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Holes for countersunk screws 1
- Thread starter kolvos
- Start date
- Thread starter
- #3
Essentially yes.
If you are working in thin material you need to consider the head thickness also. The "k" dimension here:
If you are working in thin material you need to consider the head thickness also. The "k" dimension here:
The countersink center will be coaxial with the hole. I BELIEVE the countersink diameter needs to be equal to the MMC diameter of the screw head (across sharps) plus the positional tolerance diameter of the hole. The 90 degree angle is normally assumed to be exact. The pt needs to be calculated using the floating- or fixed-fastener formula depending on which case you have. Check me on this people!
Tunalover
Tunalover
powerhound
Mechanical
Countersunk clearance holes basically constitute double fixed fasteners so calculations don't work. In other words, it doesn't matter what size you make the clearance hole, the fastener will center up on the countersink.
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
John,
What if the screws are going through the part and straight into a threaded hole? In this case the countersink diameter must account for the locus of the mating hole. The minimum LMC countersink diameter is then of diameter H=C+T where C=the MMC diameter of the screw head (across sharps) and T the pt of the threaded hole at MMC. If the minimum clearance hole diameter is F+T then the countersink diameter must be C+T.
Tunalover
What if the screws are going through the part and straight into a threaded hole? In this case the countersink diameter must account for the locus of the mating hole. The minimum LMC countersink diameter is then of diameter H=C+T where C=the MMC diameter of the screw head (across sharps) and T the pt of the threaded hole at MMC. If the minimum clearance hole diameter is F+T then the countersink diameter must be C+T.
Tunalover
- Thread starter
- #11
Increasing the diameter of the countersink to compensate for mis-position either allows the shank to bend more or the fit to be worse with only line contact between the head and the countersink.
I think the better option is to independently control the countersinks with a profile tolerance to control form and orientation as well as locate them. However, few people like doing detailed inspection on countersinks for screws.
I think the better option is to independently control the countersinks with a profile tolerance to control form and orientation as well as locate them. However, few people like doing detailed inspection on countersinks for screws.
6.94 x 90deg. Hole 3.4
Scroll to the bottom.
Scroll to the bottom.
Just noticed that DIN 7991 is now EN ISO 10642.
Therefor the table listed by DINSTOCK seems to be correct.
The c'sink for M3 is as per DIN 74, 9.94 x 90 deg.. 3.4 hole.
Now you need to specify DIN 7991 or EN ISO 10642.
The DIN standard has a tighter limit on the dia. of the screw head. Also the height of the head differs somewhat from EN ISO 10642. This should not be a problem for the c'sink.
Therefor the table listed by DINSTOCK seems to be correct.
The c'sink for M3 is as per DIN 74, 9.94 x 90 deg.. 3.4 hole.
Now you need to specify DIN 7991 or EN ISO 10642.
The DIN standard has a tighter limit on the dia. of the screw head. Also the height of the head differs somewhat from EN ISO 10642. This should not be a problem for the c'sink.
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1
- #17
As others say you need to take into account the tolerances of position - potentially of both the csk hole & the mating threaded hole (if it is fixed in the mating part). This assumes you're important goal is to keep the head flush or under flush. It may still introduce stress to the screw which is an inherent issue with csk screws in fixed-fixed patterns.
Posting guidelines faq731-376 (probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484
What is Engineering anyway: faq1088-1484
kolvos,
Where is this countersink going to be located?
If something is sitting on top of the flat head screw, you need the screw to sit invariably below flush. If you are using the flat head screw because it looks cool, you want to set as close to flush as possible. This will affect your tolerances. I do not have any size tables with me at the moment. The size tables I am aware of show that flat head screws, especially small ones, have sloppy diameter tolerances. I have been told my machine shops that if I want stuff like this fabricated properly, I should give them the screws.
--
JHG
Where is this countersink going to be located?
If something is sitting on top of the flat head screw, you need the screw to sit invariably below flush. If you are using the flat head screw because it looks cool, you want to set as close to flush as possible. This will affect your tolerances. I do not have any size tables with me at the moment. The size tables I am aware of show that flat head screws, especially small ones, have sloppy diameter tolerances. I have been told my machine shops that if I want stuff like this fabricated properly, I should give them the screws.
--
JHG
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