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Which tap/die for 7mm and 8mm OD pipe with .45mm wall? Answered

I want to thread some tube (since I can't find any to buy in the size I need).  I've never used a tap and die before.  My pipes are 7mm and 8mm outside diameter, with .45mm wall thickness.  I need threads on both sides so they'll screw together.  No, I'm not trying to screw the 7mm into the 8mm; I'm just not sure which size will work best.




It sounds like you want to thread the outside of one pipe, and the inside of another; you wrote, "so they'll screw together." In that case, you'll have to use different size pipes: the OD of one must match the ID of the other (which is almost the case with your pipes. With your tubes (7.1 mm ID vs. 7 mm OD), the threads are not going to mate properly: the joint will be floppy, and the threads will slip past one another.

An alternative is to thread just the outsides of tubes all with the same OD, and buy small spacers where you can ream out the ID to match and then tap it.

You don't need a threading machine for 7 or 8 mm tube. A manual tap and die set, which any well-stocked hardware store will carry, is sufficient. If you plan to do this a lot, or want to learn how to do it yourself for future projects, then go ahead and buy one.

If this is just a one-time project, investing in a t&d set is probably overkill. Call around to local machine shops in your town (auto body repair, motorcycle repair, metal shops) and ask if they'd be willing to thread the tubes for you.

I was thinking that maybe .45mm was a standard size wall thickness, and there would be .225mm taps and dies. Guess that's not the case. I did also find some 8mm tube with .75mm wall, so maybe it will work with the 7mm. And I think you guys are right, I am going to try calling around to see if somewhere will do it for me.

The wall size is only part of the issue.

For an outside thread, the two key parameters are the OD of the tube, and the thread depth (which you can find by doing a Google search on the OD and the thread pitch you've chosen). If the thread depth is more than the wall thickness, you're hosed -- your die will make a lovely helical cutting, but that's it.

For an inside thread, it's the same: the ID and thread depth are the standard measurements.

An 8 mm tube with 0.75 mm wall has an ID of 6.5 mm (do the math), so it won't work with your 7 mm tubing, unless you ream it out to 0.5 mm wall thickness.

Kelseymh, if "the OD of one must match the ID of the other," then won't the smaller tube simply fall through the larger? I need them to screw together. So don't I need them to either be the same size and thread them half way, or the ID of one and OD of the other must overlap?

Since there doesn't seem to be any tap/die smaller than .5 for 7mm or 8mm, could I use a 7x1.25 tap and 9x1.25 die on the 8mm tube with the .45 wall? Or should I just not be trying to do math when it's late and my brain is tired?

Okay, I am clearly not thinking straight. How do I edit my post?

You can't edit individual comments (that's a feature of the site, to avoid comment-and-edit flame wars). To edit your original posting, you should find a box labelled "Author Options" in the right hand sidebar. There will be an "Edit" button there.

Um, duh. Your question is correct (and I was writing too late at night, as well :-/). When you're making a threaded fitting (which is what you're doing), the rule is that the OD of the threaded inner piece should be equal to (or "just barely" smaller than) the ID + 2*thread-depth of the tapped outer piece.

A Google search for metric thread chart gives you the answers you need. I'm looking at the MARYLAND METRICS charts.

For the 7 mm tube, use M7x1 external (bolt) threading. The "minor diameter (that is, the diameter to the bottom of the threads) can be 5.596 to 5.891 mm (depending on the die). That requires a wall thickness from 0.56 to 0.7 mm, so the threads don't cut through.
M7x1.25 would allow for a thinner wall, but I don't find that in the chart. If you can find it, I think it would be perfect.

For the 8 mm tube, tap M8x1 or M8x1.25 (if you found M7x1.25). For M8x1, the inner diameter is 6.917 to 7.153 mm, or wall thicknesses 0.42 to 0.54 mm, which you have.

By the way, this business of trying to find multiple tube thicknesses which all fit together is a pain in the b**t. I've tried to do it more than once myself, and generally it fails dismally. There are very clear standards for tubing, in terms of "trade size" vs. true OD vs. wall thickness. Those standards do not include having the different sizes fit together. Ever :-(

There is no need to repeat your question. We need more information from you in order to help. Are you working with plastic or metal? If you are working with metal pipes, do you have the pipe threading machine and dies? Without knowing your skill level, this project could be over your head!

I wasn't trying to repeat myself. My question changed from where to buy threaded pipe to which tap/die is needed, so I thought a new post was a good idea. I'm sorry if I should have stuck to the old thread instead.

I have no tools and no experience. The pipes I've been able to find are metal. I am in over my head, but I plan to take some time next weekend to educate myself a bit and decide if trying to pursue this is practical. In the meantime, I was hoping maybe somebody could let me know if the tools are even available for the size pipe I was able to find.

You could rent the tools that you need but, if you take into consideration the time required to learn how to operate the tools, the time to bring those tools back and forth the rental center and the last minute inconveniences that might occur, I recommend that you hire a professional and relax.