Cutting an External Thread on the Proxxon PD 250/e Lathe

Cutting an External Thread on the Proxxon PD 250/e Lathe


This is the gear cutter arbor I made recently.
You can watch the full build video by clicking on the link at the top right now. In this
video I’m going to go into some of the detail of how I cut the thread for this part on the
Proxxon lathe. To ensure the thread has a clean start and
finish, it’s a good idea to cut a groove at the end of the thread. This ensures the
cutting tool is clear at the end of the cut, and isn’t left embedded in the material
at the end of each pass. It’s also useful to use a chamfer tool at both ends of the
surface to be threaded. This makes the start and end of the passes a little more gentle,
and ensures the thread start is cleaner. I use this CCMT insert tool to cut the leading
edge chamfer at an angle of 40 degrees. I cut the end groove with a 2mm parting tool And then chamfer the trailing edge with the
same CCMT tool. Before setting up the lathe to cut a thread
it’s important that the compound and toolpost are clean and stable. I start out by stripping
the carriage down and making sure each part of the assembly is in good shape. I use kerosene and this old rag, which seems
to dissolve the grime just fine. The compound is only bolted down at a single
point, so in order for it to be secure there needs to be a good metal to metal contact
between the compound the carriage. This surface gets built up with cutting oil residue and
chips on my lathe, which needs to be removed. The underside of the compound needs to be
cleaned similarly. Chips or parts hitting the carriage can raise
small burrs which interfere with the contact surface. I use a gentle abrasive stone to
make sure these burrs are removed, and the surface is flat. The underside of the compound is a softer
alloy, so should be stoned very lightly. The compound angle should be set to slightly
less than 30 degrees for metric threads. This tool is probably more accurate than necessary.
A set square or combination square would do just fine. Be careful while tightening this screw, as
the single screw design makes it extremely easy to accidentally skew the angle while
tightening. The compound should be set with a couple of
millimetres of travel available to advance the tool. The contact surfaces on top of the compound
and the base of the toolpost should be cleaned and stoned the same way as the carriage. The toolpost needs to be set at exactly 90
degrees to the axis of the lathe, to ensure the threading tool is aligned correctly. To
get this really accurate requires an indicator, which takes a while, so for this thread I
used a square to get it close enough. When tightening the toolpost it’s really
easy to knock the toolpost itself out of alignment, or the compound angle, so check both angles
once everything is assembled. The lathe tool I used is a standard external
threading carbide tool, with an 11 millimetre general purpose 60 degree insert. The next step is to fit the lead screw and
change gears to the feed screw. To select the correct gears, look them up on this table
in the manual. For a 1mm thread pitch I need a 20 tooth change
gear, and a 20 tooth lead screw gear. The lead screw gear fits onto the end of the
lead screw, with the retaining screw tightened against the flat. The change gear is more fiddly to fit, as
the bolt needs to screw into a nut held in the back of the gear arm. The gear assembly
consists of a bushing that the gear runs on, the gear itself, a washer, and a bolt through
the middle. It’s easier to assemble these before attempting to fit them to the rail. The gear spacing is also fiddly. The gears
need to be close enough to engage well, but not so close that there’s resistance. Trial
and error is required, as it’s difficult to tell whether the spacing is right without
fully tightening the bolt. The belt connects the change gear to the toothed
pulley on the back of the main spindle, and the arm can be adjusted to set the correct
belt tension. The lathe is now fully set up to cut the thread. Very lightly touch the threading tool to the
diameter of the part to be threaded. This is the starting depth. Zero the scale on the cross slide. All the passes will be done with the cross
slide in this position, so make sure it’s still exactly where it was when the tool touched
the outer diameter of the part. Take up any backlash in the compound, and
zero the scale, and flip the control to engage the lead screw. Set the lathe to a slow speed, and cut a scratch
pass. The lathe needs to be stopped while the tool
is in the groove at the end of the cut, to avoid damaging the rest of the part. This
is much easier when the lathe is running slowly, but with practice I find I can do it reliably
at higher speeds. After the scratch pass I always double check
the pitch with a thread gauge. To prepare for the next pass, back the carriage
away to clear the tool, and run the lathe in reverse to track the tool back to the start. Return the carriage to the zero position that
was set earlier. Advance the compound to set the depth of the
next cut. At this stage, only the point of the tool is engaged in the material, so the
depth can be relatively large. The sequence for the next pass is the same. Back away the carriage, run the lathe back, bring the carriage forward to zero, and advance the compound. With enough practice this sequence becomes
second nature. The tool engagement gets larger with later
passes, and it becomes important to use cutting oil to get clean cuts. As the tool engagement gets larger, the depth
of cut needs to be decreased to make sure the cutting forces don’t get too high for
the lathe. The depth of cut is now down to 0.05mm, from the
starting depth of 0.15mm, so progress is slower. It’s important to stop cutting at the correct
depth, but determining exactly where that is isn’t obvious. One way to measure precisely
is using thread wires, which I’ll describe in a future video. In this video I cut the
thread slightly too deep, but fortunately I was able to compensate for it when I made
the nut. It’s better practice to cut threads to a correct, standard dimension rather than
just cutting them to fit the matching part. One clear sign that the thread is nearing
completion is when there’s no longer any of the original outer diameter surface left. It took a lot of passes to complete this thread,
which are very similar. I’m including them here for completeness, but if you prefer to
skip the repetitive bit, jump ahead to 13 minutes to find out how the thread is finished. Woops, I overshot, and scratched the edge
of the register. The thread is now nearly complete, so this pass
is cutting at the same tool position as the previous pass. This is called a spring pass,
and it ensures that the thread is consistent and clean along the entire length. It compensates
for any flexing in the toolpost due to the cutting forces. The tool is now at its final depth, and just
to make sure the thread was cut cleanly I did three spring passes at this depth. That was the final pass, so I back the compound
out to the starting position, and take up the backlash. Disengage the lead screw, and run the tool
along the thread with the lathe at regular turning speed. The tool is at the original
outer diameter of the material, so this pass removes any burr that has been raised. Running a triangular needle file along the
thread removes any burr left inside the groove. I hope you found this guide useful. If you
haven’t watched it already, check out the full project video for this gear cutter arbor
at the link in the top right now. Let me know if you’re interested in a similar video
about cutting the matching inside thread for the nut.

29 thoughts on “Cutting an External Thread on the Proxxon PD 250/e Lathe

  1. Quando posso seguo i tuoi video.
    Sempre interessanti, e ben fatti.
    Una domanda ? Utilizzi un tornio PD 250, oppure un
    PD 230 ?.
    Io sono possessore di un
    PD 230 già usato da altra persona..
    Grazie per i video che ci mostri.
    Ciao

  2. You could flip the tool by 180 and thread in reverse (going away from the groove towards the tail – this way you don't worry about stopping the lathe on a dime; the relieve groove can be much narrower)

  3. Another terrific video, Well done!!
    Greatly appreciate your efforts in creating this video Alistair.
    Count me in for seeing more on how you went about the nut too.
    regards Colin

  4. Been a long time since I cut a single-point thread, so this was an excellent review. For depth, thread wires are good but a lot of people cut or buy the nut, then use it as a standard. Otherwise for a given thread depth D, the reading on the compound should be C = D/cos(A) where A is the offset on the compound, usually 30 deg for metric and non-Whitworth Imperial. The depth D is found in tables.

  5. Thanks for the reminder to clean the tools. I get so involved in my jobs that swarf and tools can build up on the workbench to a dangerous point.
    I try to do a 3 minute cleanup every hour. Returning tools to the box and a quick hovering of the swarth. I Learned this from my dad as a child. I set an alarm so I do not forget. Problem is the shop stays neat but never actually gets clean. So a good deep clean-up, at start up, is a good idea.

  6. The spacing between the driving and the driven gear is very easy to set, use a small strip of printer paper, I saw it once at mrpete222 aka Tubal Cain, and have used it ever since on my Myford lathe. Otherwise nice job on such a fiddly small lathe.

  7. thanks for the video. I do not understand English very well. how many degrees the trolley is tilted for a metric thread. thank you.

  8. I would like to buy a small lathe like this one or similar. I see on all the specs only tell the TPI capacity of the threads, but I live in Europe. Are all the lathes capable of both metric and imperial threads or I have to look specifically for metric thread lathes?

  9. Great instructional video for this Lathe. I know it is basically a Chinese C1 lathe, but it looks like Proxxon has tweaked it a bit.

  10. I’ve been looking a mini lathes for various RC hobby uses and just stumbled upon the Proxxon tool line. Not sure if you’ve ever used the Chinese cheap lathes but I’m wondering how they compare to proxxon? Great video.

  11. looking to purchase a small lathes,m sure its worth the money for the proxxon rather than the cheapo chinese,good tutorial and a sub.

  12. Just a suggestion – If you wire a micro switch in series with the motor fwd wiring and then mount that micro switch such that it is operated by the moving carriage then you could have a setup where it would be impossible to miss your end of thread position and you could relax a little. It would also prevent you ever accidentally going forward when you meant reverse.
    Wired properly this would not prevent you still using powered operation in reverse.
    This is a pretty simple modification and easily reversible if you change your mind but I think it would allow you to cut threads or anything else without ever having to worry about a head crash.
    I do like your videos very much. I am an electronics engineer currently wishing I was better at machining. Electronics alone is not enough any more….

  13. I never understood the business of ofsetting the topslide when screwing anything and I used to machine big stainless valve seat rings and lots of little stainless parts.

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