Minimum size of screw/tap based on steel thickness? 1

[SuperCAD79]

As a preface, I am not an engineer. I am a CAD manager who is constantly looking for ways to make my drawings more detailed. That said, here's my problem.

We're building a rather complex steel structure (decorative only) and our guys are snapping screw taps left and right. Most of the material that we are going through are either 1/8" A36 angles or 3/16" A500 tubes. The screws are mostly 8-32 and are spaced no more than 24" O.C. (this is dictated more by the aesthetic than by structural integrity). We weren't given any direction as to what size screw to use so we ended up using the same size that was used on past similar projects that were successful.

What I'm looking for, if this even exists, is a sort of guide that will state what the best minimum size thread size would be used for a given thickness of material to avoid snapping the taps. This is so I would have some direction on future projects as to what we should be using.

I know that there are many factors that go into selecting a screw thread size, but since our products are only for decoration and are not used for any structural purposes I'm not concerned with that as of yet. I'm just looking for a way to say that if we are using a given thickness of steel, 1/4" for example, that we should start with a #12 versus a #8 thread size, or something along those lines.

Sorry for the rather ignorant question, but I need some ammo when I'm confronted about this by the higher ups.

 

[berkshire]

At the thicknesses you mention you should not be having a problem breaking taps. Unless you are picking too small a tap drill.

If you are not tapping into blind holes I would suggest you use a 2 flute " Gun Tap " which pushes the chip out in a spiral forward of the tap, and allows the tap to cut in one pass without stopping.

Using a torque limiting tapping head, or a drill motor with a preset clutch, will stop the breakage if you pay attention to what the machine is telling you, and back out and clear the threads when the machine ratchets. Tightening up the torque at that point is recipe for a broken tap.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[MikeHalloran]

Like berkshire says.
... at that size, a battery operated power screwdriver should work well.
... and be helped by a drop of Tap-Magic.

If you're not going to disassemble your structures much, you might look into self-drilling screws.



Mike Halloran
Pembroke Pines, FL, USA

 

[mcgyvr]

What size tap drill are you using?
(I'd start with a #27 for an #8 machine screw into steel)

 

[SuperCAD79]

I did a little more digging, and I think I've got the answer. They ARE using the wrong size tap drill. We specified a 1/8" drill, which I know is the wrong size, but let me explain why.

A while back, we would specify our tap holes based either on the chart from EngineersEdge or the hardware manufacturer's instructions. One that sticks out was a drill size #21 (.159) for putting in a top center pivot into a steel door with 10-32 screws. We specified both the drill size number and the decimal value to avoid any confusion (I know that is wrong too, but there are a few things that we do that aren't standard in the industry).

We got a good talking too about specifying ONLY fractional size drill bits, since they claimed that they couldn't get the drill bits that we specified easily.

So here we are in present day, and we spec'd a 1/8" hole for the 8-32, based on the direction we were given, when the chart says to use a #29, and mcgyvr suggests a #27. I'm going to keep this bit of info in my back pocket when it comes time to review the project next week.

Let me know if I'm misunderstanding anything. Thanks!

 

[KENAT]

OK, sweeping rule of thumb, keep the depth of tapped holes to not much more than 1.5-2 X nominal size of thread. This is based on the idea that 1.5-2 X nominal thread size usually provides adequate holding strength.

Now this is a very crude rule of thumb that any self respecting stress person will discredit rapidly. However, for the non structural work you seem to do it may work OK.

Of course, by the sounds of it you're already in this range and it's the undersize tap drills causing the problem.

A drawing note, if working to ASME drawing standard, you don't spec the tap drill size on the drawing - just the size thread. Essentially you detail the end item.

Of course, if you're making process drawings or similar this doesn't apply.

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[SuperCAD79]

We can't really conform to any drawing standards as far as what kind of information gets put on a drawing. Believe me, I would LOVE to be able to do that. Instead, we've been directed by the owner to put as much information on the drawings that we can so the shop guys don't have to think about anything. They call it "making the drawings smarter" where I call it "dumbing the drawings down so any idiot can put it together". The irony of it all is that no matter how much information we put on a drawing, we're still the ones who get blamed if the shop doesn't make the parts correctly.

It would be nice to specify a thread size only, but the shop doesn't know what drill size to use for the pilot hole. There are no charts for them to reference. And if I try to give them a chart, the boss will come down on me for stepping on his toes.

 

[KENAT]

I suspected that might be the case, but thought I'd bring it up just in case.

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(probably not aimed specifically at you)
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[MikeHalloran]

I'm guessing the 'fractional drills only' rule came from the boss also.
... in which case, the company is doomed.



Mike Halloran
Pembroke Pines, FL, USA

 

[John2025]

Good grief, this is either a miscommunication about the fractional sizes, maybe they meant "fractional whenever possible", or else Mike is right.

 

[ak762]

A drawing note, if working to ASME drawing standard, you don't spec the tap drill size on the drawing - just the size thread

Could anyone reference to where it is stated

 

[SuperCAD79]

There was no miscommunication. We were basically forbidden from using any letter or number drill sizes. Their complaint was that they could only get a limited amount of drills from a local hardware store, yet they wouldn't tell me what sizes the store had in stock.

I spec'd a hole size of 0.159" one time and they got on me about not using fractions. When they asked what drill size that was, I said "go get a #21 drill." When the owner came back, he ridiculed me for specifying the wrong size since the drill had 0.163" on it.

So just to keep the peace we put fractional sizes on the holes. To make matters worse, they don't like seeing any dimensions that are tighter than 1/16" so I have all of my dimensioning standards set to round all fractions to the nearest 1/16". In the case I mentioned above, the closest fractional drill size needed for the 8-32 tap would have been a 9/64", but the standards rounded down to 1/8".

We just had a discussion about this yesterday at work and they want to make all thru hole diameters 1/8" larger than the fastener size, which I think is overkill, but they say is needed since we are machining stuff by hand and our shop guys can't hold a tight tolerance.

 

[dgallup]

Good grief!!!

"Buying drills from a local hardware store" !??!??!

Not only are you paying too much you are probably getting junk meant for the home handyman. What kind of an outfit are you working for?

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[berkshire]

This sounds like one of those cases where you need to breeze by that local hardware store on your way home from work and see what they do have.
It also sounds like there are some control issues going on in your company.
You should point out to your boss that if your guys are breaking taps, they cost twice as much as a drill, and that any saving on drills is going to be offset by this.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[looslib]

Fractonal drills only, go up 1/64 in size to a 9/64 drill. Right between the suggested #29 and #27 at 0.1406 diameter.

http://en.wikipedia.org/wiki/Drill_and_tap_size_chart

Tap drill charts always available on the internet with a quick search.


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

Ben Loosli

 

[BillPSU]

You will really be in trouble if you were required to produce a metric thread. Any hardware store worth anything will have number drills available.

The following link gives the tap drill size chart. There are several others available so if you don't like the supplier I chose then pick another. Google "tap drill chart".

http://www.imperialsupplies.com/pdf/I_DrillSizeDecimalEquivalent&TapDrillChart.pdf