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Bruce Caulley

drilling a deep hole close to edge of material

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Ahoy,

I have a part that has a 3.2mm hole 70mm deep that will leave only 1mm of material in the wall next to the hole. Material is 4140 at approx 40Rc and I will be using a through coolant carbide drill. If this section of wall is not supported by fixturing, what are my odds of keeping the hole straight without breaking out the side?

 

Job is only being planned at the moment, and I don't want to commit to a process that is doomed to fail.

 

Cheers

 

Bruce

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get the best drills money can buy.

guhring

 

or put out a place holder of sinker EDM as a plan b

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The amount of material containing a drill has no bearing on whether it will walk or not, as long as you have full containment. This drill is less than 22 X D, so I don't see a problem with it breaking out of a 1MM wall.

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The amount of material containing a drill has no bearing on whether it will walk or not, as long as you have full containment.

 

I've had it do just that. Same hole diameter, same drill, two different holes, same depth. I think it was a 10mm hole 260mmish deep. I had to use two different drills since as soon as the drill doing the hole close to the wall started to dull it wandered and broke through (50 or so holes) The one with plenty of material around it lasted several hundred holes. I checked all the usual suspects, TIR, holders, tapers backlash etc and am certain it wasn't the machine or holders. The job finished before I had a chance to see if clamping it so the thin wall was firm against the fixturing gave a bit of support or not.

 

Chers

 

Bruce

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^^^ I wonder if thermal plays a part here?

I know there shouldnt be any heat with thru tool, but heat disribution (sinking way from the tool) wouldn't be even if there's only 1mm compared to the rest being solid?

I don't think fixturing against the side would help because you'll never achieve a face to face lap type touch.

Drill first then mow the side wall down to wall thickness!

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Something to think about.....

Drilling forces are not just torsional and straight down. There are angled cutting surfaces that are being pushed on and are pushing back. Like a highfeed cutter that channels the cutting force up towards the z axis , the cutter is also pushing back on a downward angle. With a thin wall it seems possible to drift towards the path of least resistance. I'm sure it's all about forces and ratios that i can't do the calculations on , but I'm just thinking out loud here.....

 

When it broke thru was it just at the bottom of the hole or was it a more continuous bulge on that side. Did it wander or is it heating up that thin wall and pushing it out like some kind of thermal deformation.

Mebbe try a very shallow peck and retract to keep heat down.....or clamp it to the fixture like you said

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From my experience the drill will always walk towards the thin wall. We have had expensive parts ruined this way. Always leave extra stock and finish it after the deep hole drilling.

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Going as deep as you can with a shorter drill (or 2) before pulling out the full length drill will also help in keeping the hole straight.

I would not have thought about the heat distribution theory newbeeee raised.... but it sure seems like a capable suspect, except for the coolant being at the point of heat generation.....

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