Re: Shoptask Z-axis - quadralift?

On 5 Mar, CAD_CAM_EDM_DRO@yahoogroups.com wrote:

> This is addressed to those who have CNC'd Shoptask machines.

This is an interesting question that I am currently struggling with
myself.

I do have a CNC'd Shoptask.

I do not have a Quadralift, but I am certainly going to add SOMETHING
to raise and lower the mill head, for reasons that need no explanation
to anyone else with a 3in1.

Frankly, as I examine pictures of the QL, I'm pretty sure that a guy
could make one himself, of at least the same quality, and do it all on
the Shoptask.

The question then becomes "why use the quill at all ?"

You see, I don't agree that the quill is necessarily more accurate. I
have taken great pains to take all the play that I can out of the quill
(removing the "drill press mode" by pinning the gear, using thrust
washers on both ends on the fine feed shaft with adjustable preload,
etc) and I still get the inevitable play as a result of the lash
between the gear and the rack cut into the quill.

The issue is easily seen. Bring the quill down until it just touches
the work. Now try to turn the crank some more. You should not be able
to .. you're touching the work, right ? But you CAN turn it some more
... you're taking up the lash in the above mentioned gear train.

Now the REAL question .. is the above seen lash constant throughout the
travel of the quill ? Not on mine, and likely not on yours .. the rack
is just not machined accurately enough.

Even if it is, you're left with "zeroing" the work, by what ever means
usually taken ( rice paper, edge finder, etc ), then adding in the lash
you know is there and hope your chosen offset is correct.

There's a good reason why Chuck McGhee's ball screw quill upgrade is so
popular.

So here's the thoughts kicking around my brain on this topic.

Since I need SOME sort of screw to move the mill head up and down on my
homebrew QL, why not make it a QUALITY acme screw with QUALITY
doublenuts.

Make sure the QL goes up and down with REALLY good registration.
Thomson linear slides is my current scheme, but there are other choices.

Then just lock the quill ( it's more rigid that way anyway ) and always
use the whole head for movement, while under CNC control. You'd want a
hefty SERVO/STEPPER motor to get any sort of speed out of the deal, but
that's not a big deal.

Problem solved.

Now, as to the real gist of your question, your double encoder scheme
has a real problem. You can certainly wire something up to "add up"
the encoders, but this only makes sense if you know PRECISELY how far
your drill bit in a chuck sits below some common reference point, as
compared to how far your endmills in a holder sit below that same
reference point. God forbid you ever sharpen that bit.

Only with this precise information does your double-encoder scheme make
sense.

Here's what I would do.

Make a height adjustable electronic edge finder. ( A led, a battery a
resistor ) The "switch" to turn on the LED is made up of the machine
and the insulated top of the edge finder.

The circuit is closed when the endmill or drill bit JUST touches the top
of the edge finder.

No great precision is needed here, except to make the top of the edge
finder very parallel to your machine table.

Now, find the X and Y zeros of your workpiece, by your normal means.

Next, find the top of your workpiece also. This is your Z axis zero.
Clamp your edgefinder on some unused part of the table, screwed down all
the way, and move the quill ( under CNC control ) over to about the
middle of it.

Now slowly raise the edgefinder until the light JUST goes on. You now
have a surface in a known X-Y-Z location.

Raise the head .1 ( to avoid scarring the surface of the finder ), move
to X=0, Y=0, lower the head .1 and Z also now equals 0. Mill away.

Tool change time, raise the head ( manual or auto ), change the tool,
move under CNC to the edgefinder XY location, MANUALLY lower the tool
until the light JUST goes on. Zero EMC. Raise the head .1, move to
X=1 ... etc.

My wild idea, anyway.


Alan



--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta