Help planning a small(?) router for aluminium please

[JAZZCNC]

It's hard to come up with a design when everything I read seems to talk in 'small' and 'large' machines with no indication what this means - if it said a rail less than 300mm is small, more than 600mm is large that would be meaningful, the words alone are not...

Ok get what your saying but this is why we say research and then bit more research because without doing it you'll always be unsure of exactly whats required for your particular needs.

But to help you lets start by clearing up "Small" "Large".

Simple answer is there's No such thing really because one man's small is another man's large or vise versa.?
Ie some would consider travels of 300x300 small but if your making say wedding rings then this is massive.

So really your better just choosing the cutting area you think you'll need then design and spec from that.

Now to help you in terms of what's best Fixed or moving gantry then it basically boils down to this.

Fixed gantry is generally stronger because of the more rigid frame structure. However, this design requires more area for the same travels as Moving gantry design would require. So this design tends to suit smaller machines better because for large machine would take more space and require longer rails, ball screws etc than would moving gantry.
If wanting large cutting area ie: Router then moving gantry design is often a better option as it's more cost effective and space saving.

So from what you say my suggestion is Fixed gantry. By this I mean Moving table for Y axis and X axis moving across the Fixed gantry unlike Wals design which basicly works on same principle as milling machine with X & Y on moving saddle type arrangement.

Regards Spec then will be guided by budget mostly but it's here where cutting corners will bite you later down the line.

Can tell from lots experience if your wanting to seriously cut aluminium then you really will need profiled linear rails like Hi-win if you want best finish.
In terms of rail sizes then again depends on factors like loadings etc. But to keep things simple I can tell you for fixed gantry machine around say 400x400 cutting area then 15mm rails will easily handle any loads your likely to place on them.
However if your wanting little more ridged machine then 20mm will be better option and not lot of difference in price terms. This would be my choice.
If wanting really ridged machine then 25mm but in terms of loadings these will be massively OTT.

Now ballscrews will be next on list. This is an area where can often confuse and catch people out.
With Ballscrews you have 3 main things to consider or spec up.! Class, Pitch & Diameter. Each breaks down into sub options or classifications.
# Class = The precision and efficeincy of movement.
#Pitch = The amount of distance traveled for one revolution.
#Diameter = Well no brainer this one.!!

Class can be broken down into 4 classes. C1 C3 C5 C7. The highest class is C1 which means it will higher precision and higher efficiency than next class down. It will also have hefty price increase to match.
Also there are 2 grades of screw, ground or Rolled. Ground being more accurate than rolled. Like wise ground screws come at premium.

Now for Hobby level machines then Rolled C7 is mostly used because cost's vs spec. If want that bit better then C5 but the price increases accordingly and when gets past C5 the prices become eye watering. Also above C5 you move into ground screws territory.

Pitch is next consideration. Choosing pitch is combination of resolution required vs Feeds required. Lower pitch will give higher resolution but require more RPM achieve same feed rate as higher pitch screw.
ie: 5mm pitch with 1000 rpm motor will allow 5000mm/min but 10mm pitch will allow double this because travels double the distance for one revolution.
There are other factors come into play when choosing pitch, like how fast screw can spin before whipping but won't get into that now because will only confuse and doesn't really come into play on small machine.

Typical pitch for machine like what you require is 5mm. This will give good resolution while allowing decent feed rates which will suit variety of softer materials other than aluminium.

Diameter is obviously what it says but the importance comes into play depending on screw length and torque loadings etc. However for your needs forget torque/loadings because even smallest diameter will cover your needs.
The diameter to some degree will determine what pitches are available. Smaller diameter screws tend to be lower pitch and used on shorter machines because they whip and fibrate easier over long lengths.
Whip is factor of RPM, Diameter and Length.

Small pitch would be <=2.5mm and diameter <=12mm. Medium pitch 3-10mm D12-20mm. Higher pitch 25-50mm D25-50mm+

For your needs then C7 D16mm with pitch 5mm will be perfect.
This spec when coupled with stepper motor that will provide aprox 800-1200rpm depending on other factors like voltage which comes little further down the line when choosing electronics etc, will give feeds in the region of 4000-6000mm/min or more and resolution in realistic worse case scenerio of 0.025mm.
In reality you'll get higher than this because of other factors like micro stepping but that's worse case for stepper with No micro stepping applied which you would apply for other reasons, which again will be explained further down the road.!! . . . . Building CNC machine is like eating Elephant.? Small bites digested slowly.!

Ball screws need to be ridgedly mounted to machine and this is often done using Special bearing blocks known as BK/BF bearing blocks.
There are other types but these are most common.
Typical ballscrew will have Fixed end which holds the screw and stops any axial movement this end would use BK block. The other end is held with BF block which floats to allow for thermal expansion.

So to recap and get you going in right direction.

Fixed gantry if lower travels than say 500mm. Suggest 400x400 for your needs.
20mm Profiled linear rails for best balance of strength/cost easy of building.
Ballscrew C7 D16mm x 5mm pitch.
BK/BF end bearings depending on design.

I would also suggest Steel contruction of main frame for best balance of cost to strength.

Hope this helps. Start build thread even with out having design nailed down and ask all questions there it's easier to keep track of replys.