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rotate
07-16-2008, 10:52 AM
I'm wondering how you guys calculate the depth of countersink. I need the screw to be flush with the surface (more for aesthetics than function) and no matter how much careful measurements I do, I find that it's either too deep or too shallow. I've gotten into the habit of making a sample hole on a scrap and incrementally increasing the depth until it's just right then using that measurement. Strange enough, I find that the particular fastener that I'm using affects the depth so that I can't keep reuse the measurement.

Also, I find that when I tighten the screw down tight it sinks a little bit and I can feel the step with my finger.

Note that I'm using brass fasteners,

SGW
07-16-2008, 11:44 AM
I do it your way (by trial and error) until I get it right.

mechanicalmagic
07-16-2008, 11:52 AM
I have an early version of this:
http://www.mma-engsupport.com/5013.htm

It has an internal sliding sheet, so you get ALL the info about a single size screw at once. #0 to 1".

I wouldn't sit down to do a drawing without it.

Dave

Chipslinger
07-16-2008, 05:07 PM
If they are flathead screws, flip one over and see how it sets on the hole.

ERBenoit
07-16-2008, 05:32 PM
I'm wondering how you guys calculate the depth of countersink. I need the screw to be flush with the surface (more for aesthetics than function) and no matter how much careful measurements I do, I find that it's either too deep or too shallow. I've gotten into the habit of making a sample hole on a scrap and incrementally increasing the depth until it's just right then using that measurement. Strange enough, I find that the particular fastener that I'm using affects the depth so that I can't keep reuse the measurement.

Also, I find that when I tighten the screw down tight it sinks a little bit and I can feel the step with my finger.

Note that I'm using brass fasteners,

I do it the same way as SGW.

Remember that screws are mass produced and have a fairly liberal tolerance on head diameters. Very unlikely that a hand full of screws, even from the same lot will be EXACTLY the same.

For example from Machinerys Handbook the head diameter on a 1/4" FHSCS ranges anywhere between .480" (absolute minimum diameter) and .531" (maximum diameter which would be theoreticaly a sharp edge). Anywhere in between is acceptable.

There are different tolerances on head diameters based upon the drive style. Slotted or phillips drive have different tolerances than spline or hex drives.

Rich Carlstedt
07-16-2008, 09:26 PM
Head thickness depends on screw manufacture.
For perfect flush heads, individual fit is best.
If you countersink has a sharp point, you can touch off the top of the part, and figure your depth by taking the desired finish size, and multiplying that by 42 % for 100 degree heads, or 58 % for 82 degree heads, in order to get the depth needed
Example
For a 1/2" diameter 100 dgreee= .5 x .42= .210 " deep

Scratch this unto your cutters

Rich

Evan
07-16-2008, 10:33 PM
For setting flat head socket cap screws you need to use a countersink the same size as the screw head. The heads are standard fractional sizes for SAE parts and countersinks are made to the same sizes. Usually, to set a FSHCS flush requires sinking beyond the angle and producing a small counterbore to accommodate the head. I use a LOT of flush cap screws and find small variations from batch to batch even from the same manufacturer. I haven't tried screws from mainland China, the ones I buy are made mainly in Taiwan.

If I have a lot to do I do a trial hole and set the stop on the drill press.