Small dia Flat bottom Hole deep in a bore. Help!

So I thought I had this job all planned out ready to bang out. nope.
two broken inserts on a boring bar i ordered just for this job. Tiny guy.

Part is an aerospace bolt, 1.09 deep bore .312dia flat bottom.


Thought I had this, I drilled 19/64" dia hole with the drill bit tip hitting the 1.09 bore depth.
Then used a 4 flute endmill that I basically ground away all cutting edges except the front face. Reduced the diameter just a hair (this where I went wrong? there is a reason for me reducing the dia but I won't get into it now)and used that to produce the flat bottom.

Then used a tiny indexable boring bar I just bought(I broke the last one I had, and needed a new one for an upcoming job that has a .300" bore dia on it)specifically to run this job. That inside needs a .005-0.015 radius....so I wanted an indexable tool that used inserts with a TNR in between there to avoid having to move the boring bar down in the radius cutting, and possibly contacting on the opposite side of the cutting edge.
'
Anywayyyy...I tried different programming but the insert snapped twice! that step is measuring 1.080" deep, so that has to be where the insert is snapping... .010" shy of the endmill "bored" depth.

Any suggestions? I see there is a $153 flat bottomed drill bit at MSC....ehh...I'd rather not waste more $ on this job I'm not making much at this point as it is, and I'm not even sure that would solve my problem.

And i know there is a slight "taper" to the endmill face, hence that showing up in my part...but its a wide open tolerance bored depth, so the part will pass inspection without issue....the inspector just doesn't want to see a drill point or step in there.

Machine is a Mori SL1a. No Live tooling.

Any suggestions? I see there is a $153 flat bottomed drill bit at MSC

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MSC, so its less than $75 anywhere else, though it probably more like $30 since MSC SUCKS!!!!!

What is the diameter tolerance?

And you do realize you don't get a "perfectly" flat bottom hole with an endmill.. And quite
honestly that don't matter for fuck all 99.9% of the time... But I figured I would mention it.

I've 1000% given up on buying small boring bars.. I just use old endmills. Grind away on 'em a bit
if need be, but you get better cutting geometry, a more solid/rigid tool, and its fricken free because
you already paid for it on the mill.

Your endmill should have worked. I do it all the time.

Why couldn't you make your own flat bottom drill? I have to do crap like that all the time. When I'm home, I'll use my Monoset to chop up tools, but at work. I got a pedestal grinder, so I have to figure out how to grind it by hand (I'm getting 4 flute ball end mills that actually cut a radius!!) After you got to depth, with the first tool, use a second drill ground flat and grind one flute away so that the first flute sorta scrapes the bottom to center

whats the material

Sorry 2024 alum. Should be easy peasy. No clue why the insert broke twice!! Bore held size and surface finish.

Oh gosh. I assumed stainless or titanium.

can u use a .140 bbar with a little bit relieved? or find a junk endmill and do as suggested above..i personally would stay away from flat bottom unless u really have to

At least use a solid carbide, non insert bar for the love of god.

You can do that with a simple D bit shop made out of drill rod, simply turn the profile you want, cut / grind back to exact 1/2 dia, heat treat and grind the cutting edge ..........what 1/2 hour max and a few $ of material.

camscan said:

15-0 to you dad,now do some bloody work.

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Enough of your lip littlun ........don't forget what today is.

Harvey Tool - Carbide Counterbores - Flat Bottom
They have long reach ones also.

Evan LaCava said:

No clue why the insert broke twice!! Bore held size and surface finish.

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I'm going to guess that it wasn't on center. A flat-bottom drill or a C'bore should work fine.
JR

JRIowa said:

I'm going to guess that it wasn't on center. A flat-bottom drill or a C'bore should work fine.
JR

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Used to do a lot of like parts working in aerospace decades ago. Drill hole with twist drill and finish with a half round with a flat cutting edge as long as half the width. I seriously doubt that bottom needs to be perfectly flat. The first thing I ask is how wide of a drill point I can leave. A lot of flat bottoms at the end of a bore are just a mating surface for a compression spring.

I checked center it was dead on. No TIT when facing with boring bar. I was careful with setup.

Ohh well the job is done now. I ground an old solid carbide mini boring bar I had....it worked okay. It was kinda long for the job so I got a little chatter on the ID. I'm just being picky.

I still hate that I can't explain what happened. Maybe lack of coolant getting down there but who knows...it was 2024 damn it lol not hast or inconel.

Thank you guys!!!!

FYI Evan .when making a D bit as I described, ..making the body a good fit in the bore - say 0.002'' clearance and with just a short step on the end, say 1/4 ''long, means the work guides the cutter and no chatter, with a the coolant providing enough lube.

Limy Sami said:

FYI Evan .when making a D bit as I described, ..making the body a good fit in the bore - say 0.002'' clearance and with just a short step on the end, say 1/4 ''long, means the work guides the cutter and no chatter, with a the coolant providing enough lube.

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Thank you. And thank you to everyone else for suggestions as well.

Parts came out nice just the way I like em



Drilling those 0.062" holes on my bridgeport with half the necessary spindle speed wasn't fun lol

rpm too high often causes vibration problems as well as heat causing aluminum to stick to cutting edges
.
many long length to dia tools do better slowing down. got nothing to do with insert over heating. limit is a vibration limit

Evan LaCava said:

Thank you. And thank you to everyone else for suggestions as well.

Parts came out nice just the way I like em

Drilling those 0.062" holes on my bridgeport with half the necessary spindle speed wasn't fun lol

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I'm curious, were those 0.062 holes done with a bolt head drilling jig or end mill then twist drill ?

Limy Sami said:

I'm curious, were those 0.062 holes done with a bolt head drilling jig or end mill then twist drill ?

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I'm betting endmill, since there is one in the back ground. Regardless, nice looking part, I like the cross holes.

Why someone would design an aluminum bolt with that much structural necessity is beyond me, but hell I just make what the smart people want, what do I know, right.