RE: [CAD_CAM_EDM_DRO] minimill thoughts was Re: Dead Grizzly speed control/VFD

Ballendo -

The first problem I ran into when trying to make a long x travel with the
conventional configuration (slideways and saddle, ala Bridgeport Series 1)
was "tipping over". If you have a table that is too long for its weight,
the center of gravity of the entire machine starts getting to close to the
edge of the base footprint. Of course, you could bolt it down, but you'd
also have the problem of excessive overhang greatly increasing the loads at
the bearing surfaces (both X and Y). This would, in turn, increase the
friction forces at the bearings, requiring stronger muscles/motors, and
accelerating wear (again, both X and Y). Now, any slop (both X AND Y) in
the bearings will be exaggerated when the center of gravity of the table
goes past the end of the saddle, because it will "rock" to the other end of
the slop (the weight of the saddle will delay this a bit). All these
problems are made worse by narrow y-axis bearing spacing, and short X-axis
bearing length.

I'm trying to avoid these problems in my ground-up machine by using a
"bedmill" layout, where the table is shorter than the saddle, and the y-axis
bearing spacing is fairly wide. Ideally, the CG of the table will not
travel past the point of rocking the saddle, and since the saddle is longest
and heaviest (reverse of typical knee-mill layout), there is additional
margin before the table "teeters". If done correctly, there will never be
enough overhung load to apply a moment to the bearings. The main problem I
ran into with the bedmill layout for a benchtop mill was having a long
enough table for fixturing, etc. without ending up with a super long saddle.
I finally ended up with 14x9x10 inch travels (xyz) and an 18 x 8 table
(which I hope to leave alone since I'll be sending out the foam patterns for
castings soon). I'm posting images to the files section, in case any one
wants to see them ;^)
(http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/CNC%20mill%20design/)
Any feedback is welcome and appreciated.

Carlos Guillermo
VERVE Engineering & Design

Ballendo wrote -

What I can't figure out is why
everybody makes the x axis travel so short (about 9 inches). Joe M at
sherline says its because the extrusion warp and twist becomes
excessive to machine away in longer lengths, but what's the grizzly
excuse? poseidon? Prazi? MaxNC (could be same as Sherline)? REX?
Myford? Shipping cost of the extra cast iron?

Wm. Dubin and I have had an offlist(sherline list) conversation
regarding a "bigger" sherline, but still little. Like a sherline with
a 3 slot table (same spacing) and a 7 inch Y (2000 mill base) and a
12 inch X? I bet the person who brings this (or something like it in
cast iron) to market will do well!

The MaxNC 15 is close, but pretty expensive. And that lousy motor
setup, IMO.

Ballendo