Re: Machine construction.

Nicely said, John.
..now aren't you glad you asked that, Rob?

My first thought (for a mill) was hefty castings to withstand the
forces that can make or break the accuracy factor. So I assumed you
meant a "router type" mill of a design for less force. Inherently a
moving gantry is going to be subject to inaccuracies simply due to
its structure. One way to maintain accuracy using a gantry might be
to raise the table/frame sides high and securely (since you are
talking 12" z-axis movement, and glide the basic horizontal gantry
over these so the gantry's "arms" are as short as possible. If you
nee to have accuracy on a small object you can place a solid insert
on the table raising the Z-axis up so little of its "ways" are out
there swaying.

What type of 3D did you have in mind?

Bloy

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "John Heritage"
<john.heritage@v...> wrote:

> >> What is the most inherently accurate machine configuration?
>
> 5/6-axis gantry mills are usually used for high accuracy aerospace

type

> work.
>
> Gantry mills are designed such that the absolute minimum amount of

solid

> machinery needs to move for a given operation. This means that

they can

> achieve very dynamic movements and be constructed in a very solid

way.

>
> From memory, industrial gantrys can now manage something like 40 -

60 metres

> per minute. There are quicker VMCs using a standard XYZ

configuration, but

> they have limited table capacity for a fixed price - because the

drives need

> to push the work piece around as well.
>
> Which way you go is quite dependant on what you're machining.
>
> A gantry is useful for big heavy parts, where the part will weigh

a lot more

> than the moving surfaces. If you're only machining small model

engines or

> something similar, a standard XYZ configuration will be better in

most

> cases, since the work table's weight will be well within the

realms of

> normality.
>
> If you want accuracy, hunting around on eBay for some linear ways

with

> recirculating carriages is a good way to go.
>
> If you want to get super duper style on it, you could pick up some

linear

> motors from eBay. These things usually cost thousands new, but

they appear

> on eBay from time to time as lab surplus type gear. If you decide

to go this

> way, you'll most likely end up with a mix and match set of three,

each from

> a different auction, as opposed to three shiny matching motors.
>
> Linear motors will be the new 'it' in machine design within the

next ten

> years. They are already gaining pace on servos. Other than cost,

at the

> moment, there is little about them that isn't better. A desktop

mill powered

> by a set of linear motors would seriously move!
>
> A linear motor is a direct drive system. There aren't any belts,

gears or

> transfer mechanisms in the way. The actuator sits in a groove

between a rare

> earth magnet guide. A voltage is applied to the carriage and it

rockets

> away.
>
> You should take a look at this guy's site -
> http://www.5bears.com/cnc.htm
>
> He is very accuracy orintated in his design for his own CNC mill.
>
> Best wishes,
> John