Re: [CAD_CAM_EDM_DRO] Machine flex; How much is to much?

protman16 wrote:

>Group,
>
>I have just completed converting a mill/drill to cnc. It has a belt
>driven spindle with a round column. For the z-axis I replaced the
>riser casting with THK LM slides that now allow the mill to have
>about 11" of travel. There are a couple of reasons why I did this.
>First is that 5" travel is too short most jobs that I run, second is
>that when boring using the quill there is too much flex resulting in
>elliptical holes (same applies for side milling).
>Currently I have not measured the amount deflection but I will
>attempt to very soon.
>My question is to the group what amount of cutter/spindle deflection
>is acceptable on this type of mill that still provides reasonable
>accuracy and good repeatability?
>
>
>

It depends on what you are doing. If you are roughing out cavities in
stainless steel, or making precision bores for engine cylinders, then
.001" at 50 - 100 Lbs load might be totally unacceptable. This is the
level of stiffness for a Bridgeport Series-I head with the double-swivel
joint. That is why the BOSS and some other CNC models did not have
the swivel there, as they needed more stiffness. The Bridgeport quill
is one of the stiffest things on that machine, it is a MASSIVE piece
of steel, precision lapped to a very tight fit in the head casting.
The cheap Chinese machines are made to much looser fits.

Without telling us what you are doing, asking for stiffness is almost
meaningless. 0.00001" deflection would send a coordinate measuring
machine to the trash bin. .005" would be excellent for a wood router.

I was playing with my mini-mill that I use mostly for testing and
demos, and find it is VERY flimsy. I can see the vise vibrating about
.01" during light cuts in aluminum with a 1/4" end mill. I can also see
the table "weave" as I crank the X axis handle. The gibs are obviously
loose, but the table feels stiff when cranking it. (I have made the mods
such that the handles can still be used for manual machining.)

Jon