Hi Marty,
Gotta go with Roy on this one. You're making things overly complex.
This isn't a big lathe. The existing plates are quite adequate once
shimmed properly. I haven't shimmed yet but plan to. I am
temporarily getting reasonable performance from the stock
arrangement once properly adjusted. Admittedly many stuff them up by
twisting them all over the place because they don't understand what
they are doing. Agreed they need some thread seal to stay put. Mine
fell apart during shipping. But gee that's easy.
John
--- In 7x12minilathe@..., "Marty N" <martyn@...> wrote:
I just couldn't understand why anyone would go out of their way
(poor pun) to use hardened strips and see it as an improvement
rather than a potential risk.
John
Hi John:
I liked the pun :)
Here's my personal thought on the "why". 1.) The three cap screws
that hold
the gib opposed to the two dog points can provide zero preload to
the
fasteners, thus they are at will to move, won't hold adjustment.
This is
easy enough to take care of with a bit of blue Loctite. 2) The two
dog
points are not equidistant between cap screws, thus, even with the
most
meticulous setting the gib is bent to an arc if ever so slight in
the best
of cases and flat bowed silly in the worst of it. This means that
only the
leading and trailing edges are in contact with the ways underside,
and only
the outside "points" of that. This makes unit force over area high
enough
to, a) pierce the oil film b.) scrap the oil from the way. This is
a bit
more difficult to fix. Replacement of the dog points with shims is
an
alternative however the material the gib is made of is so soft
that in my
first attempt at it with the half hard 260 brass I used dented the
gib as
easily as a finger nail dents pine boards. I had cut the brass
into two
pieces, one each side of the center cap screw as the fine
adjustment needed
here precluded, for me, individually making holes for all shims in
the pack
required. This leads to misalignment by the depth of the dent.
Later when I
lapped the stock gibs it took nearly .004" after local contact to
remove
said dents. 3.) Even when a full shim is in play the material is
so soft
that it flexes silly under moderate pressures. 4.) When I lapped
the gibs I
took .008 of this little critters, after straightening, to get
full contact
with the lapping plate, wavy, bent, bowed, dented...junk. 5.) If
your going
to fix it, then, fix it. 5.) Made from some hardened material,
such as A2,
makes it stiff enough to loose the center cap, use a shim without
perforation where extremely fine adjustment can be made. 6)
Because my
Southbend uses a gib 10" long held with two cap screws, is .375"
thick and
is hardened, ribbed iron and doesn't flex or bow over the
cantilever. So
it's not a poke in the dark. 7.) On my machine built 5/26/1956
shows zilch
for wear after 50 years. 8) I'll copy a proven design and would
down to the
cantilever if I had enough room :) 9.) Because hard steel on iron
has 23%
less friction than iron on brass. And hard chromed (what mine spec
to) has
half the static friction of iron on iron and will not gall. In
fact hard
chrome on iron has about half the frictional coefficient dry as
brass on
iron does lubricated.
Just thoughts.
Marty
