Re: What is the weak link in machine precision - Knee Mills

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "zipdrive2k"
<zipdrive2k@y...> wrote:
Enco has a mill they state the specs are

> 0.00078/ft. error for squarness and parallelism.

Our friend Ballendo has often mentioned the overstatement of
accuracies by sellers of low end machine tools. With this in mind I
dug out the original verification documents for one of my machines.
The limit for parallelism is .00078/ft, while the limit for
purpendicularity (squareness) is .00118/ft. Notice this squareness
tolerance is 50% greater than what Enco states. The actual
measurements for my machine all fell within the stated limits. Some
were substantially less than the allowance, while others barely made
it under the wire. It's possibly worth noting that these numbers
are from factory tests on a machine that sold for slightly over a
quarter million dollars. Do you think it's possible Ballendo has a
valid point regarding overstatement of accuracy? or does Enco have
some secret no one else knows when it comes to machine
construction? My offhand guess would be some of the better
Taiwanese knee mills will equal Bridgeport's geometry numbers (I
dont know what BP's specs are), but a further guess would be that
most of the Chinese iron will fail to equal BP's numbers by a factor
of 2 or more. If the upper limit was indeed .00078/ft, then the
average machine could be expected to have the geometric accuracy of
a Moore Jig Borer. I kinda doubt that's the case.

Cliff