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Small Machine Screw Tapping Issue

CalgaryPT

CalgaryPT

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I need to tap some small machine screws into some beefy steel. The screws are 7/64 in diameter, and appear to be 40 NC. I can't seem to find a tap for these screws, or a recommendation for drill hole size. The screws are those small ones you typically buy at electronics parts stores to mount PCBs using standoffs.

It's not critical as I can find another solution I am sure. But being able to tap holes for these screw would be the cleanest solution. Any good sources for small taps in Calgary?
 
In steel you want a slightly looser hole to tap into:
1624152572655.webp
 
If you are able to make this a feature a through hole than you can get away with winding a spiral point tap straight through without breaking the chip. If it is a blind hole consider going down three times diameter maximum with a spiral flute tap. any deeper will require hand tapping. Enjoy! tapping holes takes thousands of hours to grasp. The more tapping i do the more i realize I have only scratched the surface.
 
Alexander said:
If you are able to make this a feature a through hole than you can get away with winding a spiral point tap straight through without breaking the chip. If it is a blind hole consider going down three times diameter maximum with a spiral flute tap. any deeper will require hand tapping. Enjoy! tapping holes takes thousands of hours to grasp. The more tapping i do the more i realize I have only scratched the surface.
Click to expand...
Geesh I am tired of breaking taps and making crooked threads already and I haven't done hundreds of hours yet, let alone thousands!
 
CalgaryPT said:
The screws are those small ones you typically buy at electronics parts stores to mount PCBs using standoffs.
Click to expand...

Oops didn't read this 'electronics' close enough. Unless you pulled this out of a N-Am machine from the 70's I'd give high probability to it being M3. Very common in most electronics we import.
OD of 4-40 is 0.111-0.106". Nominal OD of M3 is 0.118"
40 TPI = .025" pitch. M3x0.5mm = 0.020" equivalent pitch
I have M3 taps & screws too if you need services.

1624157441746.webp
 
I bought the screws from B&E and have run a gauge on them. Pretty sure they are in fact 4 40s. But I will triple check. At that size and even with a mag lens it is tough to see the thread gauge or feel it properly. OK, maybe my eyes are shot too.
 
DPittman said:
Geesh I am tired of breaking taps and making crooked threads already and I haven't done hundreds of hours yet, let alone thousands!
Click to expand...
I am starting to get the hang of it. I still am exploring the possibilities of form tapping. Benefits are stronger threads, more tapped holes, taps available with through tool coolant etc.
 
Similar to the chart @gerritv posted, I have a little 'pocket reference book' that shows the percentage of thread that is expected for different tap drill sizes. Unfortunately, it only shows options that are between 70% and 80%. OTOH, it does give the formula* used so I made a little Excel spreadsheet to calculate the results for a few more tap drill sizes:

Threading 4_40 options .jpg


Since @CalgaryPT is threading into "beefy steel", a 3/32 inch drill looks like a good option since it should theoretically provide 56% thread engagement. "Theoretical" since drills don't produce a hole precisely their stated size...certainly not down to tenths!

Craig

* Formula is:
Percent of thread = Number of threads per inch X (Nominal diameter - Tap drill diameter) / 1.299
 
I use a little iPhone app called iEngineer. Provides great data:

25902136-913A-4DBB-9811-0D7DB552C545.webp

Tap sizes and hole sizes. You can slide across to get all the strength data depending on the bolt grade you select.
 
Alexander said:
I am starting to get the hang of it. I still am exploring the possibilities of form tapping. Benefits are stronger threads, more tapped holes, taps available with through tool coolant etc.
Click to expand...
On the topic of form taps Alex, can you still use these when hand tapping in mild steel? I have been meaning to try these, but they are much more expensive than cut taps. The concept of form taps makes more sense to me but just wondering if they are practical in materials other than AL or maybe brass?
 
One of the greatest eye opening experiences for me in the machining world was when I finally got my hands on a quality tap.
Yes, they are expensive but they are so much better and the chance of breaking one is so much less.
My favorites (so far) are Emuge (I've bought them from Thomas skinner in the past) and as Alexander said " Spiral point for through holes and spiral flute for blind holes" - though I've used spiral flute for both no problem.
Yes, they're expensive but I've got some Emuge spiral flute taps that I've tapped dozens and dozens of holes with and they still cut beautifully.
Joe Pie... has a good video on calculating tap drill sizes (subtract the pitch of the thread from the diameter)
If its not a high stress application, consider making your tap drill hole a little larger = much less chance of a broken tap - in lots of cases you can get away with less thread engagement.
 
One more thought;
I read some ones comment regarding tap alignment a few years ago.
"one of the most common contributing factors to tap breakage is mis-alignment."
Anything you can do to "guide" the tap, ie; keep it perfectly aligned with the long axis of the hole reduces the chance of breakage.
I do a lot of my tapping on my mill with a T-handle tap wrench with the top held in a collet.
Takes longer, yes indeed but figure in the time / frustration spent dealing with a broken tap and you might be wishing you had done something differently (been there, broke the taps).
 
CalgaryPT said:
On the topic of form taps Alex, can you still use these when hand tapping in mild steel? I have been meaning to try these, but they are much more expensive than cut taps. The concept of form taps makes more sense to me but just wondering if they are practical in materials other than AL or maybe brass?
Click to expand...
Unfortunately no the cutting speed of the tap for a proper roll form thread is too high.
 
As mentioned earlier axial alignment is probably the most important factor when tapping small holes.

I've tapped a couple thousand 2-56 holes and a few hundred 0-80 holes in aluminum, bronze, brass using this tap guide and a regular T-handle tap holder. The tapping block holes are dimensioned to match the standard tap shank diameters with a few thousandths clearance. I've never broken a tap using this guide unless I accidentally bottomed a 0-80 in a blind hole - but then that one's on me:(

TAP BLOCK.jpg

Brent
 
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