removing backlash from rack/pinion drives wasRe: Shoptask Z-axis - quadralift?

Bill,

Never seen that approach. But...

What is typically done is similar. The GEAR is split (or two narrow
gears are used if you prefer<G>) and these two "halves" are sping
loaded (rotationally) against each other. You can see examples in the
stock drive products catalog, which is available free.

A 2.5 axis cnc machine would make short work of making these from a
couple of stock gears. A manual mill and a rotary table wouldn't be
too far behind...

Hope this helps,

Ballendo

--- In CAD_CAM_EDM_DRO@yahoogroups.com, ccq@x wrote:
> Hi guys;
>
> Maybe someones tried this before, and can tell us if it'd work. It
might
> require a wider than usual gear/rack, but what if you took a
slitting saw, and
> slit the rack right down the middle. lock one half down as usual,
and then
> slide the other half lengthwise, one direction or the other, until
all the slop
> is taken out. Would this be workable, or just engender extreme
wear, or
> sosmething?
>
> Bill
>
>
> On Wed Mar 5 16:20:44 2003 alan@s... wrote:
>
> >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
>
>
> --
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