30" Z Axis

[tsperry88]

Not sure if this the right forumn. I'm fairly new to CNC machines. I am pilot and have spent considerable time researching inexpensive options for general aviation aircraft. Molded composit experimentals are some of the nicest aircrafts out there. People spend large ammounts of money designing the plane, having molds built, setting up shop, ect.. They create huge overhead and then, when they realize the demand for molded parts just doesnt support there buisness, they file for bankrptcy. My goel: Build a CNC machine capable of carving a plug, for inexpensive mold production. I want to be able to produce a small 2 seat molded aircraft, while keeping overhead at a minimum. My machine needs to by 20'x5', and have a 30" z-axis. I have the mechmate plans, which should work for the x and y axis, but I need help on the Z.

[CncForMe]

You are talking about a big machine there. Lining up 240'' long rails perfectly might be you biggest challenge. What kind of accuracy are you shooting for? My recommendation for that much Z is to get a linear stage, strong accurate, and already built and ready. They come up cheap once in a while on eBay. See my build log @ Philadelphiamachineworks.com for an example of my machine. The blocks stay put and the base moves. I have around 24'' of Z travel. I know you just want to do foam but you need a lot of gantry rigidity with that much Z. fficeffice" />>>

[tsperry88]

Thanks for the reply. Im not realy sure on accuracy. Parts would be assembled by jigs. They would be on casters. Concrete is not precisely flat. The jigs would likely be stress relived and may or may not be precision ground. Part of keeping production costs down would be knowing when excessive tollerances become pointless. I got a ballpark quote a while ago to have a 5'x10' frame built, welded, stress relieved, and have the top of the 4" tubes ground to the low 1000th of an inch tollerance. It was mid 2k range. My machine as doubled since then. What kind of tollerance is normal. I would imagine a couple hundredth of an inch tollerance would be fine, and maybe overkill. There is a lot I dont know right now. Maybe I could just use the concret slab as my table and build an i beam structure which would support the gantry. I think the biggest issue is when the sun hits a glossy wing surface, there is no ripples from a poorly cut wing mold.

[tsperry88]

What you mean? "The blocks stay put and the base moves"

[tsperry88]

I googled linear stage, and cant seem to find anything that would work on a machine that big. (or small realy) What exactly are you refering to?

[CncForMe]

Did you lool at my website? Several pictures of my Z setup on there. Look for linear stage on ebay, they come up cheap all the time. Don't know your budget but most of the stuff used on ebay is great. Although 30'' could be much harder to find. I know a fair bit about fiberglass moulds for boats, airplanes are of course usally made to much higher standards. With any mold i don't think accuracy over .01 inch or even less is nesessary but it must of course be sanded to perfectly be smooth and fair.

[tsperry88]

Like this?

http://cgi.ebay.com/Parker-Daedel-50...3A1%7C294%3A50

[CncForMe]

But you need a big heavy one. The great thing about a ready built slide is everything is lined up, ballscrew installed, all set to go. You want one with four linear blocks and double rails for the most rigidity.

[MechanoMan]

You have not mentioned your required accuracy, and that changes everything.

If it is noncritical and can tolerate 1/10" errors, a relatively crude construction is possible. Does your frame NEED to be ground to a few thou?

Also, if the accuracy error is REPEATABLE, it may be possible to work it out in software. However, not all types of errors are totally repeatable. If the machine will not move smoothly due to inaccuracy in the guide rails that problem could not be resolved in software either of course.

[Oaktreeman]

I'll venture a guess that building the frame is similar to building a boat. When you double the size the cost is ten times. If you have a $2,000 quote for the small one, prepare for a shock on the large one.