Re: [CAD_CAM_EDM_DRO] removing backlash from rack/pinion drives wasRe: Shoptask Z-axis - quadralift?

Ballendo -

If you can do it with a gear, then there is no reason in principle not to
do it with a rack (or two racks & a fatter gear, if that is
easier). However, the problem would be constructing a "slideway" for the
moving half of the rack that allows linear motion in one axis while
constraining movement of the rack in the other two axes. I guess it is just
easier in most cases to split the gear.

Of course, as with any preloading scheme, applying a force greater than the
preload defeats the mechanism. I guess another approach would be to split
the gear & then clamp the two halves after adjusting out the 'lash, rather
than relying on a spring (or should it be "sping"? <G>).

Regards,
Tony

At 08:24 06/03/2003 +0000, you wrote:

>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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Regards,
Tony