I'm drilling a 1 3/4" hole 42" long on a copper bar, 21" from each end. No, tube isn't available. Anyways, it is a standard black oxide twist drill and it has done several of these parts now. The problem is the drill seems to be making a hole slightly smaller than the drill, it is tight on retraction for several inches. A present the hole is about 14" deep and when you retract it to clear chips as it re-enters the hole it goes easy for about half the depth, then gets very hard for the next several inches until it hits the depth to actually start drilling again. I mike'd the drill OD and all seems well except scuffing on the last inch of the major OD. After stoning those edges I blued them and after one short drilling all the blue was gone from the narrow OD land. The hole isn't critical for dia. or finish and I'm thinking about relieving the OD except for the last 1/2" near the tip.
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Tight drill on retraction-copper
- Thread starter Bluechipx
- Start date
- Replies 14
- Views 1,644
Mike1974
Diamond
- Joined
- Nov 5, 2014
- Location
- Tampa area
You could try sharpening it a hair off center so the drill cuts a little larger? But I think the main problem is it is copper. 
Ray Behner
Diamond
- Joined
- Mar 27, 2009
- Location
- Brunswick Oh USA
You could try sharpening it a hair off center so the drill cuts a little larger? But I think the main problem is it is copper.
Yep, just like 954 bronze. Generates heat so fast it closes the ID a touch.Keep her cool if you can.
ChipSplitter
Titanium
- Joined
- May 23, 2019
- Location
- Maybe
Are you drilling horizontally or vertically? Vertical would be better because you could get coolant down there, but I suspect you're doing it in a lathe.
Might be a bit difficult to stand a lathe on end....
I wouldn't be scared to relieve the OD. I done that myself, but never on such a long drill.
BoxcarPete
Stainless
- Joined
- Nov 30, 2018
- Location
- Michigan, USA
Having just done some deep drilling (slightly smaller scale but similar depth ratio, .404" x 3.875" deep) in aluminum bronze, I ran into a similar issue. I had to undersize the mating part down to .396 and it's still a tight fit in some spots, yes I mic'd my drill and it is .404 on the OD.
So, I think heat might be a factor but not quite the way it was described. The heat would tend to expand the material as it is cut, and then once it cools, it will contract back to form a smaller hole than the drill. Copper is such a good heat conductor that this spreads the effect out beyond the immediate cutting zone and binds the drill up, or something like that. But I can confirm that I made a hole of similar depth ratio several thou smaller than the physical size of my drill.
eKretz
Diamond; Mod Squad
- Joined
- Mar 27, 2005
- Location
- Northwest Indiana, USA
Offset grind the tip a hair. That's what I do on Ampco and similar materials, works a treat.
The drill has been extended that is used to go all the way through, luckily I have a coolant hole twist drill that is somewhat smaller dia and only about 12" long that breezes it's way for the first 12". After I have sharpened the 1 3/4" drill a few times, the point probably offset itself! I'll try the offset trick but I'm afraid that the 1 3/4" drill, after enlarging the maybe 1 1/2" hole for several inches, will be guided by it's major dia. and by the time the offset point begins to cut the drill will not be able to move off center. As I type my employee is part way through the second side on the lathe. We do a lot of long holes that must connect in the middle. I have a theory, if the part is held stationary and the drill is rotated, it doesn't cut as straight as if the part is rotated as in a lathe and the drill is stationary. Somehow a rotating part causes the drill to find center easier than a rotating drill in a stationary part. If you don't force the drill you can barely see where the two came together in the middle when done on a lathe. The long holes done on a boring mill seem to really wander the longer they are. Thoughts?
ChipSplitter
Titanium
- Joined
- May 23, 2019
- Location
- Maybe
Are you doing a lot of these?
Enough to justify a new, optimized tool?
Enough to justify a new, optimized tool?
eKretz
Diamond; Mod Squad
- Joined
- Mar 27, 2005
- Location
- Northwest Indiana, USA
Rotating part always drills straighter than stationary part. That's not theory it's a known fact. If you want them really straight, you can rotate both the drill and the part.
As regards the offset grind, it won't help you on an already drilled hole. Use it on the next part. Your comment about the first inch of the drill being scuffed is a good clue as to why you're having trouble. If the front of the drill is smaller than the back it's going to wedge itself into the hole. The lands of the drill are circular, they don't have clearance to cut effectively. If the drill is that worn you should cut off the scuffed smaller diameter part and form a new point.
Limy Sami
Diamond
- Joined
- Jan 7, 2007
- Location
- Norfolk, UK
Stupid idea time (or maybe not)
Check the drill is straight.
Check the drill is straight.
If it were me and the offset tip grind didn't work, I would weld up the margin for the first quarter inch slightly larger. I would tig weld it with high speed steel filler. Worn out small drills would probably work for filler if just drilling copper.
I wish! No, three maybe a year, I suppose there are great options if the volume warranted it. Starting from scratch with a 3 1/4" copper bar 42" long it took about 5 hours to complete the hole, 32 pounds of chips. We do have to be careful, it was over $1,000 for material and hurrying seized a drill and snapped it off once, another time a gorilla operator forced the drill so hard it went so far off center it broke through the side. The seized drill another time snapped off the tang so I let the operator take his time.
I checked the dia with a mike and it didn't show undersize near the tip, but you are probably right because something is causing a loud squealing as the drill enters the last few inches before it contacts undrilled material. I have a couple of inches of spare length left on the drill, I could cut an inch off the end a re-point it but on a 1 3/4" inch drill done completely by hand it would probably be a long ways from ideal.
Some materials like oxidized aluminium bronze do the same so fast that you are questioning your sanity.
Drilling to pilot hole seem to be the culprit in almost every case. Wears off the margins until you have self-feeding tapered reamer
Doesnt take long from a drill that is squealing and juddering in existing hole to 2-part drill
Double check that you have enough back taper and relieve if needed.
Drilling to pilot hole seem to be the culprit in almost every case. Wears off the margins until you have self-feeding tapered reamer
Doesnt take long from a drill that is squealing and juddering in existing hole to 2-part drill
Double check that you have enough back taper and relieve if needed.
