Hi Steve,
I am using 1/4-20 threaded rod with a tapped piece of Delrin
for a drive nut. It's all relative. Even if the threaded rod is
out ten thousands over a 6" length you will still get a good
board. After milling boards for over a year and selling them. I have
only found one problem. That is getting a good high speed spindle
to run the bit. I can't seem to find anything better than the Dremel
MultiPro. Let me know if you find one! Run out is the problem. Most
cheap routers have too much play in them for circuit boards. The also
have a problem if you run them too long. I have some boards that take
an hour to mill.
John
--- In Homebrew_PCBs@y..., Steve Greenfield <alienrelics@y...> wrote:
> --- n4onl <umrk@b...> 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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