What is to be expected from a CNC ROUTER ( precision VS machining centers )

[JinMTVT]

Trying to understand a few specific aspects and limitations of CNC router

Why is it that CNC routers are all ( most? ) much less precise than
even economical milling/machining centers ??

what exactly is to be expected from a high end cnc router as far as precision goes ?

we see specs like +- 1mil on some
some other ranges in the +- 4mill

how does this affect finishing on solid parts?
( mainly aluminum )

i'm no expert, but i've often seen figures of ~ +-0.005mm even on cheap machining centers and millings

heck i even seen +- 0.005mm / 3meters distance
( according to a certain test specifications )
on a 125 000$ chinese large gantry machine usgin siemens controller


is it a limitation of design ?
controller/servos limitation ?

i see all high end milling using ballscrew movements
VS most regular routers using slanted gear + racks

i personally don't see why a cnc router couldn't be as precise as any other cnc machining center ( within a comparable price range VS size of the machine )

would like to know what are you thoughts on that ..

cheers all!

[awerby]

There are no hard and fast rules about what precision a router can be built to hold. But tolerances for things made from wood aren't going to be as high as those for metal objects. Wood is inherently more difficult to machine to a precise dimension, if only because it will swell and shrink with the weather, changing dimension markedly as it does so. Metal parts, on the other hand, while they will expand and contract with temperature, do so in a more controlled and uniform way. They often need to fit into one another rather precisely - a sliding fit, for instance, is different from a press-fit. So you'll rarely find a milling machine that can't hold a couple of thousandths of an inch one way or another, and some of them can hold "tenths" (of thousandths) or even better.

It's certainly possible to build a router to that degree of precision, but every arithmetical increase in precision entails a geometric increase in cost, so one has to ask "why bother?" if the result can't be measured and doesn't matter anyway.

Andrew Werby
ComputerSculpture.com — Home Page for Discount Hardware & Software

[JinMTVT]

awerby: i fully agree and understand that for a wood working situation which is usually 95% aesthetic work, sub 1mil tolerances are unnecessary.

For plastics and aluminum works, big difference.

I've started this thread here only because "ROUTER" are woodworking things usually, and i would probably have not many interests in the metalworking forum ??? should i post it there too ?


Still, technically, it is possible to use solid router to work on soft metals
and althought they might be more of hybrid with gantry milling
i would still like to understand why there are NO cnc routers of any kind , on any commercial pages i've found so far, that have sub mill tolerances
even if advertised for aluminum cutting


controller type issue ?

[Geof]

....i would still like to understand why there are NO cnc routers of any kind , on any commercial pages i've found so far, that have sub mill tolerances even if advertised for aluminum cutting...controller type issue ?

You haven't looked at enough pages.

Haas CNC Vertical Machining Center (VMC) | Haas Automation, Inc. | CNC Machine Tools

[JinMTVT]

hey! i've looked at those already
and you let me know where they state accuracy ??

i've tried every pages their website and i can't find any info about it

[JinMTVT]

if i was to ask reverse question

where is the loss of precision in this type of machinery ?
be it milling or router

there must be many additive factors i guess ??


how do manufacturer test their accuracy ?
loaded or free ? this must also influence results ?

[Geof]

hey! i've looked at those already
and you let me know where they state accuracy ??

i've tried every pages their website and i can't find any info about it

You are correct they seem to be hiding any mention of accuracy and repeatability. They are both better than +/-0.001", I have a GR510 and it holds that quite easily.

[KOC62]

My thoughts are that a CNC router has;
- a larger cutting area to cover than a mill.
- greater flexing of moving parts.
- less mass of the moving parts.
- greater feed speeds (i.e. IPM)
- less cutting horsepower

I think these factors decrease accuracy when compared to a mill.

[JinMTVT]

usually not a good sign when a cnc manufacturer doesn't spec the accuracy/repeatability of their machines

but if you say it holds less than 1mill , seems good

can you attribute this to a specific parameter?
it is ball screws all over or gear racks for X ??

they seem to be using their standard controller panel ?

have you machined aluminum parts on it ?

cheers!

[Geof]

We have machined aluminum:

http://www.cnczone.com/forums/haas_m...eleration.html

[LazyMan]

Take a look at Datron Dynamic's large format machines.
High speed precision CNC milling engraving machine

CNTMotion claims +- .002 along the entire travel.
CNT Motion Systems has a broad selection of machines, and design automated solutions to meet your needs.

You will need deep pockets to purchases a large format machine with .001 tolerances over its entire travel.

[JinMTVT]

After having done some reasearch ...
i believe that using quality linear rails and getting a high precision
laser machine alignement is a must
and could probably lead to very high precision machine
using the correct control/servo and precision ball screws


i am looking at FANUC and SIEMENS controls and servos
what do you guys think about them ?
( the base controls )