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I've been looking for a set of metric counter-bore cutters for cap screws and I'm befuddled by the options. What to choose for "general use"?
2 flute, 3 flute, 4 flute? KEO has fixed pilots but come in three different sizes?
Before thread, Fine, Medium. So I guess you have to decide if you want to cut all your counter-bores before or after threading the hole all the time? I'm certainly not going to buy three different sets, that's for sure.
How do you guys deal with it?
My Enco set of Imperial counter-bore cutters came in one size pilot diameter. Take it or leave it. 
Willie
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I use these types of counterbores I get from McMaster.
https://www.mcmaster.com/counterbores/me...nterbores/
I buy them as I need them. I think I have them for #4-10 screws, 1/4" & 5/16" and maybe a metric one or two. Going from memory so might not be accurate.
They are pricey but once you've used one you'll want more. 
Ed
EDIT: Oops! These won't work for threaded holes. The pilot size is larger than the bolt size.
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I can't think of very many reasons to counter-bore a threaded hole. Counter-bores are mostly used to sink the head on through screws, normally socket head cap screws. I have a four flute set that is 1/64" under, with nominal size pilots, for screws up to 1/2" diameter. That's pretty much all I use. For anything larger than 1/2", I would bump the size up to 1/32" over.
Tom
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I'll agree with Tom, never heard of counterboring a threaded hole. ??????
I have a few old ones I've collected here and there. A couple have replaceable pilots, I like those, can make a pilot for any hole then.
Rarely use them, almost always use an end mill after I've drilled the hole.
Free advice is worth exactly what you payed for it.
Greg
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(05-01-2020, 01:59 PM)EdK Wrote: I use these types of counterbores I get from McMaster.
https://www.mcmaster.com/counterbores/me...nterbores/
I buy them as I need them. I think I have them for #4-10 screws, 1/4" & 5/16" and maybe a metric one or two. Going from memory so might not be accurate.
They are pricey but once you've used one you'll want more.
Ed
EDIT: Oops! These won't work for threaded holes. The pilot size is larger than the bolt size.
Interesting. I thought those short ones were "porting" tools?  For installing fittings that need a flat sealing surface for o-rings and such? Maybe they just look similar. Obviously there is more to these things than I realized. 
But the drill tip got me thinking because it looked familiar and I had to go look at my set. What I have now are actually step drills - or I guess you could call them drill / counter bore combination bits. Just a longer version of what you have Ed. 
Willie
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(05-01-2020, 04:48 PM)TomG Wrote: I can't think of very many reasons to counter-bore a threaded hole. Counter-bores are mostly used to sink the head on through screws, normally socket head cap screws. I have a four flute set that is 1/64" under, with nominal size pilots, for screws up to 1/2" diameter. That's pretty much all I use. For anything larger than 1/2", I would bump the size up to 1/32" over.
Tom
Yes I think that is where I was getting things confused. The descriptions for these things are non-existent. They say things like "before thread" 1/64" over or 1/32" over (FINE AND MEDIUM???) whaaaaa???
I thought they were talking about pilot sizes but apparently they mean the OD size of the cutter?
I went and ordered the only metric set that McMaster sells so I'll get what I get I suppose. Then a couple hours later I saw a good deal on an imperial set on Amazon so I bought that too. My imperial step drills that I been using never seem to have the right pilot size and/or the right diameter to closely match the diameter of a socket head screw. Especially metric ones! 
Hopefully having the right cutters will make things easier in the future.
Willie
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This is what I think of for a step drill
![[Image: step-drill.jpg]](http://metalshop.nobucks.net/pix/step-drill.jpg)
Is this what you've been using?
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No, although I do call those step drills as well. I always thought those were meant for sheet metal and thin plastic but I see people using them all the time for everything under the sun, even in thick metal. 
The ones I have been using are like these.
Willie
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Ohhh, them things. I have a couple of them around here. Don't remember if I ever used them tho.
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The following 4 users Like Highpower's post:
TomG (05-06-2020), EdK (05-06-2020), f350ca (05-06-2020), arvidj (05-06-2020)
The metric set I ordered from McMaster showed up. I've never heard of Alvord-Polk before but they look decent. Seem to be ground well. Made in the USA.
Willie
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