Re: [CAD_CAM_EDM_DRO] Rigid Tapping--How is this accomplished?

I'm in the same boat as you: considering cutting threads in a mill. I
haven't done it yet. Here is what I know, with questions about stuff I
don't understand:

There are 3 or 4 ways to cut threads in a mill:

a) rigid tapping
b) rigid tapping with a floating tap holder (compression/extension type)
c) thread milling: single point
d) tapping head
e) thread formers instead of taps.

a) Rigid tapping: a tap is held in a solid holder. The spindle has an
encoder and the Z axis is fed synchronized to the thread pitch of the
tap, just like the Z axis of a lathe is when single point threading.

The thing I don't understand about rigid tapping is how the software can
get the spindle to speed up/slow down fast enough such that the Z axis
stays in perfect synch with the spindle. *Any* misfeed of the Z axis is
going to cause problems.

b) Rigid tapping with a floating holder. Here the tap holder is spring
loaded so that it can compress/extend to accommodate some misfeed of the
Z axis. A holder with compression/extension lengths of 0.5" could
accommodate 10 revs of misfeed at 20 TPI.

The thing I don't understand about this setup is how the tap can totally
cut the thread without breaking. For example, if I am cutting a thread
by hand, I must advance/reverse the tap 25 times to reach the bottom of
the hole without breaking the tap. How could a milling machine power
the tap right to the bottom of the hole without reversing the tap ?
Won't it break the tap ?

c) Thread milling: single point and other...

Thread milling works like helical machining, except that the cutter is a
single point threading tool fed at a coarse pitch (ie the thread pitch)
rather than an endmill. In thread milling, the Z axis is fed downward
at the thread pitch while the X and Y axes do a circular interpolation.
Note that several passes must be done at various diameters to get to the
final thread depth.

The caveat of this method is that the tool must fit in the hole and thus
it is generally only useful for larger holes. Tooling catalogs sell
special thread mills for doing fairly small holes (ie < 5/16"), but they
get expensive, ie $200. A thread mill will work for any hole size
provided the pitch is right. A single point tool will work for any
thread that you can get it in the hole for.

It takes special software to do this operation. I've written such a
routine for TurboCNC but not yet tested it. BTW: the canned cycle for
helical milling will NOT work because most will break the threading tool
when they retract, among other problems.

Thread milling doesn't require a spindle encoder. Thread milling can do
internal or external threads.

The thread I am going to cut is a 1-9/16 x 18TPI to hold a 1" ballnut.
A tap for this size is $100 or more.

d) Tapping heads.

A tapping head is a device that feeds and reverses a tap into and out of
a hole until it is tapped. I've never used one. People tell me they
are tricky to set up. They've got a torque setting. I think they will
go as deep as they are fed.


We cut a lot of threads in our work. I want to get automated tapping
going as soon as possible.

I spoke with a CNC mill owner about tapping and he highly recommended a
floating tap holder, especially for one off holes as there is no fooling
around with torque settings as there would be on a tapping head. He
said that in a production setting he has tapped 1000s of holes without
breaking a tap. I just can't fathom how one could drive a tap from the
top to the bottom of the hole without reversing it and without breaking
it.

Kim

On Sun, 2003-10-19 at 18:14, Steve wrote:
> From: "jmorrphd2" <jmorrphd@...>
> > I would like to know how rigid tapping is implemented. I know
> > that the Z-axis feed and the rpm are synchronized such that the
> > Z-axis feed (in distance per minute) is equal to motor rpm x thread
> > pitch. How does the drive motor "measure" its angular displacement
> > or speed? Is the Z-axis speed adjusted continuously as a function
> > of motor speed variation? Or, is the motor angular momentum such
> > that there is no appreciable decrease in speed over the length of
> > the cutting of threads?
> > Finally, I imagine that there is more than one way to implement
> > rigid tapping. Any comments appreciated. Thanks, Jim
>
> On the big machines I've used, the Z and spindle just run at the speed
> you tell them for the tap cycle. I don't know how the software / hardware
> handles it. On the older machines we use to use a spring loaded tapmatic
> head, it has clutches to slip, springs to make up for the spindle and feed
> not being totaly matched and gears to reverse the tap when the head starts
> to go up. The other way we tap the holes with the cnc is to use a tapping
> cutter, IE you want to do 1"-10 tpi the cutter would be around .75 and have
> 10 theeth per inch but straight cut, you would feed it into the hole, feed
> it out .125 then do a 360 deg circle dropping .100. That way you cut all the
> threads at once.
>
>
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