Drill/countersink/counterbore

[Welderpaul]

We have several thousand holes to drill as above.
Material is 3mm ally with 3mm plastic bonded to the top. Sits on 6mm mild steel. Hole needs to be a 8mm counterbore through the plastic, a countersink in the ally and drill 5mm through the MS.
There's various bits on ebay that are countersink bits which slide over drill bits. Will the countersink bit form the counterbore ok? I don't want the plastic to tear.

More stupid jobs.

 

[scott0031]

It should do providing it's the right O.D and sharp. I would have suggested a 8mm counter sink and just plunge it to depth and the c /bore part will self generate. Cheap tools may go off quickly and begin tear the plastic, if you really do have that many to do consider getting some ground up for the purpose, not as expensive as you may think.


Kind regards,


Scott

 

[DuffyBB]

Something like this maybe?

 

[BrokenBiker]

I was going to suggest a centre drill or something similar

 

[hotrodder]

Centre drills are for making shallow conical holes to suit a tailstock centre and with the exception of doing a reasonable* impersonation of a spotting drill they suck at pretty much everything else because of their design. The sizes needed don't correspond to off the shelf centre drills anyway- length, diameters and probably countersink angle too (i'm guessing it needs to be 90°)

Scott were thinking of subland/stepped drills like http://www.phantomdrills.co.uk/shop/product/step_drill_bits or getting a modular set up ground?

* for the likes of me, probably not proper machinists