[kirk_wallace]

I forgot to mention that I tend to use the short split point screw machine drill bits with the mill. Drill Set, 115 Piece Screw Machine Length HSS 2827 - LittleMachineShop.com I haven't found an especially good source for these. I usually get just the small number of bits I need from McMaster-Carr. See "High-Speed Steel Short-Length Drill Bits" here: McMaster-Carr

[kstrauss]

Your profile doesn't give your location but from your mention of Enco I assume that you are in the US or Canada. If so I think that you will save money and frustration by forgetting metric and using number size drills. Assuming that by "4/40" you mean "4-40", I usually use a #32 drill for a close fit or #29 for a sloppy fit. As others have mentioned, using screw-machine length drills will mean that you probably don't need to spot the holes.

[UniqueMachining]

Your profile doesn't give your location but from your mention of Enco I assume that you are in the US or Canada. If so I think that you will save money and frustration by forgetting metric and using number size drills. Assuming that by "4/40" you mean "4-40", I usually use a #32 drill for a close fit or #29 for a sloppy fit. As others have mentioned, using screw-machine length drills will mean that you probably don't need to spot the holes.

I think there is a typo on the numbered drill sizes. A # 29 would typically be used for 8-32. A # 43 would typically be used for 4-40.

John

[kstrauss]

I think there is a typo on the numbered drill sizes. A # 29 would typically be used for 8-32. A # 43 would typically be used for 4-40.

John

Yes, a #43 will give about 75% thread for a #4-40 screw and a #29 is correct for an #8-32 screw. However, the original poster was contemplating using a 3mm drill (0.118 inch) which is about right for a clearance hole but is far too large for a tap drill since the OD of a #4 screw is only 0.114. In an effort to answer the original query the drills that I mentioned were for close and sloppy clearance rather than tapping. I apologize for the confusion!

I'm about 60 miles east of Toronto. Where in Canada are you?

[UniqueMachining]

Yes, a #43 will give about 75% thread for a #4-40 screw and a #29 is correct for an #8-32 screw. However, the original poster was contemplating using a 3mm drill (0.118 inch) which is about right for a clearance hole but is far too large for a tap drill since the OD of a #4 screw is only 0.114. In an effort to answer the original query the drills that I mentioned were for close and sloppy clearance rather than tapping. I apologize for the confusion!

I'm about 60 miles east of Toronto. Where in Canada are you?

Should have picked up on that.

I'm 40 minutes north of Toronto in Holland Landing (Just north of Newmarket).
If you are ever in the area feel free to pop in.

John