Thread tapping question for mechanical engineering types

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Rob Flinn

Well-known member
Joined
Jun 3, 2004
Messages
5,207
Location
Between Sussex, UK & Aude, France.
Recently I have needed to tap some M3 threads in aluminium bar about 8mm thick. So far I have managed to break 2 taps, despite following all I can remember from my O level metalwork, which incidently I got a grade A for !!

Can someone tell me the correct diameter hole I should drill to tap the thread into ? I have been drilling 2.5mm holes.

I just want to know whether the hole diameter should be bigger maybe 2.8mm, or whether I'm just being ham fisted !
 
Stainless and aluminum are two of the most difficult to tap since they are somewhat "spongy"

The size is found by this:

Diameter of thread minus the pitch equals the tap drill size.

So for metric coarse:

3 x .5 =2.5

You are using the right size drill but you will need to use a sharp tap with a lot of cutting fluid.

I can tell you from experience that you will get a "feel" for the metal as you work. You'll be able to tell when you are cutting and when you aren't.

How many flutes does the tap have? That can have a big affect on your tapping too.
 
+1 to what others have said.

The typical tap for manual use has 4 flutes (gun/machine taps have 2).

There should be plenty of tables around to look up proper hole size.

Use plenty of lubrication and reverse direction often to break the chip. Usually you will be able to feel when the tap is jamming up and flexing too much.

Taps are very hard, which is why they are so brittle.

JR
 
What others say; in/out to break chips, lubricate, remember it is a 3mm screw with half the strength fluted away. And...

> M3 threads in aluminium bar about 8mm thick.

You only need as much depth as a nut. What is an M3 nut, about 2mm thick? Nuts are sized so the bolt will fail in tension about the same time as the threads strip off. Maybe a little more for steel bolt in aluminum hole. Maybe a lot less if you are using M3 for convenience instead of Ultimate Stress.

Even for strong bolt in soft cheese, only the first part really takes load. The slender bolt stretches, the heavy casting/bar/log does not stretch. So most of the load is focused on the first few diameters of bolt length.

I dunno these funny M sizes but I think you "need" 2mm to 4mm of depth in the hole. Surely not 8mm. That's 3 times stronger than you need. And the tap-maker knows this, knows that any mature shop-person will not do long running threads with full depth.

In a "long" hole, a half-turn back-out may not be enough. Chips pile up and the flutes may not hold them all. When you can't do a half-turn back/forth very freely, run the tap ALL the way out and clean the chips.

Much past a 3mm/8mm proportion, the friction in the hole will get excessive for everyday disassembly unless you allow extra clearance.

I assume you are stuck with the 8mm bar.

You could bore 2.5mm and then counterbore with 3mm-3.5mm drill to 5mm depth, and tap the 3mm depth of small hole.

Or you could up-size the tap drill so only about 1/3rd of the bolt thread tips actually bite aluminum. Since the hole is 3 times too long, it will still be as strong as the bolt, and a lot easier to tap. I'm not sure of my M-thread math, but if you have a buncha 8mm stock then start with a 3mm drill, which will tap-in and strip-out at a touch, then try your 2.9mm 2.8mm etc drills until it A) taps smoothly B) the screw doesn't pull-out too easy.
 
I helps if you can use a tapomatic ?sp on a drill press with small taps it keeps the tap at 90 degrees and you don't get the side forces you can get by hand. If I hand tap something and the tap breaks I believe it was due to not holding the tap at 90 or unequal side force on the tap.

http://www.shender4.com/thread_chart.htm

http://yarchive.net/metal/tap_drill_chart.html
 
Another trick I know is to use a very liquid lubricant. The thick stuff generally holds the chips which get stuck in the threads and cause galling/binding and breakage. I would use something like wd-40.

You could also get fancy and buy some 4 fluted spiral taps that are used mainly in machines. they clear chips much better and are designed for use with liquid coolant/lubricants.

According to my mech eng books, 2.5 is correct for a 3mmc coarse thread screw.

Are these blind holes or through-holes? You need to make sure that your tap has the correct tip for the type of hole too, that is a common mistake.
 
[quote author="Gus"]I helps if you can use a tapomatic ?sp on a drill press with small taps it keeps the tap at 90 degrees and you don't get the side forces you can get by hand. If I hand tap something and the tap breaks I believe it was due to not holding the tap at 90 or unequal side force on the tap.

http://www.shender4.com/thread_chart.htm

http://yarchive.net/metal/tap_drill_chart.html[/quote]

After my own early experiences breaking taps I saw one of those gizmos in the machine shop and realized how the machinist never broke taps! Very cool, but probably pretty expensive, and of course you need a decent drill press.
 
[quote author="peter purpose"]Rob,
Are you doing one turn forward, half a turn back?
It clears the debris.

peter[/quote]

I sure did. In actual fact I'm making a lunchbox to house your API525s !

Thanks to everyone who replied for the advice. I think most of it I was following anyway. I guess I must be ham fisted after all.

PRR your point about the thread depth seems so obvious now I know about it !! I guess all these tips in hindsight are !!
 
[quote author="PRR"]

> M3 threads in aluminium bar about 8mm thick.

You only need as much depth as a nut. What is an M3 nut, about 2mm thick? Nuts are sized so the bolt will fail in tension about the same time as the threads strip off. Maybe a little more for steel bolt in aluminum hole. [/quote]

I'd say definitely more.
PRR's comparison to the depth of a nut seems logical, but you are tapping AL, and a bolt is not AL.
Also the whole process of hand-tapping flanges the first few threads a bit because of the tool's "wiggle" in your hand.
Blow out the hole when you're done, chips can jam the bolt/screw.

=FB=
 
I went through similar thoughts, not so analytically, recently. The school has a Gallien-Krueger bass amp, all aluminum. The heat-sink forms the top of the unit. The handle screws were threaded into this extrusion. The beast has many years of rough service. One handle screw musta pulled-out, because I found a wood-screw replacing it. (Cleverly blunted so the tip did not skewer the PCB under the heatsink!) The other screw was loose and tightening it didn't feel good. My gut said that #10 screws in ~~0.2" of soft alloy mighta been OK the first decade, but was too close to failure for "infinite duty" (defined as "until I retire from here").

My bodge was to drill for Tee-nuts. Effectively a steel insert with huge bearing on the underside of the extrusion. I know these handle screws give reasonable service in T-nutted wood.

This handled heatsink attaches to the box with a dozen amazingly small screws into C-channels in the extrusion. This looks like a weak link but all were in good shape.

> I'd say definitely more. ...comparison to the depth of a nut seems logical, but you are tapping AL, and a bolt is not AL.

True, but this is not an Engineered Joint either. Where did the proportions come from? Not from structural analysis.

Pardon me for simplifying, and using archaic units.

A 60deg thread can develop the full strength of the -area- of the base of the threads. If both components are equal, it strips on the bolt. Often the bolt is special and the block is bulk-stuff, weaker, it strips on the block. For cheese-steel in aluminum bar, we expect the AL to strip.

3mm is like 1/8 inch. The circumference is then 3/8". Say the threaded depth is reduced to 3mm or 1/8". The area is 3/64 or 1/20 square inch.

If the bulk were 20,000 pound per square inch stuff, it would take a 1,000 pound pull. We might have 10KPSI aluminum, so 500 pounds is needed to pull the bolt out of the Aluminum. 230KG outside the US.

Interestingly, 500lb is also the tensile strength of a 1/8" rod made of 32KPSI steel.

So with one 3mm screw in 3mm depth of aluminum, Peter could not pull the module out with both hands. And if the module is 5 pounds and we apply a 99 Gee drop-test, the module would not pull out (assuming on-center drop).

Anyway, 500 pound force on a 8mm square bar a foot long would put a permanent bend in most steel. Assuming the bar is 10KPSI material, as we assumed for the screw-hole, if the bar is more than an inch long the bar will fail first.

Assuming 1 pound module and four screws, we have a 20:1 factor of safety even on the drop-test. At full 8mm depth, maybe 50:1 FoS. Assuming Peter won't brutalize his modules and this is not a touring company, the threading can be exceptionally sloppy (oversize pilot hole) and still have ample margin.

The 8mm comes from Rob's estimate/experience of what he needs for the Span, and the 3mm happened because smaller screws aren't cheaper, just more awkward.

We could micro-optimize everything like a bridge, which is used by unsupervised strangers and is a major dent in the local budget. Then for stiffness in a 16" rack we might assume 1" or 1.5" beams. Since abuse can be any direction we favor round or square. Strength is a lesser issue so they may be hollow, indeed the wall thickness might be controlled by screw depth demands. Four 1mm screws 3mm deep or 1.5mm 2mm deep would resist Peter's pull. But who wants to find a dropped 1mm screw? And they are not cheaper, perhaps more costly.

> hand-tapping flanges the first few threads a bit because of the tool's "wiggle" in your hand.

A good point, and a reason not to micro-design too far. I'd actually tried to re-tap that G-K's extrusion, but without a proper setup I could see that I was making a cone not a cylinder.

BTW: the standard plastic replacement guitar-amp strap handle sold in the US now comes with Metric screws, not #10 as it used to be.
 
FWIW, In Euro-land, the required hole size should be printed on the tap. The M3 tap I have indicates "2.5" on the side of it.

I find tapping requires a lot of patience and delicacy. If you want to be ultra-conservative, tap 1 turn, completely remove the tap, blow away debris and re-insert; it'll take ages, but it's cheaper and less frustrating than breaking taps.

I managed to break a drill last week on 5mm plate :mad:

As well as a lubricant, you can also use a candle - thoroughly cover the tip of the tap in wax - I find it helps (the technique also works for sawing).


Justin
 
Back
Top