Hi gang,
I am acquiring quality drill bits for general use but mostly to fabricate and repair low carbon steel, cast iron and aluminum.
I have a drawer full of bits but, I recently purchased a good set of fractional Rushmore drill bits (short-the name eludes me). I also buy quality tap and threading dies.
My next purchase is to fill in the gaps for lettered and numbered bits.
Example, to drill a 1/4" hole, there are 6 possible drill bits required;
For 75% Thread in Aluminum, Brass and Plastics>>
1/4 x 20 .1887 =#7
1/4 x 28 .2062 =#3
1/4 x 32 .2117 = 7/32
For 50% Thread in Steel, Stainless and Iron>>
1/4 x 20 =7/32 (.2188)
1/4 x 28 .2280 =#1
1/4 x 32 .2280 =#1
For Close Fit,
Drill size .2570=F
For Free Fit,
Drill Size .2660 =H
It makes no sense to buy cheap because the drilled hole will not be round, it's anyone's guess what size you end up with.
This leads me to my confusion.
Do I really need to follow the printed guides? Is there a rule of thumb from you experienced machinists to follow?
For example, when drilling a hole to be tapped in (steel/iron) either 1/4 x 20 or 1/4 x 28, how important is the .009" difference in hole size?
When using a hand tap and watching that thing twist, the .009" seems quite significant! (don't break!)
I look forward to your advice, opinions.
Thanks,
Jeff
I am acquiring quality drill bits for general use but mostly to fabricate and repair low carbon steel, cast iron and aluminum.
I have a drawer full of bits but, I recently purchased a good set of fractional Rushmore drill bits (short-the name eludes me). I also buy quality tap and threading dies.
My next purchase is to fill in the gaps for lettered and numbered bits.
Example, to drill a 1/4" hole, there are 6 possible drill bits required;
For 75% Thread in Aluminum, Brass and Plastics>>
1/4 x 20 .1887 =#7
1/4 x 28 .2062 =#3
1/4 x 32 .2117 = 7/32
For 50% Thread in Steel, Stainless and Iron>>
1/4 x 20 =7/32 (.2188)
1/4 x 28 .2280 =#1
1/4 x 32 .2280 =#1
For Close Fit,
Drill size .2570=F
For Free Fit,
Drill Size .2660 =H
It makes no sense to buy cheap because the drilled hole will not be round, it's anyone's guess what size you end up with.
This leads me to my confusion.
Do I really need to follow the printed guides? Is there a rule of thumb from you experienced machinists to follow?
For example, when drilling a hole to be tapped in (steel/iron) either 1/4 x 20 or 1/4 x 28, how important is the .009" difference in hole size?
When using a hand tap and watching that thing twist, the .009" seems quite significant! (don't break!)
I look forward to your advice, opinions.
Thanks,
Jeff
Last edited:
