×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Countrsink lead-in for internal threads--is there a std?

Countrsink lead-in for internal threads--is there a std?

Countrsink lead-in for internal threads--is there a std?

(OP)
I'm looking for a standard or information regarding the countersink on an internal thread.

I have an engineer who insists on specifying a countersink on all internal threads. I have contended that it is standard machine shop practice to put a 90° csk as a lead-in (he always specifies 120°) a little larger than the major dia.

"Prove it!" he said.

I thought I had seen it in Machinery's Handbook, but I couldn't find it, and a search of several hours and many keywords turned up nothing--except this site, which I had never heard of before.

Anyone have any info/ideas?

Thanks,
WGC here.

RE: Countrsink lead-in for internal threads--is there a std?

I know that the 120 degrees is typical
for drilling for heli-coil inserts, but
our shop normally chamfers 90 as you
suggest for standard internal threads.
I have not seen a standard for countersinking
tapped holes for threads but have read
that the c'sink diameter should be greater
than the major diameter of the thread.
Will look forward to other opinions.

RE: Countrsink lead-in for internal threads--is there a std?

Old timers will normally csk a thread, if they are making a part for their own use.  But when making parts for someone else, any shop that has ever worked in a highly regulated industry, or is subject to inspection, will make parts _exactly_ like the print.  You can't rely on 'standard machine shop practice'; there is no such thing.  You can have any csk you want, but you have to specify it on the print.

That said, I think the 120 deg csk used to be customary, but CNC shops have adopted the practice of using the same 90 deg cutter for spot drilling, countersinking and chamfering, to save tool change time.  That's the 'new' custom.

Mike Halloran
Pembroke Pines, FL, USA

RE: Countrsink lead-in for internal threads--is there a std?

I have always used a 90 degree c'sink on threaded holes as good practice, but I know of no standard (other than a company standard) that requires this.

RE: Countrsink lead-in for internal threads--is there a std?

I use a 1" x  90 degree spotdrill and generate the chamfer using it. This gives better hole location and the chamfer all at the same time. I've never used a 120 degree as a drill will again hit on the point and wander and I have never seen a 120 degree anything as a standard tool.

RE: Countrsink lead-in for internal threads--is there a std?

If you layout the thread in CAD or on paper (if your good) you will see that a standard 60° profile thread (UNC, UNF, METRIC, etc.) will have a much cleaner entry/exit with a 120° chamfer than a 90° one. (The 90° has a longer interaction leaving a greater lenght of material to become a burr.)

Your Engineer is right, buy him a coffee then tell him to get back to work.

RE: Countrsink lead-in for internal threads--is there a std?

The purpose of c'sinking an internal thread is to prevent a raised "burr" at the imperfect lead thread.  A burr will prevent the mating part from sitting flat on the surface with the threaded hole.

RE: Countrsink lead-in for internal threads--is there a std?

It also helps to get the bolt started
or aligned.

RE: Countrsink lead-in for internal threads--is there a std?

In electronics assembly it also prevents the raised burr from breaking loose and shorting something out.

RE: Countrsink lead-in for internal threads--is there a std?

While burr prevention is an important reason to chamfer holes, the major reason to do it is to provide clearance for the bolt underhead fillet.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

RE: Countrsink lead-in for internal threads--is there a std?

Cory,
Isn't the chamfered washer supposed to take care of
the bolt fillet radius problem?

RE: Countrsink lead-in for internal threads--is there a std?

Cory is right on clearance holes.  It is not standard practice to use use a chamfered washer except in aircraft.

On tapped holes the issue is that without a chamfered hole there is a possibility of a burr that will extend above the bearing surface.  When the mating part is placed on this it can be forced down, effectively 'closing' the thread and creating an increased potential for cross threading.

I have not seen any standard practice to chamfering tapped holes and I looked for it just like you have.  IMHO the best way to do it (unless there is a definite reason to specify a specific configuration for perfomance resons) is to simply indicate "chamfer all tapped holes" and leave it open for the vendor to make them the easiest and lowest cost way he can.
We have not found a significant difference between 90 and 120 degree holes for cross threading and we have tested for just that condition.  The experimental noise was much greater than the impact of the chamfer angle.

RE: Countrsink lead-in for internal threads--is there a std?

For screws and bolts this practice is called pointing and most standards specify this as being optional by manufacturer. Since there is no standard for tapped holes you'll not see this specified anywhere. My experience is that if its not on the drawing it generally doesn't get done. Your engineer has a valid comment.

RE: Countrsink lead-in for internal threads--is there a std?

(OP)
Thanks to all. It looks like we ought to have another boilerplate note something like:

ALL TAPPED HOLES SHALL HAVE A CHAMFER OF DIMENSION MAJOR DIA+.010/+.020 X 90°-120°.

(And I was so sure I was right... darn! I probably ought to hire him back...)

RE: Countrsink lead-in for internal threads--is there a std?

If you are calling out a chamfer,
shouldn't it be 45 to 60 degrees.

RE: Countrsink lead-in for internal threads--is there a std?

(OP)
Does this sound better?

ALL TAPPED HOLES SHALL HAVE A 90°-120° CSK WITH A DIAMETER EQUAL TO THE MAJOR DIA +.010/+.020.

RE: Countrsink lead-in for internal threads--is there a std?

Yes that is better.
Sorry to be picky.

RE: Countrsink lead-in for internal threads--is there a std?

(OP)
No problem. I figure nearly all the people logged onto this forum are picky by nature or we wouldn't be searching Websites for answers—we would just do what seems expedient appears to work. I imagine most of us are also pretty opinionated!

RE: Countrsink lead-in for internal threads--is there a std?

There is a chart, have one and use it. It is a machine shop practice to chamfer all holes, break all edges, no burrs.  120° is hard to control, gets big in a hurry.

RE: Countrsink lead-in for internal threads--is there a std?

(OP)
What chart is that, Metalart?

RE: Countrsink lead-in for internal threads--is there a std?

Give me a little time to come up with the origin of this, I use a copy of all this info that an inspector put together, great sheet, I remember him saying it took him quite some time to get all the info because it was not readily available. I could fax you a copy right now if you want. This guy is good and fussy so I new it was /is good info.

RE: Countrsink lead-in for internal threads--is there a std?

Take a look at MIL-S-8879 or FED-STD-H28.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources