Drill bit size for 316 stainless

Working on a small flame eater engine. I choose 316 stainless for the cylinder. I need to tap 2, 4-40 and 2, 10-32 blind holes.

I have new Guhring taps. Green ban. Will be hand tapping.

Using Tap Magic what size hole would best work? I am “trying” not to break a tap.

I’ve looked up the recommended hole sizes, but would feel better going a bit over sized seeing as how it is 316.

I would go with a .094 drill for the 4-40s and a .159 drill for the 10-32s.

Use a VERY sharp drill bit for for each, and use coolant when drilling. You want to avoid work hardening the holes as you drill. and yes, a good sharp counter sink, before you tap is a must.

Why not form tap them?

I second form tapping if possible. OSG would be my preferred brand.

Do not hand tap them. You will break taps. Never hand tap in SS.

With the taps so small you should use a form tap and make sure it goes all the way in and all the way out with no interruptions.

If hand tapping is the only way to do it with your current set up then you may need to approach this a different way. I do not know what machine/setup you are using. SS will work harden and break your small taps.

Maybe a cordless drill with a form tap. It needs to go straight into the hole so it doesnt snap. Also form taps use bigger drills than a cut tap uses. Make sure you use the right combination.

Using my tap drill calculator, which takes all factors into account, I get the following:

4-40:
It does confirm the #43 tap drill, which is the standard one suggested for this thread. It also tells me that the actual percentage of a full thread with this drill is 71%, not the targeted 75%. This is due to the fact that a drill bit of the calculated value for 75%, which is 0.0876" is not a readily available size.

It also tells me that the 0.094" bit (3/32" or #42) will give you a 55% thread. The next size up would be a #41 (0.096") which would provide a 49% thread.

10-32:
For this thread the standard tap drill recommendation is a #21 (0.159"). My calculator shows this as an actual 76% of full thread. It is strange that Thunderjet recommended a 55% thread for the 4-40 but a 76% for the 10-32. I guess a 4-40 tap will break easier.

Other choices for 10-32:
A #19 (0.166") will provide a 59% thread.
A #18 (0.1695") will provide a 50% thread.

Unless you need every scrap of strength possible in these threads, any of the above choices would be OK. Your choice.

One thing to be sure of is that the drill bits are properly sharpened. A bit with unequal cutting edges will have the wedge point off center and it will cut an oversized hole. A good quality, 10X magnifier is a must when examining small drill bits for problems. I recommend a Hastings triplet. Also, you may want to back the drills out a time or two while drilling to full depth to keep their flutes clear of chips. Chips packed in the flutes can also cause oversized holes.

Going to try form tapping, this will be the first time. MSC has OSG tin coated thread forming taps along with precision twist cobalt screw machine bits.

Looked at some charts for drill bit sizes, the chart I looked at had two bit sizes for each tap size. I’m guessing for thread engagement. I think I am going to go with the slightly bigger size.

Any advice? Will be using the Bridgeport to do the work. What I uselly do is: dab of Tap Magic drill pilot hole then put tap in chuck and turn the quill by hand till tap starts, uselly 2-3 threads. Then finish tapping with T handle.

No idea what a "flame eater" engine is but with what I know of combustion engine cylinders that is the last material I would have considered. Are you sure this is what you want to do?

tdmidget said:

No idea what a "flame eater" engine is but with what I know of combustion engine cylinders that is the last material I would have considered. Are you sure this is what you want to do?

Click to expand...

Flame eaters are just little models like this: YouTube

When hand tapping SS I used 50/50 thread cutting (sulfur based) oil and Cool Tool for larger threads/metal working or Cool Tool alone for the small stuff. I wouldn't reuse the same tap for both sizes. New plug tap for all except the bottom tap. I'd step up to the #20 drill after the #22 drill for the 10-32. Low and slow machining SS to prevent work hardening. As mentioned, drill bits should be pristine condition.

DanASM said:

Do not hand tap them. You will break taps. Never hand tap in SS.

With the taps so small you should use a form tap and make sure it goes all the way in and all the way out with no interruptions.

If hand tapping is the only way to do it with your current set up then you may need to approach this a different way. I do not know what machine/setup you are using. SS will work harden and break your small taps.

Maybe a cordless drill with a form tap. It needs to go straight into the hole so it doesnt snap. Also form taps use bigger drills than a cut tap uses. Make sure you use the right combination.

Click to expand...

Other than the form taps, and the tap needing to be straight, I would disagree with this.

Good sharp 135° split point drills and oil(to avoid work hardening) along with form taps and some Anchorlube should be just fine(I prefer Anchorlube over Moly). Use a tap guide block to ensure they are being tapped straight. As for sizes I would use a #38 for the 4-40(60% engagement)and #16 for the 10-32(60% engagement). If the holes are deeper than 3-4xØ I would probably go one size bigger on the drills.

For the hole I would go about 0.1 mm larger in the hole D, unless it is holding engine pressure.
Charts say 0.0960 - 0.0980 steel.
2,43 mm
2,48 mm
0.0890 - 0.935 in brass, alu, plastics.
2.26 mm - 2.37 mm


Disagree a bit with most posts re: form tap and no hand tapping.
In this particular case only, of making a single piece with major value in work done before tapping.

Form tapping (machine) with the right tools, in tap, fixture, and driving, will certainly thread the 2.4 mm hole 3 mm deep in about 2-3 secs, and can probably do 1000 holes before needing a new tap, success rate 1/1000.


Because:
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4-40 is sub-near 3mm.
In SS I would always hand tap it, in the OPs position of making a one-off with no sophisticated gear.

Using a machine tap, from any industrial supplier, with a brand.
And a hand held tap wrench.

Because by hand, You can easily feel binding and jamming, before the tap breaks in the hole.
By hand, I find I can back off, clear chips, carefully try to ease past a jam, back-n-forth, and it mostly works, maybe a 95% success ratio on deep difficult blind holes in SS.
Note deep and difficult, note SS, I would expect easy steel to have 99.x% success rate.

Spiral flute taps are great and have lower friction and resistance, tap easier.
But when they jam they also snap much easier.

Over 17 years of building cnc machine tools, and tapping maybe 13.000 holes, steels, 98% in F1 calibrado (basic tool steel), I mostly power tap with cordless drills, (5) 6-12 MM d, TYPICALLY 3 - 3.5 D deep to clear the tapered tap end and leave a bit of space for chips.

So for 6 mm // 1/4" I aim for 20-24 mm deep holes, and drill-tap them until the tap hits the bottom.
Success rate is about 2-3 breaks in 600 holes.

And I know perfectly well a 6-9 mm deep thread is enough in theory, but 1-2 threads at top may get weakened or buggered, and the last 8 mm or so are not fully formed with just one tap.
And the machine taps are gently tapered and prefer a deeper hole and some clearance.
And the machine taps or industrial std taps are quite resistant to twist, they wont break easily inside the SS workpiece, by hand.

Yes, if one runs the taps fast by drill or by spindle, the resistance is maybe half/0.75 vs hand tapping.
But hand tapping saves breakage.

My personal experience has been that SS does not work harden significantly.
But it heats, fast, a lot, and the drill binds due to heat expansion.
On 303, 304, 316L, upto 6/120 mm, or 16:1 D/L ratio, very deep holes.
Or 10/120 mm 303 SS, 12:1.