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



## LVLAaron (Mar 16, 2022)

Working on this D1-5 backplate. I need to drill clearance holes and counterbore for the bolt head. For either 3/8-24 or 1/2-20 (haven't decided yet) machine bolts. 

What's the "best" process for this? is this a two step op, clearance hole with one tool, counterbore with another? or is there a specific tool for the job? If not, what tool to drill the counterbore? HSS end mill? 

Backplate is cast iron fwiw


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## ttabbal (Mar 16, 2022)

I know they make counterbore tools for socket head screws. I would imagine there would be something for standard hex head if that's what you're after. I usually just use an end mill though.


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## benmychree (Mar 16, 2022)

Two drills.


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## brino (Mar 16, 2022)

Counterbore sets are readily available. I have no direct experience with these sets but here's a few listings:

https://www.amazon.com/Accusize-Ind...eywords=counterbore+set&qid=1647448087&sr=8-3

https://www.amazon.com/AccusizeTool...eywords=counterbore+set&qid=1647448087&sr=8-5

I like to do it in this order:
1) drill the guide hole for the counterbore pilot
2) do the counterbore to depth required
3) drill final thru-hole

Brian


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## LVLAaron (Mar 16, 2022)

brino said:


> Counterbore sets are readily available. I have no direct experience with these sets but here's a few listings:
> 
> https://www.amazon.com/Accusize-Ind...eywords=counterbore+set&qid=1647448087&sr=8-3
> 
> ...



Nice. That's what I figured. I found some USA made tools on MSC direct. Wish me luck!


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## RJSakowski (Mar 16, 2022)

My counterbores all have a pilot bushing the diameter of a standard clearance hole and cut a bore for a socket head cap screw.  I am not aware of any that will cut a counterbore for a hex head cap screw.


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## benmychree (Mar 16, 2022)

brino said:


> Counterbore sets are readily available. I have no direct experience with these sets but here's a few listings:
> 
> https://www.amazon.com/Accusize-Ind...eywords=counterbore+set&qid=1647448087&sr=8-3
> 
> ...


Good advice! I guess I missed the part about counterboring for the (presumably) Allen capscrew.


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## Mitch Alsup (Mar 16, 2022)

Abomb79 uses center cutting end mills to counterbore his recessed cap-head screws.

But since you are working on D1-x fitment, you should use the D1-x threads.
But, also note: you should "index' the start of the thread so that after x turns each D1-x attachment is equally deep in its hole.
That is you want each thread to start at the same relative angle.


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## Parlo (Mar 16, 2022)

RJSakowski said:


> My counterbores all have a pilot bushing the diameter of a standard clearance hole and cut a bore for a socket head cap screw.  I am not aware of any that will cut a counterbore for a hex head cap screw.


There is no such thing as a hex head cap screw. It's either a cap head screw or a hex head.
It's not common practice to counterbore for hex head screws as a spanner won't have access and no standard for square drive socket diameters.


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## RJSakowski (Mar 16, 2022)

Parlo said:


> There is no such thing as a hex head cap screw. It's either a cap head screw or a hex head.





			https://www.grainger.com/category/fasteners/bolts/hex-head-bolts/standard-hex-head-cap-screws


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## aliva (Mar 16, 2022)

Cap screws usually have threads the entire length of the bolt. Bolts have threads on roughly 1/3 - 2/3 the bolt length


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## Parlo (Mar 16, 2022)

RJSakowski said:


> https://www.grainger.com/category/fasteners/bolts/hex-head-bolts/standard-hex-head-cap-screws


Lost in translation, in the uk, cap screw usually refers to socket head cap screws only. Hexagon head screws and bolts are usually called hex head.


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## RJSakowski (Mar 16, 2022)

aliva said:


> Cap screws usually have threads the entire length of the bolt. Bolts have threads on roughly 1/3 - 2/3 the bolt length


From Machinery's Handbook, 25th ed., pp. 1417: "A bolt is an externally threaded fastener designed for insertion though holes on assembled and is normally intended to be tightened or released by torquing a nut.   .....   A screw is an externally threaded fastener capable of being inserted into holes in assembled parts, of mating of mating with a preformed internal thread or forming its own thread and capable of being tightened or released by torquing the head."

Both Fastenal and Grainger sell both hex head screws and hex head bolts in full or partial thread configurations.  For some reason, McMaster Carr doesn't sell hex head bolts, only hex head cap screws in either full or partially threaded configuration They do sell carriage bolt which are fully threaded for the most part.   Go figure.


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## LVLAaron (Mar 16, 2022)

FYI... this is what I'm talking about 





And will be using this. https://www.mscdirect.com/product/details/43983675


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## Mitch Alsup (Mar 16, 2022)

RJSakowski said:


> From Machinery's Handbook, 25th ed., pp. 1417: "A bolt is an externally threaded fastener designed for insertion though holes on assembled and is normally intended to be tightened or released by torquing a nut.   .....   A screw is an externally threaded fastener capable of being inserted into holes in assembled parts, of mating of mating with a preformed internal thread or forming its own thread and capable of being tightened or released by torquing the head."
> 
> Both Fastenal and Grainger sell both hex head screws and hex head bolts in full or partial thread configurations.  For some reason, McMaster Carr doesn't sell hex head bolts, only hex head cap screws in either full or partially threaded configuration They do sell carriage bolt which are fully threaded for the most part.   Go figure.



If you torque it from the head its a screw
If you torque it from a nut it is a bolt

So, what is the design difference between a hex-head bolt and a hex head screw?
Is there some fillet that is on the screw and not the bolt? or vice versa ?
Is there some relief that is on the screw and not the bolt? or vice versa ?
Do you use a different washer on the screw or bolt.
Or is this something that has passed into history?


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## RJSakowski (Mar 16, 2022)

Mitch Alsup said:


> If you torque it from the head its a screw
> If you torque it from a nut it is a bolt
> 
> So, what is the design difference between a hex-head bolt and a hex head screw?
> ...


I think your last option is most likely.  The original meaning of "bolt" was a short arrow as in a crossbow bolt.  It came to mean a short metal pin used to fasten objects together in the 1400's.  Adding a head and threads was an obvious improvement.

Meanwhile, the screw traces its roots to antiquity.  It was used in Roman times as a fastener.  At some point around the eighteenth century, the two came together and the distinction fades.  It is interesting that plow bolts and carriage bolts are still called bolts.  They definitely fit the above description as the square shoulder under each prevent turning the head.

I don't know of any physical distinction between bolts and screws.


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## Parlo (Mar 17, 2022)

Mitch Alsup said:


> If you torque it from the head its a screw
> If you torque it from a nut it is a bolt
> 
> So, what is the design difference between a hex-head bolt and a hex head screw?
> ...


The most common reference is that a screw is threaded to the head and a bolt has a plain portion, any other references have no practical use.


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## Bi11Hudson (Mar 17, 2022)

Without going into technical definitions, as a rule screws and bolts are the same *except*: Screws, or machine screws are described as such when they are sized to machine screw dimensions, size 0 to 16, and have screwdriver fitted heads. Bolts are such for fractional sizes and using a wrench. There is a fuzzy area around the transition, my father used 1/8 inch 'stove' bolts that used a screwdriver. And I use machine screws, sized 0.047 or Nr 00, that have a hex head. Over time, I think the original definition has been lost. Bolts are called 'cap screws' by many, including myself. Socket head 'screws' have further muddied the water. I call 'em what I want to call 'em, and don't get hung up on technical names.


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## cathead (Mar 17, 2022)

My best way is to use an end mill after drilling the hole.   I use one set up and have my DRO on so I mill in the right spot. 
One person's best way is not necessarily another person's best way depending on what tools one has at hand.  If someone
offered to give me a bunch of counter bores or a bunch of end mills, I would go for the end mills for sure.


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## Eddyde (Mar 17, 2022)

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.


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## projectnut (Mar 17, 2022)

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.


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## LVLAaron (Mar 17, 2022)

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


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## RJSakowski (Mar 17, 2022)

Eddyde said:


> 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.


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## cathead (Mar 17, 2022)

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


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## LVLAaron (Mar 17, 2022)

Making a back plate for a direct mount fixture.


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## Eddyde (Mar 17, 2022)

RJSakowski said:


> 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.


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## brino (Mar 17, 2022)

Eddyde said:


> 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


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## Eddyde (Mar 18, 2022)

brino said:


> 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...


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## projectnut (Mar 18, 2022)

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.


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## Winegrower (Mar 18, 2022)

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.


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## Parlo (Mar 19, 2022)

projectnut said:


> 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.


Does the close fit refer to the size of the pilot on the counterbore & hence the size of the hole. Having a closer fit on the head serves no mechanical advantage whereas a closer fit on the bolt diameter gives less movement and more head engagement.


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## projectnut (Mar 19, 2022)

Both the pilot and the cutting section of the counterbore are smaller diameter on a "close fit" counterbore.  If I remember correctly the "close fit" pilot requires a hole .015" larger than the bolt, and the cutter bores a hole .015" larger than the socket head.  The standard fit has a pilot .032" over the nominal size of the bolt and the cutter bores a hole .032" larger than the socket head.


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## epanzella (Mar 23, 2022)

For a cap screw the counterbore also needs to be big enough to accommodate a socket so my piloted counterbores never seem to be big enough. I usually wind up using an end mill for the counterbore. I like a generous counterbore so I don't need a thin wall socket (if there's room, of course).


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## MrWhoopee (Mar 23, 2022)

epanzella said:


> For a cap screw the counterbore also needs to be big enough to accommodate a socket so my piloted counterbores never seem to be big enough. I usually wind up using an end mill for the counterbore. I like a generous counterbore so I don't need a thin wall socket (if there's room, of course).


Counterbores are sized for Socket Head Cap Screws, not Hex Head Cap Screws. Counterboring for HHCS is not that common and generally avoided because of wrench access problems.

I have a set of the "loose" counterbores. They frequently frustrate me because I like tighter fits. I end up using endmills a lot.


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## projectnut (Mar 23, 2022)

epanzella said:


> For a cap screw the counterbore also needs to be big enough to accommodate a socket so my piloted counterbores never seem to be big enough. I usually wind up using an end mill for the counterbore. I like a generous counterbore so I don't need a thin wall socket (if there's room, of course).


That's the advantage of having a set of counterbores that use interchangeable pilots.  You can put a 1/4" pilot in a 3/4" counterbore.



			https://www.grainger.com/category/machining/drilling-holemaking/counterbores-port-tools/counterbores-with-interchangeable-pilots


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## LVLAaron (Mar 23, 2022)

projectnut said:


> That's the advantage of having a set of counterbores that use interchangeable pilots.  You can put a 1/4" pilot in a 3/4" counterbore.
> 
> 
> 
> https://www.grainger.com/category/machining/drilling-holemaking/counterbores-port-tools/counterbores-with-interchangeable-pilots



Nice. I can see myself with a few of those in the future.


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## Cadillac (Mar 23, 2022)

To me the difference between a screw and a bolt is the head. A screw has a flat head or Phillips drive. A bolt has a hex. That’s my logic?
 You can get fully threaded bolts all day.


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