Drilling information

Chrisboon · Apr 25, 2021

Hi All
I am not learning fast enough.
Today I discovered that the holes I drill are not as expected.
I drilled 12.5mm holes in 5mm steel. The entrance side measurement is 12.67mm but the other side is 12.53mm.
My first thought was that it was due to runout, one side being closer to the chuck than the other, but then the difference would be the other way round.
Runout on the MT is 0.06mm but on the drill bit shank it is 0.26mm. Maybe time for a new chuck. I did use new high quality drill bits and I did first drill a 5mm pilot hole.
Although it does not matter much in what I do, I would still like to know why this is so.

Thanks

thermite · Apr 25, 2021

Chrisboon said:
Hi All
I am not learning fast enough.
Today I discovered that the holes I drill are not as expected.
I drilled 12.5mm holes in 5mm steel. The entrance side measurement is 12.67mm but the other side is 12.53mm.
My first thought was that it was due to runout, one side being closer to the chuck than the other, but then the difference would be the other way round.
Runout on the MT is 0.06mm but on the drill bit shank it is 0.26mm. Maybe time for a new chuck. I did use new high quality drill bits and I did first drill a 5mm pilot hole.
Although it does not matter much in what I do, I would still like to know why this is so.

Thanks

You ever think about HOW a drill "generates" a hole?

It has a great deal to do with the geometry at its tip.

And a great deal LESS to do with the spindle or chuck.

WHEN you drill a 5mm pilot hole for a 12.5 mm final hole?

You destroy the ability for the drill to do it's work.

Pivot on its tip.

There is no longer any significant substance constraining the tip but air and coolant.

It's a small hole.

Try it with NO pilot hole.

A "pilot" hole is meant to have an OD LESS THAN the a drill's web-thickness at the tip.

Any more that that, you are not "piloting".

You are trying to use a drill as if it were a reamer.

It is not a reamer.

Chrisboon · Apr 25, 2021

Thanks Thermite.
Makes sense . I will try drilling without a pilot hole and report back.

boslab · Apr 25, 2021

Short or stub drills are handy too,
Mark

Milland · Apr 25, 2021

boslab said:
Short or stub drills are handy too,
Mark

Yeah, cobalt stub drills are my go-to.

Chrisboon · Apr 25, 2021

OK so I drilled without a pilot hole and there definitely was an improvement, maybe 0.05mm. Feeding the drill at a slower rate also improved matters somewhat.
I did not drill with a punch mark but will try that tomorrow. Not sure that it will make a difference, because the discrepancy is already small.
I will update as soon as done.
Cheers
Chris

13engines · Apr 25, 2021

And let's not forget... for the most part, a drill is considered a roughing tool.

thermite · Apr 25, 2021

boslab said:
Short or stub drills are handy too,
Mark

A drill is not a very good boring-bar, either!

Even so.. intentionally grind the tip off-center when you need a size of hole you have no drill for?

It generates that larger hole. And/or breaks.

Core drills exist.

As do "D" drills.. and reamers. Boring-bars, even. Clear down to micro- sizes..

Horse for courses. Poles for holes.

"Etc."

Spruewell · Apr 25, 2021

I never center punch mark a drill hole position unless I’m using a drill press. If the hole requires positional accuracy, I will either start with a couple short pecks, or use a spot drill to guide the drill’s initial entry. I only use pilot holes for large drills over 1” without split points.

Illinoyance · Apr 25, 2021

In drilling thin material if the center breaks through before the corners of the flutes are fully engaged with the metal the result is usually a 3 lobed hole.

thermite · Apr 25, 2021

Illinoyance said:
In drilling thin material if the center breaks through before the corners of the flutes are fully engaged with the metal the result is usually a 3 lobed hole.

"In drilling thin material.." One doesn't USE helical twist drills, unaided. Too much waste of backing material to get decent holes.

Absent consumable, EFFECTIVE, and affordable-cost backing?

Naked stock?

That's what Forstner's, hole-saws, "paper drills", and similar members of the annular cutter tribes, its cousins, and hybrids are for.

Better yet, punches!

A "paper drill" is a good example of core-drilling or trepanning writ small and thinwall, actually:

Paper drilling - Wikipedia

CarlBoyd · Apr 25, 2021

Unibit for thin materials.

Illinoyance said:
In drilling thin material if the center breaks through before the corners of the flutes are fully engaged with the metal the result is usually a 3 lobed hole.

Unibit style drills work quite well for me on this materials. They do have a limited number of hole sizes that will make.

CarlBoyd

ezduzit · Apr 25, 2021

Chrisboon said:
...I am not learning fast enough.
Today I discovered that the holes I drill are not as expected...
Although it does not matter much in what I do, I would still like to know why this is so...

You are way ahead of many who have been drilling crappy holes, for years, without ever knowing it, caring or wondering why.

Use a spotting drill for critical locating. Learn to properly sharpen your drills. And ream when size and shape are important enough.

basalt · Apr 25, 2021

Coming out the other side at 12.53 is awesome. Sounds like the drill with all it's runout is gradually supporting itself as it drills. Your 12.67 is probably due to all that runout. As said drills are for roughing. Get rid of the runout then finesse the drill sharpening. Your approach to improving is admirable.

Rudd · Apr 25, 2021

As above, drills are a roughing tool. If you need closer tolerances, drill undersize and use a reamer. I'm also wondering why all the emphasis on short drills? A short drill would transfer more of the runout, a longer drill has some flex and would transfer less chuck runout.
It's going to be really hard to get a good hole with a drill when the diameter of the drill is bigger than the thickness of the material.

thermite · Apr 25, 2021

CarlBoyd said:
Unibit style drills work quite well for me on this materials. They do have a limited number of hole sizes that will make.

CarlBoyd

Yah but.. if you have a lathe, and a torch to harden, you can rapidly make a "D" drill for ONE size (or several if you care to) and have ANY size you need.

Same family. One cutting edge. They are reversible, too, so twice the life!

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Drilling information

Chrisboon

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thermite

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Chrisboon

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boslab

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Milland

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Chrisboon

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thermite

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Illinoyance

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thermite

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CarlBoyd

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ezduzit

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basalt

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thermite

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