spot drill angles

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TRX
Posts: 162
Joined: Wed Aug 20, 2008 7:30 pm
Location: Central Arkansas

spot drill angles

Post by TRX »

So, I decide I need some spot drills, and go off looking for them online.

Okay. They are available with 120 degree points, 90 degree points, and some don't say, but from the pictures look like maybe 80 degrees.

I looked through my collection of old machining books, and none of them even *mention* spot drills, much less their angles...

For use with ye olde 118 degree drille bitt, what kind of spot drill do I need?

[since somone is bound to ask, for use primarily on the drill press. Most of my "good" drills are jobber length, and despite careful laying-out and center punching the workpieces, they seem determined to walk outside the punch when they first touch the workpiece. I figured a center drill would be more rigid, and pointy without the little flat at the end of a regular drill, and should be able to start the hole without as much chance of wandering)
John Hasler
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Re: spot drill angles

Post by John Hasler »

Just use center drills. They work fine.
earlgo
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Re: spot drill angles

Post by earlgo »

I agree with Mr. Hasler. I bought a set of 4" long center drills from SHAR (#404-1359) a couple of years ago and by first indicating the prick punch dimple with a wiggler, followed by a center drill, followed by the desired drill, things come out ok. The center drills are long enough so that one doesn't have to move things up and down or use the extreme reach of the quill, as is done with a short center drill.
--earlgo
Before you do anything, you must do something else first. - Washington's principle.
TRX
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Location: Central Arkansas

Post by TRX »

For longer drills, I don't have enough quill travel to reach the workpiece with a center drill.

edit: I hate it when someone posts while I'm typing in a reply! [tries for Squint of Irritated Disapproval, winds up with nervous tic on eyelid...]

I didn't know center drills came that long; all mine are quite short. They might just do the trick.

[still curious about the different nose angles on spot drills, though!]
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NP317
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Re: spot drill angles

Post by NP317 »

Perhaps the different nose angles for spot drills allow them to be used as counter sinks?
~RN
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GlennW
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Re: spot drill angles

Post by GlennW »

Use a 90° for chamfering, a 120° for 118° drills, and a 140° for 135° drills.
Glenn

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ChipMaker4130
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Location: Ivins, Utah

Re: spot drill angles

Post by ChipMaker4130 »

TRX wrote: Sun Sep 16, 2018 8:28 am. . .despite careful laying-out and center punching the workpieces, they seem determined to walk outside the punch when they first touch the workpiece. . .
Spot drills have a very sharp 'point', even better than a split-point. They won't walk out of your punch mark, and the larger ones, say 3/8 and up will usually start right on point of first contact without any punch mark at all.
whateg0
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Re: spot drill angles

Post by whateg0 »

I have a couple spot drills, but most of the time, I find that a small drill, such as 1/8" or 3/16" will stay in the punch mark. Bigger drills tend to walk because the whole chisel doesn't fit in the punch mark, and so it may as well not even be there. FWIW, my spotting drills are 120 and get used mostly in the lathe.

I use center drills quite a bit, but they don't seem to stay sharp as long as other drills do. I bought a bunch from Grizzly when I was there a few years ago and as they get too short, I replace them. Maybe when I run out, I'll buy better ones that will hold an edge longer.

Dave
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mcostello
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Re: spot drill angles

Post by mcostello »

There have been arguements on other forums as to the reason for some of the point angles. Some say that a sharper angle is needed so the edges of the drill make contact first and center the drill up before the point touches. Others say the point needs to start first. Sort of like the old saw about chain saw bar oil or way lube, or what angle do You set Your compound at to thread. :mrgreen: :lol:
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