fixed gantry strong enough for light metal and lots of wood

[DTFuqua]

I'm trying to get my head around the strongest, easiest way to build a machine that would do 2'X3' cabinet doors for some high-end custom work and want it to be strong. I figure a little custom metal work for bikes and even some cars might make a few extra bucks seeing a Harley costs as much as a cheap house now days. I figure a foot print shouldn't be more thag 50" X 30". I figure that weight is the biggest factor to making a strong gantry and I read talk like some peoples gantrys weigh like 40 or 80 pounds and made out of aluminum. That can be a lot of weight to move with steppers unless you want to get monster motors and use two to move and keep the gantry even on both sides. I figure I can make a truss type structure that would support tons of pressure mount the Z axis running the long way of the working area of the machine and the work-piece move for the x-axcis An aluminum table of approx. 30" x 40" with bearings and thin spoil board should come in around 35 pounds even with a 24X36X1 inch work piece for making cabinet doors or maybe even a little smaller if I decide not to do over-sized doors. Well, this is just where my ideas are going. I'm basicly expecting most of my work to be the woden signs of approx 8"X24" for awhile but hope to branch out to more proffitable areas or should I just make one big enough for the rustic type signs and strong enough for milling small aluminum or brass pretty pieces for people to personalized their bikes or cars and forget about high-end cabinet doors? Anyway, Strength is important to me to be able to do small badge size or plaque size metal and come out nice.

[ger21]

You're going about it the wrong way. Heavier is actually better. The lighter you make it, the more vibrations it'll transmit. No way you can make a rigid enough machine at the light weights you're talking about. Small steppers can move a lot more weight than you think, without any problems.

Build is as strong and rigid as possible, and don't worry about the weight.

[DTFuqua]

Yes, I have no problem with weight. I just want to be able to make the gantry or bridge very strong which will make it heavy so I figure it would be easier to move the table since it would be lighter and that would avoid the gantry doing anything like racking or flexing. Also, with the table movable, I can spread out the bearings wide enough to have less chance of any un-wanted racking of the table. I realy don't need a large work area for this machine but bigger gives more capability, as long as its built strong, so I'll probably figure out a minimum useable size and then build to whatever size I can build most economicaly.

[katalin]

The fixed bridge and moving table design from the rigidity point of view is superior to moving gantry type machines but the structure of the machine is more expensive for the same working area.
The moving table will generate less inertial forces, theoretical it will have better cinematic precision(but in practice it depends on the components used, the quality of the machining and alignment).
You said earlier that a 80 pound gantry is hard to move, that is not true, in many cases it takes more toque to accelerate the screw then to accelerate the (mass)gantry.
The force is F=m*a , for an acceleration of 0.3 G and a weight of 80 pounds the force is only 120N (without the consideration of frictional forces, witch are very small when using ball or roller type linear bearing) If you are using a 5mm pitch ball screw you will need only 0.1 Nm torque to overcame the inertia of the mass.(Sorry for using metric units but I am not familiar at all with the imperial ones).
If you build the structure right to obtain performance in machining wood you shouldn't have problems machining aluminum and bras using small depth of cut and fast feed rate.
I think that is important for you to think on what type of spindle you are going to put on the machine and the type of linear guides and screw in conjunction with the rapid speeds you want to obtain.

[DTFuqua]

Originally Posted by katalin The fixed bridge and moving table design from the rigidity point of view is superior to moving gantry type machines but the structure of the machine is more expensive for the same working area. The moving table will generate less inertial forces, theoretical it will have better cinematic precision(but in practice it depends on the components used, the quality of the machining and alignment). You said earlier that a 80 pound gantry is hard to move, that is not true, in many cases it takes more toque to accelerate the screw then to accelerate the (mass)gantry. The force is F=m*a , for an acceleration of 0.3 G and a weight of 80 pounds the force is only 120N (without the consideration of frictional forces, witch are very small when using ball or roller type linear bearing) If you are using a 5mm pitch ball screw you will need only 0.1 Nm torque to overcame the inertia of the mass.(Sorry for using metric units but I am not familiar at all with the imperial ones). If you build the structure right to obtain performance in machining wood you shouldn't have problems machining aluminum and bras using small depth of cut and fast feed rate. I think that is important for you to think on what type of spindle you are going to put on the machine and the type of linear guides and screw in conjunction with the rapid speeds you want to obtain.

The precision is what I'm wanting so I will be able to make small personalized parts for bikers and hot rod owners out of aluminum or brass. The table and gantry/bridge will be made out of steel and the Z axis on a thick aluminum plate. I intend to start out with a router for a spindle for awhile, maybe for ever. Now about the formulas you are trying to help me with. I have no real education except for a life experience with building everything from stairs to power plants and wreckers to race cars. Just enough actual construction/building experience to understand things in a "seat of the pants" type way, although I did teach myself to read simple blueprints and computer repair and such.

[katalin]

If you are going to use steel for the construction i don't see any problem in machining aluminum or brass. The biggest limitation will be the router witch is designed for machining wood not aluminum, but with the right attitude I've seen people doing amazing things with this tools. If the machine will make money I don't see any reason not to invest in a professional spindle witch will increase your precision, productivity and tool life.(it will pay for itself in a shot time)
For the precision part of the machine it is important to use profiled linear guides and ballscrews (there are other types of components but this is one of the safest way for precision) . Build the machine in such way that you have the possibility to align then as good as you can.

Good luck with the build!

[cyclestart]

Originally Posted by DTFuqua An aluminum table of approx. 30" x 40" with bearings and thin spoil board should come in around 35 pounds even with a 24X36X1 inch work piece for making cabinet doors

1/8" aluminum with 1/2" mdf ? Or maybe a plywood spoil board ?
My fixed bridge (moving table) router's table is currently 3/4" MDF with no aluminum, it's a functional work in progress and the table will be based on aluminum some day. This stuff is expensive so you want to get i right the first time. I'm not convinced aluminum in 1/8" thickness adds much resistance to deflection. OTOH it will will provide a solid surface to bolt the bearings

[DTFuqua]

Originally Posted by cyclestart 1/8" aluminum with 1/2" mdf ? Or maybe a plywood spoil board ? My fixed bridge (moving table) router's table is currently 3/4" MDF with no aluminum, it's a functional work in progress and the table will be based on aluminum some day. This stuff is expensive so you want to get i right the first time. I'm not convinced aluminum in 1/8" thickness adds much resistance to deflection. OTOH it will will provide a solid surface to bolt the bearings

I am (just thinking right now) 3/8-1/2 inch aluminum plate with 1/2 inch mdf spoil board. Other than the possibility of the router mount, the plate will likely be the only aluminum piece in the whole machine, unless I go with the bearing sets from fineline automation.