"Best" way to drill and coutnerbore for a 1/2-20 bolt?

[projectnut]

If you decide to go with a counterbore keep in mind, there are 2 different sizes for SAE socket head bolts. The "close fit" size is .015" over the size of the head, and the "standard fit" size is .032" oversize. Back in the 1980's I needed a set for a shop I was supervising. At the time I'd never heard of "close fit" counterbores. I don't know if anyone sells them today, but they do come up on eBay every once in a while.

 

[LVLAaron]

Good call out. I referenced this chart, and found bores/drills to match "loose"

Tap & Clearance Drill Sizes

This tap and clearance drill chart shows tap and clearance drill sizes for standard threads. There is also a printable version that has both metric and inch threads.

littlemachineshop.com

 

[RJSakowski]

I have successfully counterbored in cast iron with a spade bit (normally used for wood) while installing the DRO on my Mill. They were shallow cuts, but I felt I could have gone deeper if needed. The spade bit was still usable for wood afterwards.
View attachment 400777

That must be a super spade. If one of mine hits a nail, it curls up.

Any high carbon steel can be heat treated to create a decent cutting edge. This increase brittleness though, and manufacturers tend to error on the side of softness.

I have taken the crappy 5mm Allen wrenches they send with self assembly office chairs which don't make it through their intended use without twisting or wearing down the corners and hardened and tempered them to make a perfectly usable wrench. I've done the same with cheap screw drivers and punches. If a spark test indicates a high carbon steel, the tool can be heat treated to perform.

 

[cathead]

So, what is the purpose of the counter bore hole?

 

[LVLAaron]

Making a back plate for a direct mount fixture.

 

[Eddyde]

That must be a super spade. If one of mine hits a nail, it curls up.

Any high carbon steel can be heat treated to create a decent cutting edge. This increase brittleness though, and manufacturers tend to error on the side of softness.

I have taken the crappy 5mm Allen wrenches they send with self assembly office chairs which don't make it through their intended use without twisting or wearing down the corners and hardened and tempered them to make a perfectly usable wrench. I've done the same with cheap screw drivers and punches. If a spark test indicates a high carbon steel, the tool can be heat treated to perform.

It was just one I had in the miscellaneous, wood bit drawer, I picked an old one that didn't have the spurs on the outer edges, like the fancy newer ones do. Otherwise, I did nothing to it, of course I used a slow speed and feed.

 

[brino]

It was just one I had in the miscellaneous, wood bit drawer, I picked an old one that didn't have the spurs on the outer edges, like the fancy newer ones do. Otherwise, I did nothing to it, of course I used a slow speed and feed.

Very interesting......
I have only seen the spurs on the spade bits I got from my grand-father-in-law.
I thought they were fancy old ones!

Brina

 

[Eddyde]

Very interesting......
I have only seen the spurs on the spade bits I got from my grand-father-in-law.
I thought they were fancy old ones!

Brina

Yeah, my time reference might be a few decades off, I bought my first spade bits in the late 1970's, they were the spurless variety...

 

[projectnut]

Both styles are available currently at Home Depot. Most of the Bosch, Diablo, and Hilti blades they offer have the spurs, while the Dewalt ones don't.

 

[Winegrower]

I’m still thinking about screws fully threaded and bolts partially threaded. So I guess I have a box or two of 3” wood bolts.