# drilling deep in 7075 Al



## jtrout13 (Apr 21, 2011)

Hi all,

I am building a modified version Elmer's Fancy engine, and am currently drilling my valve holes.  This requires me to drill a 1/16 (0.063in) hole about 0.75 deep. This is not exactly DEEP drilling, but still fairly deep for anything I've done yet. I keep breaking bits off, and after I re-make the column (bits broken off in the old one), I don't want to break any more bits.

I'm just using black oxide bits on a Bridgeport mill, 1600RPM and just standard cutting oil. I now know I should rip the RPMs as fast as the machine will go (3600) and use WD-40 for cutting fluid, pecking at about .13 inch per peck.

My question is, do you recommend any special drill bit for this? My column material is 7075 Al and I have read that it takes a lot more power to drill this alloy than regular 6061. Perhaps a TiN coated drill would prevent the standard problem of chip welding (Al galling up on HSS)? I'd like some input before I try to drill it again.

Thanks!


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## jpeter (Apr 21, 2011)

When I use a small bit I rarely tighten the chuck too tight. What I'm looking for is for the drill to slip in the chuck should the drill catch. Otherwise, peck it in, clean it every diameter, 1/16 in this case and use lube. If you hear the bit clicking pull it out quick. A clicking sound to me is a pretty good clue that bad things are coming. You could also stone the lip some so the bit's not so inclined to hog in.


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## Foozer (Apr 21, 2011)

Just drilled a couple of 1/8 holes 2 1/4 inch deep in some 7075 T6 stuff. Spun it about 3000 rpm and it would of like faster. Its a peck 0.062 at a time, in, out, clear chips. Used a small acid brush dipped in some Tapmagic to both clear chips and lube the bit. Smaller the drill, the faster the rpm, as has been my experience.


Robert


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## kcmillin (Apr 21, 2011)

jtrout13  said:
			
		

> Hi all,
> 
> 
> 
> ...



I have heard that the coating on a drill bit or end mill can minutely dull the cutting edge, and that it is best to use uncoated cutters on aluminum, since aluminum needs a very sharp cutting edge. I cannot say if this holds true for 7075 though, and I have never worked with the stuff. 

Also, A .13" peck seems a bit much for such a small drill bit. I would half that at least to no more than 1/16" With the small bits there is very little room for the chip to go. The deeper you go, the less you can cut. The first 1/4" might go really easy, but the rest of the hole can be a nightmare. 

And yes, crank up the speed. :big: WD-40 works wonders for aluminum.

Kel


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## Lew Hartswick (Apr 22, 2011)

jtrout13  said:
			
		

> I'm just using black oxide bits on a Bridgeport mill, 1600RPM and just standard cutting oil. I now know I should rip the RPMs as fast as the machine will go (3600) and use WD-40 for cutting fluid, pecking at about .13 inch per peck.
> Thanks!


In my opinion .13" peck is WAY too much for a 1/16" hole. The flutes will be solid packed.
  ...lew...


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## Swede (Apr 22, 2011)

Something worth considering... peck drilling as described, essential with aluminum and any depth, generally makes an oversized or tapered hole, larger at the entrance. Softer aluminums are especially susceptible, 7075 not so much, but if one needs an accurate hole, it'd be best to pre-drill maybe 0.005" undersize, then finish it with a 1/16" drill in one pass.

For those who have never worked with it, 7075 is awesome stuff, right up there with 12L14 as a dream metal. It cuts beautifully, is not gummy like softer aluminums, and here is where it gets really cool...

7075-T6 is stronger and harder than CR1018 mild steel! It's true.

http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA7075T6

7075-T6:
87 Rockwell B, 83,000 PSI ultimate tensile

1018 Cold Roll:
73 Rockwell B, 63,000 PSI ultimate tensile

Pretty cool stuff. Not many realize 7075-T6 is this strong and hard. I generally feel free to substitute it for cold-roll steel in many projects. It is freely available as nice rectangular cuts (scrap) as it is used a lot in the die industry. There is a guy on eBay who sells this material, advertised as FORTAL Aluminum. I stocked up years ago and love it.


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## kcmillin (Apr 22, 2011)

Swede  said:
			
		

> 7075-T6 is stronger and harder than CR1018 mild steel! It's true.
> 
> http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA7075T6
> 
> 7075-T6:



Does this mean we can make one piece crankshafts out of the stuff?

Kel


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## arnoldb (Apr 22, 2011)

John, pecking like you described should be fine, but don't go more than 0.1" per "peck" once you are deeper than 1/4". And completely remove the drill bit from the hole after each "peck". Have a look at it to see that it drops all the bits of swarf stuck to it after removing from the hole each time - use a small brush to remove stuck swarf, or stop the machine and remove by hand if needed. If the swarf does not come off easily, it means you're pecking too deep, and the chips gets packed too tightly around the drill tip.

----Edited to add:
How are you locating the start for drilling ? - if possible, use a spotting drill or center drill to spot the hole before starting with the small drill. If you have a bit of run-out in your chuck or the drill wanders just slightly on the start, it will be bending while you're drilling the hole, and will break when it goes deeper. Once started on the hole, extend the drill bit out of the chuck to leave it as long as possible, that will help if there's any flexing involved.
----

The small taper that might be caused shouldn't be a problem for Fancy's ports.

 Glad to see you started on it !

Regards, Arnold


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## Tin Falcon (Apr 22, 2011)

IIRC 7075 is nicer to drill than 6061. I used to drill deep holes in plastic a lot for pen making mostly 7mm hole almost 2 inches deep. I purchased some parabolic bits. The web is thinner for better chip evacuation . 
Tin


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## jtrout13 (Apr 22, 2011)

For those who might have wondered, I drilled the holes today.

WD-40 combined with TiN coated bits did the trick no problem, just pecked at it around 1/16 per peck and speed at 3000 RPM.

Arnold, it's looking almost as good as yours. I discovered the joys of phosphor bronze today, but pictures will come soon.


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## Swede (Apr 28, 2011)

kcmillin  said:
			
		

> Does this mean we can make one piece crankshafts out of the stuff?
> 
> Kel



Hard to say, as having a metal function as a bearing surface calls for different properties than hardness and strength. Bronze is soft, yet is a better bearing material than some harder metals. And aluminum on aluminum can gall in some cases. The expansion with temperature also needs to be considered.

But I'd say that if a small engine can function with a 1018 steel crank with no further hardening like case hardening, then it'd probably run with a 7075-T6 crankshaft, especially when we're talking about a desktop steam engine or similar. For a more powerful engine, I'd skip both 1018 and 7075 and go right to a carbon steel, like 4140 or better.

YMMV and of course there's no substitute for research.


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## Bill Gruby (Apr 28, 2011)

I am wondering why no one has mentioned the use of "Cobalt Drills" here. They stand up much better than standard drills. In my honest opinion a peck of .1 is way too much for a 1/16 .063 drill bit. Use half that and you will be OK. The deeper you go the less peck you yse. If you are indeed getting a cracking type sound change the bit. You have either chipped the edge or dulled it too much. A 1/16 drill will load up fast, be careful.

 Bill Gruby


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## steamer (Apr 28, 2011)

Ok Hang on guys,, Are you drilling a passage into another hole?

That will cause a grab that will break a bit. To get around that, put a piece of close fitting sacraficial stock into the bore, then drill into it. The drill won't grab then, it will just make a hole in the sacraficial stock.

Additionally, 3000 seems fast...I would use a decent lubricant for aluminum...kerosene works well...convienently located in a WD-40 can by the way...and as was stated....peck a little, then withdraw and clear....you should be fine then.

7075-t6 is a great alloy...I made my steamboat connecting rods from it.  It machines nice with sharp tools and leaves a nice finish.

Regards,

Dave


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## Lakc (Apr 28, 2011)

kcmillin  said:
			
		

> Does this mean we can make one piece crankshafts out of the stuff?
> 
> Kel


Probably, but then you are going to want to use steel for the connecting rods, and you would need a bigger flywheel to recoup the rotating mass. 

It sounds like the hole is drilled and the job is done. Congrats. I have not had any luck with Ti coatings and aluminum, however, they tend to stick to each other, so I dont think its recommended.


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