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

Fred Smith <imserv@...> wrote:
--- In CAD_CAM_EDM_DRO@yahoogroups.com, R Rogers
wrote:

> The cons to using the knee? you cannot accelerate as quickly as you

do with a quill. So it is a bit slower however, not a huge
difference. And you need a larger motor for Z than X and Y.


Fred smith wrote:
The knee on a bridgeport will usually lose position if not clamped
when cutting. They may also rock from side to side if the gibs are
not just right, causing chatter on heavy cuts. If your machine is
very worn at all( or dirty) you will not have accurate Z motion. This
can affect bore straightness and drilled holes may not come out
opposite the starting point.

:::::::::::::::::::::::::::::::::

You've got to be kidding! If you have encountered these problems with the knee on a Bridgeport that machine should've been scrapped eons ago. I've ran some real beaters in my time and I've never, ever encountered any of the characteristics you claim. The majority of chatter is caused by the quill being extended too far and running long length tooling that is not ridgid with its shank dia, the quill is many times less ridgid than the knee. When I have chatter and I retract the quill or go to shorter flute lengths eliminating the chatter this somehow effected the knees stability? machining with the knee I never extend the quill much over an inch and it is locked in position and run the shortest tooling possible. The next time you get chatter put your hand on the table and the quill you'll see where the chatter is coming from.

::::::::::::::::::::::::::::::::::::

Test yours by putting an indicator on a work piece, parallel to the Y
axis. Clamp the Knee and see how much the indicator dial moves.

::::::::::::::::::

I have tested it for play and it doesnt move. The ways for the knee on a mill are usually like new because if they are old manual machines they were never used like "x" and "y" were. I never use the knee lock on a BP. I've never needed to that I recall and I dont know anyone that does. Frankly, I dont know why they were even put on them to begin with. unless it was something they started doing in the beginning and never changed. The Z losing position during manual or CNC machining is something I have never experienced. I have a better idea for a test, put an interapid on your spindle extended fully like you are tramming the head with it upon a 1-2-3 block resting on the table, grab the spindle and shake it or bump it with your fist and see how much the indicator moves. Now, try to shake the knee that much.

:::::::::::::::::::

If you do this before you invest in a W axis, you may decide that
just a power feed on the knee is a better alternative, and a cnc
quill makes more sense. It does vary a lot by machine.

Fred Smith - IMService



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