Thread forming tap vs thread cutting tap

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I'm Looking to make 3/8-16 threads in a 3/8" thick chunk of copper , for some brass bolts to make my own upgrade proof busbar.

I've drilled and tapped copper before without a drill press, for 6mm metric thread ( nippondenso alternator + output stud), but was less than impressed with the results.

Researching this, I ran across the recommendation to use thread forming taps rather than thread cutting taps.

With No experience using thread forming taps, I am wondering if they pull themselves into the predrilled hole, or if they have to be lowered/ forced into the predrilled hole with precision I can never achieve by hand.

I am making my Own Bussbar, for 2/0 cable, which will rarely see a maximum sustained current of 240 amps.

So this 3/8" thick, 1 inch wide copper bar is overkill, which means its just about right.

I'd love to be able to bottom out the brass bolts, and then not have to worry about soldering them, or securing their heads so that I do not need to use two wrenches to tighten the ring terminals against the bar.
The Brass bolt heads will not be easy to access, and I'd rather not even attempt to solder such a thick mass of copper to lock them in place

The one 3/8" hole drilled so far will be accepting a 3/8-16 bolt whose receptacle is the load/source side of a 500 amp Deltec shunt.
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I would like the brass bolt's threads contacting the tapped threads in the copper bar with as much surface area as possible, even though it is not required, from an electrical resistance point of view. I know the stud is not for passing current, only securing the ring terminals against the flats of busbar or to each other, but I am going for overkill.

Anyone got experience using thread forming taps, and have some wisdom to share?

I have a 3/8-16 thread cutting tap, but am willing to click place order on a thread forming tap, or two, one for 1/4-20 as well.

Interesting note, is drilling that 3/8" hole, I was first trying to use a 1/8" bit as a pilot hole. At first with no cutting oil. Got a few copper chips then progress stopped.
Tried some 10w-30 and got about 2x as many chips before progress stopped.
One site said try using Milk. I put some 2% in the shallow hole, and got 6"+ copper pigtails the next attempt, with same drill bit.
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I have no electrical knowledge of what you are doing, .... but, cutting the threads;

For cutting threads on a thru hole, I would use a two-flute Tap.
A two-flute Tap will push the chips forward.
NO backing off the Tap, just screw in (like a bolt).

Versus a 3 or 4 flute Tap that you have to back-off every half turn (that breaks the chip up).

And of coarse you want the Tap to go in straight
and use some lubricant.
 
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For cutting threads on a thru hole, I would use a two-flute Tap.
A two-flute Tap will push the chips forward.
NO backing off the Tap, just screw in (like a bolt).

Versus a 3 or 4 flute Tap that you have to back-off every half turn (that breaks the chip up).
Respectfully, this is not accurate information.

The number of flutes has zero impact on the direction the chips move. Chip direction (forward or reverse) is driven by the geometry at the tip of the tip.

I have linked an excellent overview of tap styles and when each style is appropriate to use. Thread forming taps are typically used where very strong threads are required. Thread forming taps require a slightly larger tap drill than a standard cutting tap to account for the fact that the threads are formed by deforming the material as opposed to removing (cutting) the material. Thread forming taps are generally harder for shade tree type guys to work with and are more likely to break during use.

For OP's circumstances cut threads are completely acceptable.

FWIW tap drill sizes for the OP's application are as follows:
3/8-16 cut tap - 5/16 tap drill
1/4-20 cut tap - 13/64 tap drill

Form tap drill sizes will vary depending on the formability of the material. With copper being the material for this application, the form tap will be much more forgiving than say, a stainless steel. Manufacturers will typically provide a matrix of form tap drill sizes based on the percentage of threads that are formed. Note that recommended form tap drill sizes will vary from manufacturer to manufacturer. Use the form tap drill sizes that your tap manufacturer recommends. I have included an example of a form tap drill chart for reference.





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thanks! great detail on seemingly simple tapping. brother-in-law gave me some taps in sets of 3, just said the names but no explanation, so now i know!!! sounds like a drill press would suit the job, lots of good enough cheapies or even one that holds your electric drill
 

wrcsixeight

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Imp4,
Your response is likely the most helpful response to any question I have ever posted on the internet.

Thank you for your efforts to share your knowledge in such a precise, concise yet detailed manner, It is much appreciated.

Due to having a lot of Stainless machine screws on hand, I have been drilling and tapping wood with impressive results, even soft plywoods.
Hardening the cut threads with superglue or epoxy, and the pullout strength is impressive, as well as being resistant to moisture intrusion/ wicking.

Copper is a strange medium to work with. My previous best efforts to make my own large faced copper nuts for lots of surface area between nut and stud, and ring terminal to nut, were less than stellar, but my best results were accomplished by not running the tap all the way through, and then letting the stud itself form the remainder of the threads. The same technique I use on Wood, though I will use a slightly smaller drill bit with wood.

I've also successfully tapped fiberglass for machine threads, and this is also challenging, it seems wise to seal the cut fiberglass with more epoxy or superglue, and let cure before returning the bolt/screw. Going slow, backing off frequently and using a vaccuum to remove shavings also helps the results

I ordered the thread forming taps, as I want to experiment and I think they would be better on tapping soft and hardwoods and I have a plethora of 1/4-20 stainless steel bolts.

The one suggestion to use milk as a cutting agent when drilling and tapping copper was eyebrow raising, and even more so when 6 inch long copper pigtails formed when trying it.

Perhaps whole milk would be even better than 2%
 
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I realize I'm late to the party, but I've learned over the last thirty years of tool & die work that form taps are best for nonferrous and cutting taps for ferrous. The form taps extrude the thread and in a sense, it is forged. Plus, the tool pressure is MUCH less that a cutting tap.
 
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We don’t even go this overboard in industrial high amperage situations. Most applications use standard steel hardware with conical washer. The conical washers are the key part to deal with metal migration. Very little of the current flows through the hardware.

I assume this is for a car audio system?
 

wrcsixeight

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The milk experiment worked insanely well.

So much for reading comprehension...l.o.l

Mininmizing voltage drop to a multi kw 12v inverter at max load..., is the reason for near maximum overkill.

The big mass of copper will help keep shunt cooler and the ammeter nd columb counter more accurate.The 3/8" thick bar of copper was not much more than the 1/4" thick.

If 3/8"-16 brass bolts prove too weak...which i consider unlikely...stainless could be substituted.

My stereo is fed with 10awg. 200 watts max.
 

Job

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Not about formed threads but putting a split steel lock washer on the bolt should help keep it from turning when you tighten the nut, which is trying to loosen the bolt when tightened. How tight can you tighten a brass bolt anyway, not that tight. Stainless would be better. Higher grade steel even better. The bolt thread would have to be made on a lathe if you want a select fit. Very hard to do maybe impossible. How can you check the fit with a chunk of bar copper. You are using off the shelf bolts. When the bolt is tightened you will have lots of contact in the thread. I think the wire connector itself is the weakest link just from the pictures.
 

wrcsixeight

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Salt air is a consideration, but its a terrestrial application.

In no rush, I've been taking my time with this.

I did form one 3/8-16 receptacle in the copper bar and with the brass bolt torqued pretty well, it does not back off when torquing the ring terminal to the stud. I did not drive the form bit all the way through, but let the brass bolt itself finish the job.

I made a few 1/4-20 receptacles using the 2% milk as the lubricant. Slow going, afraid to break the form tap.
After 3 succesful taps, I tried the PB blaster instead of milk, and promptly broke the form tap in the copper bar on stud # 3. It did not feel like I was applying any more torque when it shattered, compared to the 2% milk.

Used 1/4-20 cutting taps there after, which is faster and easier in the 3/8" thick copper bar than the thread form tap was, before it broke.

Same technique though, not driving the tap all the way through, but letting the lubed 1/4"x20 SS bolt itself finish it off, and it is firm and does not back off when torquing off the ring terminal.

It's really quite a time consuming process.

I have to get some 5/16" fasteners to make a stud for that size ring terminal as well.
 
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Salt air is a consideration, but its a terrestrial application.

In no rush, I've been taking my time with this.

I did form one 3/8-16 receptacle in the copper bar and with the brass bolt torqued pretty well, it does not back off when torquing the ring terminal to the stud. I did not drive the form bit all the way through, but let the brass bolt itself finish the job.

I made a few 1/4-20 receptacles using the 2% milk as the lubricant. Slow going, afraid to break the form tap.
After 3 succesful taps, I tried the PB blaster instead of milk, and promptly broke the form tap in the copper bar on stud # 3. It did not feel like I was applying any more torque when it shattered, compared to the 2% milk.

Used 1/4-20 cutting taps there after, which is faster and easier in the 3/8" thick copper bar than the thread form tap was, before it broke.

Same technique though, not driving the tap all the way through, but letting the lubed 1/4"x20 SS bolt itself finish it off, and it is firm and does not back off when torquing off the ring terminal.

It's really quite a time consuming process.

I have to get some 5/16" fasteners to make a stud for that size ring terminal as well.
Did you use the correct size drill for the form tap? Because they require a different size than cutting taps. I've tapped a million holes in copper and most were 4-40 or smaller, 0-80 being the smallest. I never had any trouble at all.
 

wrcsixeight

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No. I did not have the perfect size bit for 1/4-20.

I went a little small then reamed it out with diamond bit on dremel a bit larger. Not large enough though.

I doubt i would have broke the 14-20 form tap....if i stayed with 2% instead of trying pb blaster instead.

Remember this is 3/8" thick copper. The succesful form tap holes threads add 3/32" thickness. The cutting taps add no thickness

The cutting taps not driven all the way through grab the screw tight enough to call it a stud. In my book.
 

wrcsixeight

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I will need to attach twelve 1/4" ring terminals to this bus bar.
3 can easily stack on each 1/4 inch stud, as shown, 4, if I flip the first ring terminal.

Too bad there is a broken thread forming bit in it, but I still have room for another 1/4-20 stud.

I have one 5/16-18 tapped( cutting tap) , but am awaiting SS 18-8 bolts to arrive before seating the new stud fully.
 

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