positions in the index.

They are very nice, and I use them a lot when I don't need (relatively) deep holes.

Enjoy, DoN.