Regarding minimum sheet metal thickness for tapping 2

[kar0]

Hi Friends,
What is the minimum sheet metal thickness for doing a thread tapping inside a drilled hole, which has to bear hanging loads?
I am designing a mounting bracket made of 304 Stainless steel sheet of 3mm thickness.
Is it possible to do tapping in 3mm thickness sheet, and if not self clinching nuts (PEM Nuts) are the only way. Or is there any other way to create internal threads?
Please give suggestions.

 

[USMechE6]

kar0,

What size screw are you using? The rule of thumb I've most commonly heard is to have the length of thread engagement be twice the screw diameter. So in this case you could get away with an M1.6 or 0-80 screw. Now I've also heard length down to 1 x the diameter, in which case your options increase. All depends on what load you're trying to support.

With 'larger' screws not only would you lose that rule of thumb but you also have much coarser threading. Going from 80 threads per inch to say that of a 1/4"-20 means you'll only have about 2 threads engaged. They'll strip before the shank breaks.

Can always weld on a nut

 

[MintJulep]

Do you think it might just be possibly important to know how heavy your bear is?

 

[kar0]

Hi USMechE6,and MintJulep
I was planning to install PEM (Penn Engineering) Self clinching flush type Unified #6-32 thread insert(Please see specification of nut in attachment)in sheet-metal drilled holes.
A Force gauge is to be mounted to the sheet metal bracket. The force gauge has 4 numbers of tapped #6-32 mounting holes provision at the rear.

The sheet metal has to butt against the flat surface of rear side of Force gaug. From behind the rear of the bracket, the screws (he force gauge manufacturer has already provided #6-32 unified screw of 4 numbers) should be inserted and engage with the Force gauge mounting holes.

The force gauge should lift around 2.5kgs of load.
Please ask further for details,or for any clarification

Regards,
Karthik

 

[USMechE6]

You can use this minimum thread engagement calculator here Link

Major diameter of a #6 screw is .138".

Based on this calculator you need a minimum length of 2.5 mm, so you're just in the suitable thickness w.r.t. thread length.

I couldn't open the attachment (don't worry pretty sure it's my end) but assuming the loading condition. A 6"-32 screw has a minor diameter of 0.0997", so just one screw supporting your load will only have an axial stress of 700 psi (not including weight of force gauge...), well below the yield strength for standard screws. You should be fine.

 

[kar0]

Hi USMechE6,
I have a doubt. How are you calculating the minimum length to be 2.5mm.
I cant understand . Please help. and also the thread insert is 2.3mm only in height, No other options for PEM insert.

 

[USMechE6]

Hi kar0,

The first input uses a second calculator's output Link for its input (either above or below 100 ksi stress), assuming the latter based on your low nominal weight. This assumes the same material between threaded hole and fastener, but given they're both steel and your nominal load is so low and distributed amongst four screws it should be okay.

The second and third inputs are given by your screw choice (#6-32) w/ its major diameter shown here Link

This is for a threaded hole tapped directly into the stainless, not using the PEM nuts. Even so, the PEM insert will probably work as well.

 

[ornerynorsk]

Also check out flow drill, pierce/extrude/tap, etc.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[drawoh]

kar0,

Look at a hex nut some time. That is how thick you need to be.

Standard screw calculations are for the screw to fail in tension. The Machinery's Handbook has calculations for failure of threads in shear. Have a sense of perspective. Your screw ought to be cheaper than your fabricated part.

--
JHG

 

[MintJulep]

Four tapped holes in the force gage.

Four tapped holes (or tapped inserts) in the mounting bracket.

How does that work?

 

[KENAT]

Mint, I was thinking the same thing after seeing the OP's 1 Nov 16 16:21 post.

Is it a language issue i.e. to us tapped implies threaded where as the OP just means "clearance holes for".

You can back calculate minimum number of thread engagement based on the thread form, properties of the sheet metal etc.

You could look at using sheet metal screws that cut their own threads.

There are other types of threaded inserts or equivalent whose suitability will vary based on details you don't give us.

Heck, why not a completely standard nut & washer on the back if you have access that side.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[berkshire]

kar0 ,
If you are still at the manufacturing stage with this part , If the part is fabricated on a CNC turret press , it would be possible for the fabricator to upset the hole and tap it with a hole more than deep enough for your min thread capabilities ,
B.E.

You are judged not by what you know, but by what you can do.