Internal & External Threading on mini lathe

[redvan22]

Hi,
I want to make a capped tube with a screw in cap out of 303 stainless. I'm not sure about the diameter at this point because I'm unsure about the threading aspect and that may change the diameter, unless I can cut any thread at any diameter (please forgive the lack of proper terminology). I'm thinking of a total length of 1 1/4" and a diameter around 1/2 - 9/16". The cap will have a turned down section at the end so I can drill a hole for a key ring. I'm hoping for a fine thread so I can get several turns of the cap to ensure a tight fit. I want the cap to have external threads so it screws into the tube leaving a seamless joint maintaining the tube diameter when finished.

Zero experience using the gears and cutting threads but I need to know how to do this for an upcoming project. I've cut threads on a lathe before in shop but then used a tap to make the matching threads in a hole. Never made both on a lathe.

I'm thinking about making the cap first, 1/4" in length, plus threads. Parting it off and then making the tube.

I figure if I make the cap first leaving enough to turn down the end for the key ring, then make the tube, I can assemble both pieces, turn down the end, and take a finishing cut across the entire length then part it off.

I know I'll have to change the gears for all the operations but that's not a big deal. (Lathe I used in shop simply had levers to set gear ratios, not a mini lathe. Need to actually change gear configurations just in case anyone reading didn't know this.)

What I need to know is this:
- Can I cut threads of any desired diameter, both internal and external or do I need to adhere to standard sizes?
- It seems to me that cutting 28 threads per inch on 1/2" diameter stock could also be cut on a 1" diameter stock so starting diameter doesn't matter, does it?
- Anything I need to know about cutting internal threads? I want to use a fine thread so I can get enough turns of the cap to ensure a tight secure fit. I realize I will need to make an internal threading tool. Buying one doesn't seem practical right now since this project is so small.

I know I'm overlooking some things but that's why I'm here.
Anyone see something(s) I'm missing and of course, answers to my questions will be much appreciated.

Michael

 

[Jim F]

You can cut whatever TPI on whatever Dia. you feel like cutting.

 

[homebrewed]

Hi,
I want to make a capped tube with a screw in cap out of 303 stainless. I'm not sure about the diameter at this point because I'm unsure about the threading aspect and that may change the diameter, unless I can cut any thread at any diameter (please forgive the lack of proper terminology). I'm thinking of a total length of 1 1/4" and a diameter around 1/2 - 9/16". The cap will have a turned down section at the end so I can drill a hole for a key ring. I'm hoping for a fine thread so I can get several turns of the cap to ensure a tight fit. I want the cap to have external threads so it screws into the tube leaving a seamless joint maintaining the tube diameter when finished.

Zero experience using the gears and cutting threads but I need to know how to do this for an upcoming project. I've cut threads on a lathe before in shop but then used a tap to make the matching threads in a hole. Never made both on a lathe.

I'm thinking about making the cap first, 1/4" in length, plus threads. Parting it off and then making the tube.

I figure if I make the cap first leaving enough to turn down the end for the key ring, then make the tube, I can assemble both pieces, turn down the end, and take a finishing cut across the entire length then part it off.

I know I'll have to change the gears for all the operations but that's not a big deal. (Lathe I used in shop simply had levers to set gear ratios, not a mini lathe. Need to actually change gear configurations just in case anyone reading didn't know this.)

What I need to know is this:
- Can I cut threads of any desired diameter, both internal and external or do I need to adhere to standard sizes?
- It seems to me that cutting 28 threads per inch on 1/2" diameter stock could also be cut on a 1" diameter stock so starting diameter doesn't matter, does it?
- Anything I need to know about cutting internal threads? I want to use a fine thread so I can get enough turns of the cap to ensure a tight secure fit. I realize I will need to make an internal threading tool. Buying one doesn't seem practical right now since this project is so small.

I know I'm overlooking some things but that's why I'm here.
Anyone see something(s) I'm missing and of course, answers to my questions will be much appreciated.

Michael

If you are single-point cutting both the inside and outside threads you can put any arbitrary TPI's on any diameter you want. They just need to fit after you're done.

 

[Bi11Hudson]

Right up front, practice quite a bit before committing useful metal until you have a "feel" for threading odd sizes. Pvc pipe is a good practice medium. Single point threading is the grand-daddy of taps and dies. You can cut any thread, both internal and external. I use 40 TPI a lot because it has a direct relationship to machines. (0.025' per rev) You wouldn't want to cut 10 TPI on thinwall conduit, the thread should be small enough to fit the tubing. Other than that, feel free to use whatever thread you need.

.

 

[WobblyHand]

You can cut whatever TPI on whatever Dia. you feel like cutting.

You are the machinist - you can cut whatever threads you like! Might take you a try or two to get nicely mating threads, but that is part of the fun and the learning.

Here's a picture of the first successful mating threads that I did. A first attempt at a sealed container. Used an o-ring in the thread groove.
Please forgive the background clutter.

 

[redvan22]

You are the machinist - you can cut whatever threads you like! Might take you a try or two to get nicely mating threads, but that is part of the fun and the learning.

Here's a picture of the first successful mating threads that I did. A first attempt at a sealed container. Used an o-ring in the thread groove.
Please forgive the background clutter.
View attachment 425058

That is EXACTLY what I had in mind only small and without the knurl. I would use the key ring instead.

 

[redvan22]

Right up front, practice quite a bit before committing useful metal until you have a "feel" for threading odd sizes. Pvc pipe is a good practice medium. Single point threading is the grand-daddy of taps and dies. You can cut any thread, both internal and external. I use 40 TPI a lot because it has a direct relationship to machines. (0.025' per rev) You wouldn't want to cut 10 TPI on thinwall conduit, the thread should be small enough to fit the tubing. Other than that, feel free to use whatever thread you need.

.

Yes, good idea. I have plenty of other material that would suffice as practice material. Wanted to use the SS to prevent rusting and for the look.

 

[redvan22]

As for cutting the internal vs external, the machine runs in forward gear and cuts from right to left when cutting external threads. The same for cutting internals (I know this sounds silly but I'm having trouble getting my head around it since I never did internals before.)

 

[Huub Buis]

For threading large diameters on the mini lathe, I prefer a small pitch, typically 0.5 mm. The outer thread is the easyest to make and check (3 wire measurement). I make the outer thread first (on a test piece) en use this to check the inner thread.
I can (and do often) manual operate the CNC lathe but not for threading. So I use CNC for all threading jobs.
For testing , learning and experimenting, I use a HSS threading tool. These are more robust than a threading insert. Threading works best if you use some kind of lubrication

 

[WobblyHand]

When I did mine, I threaded from the backside of the bore outwards. I used a left hand threading bar for this and reversed the spindle. You can also use a right hand threading tool flipped upside down, with the cutting edge at the workpiece center (and the rest of the tool above the center).

Search for Joe Pie reverse threading. Then the carriage moves toward the tail stock. (Much lower pucker factor for a beginner, than doing a normal inner thread blindly.) I made a start groove inside the part and then threaded outwards. I'd do the inner threads first, then match the cap to the inner threads.

If you want to thread with a normal right hand tool, conventionally, then I suggest mounting a dial indicator to tell you when to stop! Nothing like crashing your lathe - even a mini-lathe. I have, it isn't good. And yeah, go as slow as you can go and not stall...