Homebrew PCBs

--- In Homebrew_PCBs@yahoogroups.com, "starsnstripes_2003" 
<bwatson@a...> wrote:
> Ok, all this talk has given me the bug to build a milling machine 
> for pc boards. Something that will be semi-precision.
> Maybe some of you guys can offer some good tips for building an X/Y 
> table?
> I am a poor person and want to hold cost down as much as possible, 
> so ideas like the "Drawer Slide" linear motion bearings are greatly 
> appreciated!
> 
> Slides:
> I'm not sure how much play are in those drawer bearings. Probably a 
> lot. I'll have to look for some good ones, or maybe make my own 
> slides from brass or UHMW channels. I've dealt with different types 
> of gibs and round shaft/ball bushings before, so can probably 
figure 
> out something that is semi-precision at low-cost.


If you think of the forces involved, your main thrust should be for 
precision, not load bearing.   Drawere slides will offer load bearing 
stuff and little presision.  a simple rail, say a 1/2 x 1/2 inch 
steel piece could offer the presision needed.

make your table, roll it on the drawerslides, and put one precision 
guide down the center.  then using nylon or delrin or some such, make 
a snug fit so the table will not move side to side.  very low tech.  
remember, you only need one edge aligned to hold the table.




> Linear Motion drives:
> Stepper motors are the obvious low-cost solution, but I'll need to 
> find some good resources for cheap parts that will work properly as 
> a complete system.

forget servo's.  you need an encoder for feedback to posistion them 
and although old printers had them and are cheap, you are hacking a 
lot to use them.

steppers are much simplier.



> I can write some software (VB and Perl) but I think some 
> shareware/freeware is available to handle conversion from different 
> CAD programs...

already done and available.

but drilling is quite different than routing a board.  
drilling only requires points, and routing not only requires a 
software to figure the outline of the trace (reember, you leave the 
line, not cut it) but the much harder part and the high precicion 
part is height control.


> Then I'll need to fabricate the mechanical drive portions: 
Leadscrew 
> or Timing Belt like printers?

leadscrew will offer slow movement, but high resolutions.  a timing 
belt will offer much faster movement, but less resolution and lower 
power.

a 200 step motor with a belt rotates 5 times per inch.  or 1,000 
steps per inch.

a 1/4-20 leadscrew rotates 20 times per inch so with the same 
stepper, the motor rotates 4 times more.  That offers 4 times the 
power and 1/4 the speed.

Belt is easier and is probably good enough for drilling.
Unless the rest of the system were tight, I would go with screws if 
milling.


> 
> Design parameters:
> What are the largest boards I should allow for in the x and y axis 
> travel? 8", 12"?


The most stable set-up will be to have a table that moves back.  this 
would be the base and near full width of the board.

then make an overhead gantry so the dremel can move side to side.  
this allows the drilling part to be pretty stable.

as far as the gantry goes, most people put a verticle plate and mount 
the dremel on a large angle bracket.  a more rugged disign is to have 
the slides on the sides of a sled and the dremel in the center.  that 
way, the drilling forces are not trying to twist anything.

and the simplest way to get teh Z axis is to buy a linear slide.  
these are the pre-assemebled units that only move an inch or so.  
here a screw is deffinatly preferred.


> 
> Anyone who has experience with such things, please chime in if you 
> know of some good sources for info or parts!

parts ?

old printers !

most of everything you need are already there.  linear rails (shafts 
and bushings) steppers or possibly servo's, switches, connectors, 
ribbong cable for linear motion.  

if you are creative, you can use the smallish rails on a support to 
make your own linear rails will full lenght support.  and then a 3/4 
or open bushing.

and if you get a servo unit, you can make a chuck and use that as a 
DC motor for your drilling.


I used a full 4 ft by 8 ft sheet of 3/4 melenine to make a unit has a 
foot print of 2 ft by 4 ft with the intention of making a 12 x 12 pcb 
driller.

routing pcb's is dusty, takes a long time, and burns thru carbide 
bits.

drilling is simple with a simple machine, but you do need to get the 
coordinates from your pcb software into a file you can use for 
drilling.


Dave