Re: Knee / Quill interaction?

John,

The control doesn't have to deal with the changing position of the
knee. The machinist does. With a knee type machine there is no
fixed relationship between spindle position and table surface since
the table height is variable. Somehow when entering tool offsets
for a job the machinist has to take into account the actual table
position when setting up the job.

With a bed mill the machinist doesn't have to account for the
variable table position because it's always in the same place. In
other words the distance from "home" to table surface is constant,
not so with a knee mill.

Make sense? It can be a confusing issue.

Doug



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "jmkasunich"
<jmkasunich@y...> wrote:

>
> I posted a Shoptask specific version of this question earlier,
> but the core issues apply to many machines.
>
> How does CNC deal with two axis that are parellel to each other,
> like the quill and knee of a Bridgeport?
>
> I know the simple answer is to only motorize one of them. But
> there are pros and cons to each, and some benefits to controlling
> both. For example, the quill weighs much less, is easier to
> move, and is more accurate. But it has limited travel. On the
> other hand, the knee has lots of travel, but is very heavy, and
> on a worn machine may cause errors unless it is locked.
>
> It seems that a good solution would be to motorize both. Use
> the knee to adjust for large differences in tool length, like
> an end mill in a collet vs. a long drill bit in a drill chuck.
> Then lock the knee and use the quill for the actual machining.
> (The same thing can be done with a manual knee - adjust the
> knee manually when you do a tool change.)
>
> The question is, how does the control deal with the changing
> position of the knee?
>
> John Kasunich