Being stiffer than 2 flute 3 flute drills provide rounder/straighter holes, provided of course like all drills they are ground correctly.

Emgee

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Must be a regional dialect variation, or some-such …

Core drills, to me, are tubular things that take out cores of material.

Like Emgee, I'm of the opinion that three-flute drill make rounder holes.

[ordinary 2-flute drills often make tri-lobed holes, especially in thin materials.]

MichaelG.

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corrected typo

Edited By Michael Gilligan on 05/10/2018 15:28:40

Thanks, Jason

MichaelG.

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It does seem, however, that a referendum might show Presto and Dormer to be out of step with the rest of the world.

https://sheffieldtooling.co.uk/browse-by-category/hole-making/drills/core-drills/dormer-hss-core-drill-standard-length-metric-34mm-boxed/

Edited By Michael Gilligan on 05/10/2018 15:27:16

I would not be so sure, as Maurice said they has 3 flutes a quick google for "3 flute core drill" will throw up quite a few other using this description. You may be getting confused with s "shell core drill" which in engineering terms is the one that leaves a slug in the middle similar to Rotabroach and Mag Drill bits. Guhring list the two types.

If this were a building forum then the generic term "core drill" would be taken to be as you describe, being either the old TCT ones that mount on a K taper and use A taper pilots or the more recent diamond edged ones that use a plain shank pilot that goes into a previously drilled hole.

Also as every good model engineer who has read and reread the works of Tubal Cain will know he used a 3 flute core drill to open up the cored cylinder casting on his "Hand Maiden" engine with a brace and bit before reaming it Think I would rather go by the words of TC rather than Stub mandrel

Edited By JasonB on 05/10/2018 15:53:53

I have several three and four flute core drills. I use them in the lathe to open up smaller drilled holes but I don't think I'm getting (or needing) their full benefit.

Now, If I were machining a casting that had cored holes I think that would show their full benefit. A cored hole could, as cast, be slightly out of position but the lateral rigidity of the core drill will cope better with cutting more on one side than on the other – a common two-flute drill will try to follow the path of least resistance resulting in a hole of correct size but (slightly) in the wrong place! Of course, the context here is use in a machine that can position the drill axis in the right place by some means other than needing a centre punch mark.

The core drills I have do not have a point but have an almost flat business end.  They cannot start a hole on their own.

Best regards,

Swarf, Mostly!

Edited By Swarf, Mostly! on 05/10/2018 17:01:28

Hi,

Many years ago when I was an apprentice we had u-shaped steel castings that had cast in holes through the two lugs. The holes were 35mm (actually 1 3/8" !!) in diameter. As cast they were about 1" diameter. We used a 1 3/8" 3 flute drill to begin with but later we got 3 flute drills with a reamer section half way along. This enabled us to bolt the casting in a jig, drill and ream in one operation in an old Ward capstan in about 3 minutes, the previous time was nearer 10 minutes. It was so efficient we only needed to make them for 3 days a week to keep up with production instead of a full time job!