Deep Hole Drilling In Aluminum

[prasad]

Guys,

I want to drill a deep "through hole" in a 2.65" long aluminum block. The hole will be 0.25" in diameter. The alu block is 1"x1" square. I know I must start with a center drill. After that I am not sure. What should be done to prevent the drill bit from wandering? I have drill bits that are 4" in length and I hope they should be sufficient?

Next step (not immediate) I want to attempt will be to make the same hole into an elongated hole (oval shape) of similar depth - 0.25" x 0.4". I think that will be even more tough.

My machines are G4000 9x19 lathe and G8689 Mini-mill by Grizzly.

Regards
Prasad

 

[kd4gij]

I would use a 4 jaw chuck and drill it in the lathe. As long as the drill is ground properly it should drill strait.

 

[4GSR]

Start your hole with a drill bit long enough to drill the hole in one setup. A standard jobber length drill bit should do the trick. Use a cutting oil to lubricate the drill bit so it does not gall up the aluminum and get stuck. Retract your drill bit often to remove chips and add cutting oil.

 

[Round in circles]

I've done a similar hole job a few days ago in 6 mm & 5 mm diameters . My drills are hand ground and fairly accurate even if I do say it myself .
I'd prefer to drill the long hole first so nothing jumps or moves around .

First time was in the 4 jaw chuck with the 5 mm drill , it was a doddle , plenty of WD 40 to lube on the drill bit as the clean drill goes in and a small brush to keep the drill clean as it's withdrawn . Never sink the drill in past the end of the flute unless you are desperate because this often traps swarf & leads to a broken drill bit . Only go in for say 1.2 " maximum at a time withdraw and use a paint brush brush to clean the drill bit don't use your fingers as the swarf is usually razor sharp . Never used a rag on a rotating drill or lathe

Drilling the 6 mm holes in the aluminium was done with it sitting on the drill table under the drill press as it was four inches thick and would not go in the four jaw chuck because of the length of the pieces .

I turned up a small parallel test bar three inches long & an inch and quarter in diameter undercutting the faced end so that when it sits on the table only the outer edges touch the table . I used this to check & set up the table so it is at right angles to the vertical doing it fore & aft and again left & right to the spindle into which I'd put in a four inch long turned half inch round test bar & gently tightened it up using all three key holes so it sits as near to perfect as possible ...sometimes if you only tighten it on one key hole the part will be ever-so off centre and at a very slight angle too boot. I needed to make a slight adjustment to get the table set correct


I then carefully found the centre of the bar using a steel ruler across the diagonals , then checking it with a pair of dividers by drawing the biggest circle possible to see it almost touch each side before giving it a decent clout with a hammer & center punch to fix it & then marked the other end face in the same way . I set it up again this time using the table vice & found it wouldn't fit as the overall height was too big for the throat available.

Had to do vertical rechecks from the foot plate ( OK ) and then set it up in the vertical in the table vice and bolted it down to the table did a vertical recheck then did a cold run at the drilling to see if it did actually hit the desired point .

With everything OK I went a head and drilled the first hole with caution & care.
I drilled in with a 5.5 mm drill from each end face so as not to have to worry too much if the drill wandered off a tad internally . Then followed through with the slightly longer 6 mm drill from each end to take out any deviations . There was very little deviation with the 5 mm drill runs to the middle which really helped a lot .

 

[Wreck™Wreck]

Get yourself a parabolic flute drill, this will work a charm in high D/L ratio holes in aluminum, use a spotting drill rather then a center drill to start. this should give you a fairly straight hole then ream or bore as needed.

 

[prasad]

Thank you all. Great ideas. I will try that and will report back on how it went.

Any ideas on how to make the hole into an oval?

Thanks
Prasad
AB3EH

 

[RJSakowski]

When drilling deep holes, it is easy for the drill to wander. A less than perfect grind, on the drill, drill not perfectly aligned with the start point, not clearing chips properly all can cause wander. If I need drill to come out exactly on center on the far end, I will drill approximately half way with a smaller drill, reverse the work piece and drill completely through with the same drill. Then I will finish with the final size drill. When the second hole hits the first, the drill will follow it and if there is a misregistration at the intersection, the second operation will ream the surfaces slightly creating a straight through path. The finishing drill then drills a straight hole on center on both ends.

Your hole has a 10:1 depth to diameter ratio which is not too extreme and it should not be a problem. I have successfully drilled through holes using the above method with as much as a 20:1 ratio.

Bob

 

[RJSakowski]

Thank you all. Great ideas. I will try that and will report back on how it went.

Any ideas on how to make the hole into an oval?

Thanks
Prasad
AB3EH

I think HF drills cut oval holes.

Seriously, you would want to use an end mill for precise holes.

 

[Wreck™Wreck]

I then carefully found the centre of the bar using a steel ruler across the diagonals

This I find fascinating and perplexing at the same time, correct me if I am wrong.
You machined tools and fixtures to make a part as accurate as possible then the last operation is eyeballing the hole position with a rule, what am I missing?

 

[T Bredehoft]

Yes, I'd centerdrill both end on a mill, after locating true center. Then drill through on a drill press. Neither my lathe or mill has two inch stroke.