--- n4onl <umrk@...> wrote:
> Hi John, All questions comments, ect. welcome.
-
> Resolution and repeatibility (accuracy): Good points, and no I
> don't
> expect extreme accuracy and repeatability out of this setup,
> especially using Acme rod, but i think it will be rather
> accurate.
> What Accuracy (resolution and repeatibility) is Really Needed?
Good point. Anyone have the answer? Repeatability I would imagine
will be mostly affected by backlash and stiffness. Resolution by
the thread pitch and whether you use pulleys to "gear down" the
steppers, and if you use full step or half. Using pulleys to gear
the steppers down has the advantages of both higher resolution and
more resultant power, at the cost of speed. For me, I can take the
cost in speed. Accuracy is not just repeatability or linearity. It
is also important that if the calculations say X steps move the
head 4 inches, that it not be 3.9 inches.
> Going by lathes, mills... stiffness of the frame lends to
> accuracy,
> this is why i'm using 8020 and aluminum plate ($0.35/lb scrap
> price).
> It will be rather hefty. Also backlash is a factor so I'm going
> to
> use antibacklash nuts that I will make out of Delrin (for reduced
> friction and wear of the Acme rod lead screws).
Boeing Surplus... Yeah! I live about 30 minutes from Boeing
Surplus. What they don't have, there are metal sheet suppliers all
around them.
> My Guides: For X and Y axis (right and left), Thompson TWN 10
> (5/8")
> Super Pillow Block Bearings and Precision Ground Rod. their used
> but
> are in Excellent mechanical condition, smooth and tight.
http://www.smallpartsinc.com for new stuff. I had a project
cancelled to build an ultrasmooth sliding door, I did a lot of
research for bearings like this. I should dig out the catalogs
again and post the links.
> For Z axis (router verticle), used Robohand Inc. model PS-4-4-SH
> (similar to these http://www.robohand.com/dlbb10.htm) with 12mm
> shafts and 4" travel. Very smooth and tight. I also have some
> 3/4"
> supported linear rods and bearings but I think they would be
> overkill. :)
Hey, if you have them already and don't have another project for
them, why not?
> The part that I have the least knowledge about are the stepper
> motors
> themselves, so I'm going a bit larger than the ones I've seen on
> commercial equipment I've found on the web. I have 2 new (old
> stock)
> Vexta 83 oz/in, 1 used 100 oz/in and 1 used 120 oz/in, all 200
> full
> steps per rev and NEMA23 frame. This one ( http://www.t-
> tech.com/products/quickcircuit/ ) appears to be using 60 oz/in
> motors, .00025" resolition and .00025" (one halfstep ?)
> repeatibility. Check out the PCBs it makes. :)
What makes you think they are using 60 oz-in, besides the size of
the steppers?
Here's that link fixed:
http://www.t-tech.com/products/quickcircuit/
BTW, here's my secret source of good battlehardened 60 oz-in
steppers- Apple Imagewriter II! Two per printer. I checked them
with a constant current power supply and a balance, at 60 oz they
held, at 61 oz they'd slip one or two notches.
> Speed of the cut is also a big factor in accuracy, so I intend to
> keep my cutting inches/sec on the slow side, at least until I see
> how
> fast I can move and not affect accuracy to a great degree.
> What Accuracy is Really Needed?
From the CAD_CAM_etc list, I think it is important to match cutter
speed to movement speed. IE, while it is bad to move too quickly
and so take too big a bit on each pass of the cut, it is also bad
to move to slowly and take too small a cut. So it would follow that
you'd need to slow down cutter rotation if you slow down inches per
second.
Don Lancaster http://www.tinaja.com has a great idea for
lightweight milling heads- dental turbines. Tiny flexible air hose,
tiny dental turbine, and you have a cutter head that is extremely
lightweight. Very high speed but the cutting head is small so
cutting speed isn't quite as high as it might seem. Maybe something
that could even be retrofit to an HP flatbed plotter? Very cheap so
when they wear out you just toss the turbine and install another.
Perhaps use the exhaust air to make a vacuum system to suck the
particles up.
> BTW using a RTX for the spindle has been changed to using a small
> laminate trim router. I found one (and a spare) at Harbor Freight
> for
> half the cost of a RTX ($20 ea.). Thats cheap enough to replace
> as
> needed. :)
Makes sense. A friend of mine bought a very expensive sabre saw a
few years ago. Since it was so expensive, he felt obligated to
repair it when various parts wore out. In that time, he's spent
twice the cost of the saw on parts. He's cutting 3/8" aluminum with
it, so it is hard on it. In that time I wore out three $25 sabre
saws and spent a fraction of what he did.
Steve Greenfield
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