Countersink dim 1

[Dockken]

The question is Countersink dim. for a 4-40, 100 deg. c'sink

Helical sharp dim. for this screw is .225

print size callout is .231 + - .005 = .226 to .236

is this a aerospace standed

the low end seams to close to helical Max dim. of .225

or a case of lazy engineering, just use = + - .005

 

[KENAT]

What's the position tolerance or equivalent on the hole?

What's the assembly condition, fixed or floating?

What's the primary design consideration, flush head?

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[Dockken]

What's the position tolerance or equivalent on the hole?


+ - .005

What's the assembly condition, fixed or floating?

Fixed

What's the primary design consideration, flush head?

Flush

 

[KENAT]

So, as it's fixed, what is the position tol on the mating part hole, .005?

If .005 this corresponds to pos tol dia .014 (give or take a tenth of a thou)

So, to guarantee flush you need the minimum counter sink size to be the 'max sharp diameter' of the screw head, + pos tol of csk hole + post tol of threaded hole.

So if your numbers and my assumptions are correct that'd be:

.225+.014+.014 = .253 minimum.

My guess, it's a case of lazy/uneducated engineer.

Note - that while this guarantees flush, it does not optimize stress under the head or anything else, just guarantees the head should always be flush or below flush.

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[Dockken]

Thanks Kenat

You put in black and white the hole picture as I also see it

Dockken

 

[KENAT]

Very drole!

Are you in a position to go back to the person that came up with the .231+-.005 and question/educate them?

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[ewh]

Sharp as ever there, KENAT.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[Dockken]

They have been making it for year that way
If I get an opportunity will try, but they are reluctant to change

 

[KENAT]

Well there's the rub, because of how good a lot of C&C processes are these days etc they're probably hitting near the middle of the tol ranger fairly repeatedly. The above is for worst case.

Plus the head of the screw and/or the material it's going into probably deform a bit.

Plus, unless they're really looking for it they may not notice heads just a few thousandths above flush.

However, there are situations where not being truly flush, or generating more stress under the screw heads can be a real problem.

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[Dockken]

I wish it was that simple to have it done in C&C.

Generly around 1/8 covers, done manualy

no fun

using

Micro-Stop Countersink Units

http://www.newmantools.com/severance/ToolCatalogstopcountersink_nt.pdf

Dockken

 

[KENAT]

OK, made to look silly by assumption. Still I'd think even manual machines would probably tend to be better than +-.005 in a single set up - or are they literally scribed out and drilled?

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[Dockken]

The part is machined to size and thinkness and drilled, 6 to 12 holes per part, may be 20 to 50 parts, but the C'sink is done manualy

dockken