Drilling question

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jscarmozza
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Drilling question

Post by jscarmozza »

I'm trying to drill three mounting holes in a bronze worm gear that was cast onto a steel hub, and as luck would have it the holes are right on the interface of the bronze and steel. The first hole walked off into the bronze and is out of alignment, which I will fix by plunging an end mill through the hole for a shear pin, but I need the other two hole to be aligned for tapping so I can fasten the gear. Any tips on how to drill these holes will be appreciated. Thanks, John
SteveM
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Re: Drilling question

Post by SteveM »

I think anything that generates a side force is going to get off center, so a drill is not going to work (as you found out). It's just taking the path of least resistance.

Is there an end mill that is the size of the tap drill you would need?

Steve
jscarmozza
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Re: Drilling question

Post by jscarmozza »

Steve, the hole is an F drill hole for a 5/16 X 18 tap. I can look around for an end mill that's close, but I'm still going to have to drill a pilot hole.

When I drilled the first hole I figured it was going to be a problem so I clamped the piece I wanted to mount onto the gear, transfer punched it, center drilled it, then drilled it with the piece still in place to act like a drill bushing...it still walked off. I can't drill a very small pilot hole either, the center of the hole is right on the seam.
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GlennW
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Re: Drilling question

Post by GlennW »

"F" is ,257"

Use a 3/16" center cutting end mill followed by a 1/4" end mill, then bore it to final size if you need to.
Glenn

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jscarmozza
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Re: Drilling question

Post by jscarmozza »

Got it!
Thanks guys, John
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Re: Drilling question

Post by whateg0 »

GlennW wrote: Thu Jul 26, 2018 9:20 am ...then bore it to final size if you need to.
That's the part that is key, I think!

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Re: Drilling question

Post by LIALLEGHENY »

I would use an end mill to make the hole, stepping up in size if need be. A letter "F" drill will give you a theoretical thread depth of approx. 77% . If you were to stick with the 5/16 hole generated by the end mill, the thread depth would be around 86%. A little more difficult to tap but it would work. Chances are the endmill will cut oversize as will any drill. You also have to figure that even if the hole is dead on, the tap is going to try to deflect over into the softer material.


Nyle
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Re: Drilling question

Post by Glenn Brooks »

John, it strikes me you could put helicoils in the bronze worm gear in lieu of drilling and tapping on top of the interface. Certainly if your attempts to tap the hole go awry, you could use the inserts and do just fine, I think.

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SteveHGraham
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Re: Drilling question

Post by SteveHGraham »

I would start with an undersized end mill and then use a drill bit to open it up before tapping. This may be wrong, but it's what I would do.

Too bad they don't make annular cutters that small. Or do they?
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Harold_V
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Re: Drilling question

Post by Harold_V »

LIALLEGHENY wrote: Fri Jul 27, 2018 8:17 pm I would use an end mill to make the hole, stepping up in size if need be. A letter "F" drill will give you a theoretical thread depth of approx. 77% . If you were to stick with the 5/16 hole generated by the end mill, the thread depth would be around 86%
Woops! Somewhere the actual size has been lost. Conversation, thus far, was in regards to a 5/16-18 thread. If a 5/16 end mill was used, the resulting thread would amount to, for all practical purposes, 0. I suspect you meant ¼", but things like this have a way of being translated such that scrap is made. Just wanted to avoid that possibility. I know you know the difference.

H
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Harold_V
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Re: Drilling question

Post by Harold_V »

SteveHGraham wrote: Fri Jul 27, 2018 10:51 pm I would start with an undersized end mill and then use a drill bit to open it up before tapping. This may be wrong, but it's what I would do.
In spite of drills having a margin without relief (they are reduced, but no further relief applied), they will still deflect, resulting in the hole being mislocated to some degree, as well as it no longer being perpendicular. Same thing goes when the hole is tapped, as has already been mentioned.

Any time dissimilar metals are machined, as they are in this instance, you can expect tool deflection, as the greater cutting pressure for the harder to machine material will push the cutter away. It can be managed by using very short cutting tools, and rigid setups. To ensure location, I'd stick with an end mill, as it has the ability to remove metal without excessive deflection, given the chance. Much lower cutting pressure, and without the angled cutting face found on drill bits, which exacerbate the deflection problem.

Boring, as Glenn suggested, is always an acceptable solution. It just takes more time.

H
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LIALLEGHENY
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Re: Drilling question

Post by LIALLEGHENY »

Harold_V wrote: Sat Jul 28, 2018 12:12 am
LIALLEGHENY wrote: Fri Jul 27, 2018 8:17 pm I would use an end mill to make the hole, stepping up in size if need be. A letter "F" drill will give you a theoretical thread depth of approx. 77% . If you were to stick with the 5/16 hole generated by the end mill, the thread depth would be around 86%
Woops! Somewhere the actual size has been lost. Conversation, thus far, was in regards to a 5/16-18 thread. If a 5/16 end mill was used, the resulting thread would amount to, for all practical purposes, 0. I suspect you meant ¼", but things like this have a way of being translated such that scrap is made. Just wanted to avoid that possibility. I know you know the difference.

H
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