Re: [CAD_CAM_EDM_DRO] Re: Knee for z-axis control?

Fred Smith wrote:

>--- In CAD_CAM_EDM_DRO@yahoogroups.com, "jlsmith269"
><jlsmith269@y...> wrote:
>
>
>>tkis is a "sunk cost" or a condition of hundreds of pounds, fred...
>>
>>that difference would be EXACTLY the same, knee or quill, as the
>>impact would be on the TABLE..
>>
>>if you move 300 # of weight (which a home guy would do how often?)
>>
>>
>to
>
>
>>any extremes, it would have the SAME effect.
>>
>>
>
>I'm not sure if you are asking a question or making a statement, but
>a 300 lb part on the table won't have any impact on the quill
>accuracy, if the table is locked. I don't think the table will bend
>either.
>
>

The problem is that the knee screw is 9.5" away from the knee ways on my
machine (an old round-ram base) and I think it is even farther away on
the later vee-ram bases. This is a pretty large moment arm, even given the
wide vertical span of the knee riding on the ways. When the knee is
settled,
the weight is probably balanced fairly well, but when the knee is being
elevated
the force on the ways is all against the top of the knee. When the knee is
being lowered, the weight all falls on the bottom of the knee. This
accelerates
wear on the knee sliding surfaces. It is probably no concern on a Chromed
Bridgeport - that Chrome is HARD stuff! But, on plain cast-iron ways, like
my ancient Bridgeport, wear takes its toll, and the knee now rocks when you
change directions on the knee. I always lower it farther than I want,
and make
the last move upward, on the theory that it is best to have the knee screw
pushing the knee against the flat way surface.

>
>

Jon