Cutting BB threads before vs after welding in practice

[hans736]

I'd appreciate feedback from anyone who has practical experience with cutting threads into bottom brackets before and after welding the connecting tubes. I know that in theory the welding heat can distort the bottom bracket (unround, misalignment). As far as I know in particular a lack of alignment of the threads on the drive side vs non drive side can cause creaking and the crank to not spin freely.

Anyone with experience how much cutting threads before vs after welding actually effects creaking and free crank spin (assuming one uses basic heat dissapation techniques)?

And does a tool like this https://ibb.co/bH07Z2S help to reliability tell whether drive side and non drive side threads are aligned / have been effected by the welding heat (aligned enough at least to not matter in practice)?

Context: It's a titanium frame.

[Rich Beaudoin]

I only have experience with steel and I use the same Park tool to chase/face bottom bracket threads.
With that said, I’ve never had significant warping. Never had any difficulties getting the tool into the threads (unless you count powdercoating at the edge). And that tool is fine to get the faces parallel to the tolerance needed for bikes if you swap to the facing cutter and leaving the opposite side inserted for alignment.
I have a friend who welded only one frame (titanium), blew holes in the seat tube from too much heat, and the bottom bracket was still fine and doesn’t creak.

[VertigoCycles]

I'd appreciate feedback from anyone who has practical experience with cutting threads into bottom brackets before and after welding the connecting tubes. I know that in theory the welding heat can distort the bottom bracket (unround, misalignment). As far as I know in particular a lack of alignment of the threads on the drive side vs non drive side can cause creaking and the crank to not spin freely.

Anyone with experience how much cutting threads before vs after welding actually effects creaking and free crank spin (assuming one uses basic heat dissapation techniques)?

And does a tool like this https://ibb.co/bH07Z2S help to reliability tell whether drive side and non drive side threads are aligned / have been effected by the welding heat (aligned enough at least to not matter in practice)?

Context: It's a titanium frame.

Just to clarify, threads are typically already cut when the bottom bracket is machined and then they're chased/faced after being welded. They WILL need to be chased and faced after being welded. It's completely possible to build a frame where you can easily thread in a BB after the welding is complete BUT I'd need to see some real proof if someone claims that the drive/non-drive bores are concentric and the faces are parallel after they've welded a frame.

The tool you linked will chase the threads and will make them reasonably concentric, but without a face cutter it'll only do half the job. Most BB's use the face of the shell as a stop. The threaded bore positions the bearings to be nearly contentric but it's the face, with the correctly torqued applied, that holds the BB in position. If it's not faced, chasing the threads won't help a whole lot.

I'm curious why you're asking the question.

[progetto]

When building with aluminium, I always use a blank shell and cut the threads after welding, aluminium generally see's more distortion than steel and cutting threads after makes for a much crisper thread.

[hans736]

I only have experience with steel and I use the same Park tool to chase/face bottom bracket threads.
With that said, I’ve never had significant warping. Never had any difficulties getting the tool into the threads (unless you count powdercoating at the edge). And that tool is fine to get the faces parallel to the tolerance needed for bikes if you swap to the facing cutter and leaving the opposite side inserted for alignment.
I have a friend who welded only one frame (titanium), blew holes in the seat tube from too much heat, and the bottom bracket was still fine and doesn’t creak.

That sounds like the threads on your bottom brackets were cut before you welded. After welding you chased and faced the bb. That process leaves you with no creaking and free spinning cranks. Did I understand that right?

Do you know whether titanium is more or less prone to heat distortion than steel?

[hans736]

I'm curious why you're asking the question.

I guess "out of ignorance". I thought that threads would be cut into the bb after welding and that that was the only way to guarantee a creak free and free spinning bb. This is certainly the impression one gets when "googling" something like: https://www.google.com/search?q=cutt...r+welding#ip=1

The bigger context is this: I am trying to assess the quality of two different titanium builders.

The following is a short summary of the terms as I understand them:
Tapping = cutting threads or recutting threads.
Chasing = Reaming = removing paint and metal shavings.
Facing = making the BB surfaces level.

It's completely possible to build a frame where you can easily thread in a BB after the welding is complete BUT I'd need to see some real proof if someone claims that the drive/non-drive bores are concentric and the faces are parallel after they've welded a frame.

I thought cutting threads helps exactly in such a situation (when the BB was distorted)?

The tool you linked will chase the threads and will make them reasonably concentric, but without a face cutter it'll only do half the job.

Can you elaborate what you mean by "concentric"?
I known that the term is used when two circles have the same center, one bigger than the other. But I don't know how it makes sense in this context. Do you mean alignment, i.e. if you drew two lines perpendicular to the face of the bottom bracket and through the center of each bb bore (drive side and non drive side) then these lines match / are identical?

Overall you are saying that, as long as I chase and face a titanium bottom bracket after it was welded, I am fine if the threads are cut before the welding and that this is the standard process. Did I understand you correctly?

[hans736]

When building with aluminium, I always use a blank shell and cut the threads after welding, aluminium generally see's more distortion than steel and cutting threads after makes for a much crisper thread.

I'm curious what kind of distortion you see after welding:
- lack of roundness
- lack of alignment which, I guess, goes along with the faces of the bb not being parallel to each other
- other?

I still don't 100% understand which if these distortions, if they occur, could be fixed and with what tools.

Here are two examples of what I mean by "out of alignment":
https://ibb.co/dcB5Gp9
https://ibb.co/chCScGZ

[Rich Beaudoin]

That sounds like the threads on your bottom brackets were cut before you welded. After welding you chased and faced the bb. That process leaves you with no creaking and free spinning cranks. Did I understand that right?

Yes, that’s how I build and many builders use this process successfully.
Pre-threaded bottom brackets are cheap and easy to use to source.
Cutting threads into a blank pipe is a process best done on a lathe. The tool you have is good for cleaning ‘chasing’ threads and for facing the ends (if using the correct part not shown in your pic).
And I don’t know which material is more prone to heat distortion.

[hans736]

Yes, that’s how I build and many builders use this process successfully.
Pre-threaded bottom brackets are cheap and easy to use to source.
Cutting threads into a blank pipe is a process best done on a lathe.
And I don’t know which material is more prone to heat distortion.

Great, thanks for clarifying!

[progetto]

I'm curious what kind of distortion you see after welding:
- lack of roundness
- lack of alignment which, I guess, goes along with the faces of the bb not being parallel to each other
- other?

I still don't 100% understand which if these distortions, if they occur, could be fixed and with what tools.

Here are two examples of what I mean by "out of alignment":
https://ibb.co/dcB5Gp9
https://ibb.co/chCScGZ

out of roundness and bowing of the shell, aluminium is more prone to this, not so much with steel and titanium, heat sinks help.

[Chik]

The bigger context is this: I am trying to assess the quality of two different titanium builders.

I know next to nothing about building bike frames but have been in a trade where I work with makers in other product categories. And, here's my thought on your situation:

If you're having to assess a builder based on what he does with the BB bracket, strike that builder off your list. If he's causing you concern about a detail like that, it's likely that that'll be the least of your problems.

If you're focussing on a detail like that in order to select/eliminate one of the two builders, I'm going to guess that you're misdirecting your decision process from whatever that's actually driving your decision, eg, price. If that's the case, be honest with yourself. Not trying to be presumptuous here even though I realise I do sound like it.

[steve garro]

Who cuts threads in a welded Ti bike, huh?
Who?

Only thing near this I have heard of was IIRC a press-fit to T47 retrofit which from the sound of it was dicey as hell and expensive

- Garro.

[false_aesthetic]

I know next to nothing about building bike frames but have been in a trade where I work with makers in other product categories. And, here's my thought on your situation:

If you're having to assess a builder based on what he does with the BB bracket, strike that builder off your list. If he's causing you concern about a detail like that, it's likely that that'll be the least of your problems.

If you're focussing on a detail like that in order to select/eliminate one of the two builders, I'm going to guess that you're misdirecting your decision process from whatever that's actually driving your decision, eg, price. If that's the case, be honest with yourself. Not trying to be presumptuous here even though I realise I do sound like it.

What Chik said.

If there's a doubt with this part of the frame building process there should be other doubts leading up to it (e.g. tubing prep + cleanliness.)

[hans736]

What Chik said.

If there's a doubt with this part of the frame building process there should be other doubts leading up to it (e.g. tubing prep + cleanliness.)

I didn't quite understand this argument.

[hans736]

Overall my conclusion, based on what Vertigo, Steve Garroa and Rich Beaudoin said, is that having threads cut into a titanium bottom bracket before welding is fine and actually the standard. But that facing is essential after welding.

[steve garro]

Overall my conclusion, based on what Vertigo, Steve Garroa and Rich Beaudoin said, is that having threads cut into a titanium bottom bracket before welding is fine and actually the standard. But that facing is essential after welding.

You don't cut them.
You buy threaded shells, then you run a tap through them to "chase" the distortion then a tool which "faces" the outer faces of the BB shell.

I'm of an era where we would thread steerer tubes and such and it was always last option and quite expensive

You / everyone buys threaded shells like this one here unless it's press fit :

https://www.paragonmachineworks.com/...-1-625-od.html

- Garro.

[progetto]

Overall my conclusion, based on what Vertigo, Steve Garroa and Rich Beaudoin said, is that having threads cut into a titanium bottom bracket before welding is fine and actually the standard. But that facing is essential after welding.

This has always been the norm with titanium and steel

[VertigoCycles]

Overall my conclusion, based on what Vertigo, Steve Garroa and Rich Beaudoin said, is that having threads cut into a titanium bottom bracket before welding is fine and actually the standard. But that facing is essential after welding.

There has already been a lot of great follow-up to your original question but I'll add a bit more that might help you understand the "why's" a bit more.

This is what most of us buy: https://www.paragonmachineworks.com/...-1-625-od.html It's a pre-threaded shell

The reason is simple...these are made on CNC machines. They're "cheap" and accurate. The "why" part of it is a little more complicated and it has to do with the actual machining part of the process and what type of tooling is available to the majority of frame builders. The Paragon BB's are machined with a "single point" tool over multiple passes. Google "single point threading" and you're likely to find a ton of information, usually involving lathe turning. You'll see how it works and then see why it's impractical to do this after a frame is welded.

I can't recall off the top of my head what the thread depth of a BSA bottom bracket is. Let's say it's 0.040". If I were to machine one of these, I'd chuck it up in my lathe and would do maybe 6-7 passes on each thread, starting with about 0.008" depth of cut and finishing with a 0.002" depth of cut and a final spring pass. It keeps the cutting pressure down, keeps tool deflection down, zero chatter and a nice finish on the threads. There's ONE cutting edge in this process (the way I do it, it can be done using both sides of the tool and still work out fine) and it's only engaging a max of 0.008". My single point tools are all carbide and I imagine that anyone else who's doing that type of work is using carbide as well. This has some relevance

Another option is to use a taps tool. I want to be clear, this isn't really a viable option in titanium or even other metals, due to a nearly zero chance of success. Before anyone calls me out, the Park, Cyclus, Bringheli BB taps are not intended to cut new threads, only chase them. The reasons why this has almost a zero chance of success is due to the cutting pressures involved. There are multiple cutting teeth on the circumference of the tool and you'd be asking them to cut the full 0.040" in one pass...not exactly because there's a lead-in, but it's still too much to ask of the tool. You can't manage the amount of heat generated by the pressure without heroic measures and the likelihood of the tap getting stuck in the BB is very high. I know more than several framebuilders who have gotten a tap stuck in the BB while just chasing it, let alone trying to cut an entirely new thread. Then there's the fact that you'd have to start with a good bore at the start, which the builder wouldn't have after welding anyway, so the BB would need to be reamed first which adds another unnecessary step to a process that doesn't make sense

The last option that I know of would be if the builder had a CNC mill with a big enough table that they could single-point bore and then threadmill or single-point the threads into the BB shell. With some fancy tooling and programming, this might even be ideal but unnecessary considering the first option. I believe Seven Cycles did it this way

There's your long answer. The short is this: does your builder face, chase and ream all the important places (head tube, seat tube, BB at a minimum) before delivery? If yes, don't worry about it. If no, find someone else.

[hans736]

There has already been a lot of great follow-up to your original question but I'll add a bit more that might help you understand the "why's" a bit more.

This is what most of us buy: https://www.paragonmachineworks.com/...-1-625-od.html It's a pre-threaded shell

The reason is simple...these are made on CNC machines. They're "cheap" and accurate. The "why" part of it is a little more complicated and it has to do with the actual machining part of the process and what type of tooling is available to the majority of frame builders. The Paragon BB's are machined with a "single point" tool over multiple passes. Google "single point threading" and you're likely to find a ton of information, usually involving lathe turning. You'll see how it works and then see why it's impractical to do this after a frame is welded.

I can't recall off the top of my head what the thread depth of a BSA bottom bracket is. Let's say it's 0.040". If I were to machine one of these, I'd chuck it up in my lathe and would do maybe 6-7 passes on each thread, starting with about 0.008" depth of cut and finishing with a 0.002" depth of cut and a final spring pass. It keeps the cutting pressure down, keeps tool deflection down, zero chatter and a nice finish on the threads. There's ONE cutting edge in this process (the way I do it, it can be done using both sides of the tool and still work out fine) and it's only engaging a max of 0.008". My single point tools are all carbide and I imagine that anyone else who's doing that type of work is using carbide as well. This has some relevance

Another option is to use a taps tool. I want to be clear, this isn't really a viable option in titanium or even other metals, due to a nearly zero chance of success. Before anyone calls me out, the Park, Cyclus, Bringheli BB taps are not intended to cut new threads, only chase them. The reasons why this has almost a zero chance of success is due to the cutting pressures involved. There are multiple cutting teeth on the circumference of the tool and you'd be asking them to cut the full 0.040" in one pass...not exactly because there's a lead-in, but it's still too much to ask of the tool. You can't manage the amount of heat generated by the pressure without heroic measures and the likelihood of the tap getting stuck in the BB is very high. I know more than several framebuilders who have gotten a tap stuck in the BB while just chasing it, let alone trying to cut an entirely new thread. Then there's the fact that you'd have to start with a good bore at the start, which the builder wouldn't have after welding anyway, so the BB would need to be reamed first which adds another unnecessary step to a process that doesn't make sense

The last option that I know of would be if the builder had a CNC mill with a big enough table that they could single-point bore and then threadmill or single-point the threads into the BB shell. With some fancy tooling and programming, this might even be ideal but unnecessary considering the first option. I believe Seven Cycles did it this way

There's your long answer. The short is this: does your builder face, chase and ream all the important places (head tube, seat tube, BB at a minimum) before delivery? If yes, don't worry about it. If no, find someone else.

Interesting.

Regrading your last paragraph: I thought chasing equals reaming (removing paint (unusual with titanium) and metal shavings from the threads). Do you understand it differently?

[Rich Beaudoin]

Think of chasing like sharpening a blade only you’re sharpening the tiny threads.
So when it is finished, the threads look clean. If there was paint or rust or anything that would inhibit installation of the BB, it is removed by the chasing tool.