# Countersinking flat head machine screws



## Philjoe5

I recently needed to countersink some #6 flat head machine screws. I consulted Machinery's Handbook for dimensions and found most of what I needed. They give the screw head thickness and width. In the case of a #6 an "H" drill seems about right for hole width but I wanted to know how deep to go to just sink the screw head below the surface. I can index the z axis toolbit to the workpiece (no DRO, just a piece of paper) and found by trial and error that going 0.12" deep does the trick. Now my question: Has anyone published a table that tells you how deep you have to go for each screw size? That sure would be handy. 

Cheers,
Phil


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## zeusrekning

I just trig it out. I belive imperial is 82 deg and metric heads are 90deg. Tan41=.8692 . Head diameter / 2 = H . So, H/.8692 = c-sink depth. 

Marv is this correct. I'm sure there is an easier way.I seem to always do math the hard way.
Tim


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## BobWarfield

One of the "pro" CNC machinists over on CNCZone gave a worthwhile tip. He just uses his spotting drill as a countersink. This saves a tool change and extra cutter inventory. Since most spotting drills are 90 degrees, not 82 degrees (though you can get 82 degree spotting drills, probably so they can be used in this way!), you will have some error. When I wrote this tip up on my cnccookbook blog, I did a table of the error between the two:







That table is all trigged out for both cases, so that answers your first question.

Second thought after getting this far was to say, gee, if I'm using a 90 degree cutter anyway, I'll just make my depth 1/2 the diameter of the head or a little more. This way, I don't have to refer to a table, I just grab the calipers and see what I need.

See what I mean?

Cheers,

BW


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## Stan

I must be doing something wrong since I haven't found any way to tell within a few thou of where the countersink touches off in the screw clearance hole. I seem to find the correct depth for the first screw by trial and error and then go the same depth for the rest of that size.

BTW: 90 degree countersinks are for 90 degree screws which should be used in thin metal. An incorrect countersink angle has the screw head hanging on the edge or else sitting with no contact at the edge giving very little holding strength.


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## te_gui

Metric flathead capscrew use a 90 degree countersink as well.

Brian


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## BobWarfield

Stan  said:
			
		

> I must be doing something wrong since I haven't found any way to tell within a few thou of where the countersink touches off in the screw clearance hole. I seem to find the correct depth for the first screw by trial and error and then go the same depth for the rest of that size.
> 
> BTW: 90 degree countersinks are for 90 degree screws which should be used in thin metal. An incorrect countersink angle has the screw head hanging on the edge or else sitting with no contact at the edge giving very little holding strength.



Well if you used the spotting drill as the countersink, the hole is countersunk first, and so your touch off is against a flat surface and will be easier.

RE how much less holding strength? It will certainly be somewhat less, but one wonders about it being "very little". After all, Machinery's Handbook says the spec on the bolt head itself allows a variation of 78 to 82 degrees. So an 82 degree countersink is pinned at the high end of the range. This makes me think a couple of things. First, if that much difference is allowable without changing the spec of what the bolt is supposed to be capable of, an extra 8 degrees at the other end probably won't be the end of the world. Second, it's evidently more desirable for the countersink to have a greater angle than the bolt head, hence if I didn't have an 82, I'd rather a 90 than a 60. 

And speaking of 60's, Machinery's Handbook says you should slightly chamfer the bolt clearance hole with a 60 degree tool to account for the slight radius between the socket head cap screw and the threaded portion of the bolt. Learn something new every day!

While I was looking through the Handbook, I saw all kinds of angles. As Tim, and then Brian said, metric is 90. But, Instrument screws in Imperial, for example, are also 90 degrees. Seems like the sort of thing a fellow who worked on small engines might possibly come across. There were a number of others, and all had a range of angles within which the bolt was "in spec."

I even went over to Practical Machinist and had a look around. One group of fellas opined as how most aerospace work uses 100 degree screws. They hold better on thin aircraft skins. As Stan mentions and as the Handbook mentions, more angle = better for thin stuff. Kinda makes you wonder about metric when small fasteners are involved because you have 90 instead of 82.

Times like this it sure is handy to have a grinder and the ability to make the tool do whatever angle you'd like.  ;D

Cheers,

BW


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## Bogstandard

You lot don't half make life difficult for yourselves.

On these small engines, nothing about fasteners is very critical.

If you are doing super large amounts, OK, get the exact and correct countersinking bit. 

But if just doing a few, use a bit of lateral thinking, and skim the underside of the head to whatever your favourite countersinking bit angle is. Job done in ten minutes, rather than scratching your a##e, with brain in neutral, worrying about what angle is best. BTW, it is always better to go for a shallower angle, say for your supposedly 82 deg, go for 75 deg angle countersink.

This might upset the purists, but this is all about getting the job done in an easy and uncomplicated way, and ending up with a safe fixing in the end.

I can now see why some of these engines take tens of years to get built. You are worrying about perfection, when the only one who will really cares about it is yourself. Lighten up a bit and start to enjoy the hobby, rather than getting uptight about if the countersink is 0.001" underdepth, the counterbore for the head is 2 thou oversize, or the clearance hole isn't exactly what is called for. Make your engines rather than beating your gums about them, trying for perfection.

John


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## tel

Well said Bog!


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## Bernd

What Bog's said, now works for me. Makes parts is now faster and funner. :big: :big:

Bernd


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## Stan

I use almost all socket head cap screws for assembly which are too hard to easily skim the angle on the underside of the head. Since I use both 90 and 82 degree screws, I know that if I put a 82 degree screw into a 90 degree countersink, it is an obvious misfit and you see the gap around the head of the screw. I don't know if the plus or minus 2 degrees makes a difference but the 8 degree difference is quite visible.

Although my abilities are quite limited, I am not into mass production of models (no CNC) and I try to do quality work. After twenty years of making a few models a year, I have run out of room and have to give them away or just scrap them. I don't have any desire to make them any faster. I just bought a better lathe so that I can improve the quality but not the quantity.


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## SmoggyTurnip

Flat head screws should be baned from the earth!


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## DickDastardly40

This down load is an addendum for designers etc which can calculate for you, if this method floats your boat, the depth required for countersinks and counterbores dependant upon screw size and included angle amongst other things. I have it on my home computer though I've not used in in anger it looks to be well done. As it is at home and I'm not until tomorrow, I can't have a quick look see to relate what else is included.

http://www.cwattsdesign.com/Sidekick.htm

I'd be interested to hear what other members think.

Al


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## BobWarfield

Here, here, Boggy!

And I like Dastardly's free software. Very nifty.

Cheers,

BW


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## Lew Hartswick

Stan  said:
			
		

> BTW: 90 degree countersinks are for 90 degree screws which should be used in thin metal. An incorrect countersink angle has the screw head hanging on the edge or else sitting with no contact at the edge giving very little holding strength.



Ahem! 100degree heads and countersinks are for thin metal .I've used them often The 90deg, as others
have said, are the metric standard.
  ...lew...


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## Stan

Right you are Lew. I guess I don't have any 90 degree countersinks. They are 100 degree bought from the aircraft industry.


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