Why enlarged hole will happened randomly after thread taping 1

[allenchu]

Hi,
I’m new in this Forum. I have a tapping problem, hope somebody can help me.

Problem: may have one or two enlarged taper hole on the far side after taping.
Material: Al 5052 H112, Thickness: 12 mm
Assignment:
Taping (through) (21) M3 Threads, (38) M4 Threads, (52) M3 Threads
Machining parameters:
Drill hole diameter before taping:
M3 2.5 mm Dia. M4 3.3 mm Dia. M5 4.2 mm Dia.
After taping, one or two tapered and larger hole may be found on the far side. This may occurred every 5 to 7 pieces part, and the enlarged hole’s position was randomly. The largest diameters of each are as below:
M3 2.7 mm Dia. M4 3.6 mm Dia. M5 4.35 mm Dia.

I checked the drill and tap’s runout after install them in the Vertical Machining Center’s spindle, the runout is 0.015 mm. Coolant can flush the tap’s tip to ensure there is no residual material on the tip. Tap is new.

Your solution and suggestion is appreciated.

Thank you.

Allen

 

[BillPSU]

You haven't given enough information about your current process so first check these:

What size is the drill producing?
Are you using a suitable coolant and is it the correct concentration?
Are you using drills and taps designed for aluminum. Fast spiral polished flute drills and high hook angle taps?

Going beyond these the following are some other situations that could be causing the problem.

One possibility is you are getting build up of chips in the flutes of the tap which is rubbing on the minor diameter of the hole. Change your tap to either a spiral flute tap and pull the chip up out of the hole or to a spiral point tap which pushes the chips through the hole.

Another possibility is because of the length of engagement is causing a problem with the tap on the smaller diameters.

 

[tripleZ]

I'd have to agree with BillPSU on the chip build up. Gummy material will build in conventional flutes and blow out holes unless they're removed.