Need Help!- Drilling 75mm

[grf]

Hi everyone
i need some help for drilling 6mm wide 75mm deep holes in aluminum.i the problem i have is that my holes are not straight. from end to end they are out by 2mm. it is not a tram problem more like a drill bend issue. does anyone know what process i should follow or what tool to use to make deep holes? a friend that owns a lathe warned me about it but i thought i might get lucky.
i forgot to mention i am using a bf30 drill mill and simple hss drill bits.
thanks

[Geof]

Are you spotting the holes first to make sure the drill starts on center?

75mm is deep but you should be able to get them straight to better than 2mm.

[grf]

yes i am but i am not using a center drill.
is it possible that cause i am not using a center drill the normal drill deflects a tiny amount at the beginning that i did not notice and ends up at around 2mm at the end of the hole?

[grf]

yes i am but i am not using a spotting drill.
is it possible that cause i am not using a spotting drill the normal drill deflects a tiny amount at the beginning that i did not notice and ends up at around 2mm at the end of the hole?

[christinandavid]

Hi,

Centre Drill the hole so your 60deg cone is drill diameter. Use a standard length drill and drill as deep as you can before resorting to long series drill. Make sure your drills are nicely ground. Small pecks.

DP

[Geof]

You can try using a center drill for starting the hole location but a spotting drilling can sometimes give a better result.

One problem with using a center drill and going deep enough that the 60 degree angle is out to the drill diameter is that the drill can flop around in this taper as it enters because it does not have the web in contact with anything to hold it steady; this can result in an rough oversized entry hole. A spotting drill simply makes a shallow depression and the drill finds its center in this more reliably. Ideally a spotting drill with the same angle as the drill should be used because then the drill commences to cut along its width right from the start.

[grf]

i new i was doing something wrong.
thank you for your replies