RE: [CAD_CAM_EDM_DRO] A few simple engineering questions

-----Original Message-----

>From: Charles Knight [mailto:charles@...]
>Sent: Friday, May 23, 2003 11:49 PM
>
>Z-axis. On most designs I see, there is a vertically oriented axis,
>which has the spindle (usually a router or a Dremel...in any case a
>non-negligible weight) cantilevered at the end of an arm. [...] Doesn't

this type of loading cause the

>bearings to wear prematurely, and in a predictable pattern?

I would be more concerned that the off-center loading could cause
unpredictable take-up of any Z backlash. With the weight balanced on the
guides, the head will drop smoothly under the effect of gravity whenever the
Z leadscrew lets it. With unbalanced weight, the motion may stick, and not
move down until the backlash is taken up and the leadscrew actually *forces*
the head down, at which point it may drop by the amount of backlash etc.

Also, the off-center cutting loads reduce the stiffness in the Z direction:
as the Z guides flex under load the actual cutting tool moves up and down.
When the leadscrew(s) and guides are symmetric around the spindle, Z guide
flex can only cause X and Y error (and even this is reduced, as described
below).

>Has anyone
>come up with a better design...one which places the router's center of
>mass between the slides, for instance?

I think part of the difficulty is that it is keeping the leadscrew/guide
design independent from the spindle cradle/mounting design. You are almost
forced to come up with a cradle design where spindle removal is done axially
because, with the spindle surrounded by the guides, there is no place for a
side clamp to come off.
Also, to fully benefit from the stiffness of the balanced guides, you also
want multiple balanced leadscrews, and you then have the problem of
synchronizing these to each other.

>This seems to be a relatively
>simple task...what's the down side to it? This graphic,
><http://www.cnczone.com/attachment.php?s=&postid=3691> is perhaps the
>best one I've seen, to illustrate this cantilevering. Is this a problem
>in real life? Theory says it should be...but theory sometimes
>complicates things unnecessarily.

This particular design seems to have the cradle unnecessarily "tall"; it
looks like the router could be almost an inch closer to the guides. At
least, that's how things appear in the photo

>Y axis. I've decided to place the rails next to each other in a
>horizontal plane...mass of the Z axis centered between them. I
>understand that this type of layout is used in the ShopBot, and the
>ShopBot is known for Y axis flex. Is this a function of this type of
>design, or is it just a problem with underspecced parts?

If you mean your would be using two round guide rods only supported at the
ends of the Y axis, it turns out the rods have to be really beefy to get a
rigid design. This setup (regardless of how you position the guides relative
to each other, only their spacing matters) amplifies the lack of stiffness
in the guides. This is because, in addition to lateral (in this case,
X-direction) forces flexing the rods directly, there is also a torque
involved so that one guide rod flexes up and the other flexes down thus
allowing the cutting tool to displace much further than either of the rods
is flexing. If, for example, the cutting tool and the two guide rods happen
to form an equilateral triangle, the tool displacement from twisting will be
3 times greater than the displacement from plain lateral flexing (for a
total displacement of 4 times), so if half your expected cutting load would
displace the guide rod 0.001", your total cutting tool displacement would be
0.004".
To reduce this amplification effect you have to place the guide rods far
apart and low to the work.

>
>Antibacklash nut design. While this is far from a commercial grade
>antibacklash nut, it seems that it could be implemented by using off the
>shelf PVC as an adjustable spacer, between 2 tapped nuts. In fact the
>whole thing could be implemented by tapping the PVC, but I would doubt
>the material's longevity in use. Take a female threaded cap and a male
>threaded plug...screw together. Drill holes in both ends and epoxy nuts
>to the ends.

If you mean PVC pipe fittings, these have tapered threads which might give
you grief.
You might consider instead the (continuous) threaded tubing used in things
like table lamps. This is available from the electrical department of most
hardware stores, in the same area as the replacement lamp sockets and
switches for table lamps. They will also have caps, nuts, and klurled
collars to fit.
This sort of design needs enough give to allow for variation along the
length of the leadscrew. If there is not enough give, you will have trouble
adjusting it.
-Kevin Martin