Re: [CAD_CAM_EDM_DRO] Poor CNC Machine Finish

bryantmorgan wrote:

> I don't know if this is the correct forum but thought i would give it
> a shot. I am new to the forum and am the owner of a republic lagun
> machining center that I purchased at an Auction it is a 96 with about
> 8k hours on it. I set the machine up myself leveled it using a
> starret 12 inch precision level, put an indicator in spindle and ran
> across table to check for levelness and adjusted the lag on the axis
> motors all by the manual. After doing all this i still get a poor
> surface finish which looks like chatter and i know for a fact the
> machine is capable of better because i have a few machined parts that
> where with the machine when i got it. With all this in mind i have
> determined it must have to do with my setup of the machine and am
> wondering what else i could try and or what I am neglecting to do.

Poor surface finish is hard to diagnose at a distance. Are you an
experienced manual machinist, or are you somewhat new at that, too?
The problems are going to be roughly the same, I think it is unlikely
the CNC part is at fault, directly. It is most likely due to the wrong
cutting tool for the workpiece material, a worn cutting tool, wrong
surface speed of the cutting tool, wrong feedrate, or a difficult to
machine workpiece. Sometimes, when you grab a piece of scrap
to try a machine out, you end up with a mess, because that just
happens to be a piece of scrap that is some unmachinable alloy.

Climb milling generally produces better surface finish, but isn't done
on manual machines due to backlash in the Acme screws. Conventional
milling causes the re-cutting of chips, that plays hob with surface
finish, especially on aluminum.

If the fixturing of the part was not very solid, that could easily cause
such a problem. Vibration of the workpiece can easily look like chatter,
as that is just what it is.

The only problems in the machine that I can think of that would
cause a surface finish problem would be worn spindle bearings or
a loose quill, excessive vibration in the head, or a twitchy servo
axis. The twitchy axis should be pretty obvious, and loose quill
or spindle bearings should be pretty easy to check for.

If you could describe the nature of the finish, the part and how it was
fixtured, and the cutter used, that might help to diagnose the source
of the problem.

Jon