Questions for starting and a formula ...

[elenabux]

Hello,

I'm a new user and I learned a lot from this forum in the past two months but mostly I saw many people making their own cnc machines which made me think it's possible.
So I'm interested in building my own cnc machine.
Maybe you can help me with a few questions I have.

First, the specifications:
-it will have about 50cm x 30cm x 20cm of useful area (20in x 12in x 8in)
-it will be used for making model parts and maybe pcb's. In model parts, I'm interested in cutting wood plates (balsa especially) in 2d and also making small metal pieces (not more than 3in). Ideally I would like to be able to make components for model engines, which would imply the ability to cut aluminum and even soft steel.
-the speed doesn't matter, especially for cutting metal i.e. it's ok if it takes a long time to make a small part
-the construction should be lightweight
-last but not least, the cost should be low

So, I started to think about this machine. I found a formula to calculate the spindle power needed - this link is an Excel file in French:

http://wapics.free.fr/outil/puisbroche.xls

To summarize:

Power=K*Q

where Q is the volume cut in a minute (in cube mm)
and K is coefficient between 0.017 for aluminum and 0.11 for hard steel.
If we take
-the linear speed to be 120mm/min (2mm/sec)
-the cutting depth to be 1mm
-the tool width to be 5mm
than we have Q=600mm3/min and
Power=10w for aluminum
Power=66w for steel
That is the useful power, but the efficiency is only 65% to 85% therefore 100-150w of power should be enough.

I thought I could use for spindle:
-either a Dremel type drill with power ~100w and 5000-20000 rot/min
-or a manual drilling machine with power ~500w and 300-3000 rot/min
The first one can hold a tool (end mill ?) of 1-5mm diameter, and the second one, 3-10mm.

Which one should I use?

I like the first one better as it's lighter, but can it do the job ?

That's it for now - more questions to come
I would really appreciate any tips and suggestions you have.

[DareBee]

I relly dont think the dremel will have enough power and the drill concerns me because they are not designed for that kind of duty cycle or side load. Example-usually you can push pull the spindle of a hand drill in and out 10 - 20 thou. when you are drilling the pressure always holds the spindle agains the rear bearing. side loading would cause it to slop up and down.
I would think a router head would be better choice for spindle/tool holder.
To get that kind of travel out of a conventional mill you need to buy a 9 x 42 which is at least 2000 pounds and has a 2 Hp head.
Vertical bench mill in this catalogue is $2000 weighs 650lbs has travels of 18 x 7 x 5 inches and a 2 hp motor.
$ are CDN.
I suggest buying a used 9 x 42 for under $1000 and convert it.
Other option would be to build a router table but I dont think that would be rigid enough to mill steel.

[elenabux]

DareBee:
Thanks for the answer. I should have mentioned that I'm thinking about router type machine not a mill. Of course for a mill that kind of range is huge and it takes a big mill to do it.
Cutting steel is not really important, and anyway for steel or aluminum I would only need about 4in x 4in x 2in of range. The big range would be only used for thin wood (balsa) sheets which are a lot less demanding that metal.

Your point about a router table not beeing rigid enough brings me to my next question: if the machine takes light cuts than it doesn't have to be very rigid, right ?

Is there a formula for estimating the lateral force on the spindle ?

Thanks.

[ger21]

Your point about a router table not beeing rigid enough brings me to my next question: if the machine takes light cuts than it doesn't have to be very rigid, right ?

It does if you want to cut aluminum, and especially for steel. Even Hardwoods need a rigid machine for clean cuts. Rigidity is very important, if you'll be cutting anything other than balsa.