- Joined
- Oct 30, 2019
- Messages
- 225
Ever used a tap as an external threading tool?
I am helping someone out with a project that involves a carburator from a very specific name-brand generator from the 60s, which was never sold commercially and the carburator (or replacement parts) is impossible to find.
The issue this person is running into is that the carburator is missing the mixture adjustment screw, it just wasn't in the carburator when it came into their possession. Looking through bins of spare carbs and parts, they found one needle that threads in, but the threads bottom out before the needle reaches far enough in to impact the mixture.
They provided me with that needle, with the goal of duplicating it but extending the primary taper of the needle by 1/4" or so.
Here it is under magnification, as I was trying to figure out what the thread pitch was. None of my thread gauge leafs settled in fully, 0.5mm was closest (it turned out the very tip of that leaf was dinged, but I wanted a second means of verifying the pitch)
I counted 8 threads over 5/32" - which is near spot-on for 0.5mm pitch
These magnified pictures were taken by zooming in on my phone, which was aimed through a magnifying visor
A few challenges to this part to solve for/test:
- Measuring or setting up for the taper
- Material and tool choice to cut such a fine taper without deflection causing issues
- Cutting this metric thread when my lathes are both inch machines and I don't believe I have a transposing gear for either currently
- The major diameter of the threaded section is nominally 5mm (slightly undersized); 5mmx0.5 is not a standard pitch I would have a die for without ordering one, and even then I may not get close enough to the shoulder on the part. That might be solvable by making the part longer, but that could pose challenges with installation and usage on the compact machine.
To set up for the taper, I made a small set screw collar to hold the part, with a wad of paper under the set screw so that it could bear on the threads without damaging them.
Put that in the collet chuck, and indicated the top slide in to the taper which turned out to be nominally 5 degrees.
I decided to make the new needles out of bronze rod which I have on hand from another project, and used a very sharp HSS tool to test a taper, went fine
For the threads... any solution at the lathe would hinge on being able to get a 0.5mm feed rate. I ended up finding a gearbox configuration that was 0.019"/rev, whereas 0.5mm is more like 0.0196"/rev. That's a 3% error which is quite large, and realistically I am not thrilled to single-point thread such a small thread form.
What about something that might compensate for the timing error (at least for 5-10 threads), and let me thread this at the lathe without having to buy or grind a super-clean tiny threading tool?
Like holding a tap in the tool holder, and using it as a form tool to thread the OD of the part?
I have this oddball M8 x 0.5mm ultra fine-thread tap that has been in my drawer of assorted taps for a few years. So it's the right pitch, and the diameter is relatively huge - benefits to make it rigid and make it easier to see to clock it in the toolholder
I clocked it with one of the flutes basically horizontal, just eyeballing where it should have the right rake and clearance.
Here's a test:
This was on an arbitrary diameter I cleaned up on a piece of stock - it worked fine, just needed to get better at deciding my DOC per pass and what depth would result in fully-crested threads. The first result wasn't fully formed but progress:
With those issues solved, onto the first real part (first of two, with different needle lengths to test).
Taper turned, nothing special there - just a very sharp 1/4" HSS tool, and turned the threaded section to 4.8mm. I found in my testing that the tap process would expand the diameter slightly, I think maybe I had it mis-clocked and it was forming the threads instead of just cutting them. If this was for something that needed thread strength, I'd have gone back and adjusted it since any "flowed" material was probably weakened.
I was originally worried that due to the 3% error accumulating, the threads near the end of the part would start getting eroded by the tap as it fed further in, but this does not appear to be the case. Perhaps if the tap was flowing material, it was compensating for any cutting that was occurring. Or, the .0006/rev effective feed rate was so small that even the super-sharp tap couldn't take a cut and was just rubbing the oncoming face, leading to the appearance of flowing.
After that, knurled and parted off.
Extremely happy with the results, made a second one. The second one only took 30-45 minutes now that the process was all ironed out
Full, clean threadform. All of the "is this fully formed" ended up being done by eye, I don't have thread measuring wires nearly small enough for this
If I need to make more, I'll do some more tests by clocking the tap differently (in the view shown above, I'd go slightly counterclockwise so that it has slightly more rake and clearance to reduce the chances that it is flowing material)
I would also use a M3x0.5mm tap to cut internal threads on an M5 hole in a test fixture/gauge, cutting until the original needle threads in. Then I could use that as a go/no-go gauge to test the new parts. If the needles I made so far have any issues threading in, I will need to do this; and if it just turns out I need to do different needle lengths, I'll still do this if the person sends me the original needle back again.
Would love to hear thoughts - thanks for reading
I am helping someone out with a project that involves a carburator from a very specific name-brand generator from the 60s, which was never sold commercially and the carburator (or replacement parts) is impossible to find.
The issue this person is running into is that the carburator is missing the mixture adjustment screw, it just wasn't in the carburator when it came into their possession. Looking through bins of spare carbs and parts, they found one needle that threads in, but the threads bottom out before the needle reaches far enough in to impact the mixture.
They provided me with that needle, with the goal of duplicating it but extending the primary taper of the needle by 1/4" or so.
Here it is under magnification, as I was trying to figure out what the thread pitch was. None of my thread gauge leafs settled in fully, 0.5mm was closest (it turned out the very tip of that leaf was dinged, but I wanted a second means of verifying the pitch)
I counted 8 threads over 5/32" - which is near spot-on for 0.5mm pitch
These magnified pictures were taken by zooming in on my phone, which was aimed through a magnifying visor
A few challenges to this part to solve for/test:
- Measuring or setting up for the taper
- Material and tool choice to cut such a fine taper without deflection causing issues
- Cutting this metric thread when my lathes are both inch machines and I don't believe I have a transposing gear for either currently
- The major diameter of the threaded section is nominally 5mm (slightly undersized); 5mmx0.5 is not a standard pitch I would have a die for without ordering one, and even then I may not get close enough to the shoulder on the part. That might be solvable by making the part longer, but that could pose challenges with installation and usage on the compact machine.
To set up for the taper, I made a small set screw collar to hold the part, with a wad of paper under the set screw so that it could bear on the threads without damaging them.
Put that in the collet chuck, and indicated the top slide in to the taper which turned out to be nominally 5 degrees.
I decided to make the new needles out of bronze rod which I have on hand from another project, and used a very sharp HSS tool to test a taper, went fine
For the threads... any solution at the lathe would hinge on being able to get a 0.5mm feed rate. I ended up finding a gearbox configuration that was 0.019"/rev, whereas 0.5mm is more like 0.0196"/rev. That's a 3% error which is quite large, and realistically I am not thrilled to single-point thread such a small thread form.
What about something that might compensate for the timing error (at least for 5-10 threads), and let me thread this at the lathe without having to buy or grind a super-clean tiny threading tool?
Like holding a tap in the tool holder, and using it as a form tool to thread the OD of the part?
I have this oddball M8 x 0.5mm ultra fine-thread tap that has been in my drawer of assorted taps for a few years. So it's the right pitch, and the diameter is relatively huge - benefits to make it rigid and make it easier to see to clock it in the toolholder
I clocked it with one of the flutes basically horizontal, just eyeballing where it should have the right rake and clearance.
Here's a test:
This was on an arbitrary diameter I cleaned up on a piece of stock - it worked fine, just needed to get better at deciding my DOC per pass and what depth would result in fully-crested threads. The first result wasn't fully formed but progress:
With those issues solved, onto the first real part (first of two, with different needle lengths to test).
Taper turned, nothing special there - just a very sharp 1/4" HSS tool, and turned the threaded section to 4.8mm. I found in my testing that the tap process would expand the diameter slightly, I think maybe I had it mis-clocked and it was forming the threads instead of just cutting them. If this was for something that needed thread strength, I'd have gone back and adjusted it since any "flowed" material was probably weakened.
I was originally worried that due to the 3% error accumulating, the threads near the end of the part would start getting eroded by the tap as it fed further in, but this does not appear to be the case. Perhaps if the tap was flowing material, it was compensating for any cutting that was occurring. Or, the .0006/rev effective feed rate was so small that even the super-sharp tap couldn't take a cut and was just rubbing the oncoming face, leading to the appearance of flowing.
After that, knurled and parted off.
Extremely happy with the results, made a second one. The second one only took 30-45 minutes now that the process was all ironed out
Full, clean threadform. All of the "is this fully formed" ended up being done by eye, I don't have thread measuring wires nearly small enough for this
If I need to make more, I'll do some more tests by clocking the tap differently (in the view shown above, I'd go slightly counterclockwise so that it has slightly more rake and clearance to reduce the chances that it is flowing material)
I would also use a M3x0.5mm tap to cut internal threads on an M5 hole in a test fixture/gauge, cutting until the original needle threads in. Then I could use that as a go/no-go gauge to test the new parts. If the needles I made so far have any issues threading in, I will need to do this; and if it just turns out I need to do different needle lengths, I'll still do this if the person sends me the original needle back again.
Would love to hear thoughts - thanks for reading
Last edited:
Do your parts fit, or do you know yet?
