Scratch Design Questions

[danowar]

I have recently started designing a CNC Router. I have the X axis and Y axis all planned out, and nearing completion on the Z axis.

While I wish I could post pictures of the design so that I could get input on what I am doing wrong or right, it is not possible. I am currently deplyed oversees with the Army, and don't have the capabilty to do that. The reason I even started designing it was to pass the time, but now I am hooked on getting it to work.

I didn't discover this webpage until very recently, so after veiwing many threads, I have already made numerous changes to my design.

The requirments I would like to meet for my design is to handle a 36" by 12" sheet of wood. The wood would be basla wood and lite plywood used for model aircrafts. To achive this requirement I have decided that a the Y axis would be a moving table, since it only would have to move the 12" or so. The X axis would traverse on overhead rails with the Z axis attached to it.

My biggest question at the moment, is what to make the table surface out of, and how to hold down material. I will be wanting to cut through the wood, to cut shapes out. This poses 2 problems that maybe you have and answer to.

#1 How will I ensure the bit cuts deep enough to cut through the material, but not into the table? Standing off the material would fix this problem, but if you have ever worked with balsa, you know it isn't very rigid, and standing it off would be difficult. Another thought is to put a scrap piece of wood under the material, so that it doesn't matter if the cut is slightly to deep.

#2 How will I hold down the material? I will be cutting parts out, so how do I insure those don't move around during the final part fo the cut? I have thought vaccum, but then I wouldn't be able to put a scrap piece of wood under the material. I also don't know how to implement a vaccum, but I could learn I am sure.

This got a little long winded, so I will stop here. I am sure I will come up with many other questions before the design is done, but I can ask them later.

Thanks for your time.

Dano

[toolmkrman]

I would put wood top and bot. with the good in the middle.
Then you could just toe clamp it to the table.

[balsaman]

I had the machine drill a grid of holes in it's own table, and then installed bilind nuts.

I just clamp the edges of the wood, and cut. The surface of the table slowly gets cut up, then you change it after many many airplanes.

You can leave little tabs to hold the parts in. Cut everything inside to outside.

Eric

[samualt]

1. Yes, that is why they call it a sacrificial top. It is made to get whacked. Usually MDF (Meduim density fiber board...cheap stuff). I have seen people use nice metal tops out of aluminum. But I bet they get scratched and wacked just the same. You might break a few wood router bits on a metal top.
2. Vacuum tables are expensive to build, or at least can be, and a bit tricky. They might not work well with balsawood that is pretty porous (don't know really). There are two ways I've seen to hold stuff down easily.
A. Have your router, once built, to drill a bunch of holes in your sacrificial top so you can use screws/bolts or whatever to hold stuff down. A bolt tightened down with a metal-rectangle hanging off the top of it or an actual clamp of some kind.
B. I have seen people use a kind of T-Sloted wood top (Think of an upside T carved in rows in the wood). That way they used T-nuts and clamps or metal rectangles again to hold stuff down.

Just some ideas I've seen. I'm a complete newbie myself.

[boxwood]

Hi all

2 sided tape can also work pretty well for thin light materials
get the good stuff 3M

Regards

[sol]

I made my own T-slots, since they were not locally available, by ripping MDF into strips and stacking them as shown; it also adds significant mass to the table.
In regard to extracting a little more life out of sacrificial table tops, I fill the troublesome voids with Bondo® car body repair epoxy. It is solid enough to support the parts but soft enough to be easily sanded flush.

[steveald]

Hi

I used 1/2" sintra for my 2x2 table top. I created a T-slot table by running a keyhole router bit down the length of the sintra every 2" I then cut strips of 1/4" sintra and double sided taped them between the slots, then milled the 1/4" sintra flat. The keyhole bit only allows a 3/16" bolt, so I had to cut one direction, move over 1/16" and cut back to allow for 1/4" bolts to clamp with. I did this on a larger CNC machine, but it could be done on its own machine if the axis allows the bit to start and end past the table.
I built a vacuum system for a 4x8 table several years ago using 1x2 aluminum tube. I plugged the ends with sintra, built a manifold out of pvc pipe and ball valves, wrote a program to perforate the tubes with 1/4" holes, taped 1/4" sintra to the tubes, ran the program again with a 1/8" bit to perforate the sintra, and milled the whole thing flat. Worked well with a shop vac, and great with a regenerative blower. For 12" x 36" and balsa, I would use a shop vac and skip the manifold and valves.
You might have a problem getting double sided tape off balsa without breaking it, so I would recommend Anchor brand paper backed tape, which is like double sided masking tape. It holds well, and can be removed easily. Just don't get it wet

I seem to be running on a bit here, but I hope this gives you a few more ideas to pick from.

Steve

[HomeCNC]

Dano,

Nice to see you made it over here!