Question regarding counterbores.

[Ccarrigan]

Hi
I purchased a set of metric and standard counterbores. When I went to use the 10 mm it would not fit in the 10 mm hole I had drilled. I measured the pilot and found they are all larger than the stated bore size. Is this normal and if so I think I am missing something in the procedure to counter bore a 10. mm hole. ( eg. the 10 mm bore pilot measures 10.75 mm) Any help would be appreciated. The counterbores are made by accusize.

 

[migrovon]

Hi,
the 10mm is the rated thread size - the corresponding through bole size is bigger and so is the pilot of the counterbore.
Durchgangsloch bzw. Durchgangsbohrung M1 bis M64 <- shows a table of through hole sizes. To make it a little more complicated there are 3 qualities of through holes (fein = fine, mittel = medium grob = coarse).
Regards
Thomas (from the metric part of the world)

 

[danallen]

Most counterbores have a pilot sized to fit a slightly oversize hole. Most holes for cap screws are drilled a bit over the nominal size of the screw.

 

[BaronJ]

Hi Guys,

Cap head screws have a radius under the head so the hole size is enlarged so that the flat parts sits squarely together. You should drill clearance size. Though I often cheat and use an ordinary drill just for the head and rely on the drill cone to create the clearance. If I really need a flat bottom I use a counterbore.

 

[timo_gross]

Hello,

Is this normal and if so I think I am missing something in the procedure to counter bore a 10. mm hole. ( eg. the 10 mm bore pilot measures 10.75 mm) Any help would be appreciated. The counterbores are made by accusize.

It seems it is normal. My sequence, e.g. M6 clamp.

1. First spot drill
2. 6.2 mm dia drill 10 mm deep
3. 5 mm dia drill 20 mm deep
4. Counterbore deep enough to hide the head of bolt
5. Champfer top of counter bored hole
6. Tap with spiral flute tap
7. Cut slot a little above thread

So seven tools for one measly hole...
The commercial counterbores I found in my tool box are all 0.2 mm over the thread nominal size. M3 3.2 mm, M4 4.2 mm, M5 5.2 mm and so on.
But it is mentioned on the lable of the plastic tube.
My book says M10 clearance hole, fine 10.5 mm medium 11 mm and coarse 12 mm.

Greetings Timo

 

[Ccarrigan]

Thank you very much. Learned something new today from all of you.

 

[Michel ROBIN]

Hello,
I am a new member and french, for this raison my english is poor.
I understand that Ccarrigan has problems with conterbore sizes. I join you a file with these sizes (metric and imperial).
Best regards
Michel

 

[Ccarrigan]

Thankyou Michel
In English as well, many thanks

 

[SmithDoor]

I use count bores be for tapping.
The tap can make smaller hole size after tapping.
You can re-drill hole too.

Dave

Hi
I purchased a set of metric and standard counterbores. When I went to use the 10 mm it would not fit in the 10 mm hole I had drilled. I measured the pilot and found they are all larger than the stated bore size. Is this normal and if so I think I am missing something in the procedure to counter bore a 10. mm hole. ( eg. the 10 mm bore pilot measures 10.75 mm) Any help would be appreciated. The counterbores are made by accusize.

 

[Brian Rupnow]

Something you may not know--Counterbore tools come with different size pilots on the end. How do I know this?--Because I had bought a couple of counterbore tools that had a pilot larger than a standard clearance hole. I "fixed" them by grinding down the pilots to fit my standard clearance holes.