RE: [CAD_CAM_EDM_DRO] Re: Sherline Spindle Pulse disk for threading

Hi,

> The comment about the spindle slowing is the exact
> reason why
> >threading with one pulse per rev "shouldn't work well".
>
> But it does :)

Yes. As long as the spindle speed remains constant. If it varies the
thread may end up looking pretty awful. But here's the rub. If the
spindle is under powered and slows down under slight loads the issue
isn't with the one pulse per spindle rev but with the speed control on
the motor which changes the SFM and can also screw up surface finish.

So it a lathe has a problem maintaining speed, it's better to spend the
energy to fix that than worry about one pulse per rev.

<snip>

> >I sure hope it keeps adjusting speed over the threading pass
> (more like
> >"locks until it sees another index pulse").
>
> Nope it doesn't, but it works.

You don't want it adjusting over the threading pass or even a different
speed per pass. You want a stable spindle speed in the first place.
Here's why.

Like engaging the half nut when the threading indicator lines up, the
software has to know where the spindle is. So it needs at least one
'unique' pulse per revolution to do this. Now imagine you told it to
start threading. The software waits for the spindle index indication and
then accelerates the lead screw stepper/servo motor from stop up to
threading speed.

It may well be that the spindle has rotated 1/2 turn by the time the
lead screw motor is turning at threading speed. It doesn't matter that
there is this delay because each time for each pass, the spindle turns
the exact same distance, the carriage starts from the exact same
location, and the lead screw motor takes the exact same time to get up
to speed. The tool bit always enters the thread at the same point on
the thread.

Now, feed hold (if possible) halfway through threading after a pass is
complete, double the spindle speed. The lead screw has to turn twice as
fast too. But the lead screw motor hasn't had the acceleration
parameter changed since that's a machine configuration parameter.
Depending on the acceleration, the lead screw may or may not be up to
speed after 1/2 rotation of the spindle. Physically, the coupling
between the two is no longer at the same point and the tool bit no
longer enters the same spot for threading.

Now that's a pretty extreme example but now imagine, on each pass, the
spindle speed is different. The lead screw speed and the spindle speed
will end up being correct for the thread pitch, but not at exactly the
same time as with a different spindle speed.

So if the spindle speed varies, fix that and don't worry about one pulse
per revolution.

>
> I've cut thousands of threads over the years with Mach
> without a problem
> :)

I haven't cut thousands with the ELS yet but it's definitely more than
100 now.

>
>
> Steve Blackmore

I do have a question for you Steve. If you are using the G76 to cut the
threads, one of the parameters is thread depth. If, after the G76 is
done, the nut doesn't fit because the lathe and work flexed such that
the depth wasn't cut, how do you fix this? Set a deeper depth and run
the G76 again? Do some sort of special G76 as a single spring pass?

Thanks

John Dammeyer

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