Drill bit sizes

[Moonlite37]

In some magazine such as Popular Mechanics, the writer stated sizes that actually were not needed, as they were not the proper size for drilling holes for tapping or clearance holes for bolts to go through; for example the 1/4 drill bit could be replaced by some other size bit. By theory this was exactly right, but most of us use the so-called unneeded bits are frequently used. I have number size bits. letter size and fractional. plus a drawer full of mixed sizes. Most of my drilling is with sizes I have convenient on a small shelf at each drill press. If someone would ask what sixe they were I would not be able to tell them. They are oftentimes abused and sometimes used in hand held drills. I only use my better bits when drilling holes to be tapped.

 

[Brian G. NY]

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So......I have been wrong all these years for using a 1/4" drill bit to drill a hole for a 1/4" bolt?

 

[Steve@Advance]

Brian, not necessarily wrong, just making things harder than the need be.

The typical clearance size for a bolt is 1/32" over the bolt size.

It just give you some wiggle room in case the alignment isn't exactly on, and if the bolt is tightened down without a washer, it can flare the hole inward, making the hole even tighter.

 

[Moonlite37]

My drill index has about a dozen 1/4 bits but no spare #7. I always put the #7 back in the drill index and not worry about the 1/4 as I have more. I have a fractional set I take to the field and leave letter and number in the shop, Also only lend the fractional sizes because neighbors do not know anything about numbered sizes and letter sizes. Also never leave letter or number sizes in drill press or hand held drill.

 

[MarkB_MI]

It's pretty difficult to drill a hole that's not oversize. So drilling with an undersize bit, then redrilling with the correct size drill (or better yet, a reamer) will result in a tighter hole.

 

[Dick L]

I have never worried about a 1/4" and larger fractional holes being slightly oversize when being drilled for bolts to hold two or more pieces together. When it is necessary to hold two pieces or more in alignment I hold the pieces clamped to an angle plate or flat edge and drill undersize and ream using a shoulder bolt or a dowel pin away from the normal oversize bolt hole if there is room.

 

[4wdtom]

The fit or clearance is dependent on what you are doing and how precise you want it to be.

 

[Little Ed]

The recommended tap drills are for 75% thread engagement. If I am drilling Ampco bronze I usually drill the hole to a 60%-65% engagement, to avoid tap breakage. Most of what I do with it requires hand drilling and tapping in #6 and #8 bolt sizes. If working in teflon or delrin, I close the hole up to a few thousandths smaller for more holding power.