# How To Counterbore?



## RVJimD (Jan 28, 2015)

Guys,

two questions (sorry about all my typos...)

1 how do you put a counterbore on a hole?  Drill the hole, then use a counterbore cutter that has a pilot?  Can this be done accurately without a pilot cutter?  I have drills and end mills.

2 is there a list of this sort of stuff for me to find answers to a simple question like this without bothering everyone?  I put "counterbore" into the search box, but didn't find it on the first page so here I am...


thanks

jim


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## JimDawson (Jan 28, 2015)

RVJimD said:


> Guys,
> 
> two questions (sorry about all my typos...)
> 
> ...




Normally you would drill the hole then use an endmill or a counterbore with a pilot.  The end mill is stiff enough to do the job without a pilot, the normal long shank counterbore generally is not.  There are also drill/counterbore combos that allow you to do the operation in one pass.

The search feature doesn't always work as expected.  As far as I know there is no list.


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## wrmiller (Jan 28, 2015)

I find endmills to work very well for this. I just make sure I don't alter the setup so the counterbore stays true to the drilled hole.


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## JR49 (Jan 28, 2015)

RVJimD,  I, for one, thank you for asking the "simple questions". I wish more members would do so without feeling intimidated. Certainly the "pro" members on here have gone out of there way to assure beginners, like us, not to feel embarrassed, and I thank them for that. Good luck with your counterboring, JR49


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## Wreck™Wreck (Jan 28, 2015)

RVJimD said:


> Guys,
> 
> two questions (sorry about all my typos...)
> 
> ...



If you are making a counterbore for a socket head cap screw an endmill is more then accurate enough. The counterbore has zero effect on the fastened joint, it is simply clearence for the head. SHCS's are cold headed, the concentricity between the threaded portion and the headed portion is not terribly close so a good deal of clearence is recommended. The reason that counterbores with pilots exist if for production work. You're not doing 2000 per day are you (-:


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## Bill C. (Jan 28, 2015)

RVJimD said:


> Guys,
> 
> two questions (sorry about all my typos...)
> 
> ...



In a modern shop the company would furnish a counter-bore. 

You would drill a clearance hole then the counter-bore would have pilot that guides it as the counter-bore cuts the larger hole. I usually drill it a little deeper then the length of the bolt head.  But it you can spot counter-bore as well if the surface is rough or rounded.  If you are cutting one size socket head cap screw then buy one size tool.  They do come in sets as well.


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## RVJimD (Jan 28, 2015)

Wreck,

i hadn't thought about it like that.  Makes it seem pretty simple since the concentricity is not a big concern.

thanks for the help everyone!

Jim


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## robertw78368 (Feb 16, 2015)

RVJimD said:


> Wreck,
> 
> i hadn't thought about it like that.  Makes it seem pretty simple since the concentricity is not a big concern.
> 
> ...


nice airplane


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## RVJimD (Feb 16, 2015)

Robert, thanks!  It took me a long time to build, but it was worth it...


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## FOMOGO (Feb 16, 2015)

Not a lot of experience myself, but why couldn't you use an end mill and cut the counter bore first an then drill your thru-hole. Wouldn't that make sure you were centered as long you didn't move the work piece. I'm sure there's something I'm missing, but that's how we learn. Thanks, Mike


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## bladehunter (Feb 16, 2015)

From my limited experience.

You would have to use a centre cutting endmill or slot drill.
If you drill the hole first there is less material to remove, drills are cheap, end\slot mills not so.
You can fine tune the counterbore depth, set your DRO to zero then easy to have equal depth counterbores.
As I said I have limited experience, and I could be wrong.

BTW I have found these useful:
https://www.carbidedepot.com/formulas-cb-metric.htm
https://www.carbidedepot.com/formulas-cb-inch.htm


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## shorton (Feb 18, 2015)

OK, lets expand this.  I use a 1/4" piloted CB to do a bunch of CB's "freehand" on my mill.  Depth is set but the part "floats" on a table like a drill press.  How does one determine the best RPM for the CB?  In Alu I can usually find one that works, but if I try steel I'm prone to break the CB.  The pilot hole is 1/8".


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## Mark_f (Feb 19, 2015)

I counter bore at a slower speed than I drill usually. Just feed slowly at first and see how it cuts. If necessary, slow it down.


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## Wreck™Wreck (Feb 20, 2015)

FOMOGO said:


> Not a lot of experience myself, but why couldn't you use an end mill and cut the counter bore first an then drill your thru-hole. Wouldn't that make sure you were centered as long you didn't move the work piece. I'm sure there's something I'm missing, but that's how we learn. Thanks, Mike


The less material that you have to remove using a center cutting end mill the better,(also a center cutting tool is not required if the center of the hole is absent) drill the hole first, chamfering the hole at the  bottom of the spotface would be a second operation anyway.


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## FOMOGO (Feb 20, 2015)

Thanks for the explanations, all makes sense. Mike


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## Ulma Doctor (Feb 20, 2015)

mark_f said:


> I counter bore at a slower speed than I drill usually. Just feed slowly at first and see how it cuts. If necessary, slow it down.



Good Info Mark!
Run reamers and counterbores as slow as possible and generously lubricated (unless machining cast iron or brass ,but still turn it slow)


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## 18w (Feb 20, 2015)

shorton said:


> OK, lets expand this.  I use a 1/4" piloted CB to do a bunch of CB's "freehand" on my mill.  Depth is set but the part "floats" on a table like a drill press.  How does one determine the best RPM for the CB?  In Alu I can usually find one that works, but if I try steel I'm prone to break the CB.  The pilot hole is 1/8".



I think you will have better results with the part clamped, not floating. The pilot needs clearance so it doesn't try to seize in the bore also.. Counterbores and countersinks work best if everything is rigid. As everyone has said ,slow speed, moderate feed, and lots of cutting oil.

Darrell


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## Mark_f (Feb 21, 2015)

18w said:


> I think you will have better results with the part clamped, not floating. The pilot needs clearance so it doesn't try to seize in the bore also.. Counterbores and countersinks work best if everything is rigid. As everyone has said ,slow speed, moderate feed, and lots of cutting oil.
> 
> Darrell



Darrell has it right,Definitely clamp the part in position if ,possible, especially if you are using an end mill or the part will scoot around when the end mill hits it, making an off center, crappy counter  bore. ( learned from experience). A piloted counter bore won't do that as easily and does a better job.


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## bfd (Oct 9, 2016)

2 things, the end of an endmill is not flat. there is a slight angle with the outside dia. hitting first. otherwise as you went across the part the inside would rub and overheat. also a bad finish would result. if you were using shcs. then you would point load the area next to the bolt shank. this would cause galling and loss of torque. maybe this would not matter. second thing is to make a flat bottom drill the size of the required counterbore. go slow with this method as the drill will tend to walk around, you can make more hole sizes as drill bits come in many more sizes then endmills. just grind the 118 degree point off and grind back clearance on the flat ends. bill


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## 4GSR (Oct 9, 2016)

Back in the days when we didn't have counterbores and used end mills to create the countersunk hole, we would seat the socket head cap screws at the required torque.  A 1/2 SHCS would get seated using a long Allen wrench and about three to four hits from a engineers hammer.  Break it out, check the seating pattern.  Generally you get a good even pattern across the face of the cap screw.  The back angle of the endmill is so small that it really matches the face angle of the SHCS, which has a slight back angle, too from the upsetting operation when it is made.  You don't notice it on smaller ones as you do on larger ones.  Also got to remember, the SHCS is much stronger/harder than the material that it is seating into.  A yes, it will yield the material a bit to seat correctly.  In over 40 years, I have never seen this come up in question as being an issue.  Ken


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## uncle harry (Oct 10, 2016)

FOMOGO said:


> Not a lot of experience myself, but why couldn't you use an end mill and cut the counter bore first an then drill your thru-hole. Wouldn't that make sure you were centered as long you didn't move the work piece. I'm sure there's something I'm missing, but that's how we learn. Thanks, Mike



If you drill  first you can use a non-center cutting end mill and cut more quickly and with less power. 

Edit:   I replied before moving on. Sorry for the duplication.


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## Andre (Oct 10, 2016)

If you're using a drill press, use the piloted counterbore. The endmill will just chatter and wobble around with most dill presses, creating a eccentric hole.


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## 4GSR (Oct 10, 2016)

One thing I forgot to mentioned.  When you use a end mill to make a counterbore with.  Especially one that is a little dull on the ends, lock your X-Y table locks on your mill.  Things can get a little excited and the end results, well lets say it still worked.  Ken


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## T Bredehoft (Oct 10, 2016)

Andre above mentioned it, without a pilot you/we can't counter drill with a drill press. An end mill will only work in a milling machine. (or a lathe.)


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