Ring at the bottom of drilled hole????

[Mark R R]

Drilling 6061 aluminum, 1/4" thick, 1/4" hole drilled through the material. Drill speed about 450 rpm. Looking through the hole there seems to be a ring or ledge about 1/32" thick at the far side of the hole. Hole is otherwise smooth. Drill bit is new. Ideas anyone?

 

[Tony Wells]

Despite being new, I have seen that "feature" on drilled holes when the point is ground a bit off center. Measure the hole. If it is oversize, that's the likely cause. Once the drill breaks through, nothing causes it to rotate off center and the "ledge" or "step" you see will probably be close to size.

 

[Mark R R]

Thanks! I would not of thought of that.

 

[Chuck K]

That's interesting. I just had the same thing earlier today. It was a large used silver and demming drill. I just needed a large hole to press a bearing through so I wasn't real concerned about the finish. I had sort of forgot about it before reading your post. I'll have to take a look at the drill tomorrow and see what it looks like.

Chuck

 

[Mark R R]

Ok, can anyone tell me how to reshape a bit so it does not do this but it drills a truer sized hole? Or is it a case of replacing the bit?

 

[Terrywerm]

You will need to resharpen the bit, making sure to get the chisel point exactly centered. If the cutting edge of one side is slightly longer than the other, the drill will cut oversize.

Drilling a pilot hole also helps to reduce this problem, but will not eliminate it if the drill bit is ground off center. When I have this problem but am unable to regrind the bit, I will often drill nearly up to the finished size, then run the finish size drill through to finish the hole. For example, I desire to drill a hole 3/4" in diameter. Drill a pilot hole of about 1/8", then follow with a 1/2" or 5/8" bit, then finally drill to size with the 3/4" bit.

Another thing that helps is to drill the hole from one side, so that the finish diameter is cut about 1/8" deep, then flip it over and drill from the other side.

 

[Carl]

Drill sharpness, drill tip/end geometry, etc are all important issue. It's already been covered.

Just a thought, hope it will be useful.

For this material & thickness, and the size of drill, I've eyeballed touch up (grinder) drills for sharpness, without this issue. Aluminum is/can be gummy, "sharp" drill major plus, and lube.

Seems that for 1/4" thick aluminum and a 1/4" drill 450 rpms is a bit slow.

If I have done my math correctly. I've only got 3 brain cells and I'm over 60 yrs, so reading, thinking and typing are really pushing the juice.

AL cutting spd is 350-400.
4 x 350 = 1400; 1400 divided by drill size .250 = 5600 rpms.
4 x 400 = 1600; 1400 divided by drill size .250 = 6400 rpms

Maybe in the CNC world with coolant feed - 5600+.

But at 1500-2000 and a bit of cutting oil, a 1/4 drill should go very nicely.

Try increasing the rpms, a little cutting oil, and avoid "busting" thru.

Side note: I have 2 lathes and a BP mill. Applying cutting oil. Get a small 1/2" best, 1" is good,
non-synthetic paint brush. A "small" can (fruit/juice type) pour in cutting oil. Use the brush to apply cutting oil to the drill, also works very good when threading (int/ext) in the lathe.

Best regards.....carl

 

[Mark R R]

Thanks everybody! Very helpful!

 

[george wilson]

Old time machinists who often did not have the money to have number drills,letter drills,etc.,would use the practice of grinding a drill a bit off center to produce an oversize hole for a special need. They developed skills that today are largely forgotten in order to get by with nearly nothing.