Combination Drills (center dills)

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earlgo
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Combination Drills (center dills)

Post by earlgo »

Is there a rational explanation why center drills are not made in 'standard' sizes to fit 'standard' collets like R8 and C5?
In my horizontal mill I cannot use a large drill chuck with a MT shank because the table doesn't have enough 'z',(in and out), so I end up using spotting drills that fit the collets. It works, but it made me wonder, because screw machine length drills are nearly as expensive as center drills.
--earlgo
Before you do anything, you must do something else first. - Washington's principle.
SteveM
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Re: Combination Drills (center dills)

Post by SteveM »

Not sure what the answer as to why the sizes, but maybe they were sized to deliver a certain size center hole, and the drill would have to be larger in diameter than the center hole, as you have to stop before the straight shank enters the hole.

If you are just using it as a starting hole for a drill, then a spotting drill works better anyway. The center drill is required when you need to stick a center in the hole, as you need the clearance at the end for the center's tip.

Get yourself a chuck with a straight shank in a size such as 1/2" or 3/4", Cut the shank short - to the grip length of the collet. There's no point in having a 4" shank when the chuck will only grab 1".

With that, you can swap the chuck in and out without having to drop the table, or at least not very far.

I made a JT to 5/8" adapter for a chuck when the largest collet I had for the P&W mill was 5/8".

Steve
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GlennW
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Re: Combination Drills (center dills)

Post by GlennW »

Use your surface grinder to step the shanks down on a couple of center drills so they fit in a collet that you have.
Glenn

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choprboy
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Re: Combination Drills (center dills)

Post by choprboy »

Center drills not fitting standard collets? Never heard of such a thing, seem to fit in my R8 collets. Checking a drill chart quick:
#1 3/64 drill, 60 angle, 1/8 body
#3 7/64 drill, 60 angle, 1/4 body
#5 3/16 drill, 60 angle, 7/16 body

Maybe you have metric-sized center drills?
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tornitore45
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Re: Combination Drills (center dills)

Post by tornitore45 »

Center-Drill-Dimensions.pdf
(148.82 KiB) Downloaded 96 times
My first cheap set Chinese of center drill were not standard imperial sizes, no idea whether they were metric. My next purchase of quality USA made are all standard.
Mauro Gaetano
in Austin TX
earlgo
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Re: Combination Drills (center dills)

Post by earlgo »

choprboy, you are right about the sizes of the drills. I was walking around with a 3/8 R8 collet looking for a center drill and just got frustrated. My cheapo 6pc R8 set doesn't have a 1/8, 3/16, or 7/16 collet but I did buy a 5/16 size.

GlenW, excellent idea BUT I don't have a collet for the Spindexer to grip the center drill. Hence the original issue. :(

SteveM: I guess you are right and I need to pry open my wallet and do as you suggest and replace the crappy 3/8 pistol drill chuck I currently use, which is why I wanted the spotting drill to fit a collet.

I have been trying not to spend more money on tooling because it just means more stuff to pitch later, but in this case, I guess it is justified. At least that is what I'll tell SWMBO. For the time being, though, I did buy a 3/8" cobalt spotting drill.
--earlgo
Before you do anything, you must do something else first. - Washington's principle.
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GlennW
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Re: Combination Drills (center dills)

Post by GlennW »

earlgo wrote:GlenW, excellent idea BUT I don't have a collet for the Spindexer to grip the center drill. Hence the original issue.
I guess I missed something here.

I checked a set of center drills that I have and they are 1/8", 3/16". 1/4", 5/16", 3/8", and 7'16".

I thought that you were just limited on the collet sizes for your horizontal.

I keep and use 5C "Emergency Collets" around as they are used like soft jaws and can be bored (or threaded) to the size needed. Not inexpensive, but very handy if your lathe is set up to accept 5C collets.

A split reducing sleeve for a 5C collet that you do have would work too to hold it for reducing the shank if needed,
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
John Hasler
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Re: Combination Drills (center dills)

Post by John Hasler »

GlennW wrote:
earlgo wrote:GlenW, excellent idea BUT I don't have a collet for the Spindexer to grip the center drill. Hence the original issue.
I guess I missed something here.

I checked a set of center drills that I have and they are 1/8", 3/16". 1/4", 5/16", 3/8", and 7'16".

I
I think the question is, why so many different shank sizes for center drills? Seems like a few would suffice.
SteveM
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Re: Combination Drills (center dills)

Post by SteveM »

John Hasler wrote:I think the question is, why so many different shank sizes for center drills? Seems like a few would suffice.
On each of those sizes, the center point is a different size. That probably gives you some ratio of clearance hole to surface area of where a dead center will rub.

Steve
John Hasler
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Re: Combination Drills (center dills)

Post by John Hasler »

SteveM wrote:
John Hasler wrote:I think the question is, why so many different shank sizes for center drills? Seems like a few would suffice.
On each of those sizes, the center point is a different size. That probably gives you some ratio of clearance hole to surface area of where a dead center will rub.

Steve
That establishes a minimum shank size for a given center point size. Why would the one with a 1/4" shank not still work fine were the shank to be increased to 3/8" without changing the point?
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BadDog
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Re: Combination Drills (center dills)

Post by BadDog »

That little point is your worst enemy when it comes to center drilling, and generally you want it as robust as reasonable for the application. Smaller center drills beak the tips if your even a few thou off center, hence the tip-grip advice to let the float to center. On the bigger ones suitable for larger bars, I don't have to pay much attention. So I grab the largest practical for the needs of what I'm making.

So just having an enlarged shank would require more material, thus more cost, and take up more space, and be less likely to use the same center drill for all 1/4" needs since a 1/4" shank will fit in the chucks of a very small lathe, but a 3/8 or 1/2 over sized shank might not. At least that's what comes to mind.
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earlgo
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Re: Combination Drills (center dills)

Post by earlgo »

My apologies for getting this started. It started because I was using a 3/8 R8 collet in my Horizontal. The wiggler, edge finder and drill chuck shank are all 3/8" and I wanted to have a spotting drill with a 3/8 shank, too. (I don't have a 4 1/2 center drill.) As mentioned, the 3/8 drill chuck is a miserable pistol drill chuck and not suited for spotting a hole. It will follow a spot reasonable well though. So my frustration was in having to change collets to use a spotting drill. Now, after a trip to McMaster-Carr, I have a 3/8 spotting drill and the problem of changing collets has mostly gone away.
Interesting discussion, though. Thanks.
--earlgo
Before you do anything, you must do something else first. - Washington's principle.
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