[originator]

I am building a 36" x 24" machine to make a giant PCB for sign use. The machine will route traces for leds and other fixtures mounted on the sign. The table and frame is all 8020. In past experience the variance in the table surface caused problems when trying to mill PCBs, and especially on a large board there will be flex in the board all over the place without a vaccuum addition, and even with a vaccuum it maybe not be tight enough.

So I was making some drawings of a spindle that would float(spring loaded) on the board with some ball rollers located near the end mill that rolled on the boards surface. This idea would allow the Z to come down and put some sligt pressure on the board, and the tool rides consistently relative to the boards top.

Then today I thought to even simplify this further, forget the floating spring loaded spindle, and just place a 1/2" layer of rubber on the table. The spindle would still have rollers on it for setting the depth of cut, but this way the rollers just put a slight pressure on the board with the rubber acting as a spring. In theory this would allow for a consistent depth across any length.

I thought I'd see if anyone had any better ideas or suggestions on the idea, as I fulyl anticipate headaches trying to maintain such a small cut across >30" inches.

[mxtras]

My advice would be to secure a sheet of maybe 1/2" or 3/4" MDF to the table and secure it with recessed screws, then skim cut the surface true and call it a day. This assumes you would use vacuum for hold down - vacuum will transfer through the MDF pretty well.

Otherwise, your floating head idea would work - there are commercial machines with such a system that use a nylon shoe to run on the material.

Scott

[originator]

The mdf is a great idea, but then I lose the t-nuts in the 1030 surface. I think a good solution is to do the mdf, mill the surface(with what type tool?), and drill 1/4" holes across the surface to line up with the tslots underneath for the air to flow through. Then, if some other material besides PCB is being milled, just remove the mdf as needed.


I like the nylon shoe idea better than the roller balls, I'll look around for them.

[pastera]

look at the t-tech PCB mills.
For a large PCB you really need to use a floating head as the core thickness is not really well controlled.

Aaron

[mxtras]

... I think a good solution is to do the mdf, mill the surface(with what type tool)....

I don't know what your spindle will hold, but just cut it with as large of a diameter, regular end mill that your spindle will hold. It might take a while to do if you can only hold a 1/16" cutter, though.

As far as the nylon shoe - I doubt you will find one on the market that suits your needs. You may end up making one yourself.

Scott

[originator]

Thanks for the good info fellas. I haven't bought the spindle yet. MY past experience with trying to cut a flat surface with a 1/4" square 2 flute in aluminum is that it never leaves a nice flat surface, but I am using Micro100 and they are all center cutting. Maybe there is a non-center -cutting mill that will work better. I'll try one and see. I suppose a light sanding after the milling could improve the finish imperfections.

[joecnc2006]

To be honest with you i would not go the floating head type or spring loaded, any variance in presure will affect the cut.

If you could get a piece of granite and use it as an adjustable height surface, this will ensure a flat surface, and you can use the tape meathod for hold down.

[pepo]

On the nylon shoe idea,teflon may be a better choice,its slippery like wet soap.