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

Fred,

Each of us has a right to an opinion. Some are based on suspicion, others are based on fact. This .003 -.005 tolerance claimed for the ways of knee? Where else would one find those specs? I was walking through the shop earlier and noticed my surface grinders use dovetail ways on Y and Z and they can hold tenths all day long. Some apparently have the misconception that the knee "hangs" off of the Bridgeport and is imposing a large side load on the ways, when in fact it is actually balanced atop the lead screw and stantion. And guided along by the very precision dovetail ways of the machine. Thats why the "test" that was mentioned would not yeild the results that one would "suspect".

As far as the knee-lock is concerned, I dont know anyone that uses it. I've worked in alot of different shops and have never seen anyone use it. Does cause one to wonder if it had some purpose it would be used occasionally.

Sorry about wandering off-topic. This is my last post regarding this subject.

Ron



Fred Smith <imserv@...> wrote:

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "pondindustrial"

wrote:

> I'll
> also say that with the input from the `experienced' CNC-knee users,
> none have suggested the knee couldn't hold tolerances or had
> chattering problems that couldn't be compensated for; therefore

again

> I'm sold! My first attempt will utilize the existing acme screw in
> hopes that it will provide sufficient accuracy and drive, if it
> proves not worthy, I'll convert to a ball-screw and add a brake to
> the drive.

There is a reason that a Bridgeport knee mill has an expensive
saddle lock and it's not because it looks good. Many machines that
move the knee have clamp and unclamping for repositioning, some even
had heads with the same kind of mechanism. Typically the vertical
slides would hold .003-.005, and the quill would hold .001 or less on
vertical positioning.

>
> I share the reluctance to drive the mass of knee up and down on a
> regular basis, but I think with the proper drive and counterweight
> the weight can be handled effectively. Besides, that's why they

make

> a variety of motor sizes, to match the load of the application at
> hand! I'm not afraid of hard work, as long as I'm not the one doing
> it :).

As long as your machine will hold the tolerance for the parts you
want to make, this is fine. I described a simple method to check it
(before you start your retrofit) in response to an over simplified
list of pro's and cons. Did you check before you made your
decision? A popularity poll will not always result in a better
machine conversion.

More weight = more $ for mechanisms and motors, and for the
powersupplies and drivers to match.

Fred Smith - IMService



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