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What to do about shallow threads? (help!)

fafrd

Solar Wizard
Joined
Aug 11, 2020
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I just got in two new 280Ah cells which look great but the tapped aluminum threads are very shallow (3 to 3-1/4 threads deep).

After already having stripped a thread that was only 4 threads deep and having characterized that all of my other threads were ~5 threads deep, I’m pretty certain that even if I glue a grubscrew into these pristine shallow threads, it’s unlikely to take 35 inch-lbs and will probably pull out.

So I am very eager to hear anyone’s advice as to how best to salvage these shallow-tapped threads.

I’m thinking I may try to still deeper holes and modify an M6 tap to make it a bottoming tap in an attempt to get another 2 threads down.

Has anyone else gotten cells with so few threads? Anyone else either had no issues with 3 full threads withstanding 35 inch-lbs or had such shallow threads strip?

Anyone else attempted to tap shallow M6 threads deeper?
 
I vote you drill it slightly deeper using a drill press with a depth gauge. Set the depth by bottoming out the bit in a good cell. Buy a bottom tap (and a tap holder, if you don’t already have one) and tap it by hand, it’s very easy for aluminum.
 
If you can find a friendly person with a laser welder I would suggest you laser weld studs into these and all your cells. It eliminates the concept of stripping threads. There is some controversy about then having a smaller direct contact area, so you can consider that too. It would not concern me.
 
If you can find a friendly person with a laser welder I would suggest you laser weld studs into these and all your cells. It eliminates the concept of stripping threads. There is some controversy about then having a smaller direct contact area, so you can consider that too. It would not concern me.

Yeah, I think I’m the one that first raised those concerns (valid or not).

All of my other cells have grubscrews Loctited or JB Welded in, so I’m going to have to find a way to secure a grubscrew into these terminals as well...
 
I vote you drill it slightly deeper using a drill press with a depth gauge. Set the depth by bottoming out the bit in a good cell. Buy a bottom tap (and a tap holder, if you don’t already have one) and tap it by hand, it’s very easy for aluminum.

I’ve already drilled one 15/16” hole to 7mm without much difficulty (using a drill press & turning by hand).

The problem with that hole is that it was not perfectly straight (defective drill-press).

I’m not too concerned about drilling too deep but I am concerned about chewing up the existing M6 threads in the process...

M6 is supposed to be 4.917mm inside diameter, so I may use a 3/16” bit (4.7625mm) and even to protect the existing threads with a cylinder of paper or tape (standard printer paper is 0.1mm thick so may need to use thermal paper or drop down to a 4.5mm drill bit...

And instead of using the drill press this time around, I’ll probably try using my new tapping guide and tap handle.

If the existing threads are protected, they should keep the bit centered.

I’m worried an M6 tap will also screw up the existing threads, but there is nothing I can do about that (except be careful).

If I can drill down to 7mm with a standard (pointed) drill bit, I should be able to add a minimum of 2 more usable M6 threads (with 2 slightly tapered threads beyond that).

What fun. ;).
 
If the diameter allows you could just drill out the threads to something bigger than M6, like M7 or M8 (duh!). Taps are cheap, like $5-10 even for a good one. Or $0.25 for a bad one which would also work fine in aluminum.
 
If the diameter allows you could just drill out the threads to something bigger than M6, like M7 or M8 (duh!). Taps are cheap, like $5-10 even for a good one. Or $0.25 for a bad one which would also work fine in aluminum.

That’ll be my fall-back plan if extending the existing M6 threads fails. I’ve already had to go that to one cell I stripped and understanding how much larger holes reduce the terminal contact surface, I’m loathe to drill bigger holes unless there is no other choice.
 
If you can find a tear down picture of the inside of the battery you might be able to judge how big of a chunk of aluminum you’re drilling into. I have only seen a handful of such pictures but my sense is that M6 to M8 is one more millimeter in each direction which is insignificant and won’t compromise the structure of the block of aluminum you’re tapping into.
 
If you can find a tear down picture of the inside of the battery you might be able to judge how big of a chunk of aluminum you’re drilling into. I have only seen a handful of such pictures but my sense is that M6 to M8 is one more millimeter in each direction which is insignificant and won’t compromise the structure of the block of aluminum you’re tapping into.

I’ve already drilled down a hair over 7mm (into another cell I stripped), so drilling (carefully) to that depth is not an issue in these 280Ah cells.

On hole diameter, the entire terminal is 13mm wide all the way down, so no structural issue at all going from an M6 hole to an M8 hole.

The concern with a larger hole is electrical - an M8 hole reduces accessible terminal surface area by over 21% and increases contact resistance at equal clamping force by 27%...

I may try to characterize contact resistance at lower torque levels to assure reaching 35 inch-lbs really makes a difference before going to all of this trouble.

A terminal with 5 full threads can withstand 35 inch-lbs of torque, so these shallow holes only 3 to 3-1/4 threads deep should be able to withstand 20 inch-lbs without stripping.

I may attempt to use the dV/dI measurement technique to see what difference I get in contact resistance (voltage drop) running 10A through a lug/busbar clamped down with 35 inch-lbs versus 20 inch-lbs...
 
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Go ahead and drill, I've gone a 3/8" into a batch of 280ah cells that didnt come with threaded holes. The first one I drilled and tapped, second cell and on I just drilled and ran the stainless bolt directly into the untapped hole. The aluminum is so soft it easily self-tapped. And it gave a hell of a good bite to the metal too.

It does make a bit of a mound next to the bolt but a quick bit of cleaning up with a file fixed that. Skipping the tapping portion made it a hundred times quicker and easier.
 
Go ahead and drill, I've gone a 3/8" into a batch of 280ah cells that didnt come with threaded holes. The first one I drilled and tapped, second cell and on I just drilled and ran the stainless bolt directly into the untapped hole. The aluminum is so soft it easily self-tapped. And it gave a hell of a good bite to the metal too.

It does make a bit of a mound next to the bolt but a quick bit of cleaning up with a file fixed that. Skipping the tapping portion made it a hundred times quicker and easier.

Interesting idea, thanks.
 
Put a 2 or 3 cm, m6 bolt or all thread in the threads.

On the bolt or all thread put two separate nuts. You DO NOT need more than three turns to secure the bolt or all thread. It just needs contact. with the terminal.

Use the double nuts to be tight with the wire leads.

20210327_235551.jpg 20210327_235619.jpg
 
Put a 2 or 3 cm, m6 bolt or all thread in the threads.

On the bolt or all thread put two separate nuts. You DO NOT need more than three turns to secure the bolt or all thread. It just needs contact. with the terminal.

Use the double nuts to be tight with the wire leads.

View attachment 42859 View attachment 42858
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I’m interested in any evidence you have you support that assertion.

I’ve only stripped one out of 32 terminals, and that terminal was shorter than all the others by ~1 thread (4 full turns versus ~5 full turns in all of the others).

I was torquing all of those terminals to the recommended 35 inch-lbs / 4Nm and while all of the deeper 5-turn threads held, the ~4 turn thread slipped and stripped.

So I’m pretty sure these even shallower threads with only 3 full turns are not going to take much over 20 inch-lbs without stripping / pulling out...

[EDIT: p.s. and reason grubscrews are far superior than a long bolt with two nuts is that if/when that battery needs to be disassembled, removing the stainless bolts will result in galling which will quickly wear out the soft aluminum thread. Ideally, you want to want to exercise those soft aluminum threads once and only once - to Loctite in / permanently secure a stainless grubscrew.]
 
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Who did you buy your cells from? 3 threads is ridiculous.

I'd still try red loctite. But I have no problem fixing it if the threads tear out.

Red loctite should help a lot. But 3 threads is weak.

I think you will get 35 in lbs with loctite.

But if you push the stud sideways the threads will tear out. You could probably do it with your thumb.

So, shorter studs would be better
 
Who did you buy your cells from? 3 threads is ridiculous.

I'd still try red loctite. But I have no problem fixing it if the threads tear out.

Red loctite should help a lot. But 3 threads is weak.

I think you will get 35 in lbs with loctite.

But if you push the stud sideways the threads will tear out. You could probably do it with your thumb.

So, shorter studs would be better

These cells came from Shenzen Basen. They look much better than the earlier cells I’ve gotten from them - hopefully true grade A.

The terminal threads are all uniform depth now and they shipped with grubscrews and nuts. Everything looks shines and brand new compared to the earlier cells that had some markings / scratches / oxidation on the terminal surface.

The only issue is the threads have been tapped to shallow.

I’m happy to hear you think Loctite in 3 threads would hold 35 inch-lbs, but I think I’m going to try drilling deeper.

I’m still working on salvaging my stripped terminal and once I’ve succeeded at that, I’ll hopefully have your same confidence about repairing any terminals that strip.

But these shallow threads are even shallower than the one I stripped earlier, so I feel like I need to do something to strengthen / increase them...
 
I've got the same cells from Basen. Grub screws would thread in about 3 turns. I made a bottom tap by cutting down a standard tap and cut the threads until the tap bottomed in the hole (I measured the depth of each hole before tapping). After cleaning, I could then get between 3-1/2 to 4 turns of the screw (not much better but better than not doing it). I then used red Loctite and let it dry for 4 days. I haven't tested all 8 of my cells, but the few I have tested hold 35inlb without stripping. Hopefully the others will hold too. If not, I'll then resort to helicoils.
 
These cells came from Shenzen Basen. They look much better than the earlier cells I’ve gotten from them - hopefully true grade A.

The terminal threads are all uniform depth now and they shipped with grubscrews and nuts. Everything looks shines and brand new compared to the earlier cells that had some markings / scratches / oxidation on the terminal surface.

The only issue is the threads have been tapped to shallow.

I’m happy to hear you think Loctite in 3 threads would hold 35 inch-lbs, but I think I’m going to try drilling deeper.

I’m still working on salvaging my stripped terminal and once I’ve succeeded at that, I’ll hopefully have your same confidence about repairing any terminals that strip.

But these shallow threads are even shallower than the one I stripped earlier, so I feel like I need to do something to strengthen / increase them...

The vast majority of the load in a simple threaded application under tension is handled by 3-4 threads. However, the LAST 1.5 threads of a tapped hole do not count as there is less backing material, so you would want at least 5 visible complete threads from the cut entrance.

Threads are not perfectly spaced, nor are their mating components. Even if you get 10 turns engaged, you're not going to have all 10 thread forms in contact.

Lastly, it helps to think of it in terms of tooth shear vs. tensile failure. The cross section of a bolt can handle X load. 3-4 thread teeth have an equivalent shear strength.
 
I've got the same cells from Basen. Grub screws would thread in about 3 turns. I made a bottom tap by cutting down a standard tap and cut the threads until the tap bottomed in the hole (I measured the depth of each hole before tapping). After cleaning, I could then get between 3-1/2 to 4 turns of the screw (not much better but better than not doing it). I then used red Loctite and let it dry for 4 days. I haven't tested all 8 of my cells, but the few I have tested hold 35inlb without stripping. Hopefully the others will hold too. If not, I'll then resort to helicoils.

I just ordered an M6 bottoming tap and will try to tap deeper.

I’ve drilled another terminal to 7mm, so I’m going to try to drill deeper before tapping.

The fact you reached 35 inch-lbs with only 3-1/2 threads is encouraging.

My earlier cells from Basen all had ~5 full threads and have taken 35 inch-lbs after being Red Loctited in without fail.

I inserted a Helicoil into one stripped thread (that had less than 4 full threads and stripped at 35 inch-lbs) but it stripped out at 25 inch-lbs with brass nut (corresponding to just under 30 inch-lbs with a stainless nut).
 
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