Interference issue when adding flat head cap screws

[dtharrett]

When adding standard flat head scews to an NX countersunk screw clearance hole feature, the flat head screws do not sit "flat" or flush (they protrude from the surface). The screws I am using conform to ANSI/ASME standards. For example. I have a 6-32 FHCS which has a head c-sink diameter of .307". The hole feature for a #6 (ANSI inch setting) has a c-sink diameter of .279".

The work around I have been using is to alter the hole feature ("custom" selection) and manually enter the appropriate c-sink diameter.

Is this the generally accepted way to get a c-sink hole feature to fit a standard flat head screw?

 

[looslib]

Adjust the hole table files to what you want them to be. I found that most of the dimensions are theoretical in size and needed adjusting to shop sizes.


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[JohnRBaker]

Where are you getting your Flat Head screw models from?

I just checked our Machinery Library and the ANSI standard 82° Flat Head Machine Screw which we supply has a head diameter of 0.262, which is within the tolerance allowed according to the Machinery's Handbook (note that I'm working from home at the moment and only have access to a 1953 copy, but I've got the latest copy at the office which I'll check later). Per the handbook, an American Standard 82° #6-32 Flat Head Machine Screw has a maximum/minimum head diameter of 0.279/0.257.

John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA

http://www.siemens.com/plm

http://www.plmworld.org/museum/

To an Engineer, the glass is twice as big as it needs to be.

 

[dtharrett]

Hmm,
I might be missing something but my machinery handbook is showing .307/.263 for the head diameter of a #6. I am referencing the 28th edition on pg 1590 which references ANSI/ASME. These are the same numbers I am getting from Holo-Krome & McMaster as well...

 

[JohnRBaker]

Like I said, I was referencing the 1953 edition which referred to what was then called the "American Standard (revised 1947)" but will check the latest copy of the handbook when I get to my office later.

John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA

http://www.siemens.com/plm

http://www.plmworld.org/museum/

To an Engineer, the glass is twice as big as it needs to be.

 

[looslib]

This webpage claims to be to the ANSI B18.6.3 - 1998 standard.

http://www.smithfast.com/msflathead.html#82deg

It lists #6 as .262/.238 head diameter.


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[dtharrett]

The smithfast link shows only part of the spec. The screw in question is a flat head cap screw.

 

[JohnRBaker]

I have never heard of a #6 CAP SCREW[b/]. I was under the impression that any 'numbered' screw was automatically considered a MACHINE SCREW. Cap Screws always started at 1/4 dia.

Note that the standard from the 1953 handbook was for MACHINE Screws. When I looked at the section on CAP Screws, it started at 1/4 inch, where I expected that it would.

John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA

http://www.siemens.com/plm

http://www.plmworld.org/museum/

To an Engineer, the glass is twice as big as it needs to be.

 

[JohnRBaker]

I have also found a reference which states that the ASME standard B18.2.1-1996 covers Cap Screws from 1/4 in to 3 in in diameter.

John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA

http://www.siemens.com/plm

http://www.plmworld.org/museum/

To an Engineer, the glass is twice as big as it needs to be.

 

[JohnRBaker]

OK, I just checked my 1996 version (25th Edition) of the Machinery's Handbook and it turns that there actually is ONE situation where you will find a #6 CAP Screw and that is IF it's a Hexagon or Spline Socket Head Cap Screw. In only this instance will you find 'numbered' sizes included in a standard covering Cap Screws, in this case ANSI/ASME B18.3-1986.

Now this standard covers the normal Socket Head Cap Screws along with Set Screws as well as Button Head and Flat Countersunk Head. And in this last instance, a #6 American National Standard Hexagon and Spline Socket Flat Countersunk Head Cap Screw does have a maximum head diameter of 0.307 inches.

However, all other situations were you have 'numbered' screws, they are considered part of the standard covering Machine Screws, ANSI B18.6.3-1972, R1991, and in that case, the maximum head diameter is 0.279 (this is for both 82° and 100° styles).

And if that's not confusing enough, YOU'LL JUST LOVE THIS:

It turns out that even within the family of CAP Screws we have disagreement. For example, the Machine Screw standard goes up to and includes 3/4 inch and if you look at normal 'Slotted' Flat Head Cap Screws, which start at 1/4 inch, they are in complete agreement with the Machine Screw standard for equivalent diameters. That is that Diameters of the heads are the same. However, if you look at the Socket Head Cap Screws from 1/4 inch and up, they do NOT match Slotted Cap Screws, in each case being a slightly larger value. It would appear that this is the 'standard' for any Socket Head screw, I assume this was done to allow the maximum sized socket with the maximum possible depth and still provide the strength and durability needed, i.e. they simply had to make the head larger to accomplish this. Which seems reasonable considering that the advent of Socket Head screws is much more recent than Slotted, so we should not be surprised that they chose to 'violate' the prevailing standard to get this to work. But you will notice that they did NOT go back and change the standard for traditional Slotted Screws.

Don't you just LOVE standards?

Anyway, mystery solved.

Now as for what should be done, I guess you could ask that we enhance the Hole function to accomodate BOTH Slotted and Socket Head Countersunk Screws by offering two different options for the 82° style (there are NO 100° Cap Screws). Or optionally, if this does not happen often, just use the 'Custom' option on the Hole Feature dialog and enter the desired size yourself.

John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA

http://www.siemens.com/plm

http://www.plmworld.org/museum/

To an Engineer, the glass is twice as big as it needs to be.

 

[JohnRBaker]

And just so that there are no loose ends, if you DO use a #6 Socket or Spline Head Countersunk Screw downloaded from our Machinery Library, it will have a head diameter of 0.307 inches, so we are in compliance with the 'standard', despite it's non-standard nature, at least when it comes to providing the fastener.

John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA

http://www.siemens.com/plm

http://www.plmworld.org/museum/

To an Engineer, the glass is twice as big as it needs to be.

 

[dtharrett]

My machinery handbook referencing the same spec shows FHCS all the way down to size 0 but I do not think that is the problem here.

Let's take a 1/4-20 FHCS for example. The ANSI spec for a 1/4 FHCS is .531/.480

The NX hole feature puts in a .507

With a better understaning of the two different sizes (machine vs. cap screws), I am thinking that I am trying to use the feature not as intended (i.e putting a cap screw in a machine screw feature)...

 

[looslib]

With the 2 different head diameters, and depending on what your company uses, you can modify the hole tables if you only use the FHCS.
Another option would be to make a second table for the FHCS holes.


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[dtharrett]

Well it appears I have learned a lot about machine vs socket head standards today. More importantly, I think I found my problem with the NX feature.

There is a sub menu under ANSI inch countersunk screw clearance. The sub menu gives you the option for "machine 82 degree", "machine 100 degree" and "socket flat head". My default was machine 82 degree hense the small hole. Switching to the socket flat head gives me exactly what I am looking for.

 

[JohnRBaker]

Yes, after my 'dissertation' above, I got to thinking about this as well, particularly after I confirmed that the Machinery Library does include Socket and Spline Countersunk Screws. I went back to the Hole Feature dialog and found that we indeed have provided the solution by giving you a specific option for this albeit limited, but not all that unusual situation.

And as a side note, if you look at the Metric Standard, they too have a difference in head size when it comes to slotted versus socket head, so this issue is not limited to only the ANSI/ASME standards, but appears to be universal in nature.

Now I guess it is indeed safe to say, problem solved!

John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA

http://www.siemens.com/plm

http://www.plmworld.org/museum/

To an Engineer, the glass is twice as big as it needs to be.