Hi everyone. I'm gonna describe the rigid tapping setup I've made for the PM 25
CNC conversion. Rigid tapping holds the tap securely in a tool holder like an
end mill would be and if you compare that to a tapping compression head it's
easy to see why rigid tapping gives better control over depth especially as
the tool begins to wear. And depth control is especially important if
tapping into a blind hole. To perform rigid tapping there needs to be
synchronization between the tool height and the angular position of the tool and
that's measured by a rotary encoder on the spindle. In this case, I attached the
rotary encoder to the drawbar. The rotary encoder is attached to a differential
receiver which is attached to a Pico Systems Universal Stepper Controller
which LinuxCNC uses to coordinate all of this. The other thing
that's needed is sufficient reserve capacity in this spindle motor and the
axis motor. Ideally, during tapping the spindle would maintain velocity but any
change in velocity has to be met with acceleration of the axes motor. The
rest of the video is the construction of this assembly, testing using a spiral
point and forming tap, some examples of problems that occur with the
system if it's run too fast and then a comparison to thread mill. Now, rigid
tapping is just another tapping tool and how I plan to use this tool is when I
have parts that have a lot of small threaded holes that I can tread with a
forming tap, because it is kind of inconvenient to take this assembly off
to change tools, otherwise, especially with external threads, I'll use single
point treadmills, and with popular threads like m6-1
which is the most popular thread that I use, that I make, is to use a
multi-point thread mill. If you have any questions let me know otherwise
let's get started. I previously made this drawbar and I'm
taking this chance to modify the hex to be compatible with the 17 millimeter
wrench that I use for my clamping kit. The hex is then turned down to receive a
flex coupler which will attach the drawbar to the rotary encoder. This isn't
terribly convenient especially when wanting to change tools but it gives a
nice easy way to demonstrate rigid tapping. I considered also using a magnetic
base encoder on the top of the spindle motor and also using a timing belt to
move the rotary encoder to the side of the head but I'm planning to change a
spindle motor and until that's done I don't want to do anything more
complicated. The rotary encoder gives position feedback to LinuxCNC which I
use to calculate velocity and position of the C axis. This might be useful for operations
like broaching where you'd want to know the angular position of the spindle
precisely but you would have to come up with a means to lock the spindle at that
position.
Rigid tapping feels pretty sketchy the first few times and before recording
this threading test in aluminum I did several threading tests using wood. Takes
a little bit of a leap of faith I guess. I think this is so cool that even on
do-it-yourself CNC machines that two axes can be synchronized and controlled
so precisely that rigid tapping is possible. From the side you can see how
far the spiral point tap needs to come down in order to fully tap this
through-hole. Besides a spiral point tap we'll also try a forming tap. The forming
taps require a different size drill bit and they deform the metal instead of
cutting the metal. More of the tap is also available for threading as well. To
compensate for the threads that are not usable you have to change the
breakthrough amount in Fusion 360 and that can be parameterized by the thread
pitch.
The threads created by a forming tap should be stronger than those created by
a cutting tap because the forming tap deforms the metal to create the threads.
Also with a forming tap there are no chips created so tapping to a bottom of
a blind hole is more practical. Unfortunately a forming tap requires two
to three times a torque than a spiral point or spiral flute tap does and in a
high speed belt setting the PM25 just does not have sufficient torque at the
low end speeds. Since the height of the head is synchronized to the angular
position of the tool in the event of a spindle stall the head will stop moving
and they'll be minimal or no damage to the part but unfortunately a recovery is
quite a pain.
Peck tapping may not be available in LinuxCNC and if generating that code
in Fusion 360 there are no warnings shown unless the NC editor is enabled.
Other than demanding too much torque during tapping the other type of problem
that can come up is if tapping too fast there's considerable overshoot because the
head has to remain synchronized with the spindle motor as it free spins to
slowdown. Also, with a 2,500 pulse per revolution encoder, in quadrature mode, at
800 RPM, you need a very fast sampling rate. So if the spindle is run too fast, even
the high megahertz clock of the Pico Systems controller board isn't able to
fully sample the rotary encoder. 400 RPM seems to work pretty well, especially for
fine pitch threads. Another gotcha is since the final upward motion after
leaving the hole is uncoordinated, it isn't possible to re-thread a hole. So you
only get one chance to thread a hole and if you try to go back and thread it
again it's going to destroy the thread. We can also a thread with thread mills.
Here are two single point and one multi-point thread mill. Thread
mills use coordinated motion of the XYZ axes and don't require a rotary
encoder on the spindle. To use a thread mill, a hole is first created to the
minor diameter of the thread. Then some thread relief is created with a chamfer
tool to help the head of the screw set properly. In Fusion 360 I use a 2d
contour to create that chamfer and this small Python script I wrote to calculate
the chamfer width. And this is what the treadmill looks
like in Fusion 360.
Probably every time an expensive treadmill drops down into a hole is
stressful but these tools are so cool.
This hole didn't get threaded quite all the way through because Fusion 360 is
looking at the tool length and bringing the bottom of that tool to the bottom of
the hole. For this through-hole, we need to add a slight amount of breakout and
that can be parameterized by the tool pitch.
A huge advantage of thread mills is that a single thread mill can cut a variety
of internal and external threads of different diameters, and if using single
point thread mills a variety of thread pitches. But the interpolation required
for thread milling is going to always take more time than using rigid tapping
with a specialized tool. I forgot the chamfer on these holes but
I hope otherwise this demo was clear. Okay, recapping we looked at adding a
spindle encoder to a CNC conversion of a PM25 milling machine, some successes
and some failures of performing rigid tapping using the stock motor, and
then compared rigid tapping to thread milling. I hope you find this as
interesting as I do. Coordinated motion between multiple axes is really
interesting subject. If you have any suggestions or questions please let me
know otherwise thank you for watching and I'll see you in the next video.