Slanted holes

[compact8]

I used to take it for granted that holes are always perpendicular to the table until I checked a couple of 8mm holes drilled on a 7075 Aluminium block last night. Both were slanted. The way I did it was drilling out the center with a 4mm center drill ( the biggest I have got ) , then a 4mm through hole followed by 6mm and finally 8mm. What I used to believe is that the first hole should guide the drills used in later stages and the final hole should be just as straight but it was not. Is this my misunderstanding or something else caused the problem ? The other two smaller holes I drilled on the same block are all good. ( 4 mm center drill -> 4 mm through hole -> 4.2 mm final hole )

 

[Reddinr]

If the aluminum block was hand-held on the table, anything can happen. Slight upward movements could get a bit to grab slightly on one side of the hole and maybe take off at an angle from there. If you can clamp the piece to the table you might have better luck. Did you check the straightness of the pilot holes you drilled? Maybe one was off a bit to start with.

 

[Lo-Fi]

Could be a few things. What machine are you using? Some drill presses are not so sturdy and can flex.
You're also allowing errors to creep in by going up in small increments. Best practice is to have the pilot hole just big enough for the chisel point on the largest drill. When shaving off 1mm, particularly in ali, the larger drills are liable to grab and wander. In this case, 4mm then straight to 8mm should be pretty good

 

[RJSakowski]

Are the slanted holes going in the same direction? If so, you probaly have a setup issue, sagging table,e.g.. If in different directions, look at the drill bit(s).

 

[compact8]

.... Best practice is to have the pilot hole just big enough for the chisel point on the largest drill. When shaving off 1mm, particularly in ali, the larger drills are liable to grab and wander. In this case, 4mm then straight to 8mm should be pretty good

Thanks ! that makes a lot of sense. Will definitely try it out.

 

[compact8]

If the aluminum block was hand-held on the table, anything can happen. Slight upward movements could get a bit to grab slightly on one side of the hole and maybe take off at an angle from there. If you can clamp the piece to the table you might have better luck. Did you check the straightness of the pilot holes you drilled? Maybe one was off a bit to start with.

The block was clamped by a vise. I haven't checked the straightness of the pilot holes but the other smaller holes I drilled on the same workpiece in the same set up are all straight ( 4mm center drill -> 4 mm drill -> 4.2 mm drill ).

 

[compact8]

Are the slanted holes going in the same direction?

Overall no, they are inclined to the same direction along the X axis but opposite along Y so it seems to be caused by something random

If in different directuions, look at the drill bit(s).

That's why I asked about center drills in this post : https://www.hobby-machinist.com/threads/do-center-drills-have-runout.95881/ .

 

[Flyinfool]

Are you using a spotting drill to start the hole.
A small drill can easily walk before cutting and will then drill off at an angle. The small increment of bigger drills will try to follow that crooked hole.

 

[compact8]

Are you using a spotting drill to start the hole.
A small drill can easily walk before cutting and will then drill off at an angle. The small increment of bigger drills will try to follow that crooked hole.

Yes I did. It was a 4 mm center drill.

 

[Flyinfool]

A center drill is better than not having any start, I'll admit, I have also used a center drill many times to start a hole, but is not the same as a spotting drill. If a hole is critical I use a solid carbide spotting drill with a point angle to match the drill bit that I will use. Solid carbide is much stiffer than HSS.