Calculating force required to seat self tapping thread?

[jxc]

Evening,

My general question to the forum is stated in my title. I am using a self-tapping thread to hold a to objects. One has a clearance hole and the thread is cut into the 2nd piece. I am having issues seating the screw properly and require more than one attempt. I work in a high volume manufacturing facility and this effects the efficiency, thus cost of this assembly. I am using pneumatic drivers and want to verify the force or torque I am using is adequate for the process.

Thamks...

 

[MintJulep]

Since it seems apparent that you don't have the torque to do the job, what do you think you will gain from a dubious calculation?

Any calculation will have a bunch of assumptions built in to it.

Test.

 

[bvanhiel]

I agree with MintJulep. Measure the torque to sink 30 screws and calculate the mean and standard deviation. this should give you some idea of whether or not your manufacturing process is feasible, as it may be that the +3 sigma torque is enough to strip the screw.

-b

 

[jxc]

At this point I feel that is not a true assumption. We have increase the torque on numerous occassions and continue to hit and miss seating the screw.

But, B I will use your trial and give an update regarding the results.

 

[IRstuff]

Perhaps you need to explain more clearly by what you mean when you say, "miss seating." Do you mean that the fastener goes all the way in, but is crooked?

If so, that sounds more like a tooling problem than a torque problem. That would suggest that you should either pre-tap the holes with a bit that has the same threads as the fasteners. The bit would be held in a chuck, and should always tap true. The other choice would be to have some sort of constraint on the fastener, to keep it from rotating, a seemingly more difficult problem.

TTFN

FAQ731-376

 

[IRstuff]

Oh, a 3rd possibility is that you have sufficient torque, but too much force, i.e., you're attempting to advance the fastener too fast, causing its mechanical contact with the driver to essentially buckle.

One would think there should be only minimal force pushing on the fastener, only enough to maintain mechanical contact long enough for the thread to catch in the material.

TTFN

FAQ731-376

 

[CorBlimeyLimey]

Or the tapping hole is just too small.

What material is being tapped?
Which screw thread type? (forming or cutting)
Sizes involved?

 

[proinwv2]

Considering the previous comments, it seems very possible that the torque due to friction is going to vary widely. Then you have the added possibility that different thread surface conditions and lube further vary this. It has been shown that the torque to overcome friction is a major component of the torque used to tighten a thread.

Have you asked if the design is appropriate for your assembly?

Paul

http://www.ostand.com

 

[dimjim]

I wonder what the tolerance is on a self tapping screw.
I would imagine a lot. That may compound your problem
in finding a solution.

 

[jxc]

I appreciate the input, unfortunately my plant is on a holiday break I will have the pertinent info. tomorrow.

I would like to address the comment on design. This application is on a small magnetic motor armature and this tap is spot welded in place and holes drilled for the self-tapping screws. The material is a mild steel. Cost is a major consideration in our operation and that was one of the reasons self-tapping screws were used. The operations I laided out previously are done in an automated piece and adding the tapping operation would mean handling the pieces additionally.

This evaluation I am currently undertaking will address the cost of rework and the additional operation.

Thanks again for the input.

 

[Cockroach]

Compute the shear on threading for the imperfect threads, typically three (3) thread stand-off or so. The shear value will permit the calculation of force necessary to remove these threads, then Newton's Law, the force equal and opposite which is what you need.

If you need torque, then it is simply the normal force times the radius of your tap, i.e. half the major diameter of the threading.

I use this on our NC machines in order to figure out if we're going to bend small size drills during tapping operations. Works to about 2.8% error, pretty bang on for scientific error acceptability which is 5%.

Hope this helps.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Tmoose]

Potato chips are bad for me, and steel chips are probably bad for motors, so I wonder if these aren't thread forming screws.

I think thread lengths over 1.0 are kind of rare for a reason for self tapping screws, and each manufacturer claims the other guys' thread forms are problematic, but these guys make pretty nice screws.

http://www.camcarsolutions.com/taptite2000.asp

 

[jxc]

Kenneth (have a hard time addressing someone cockroach), nice suggestion. Force is what I am after, the theoretical value in particular. Operators have been compensating by pushing down on the driver, I think some earlier addressed that issue and training operators on what to expect the driver and fastener to do is an issue I will work with also.

Again all ideas and suggestions appreciated.

 

[IRstuff]

Oh, it's a manual operation. That's a problem all by itself.

You might consider if it's possible to make a combo drill bit, bottom part drill, upper part tap to automate the tapping process. I assume that the drilling takes place on a drill press, at least, and isn't done by some guy holding a portable drill?

TTFN

FAQ731-376

 

[jxc]

No, the driver is a pneumatic driver and the holes are drilled using equipment that welds and drills these holes and others on a dialed mechanism, then components are fixtured, location and depth is controlled. The tolerances the controls are based on may require review.