Metal-cutting CNC gantry build

[martin_05]

OK, I've given up on the idea of converting a Bridgeport knee mill to CNC for various reasons given in an earlier thread I started as well as others. I'll get the mill (which is free) and keep it as a manual tool.

I've been bouncing around looking at what to do. Some suggested finding a small used VMC suitable for a home garage installation. I looked around. Frankly, the stuff I've been able to find so far scares the crap out of me. What I mean is that I can see landing a "bargain" that ends-up costing another $10K to fix and really get working.

One idea that occurred to me was to actually build my own machine. I only need it to machine aluminum and be able to go through dozens of design iterations until I can zero-in on a final product design. I don't need to cut steel.

Here's what I am thinking:

I still own a very nice subplate I made back when I owned a VF-3SS. It is made from 2 inch thick MIC-6 and is compatible with Steven's Engineering plates and related accessories. While it isn't steel it is one solid surface to start with.

I already own a bunch of stepping motors, related motor drivers and computers. I'm an EE, so electronics and software are not a problem at all. I would use LinuxCNC. No worries.

So, the idea at this point is to design a steel "cube" made from welded 3 x 3 x 0.25 in structural steel tubing as the frame for the gantry mill.

The base would support what I'll call my subplate. Four columns at the corner would then support a rectangular structure where all the CNC magic would take place. This would be a moving beam gantry, like this but smaller:

Gantry CNC milling machines and machining centers | Memphis/memphis u

In other words, all of the mechanics would be well above the subplate and only the beam would move. A lot of wood router designs consist of a huge bridge that moves back and forth. I can see a lot of issues with this when cutting metal, dealing with coolant, etc.

In looking around for spindles I found a number of options. I might go with a simple 3KW VFD at first just to get going and look at integrating a CAT30/40 setup with a tool changer later on. I'd design the available travel envelope with this in mind. I own a bunch of CAT40 tooling already. However, I don't see a lot of economical options when it comes to putting together a CAT40 spindle (and it has to run off household 220 single phase).

The loose gameplan would be to weld the "cube" and then have the reference surfaces ground flat and parallel. The "cube" minus the subplate will probably weigh in the 400 to 600 pound range depending on construction. The subplate is another 200 lbs or so if I remember correctly (it's got hundreds of holes).

My first level questions have to do with the very idea of fabricating such a weldment. I can weld it together and do my best to keep it all square. It might make more sense to get a welding shop with a fancy calibrated welding table to do it. Then someone would have to grind the subplate support flat and the top of the "cube" flat and parallel to the subplate support. I really don't want to modify the subplate, so the carriage surfaces will have to be parallel to the subplate support.

Well, how realistic is this? I know structural steel tubing isn't machine-grade straight. Starting with 0.250 in walls there's a lot of meat that can be shaved-off to make it flat. At some level I am not worried about that part. Just need to understand the best approach to fabrication. Then again, there's probably something I don't know that will make this a stupid idea. That's where you guys come in. Let me have it. Please.


Thanks!

 

[CarbideBob]

....... That's where you guys come in. Let me have it. Please.

No, not gonna do it.

Size?
Gantry machines have their own sets of mechanical oh-poops.
I have no problem with making it from weldments but you need to think about stress-relive or the thing twists and warps after a year or two.

IMO, it is always a bad idea to work with motors that you have laying around or find cheap.
You need to size these things from your design. (or maybe design around the motors you have)

Desktop is rather easy,even small VMC size is not the same.
.... and then there is the spindle.

You will only learn by doing so what the heck, give it a try, just start small.
Not sure what you are thinking on ways but I'd glue (weld) some thick strips on the surfaces which would be ground/scrapped for linear rails.
Sag as the head or in/out load structure moves across the beams? Lots of bracing needed if it is of decent size.

You can buy old iron (nicely aged) for less but you won't learn as much.

The question is do you just want to make chips or run through the frustrating gantlet of machine design.
Doing things the hard way has taught me a lot even though many times it does not work out very well.
Many hours and many thousands spent on ideas that now seem very stupid. Each one a lesson learned.

You have to think about where the loads try to bend and twist things here.
All moving components have some clearance and pitch,yaw and roll errors when loaded.
You stuff this "flex or twist" in with how far it is from the tool tip as it becomes an angular error and gets big with distance.

Never think of your machine structure as "rigid", better to think as if the thing is made of rubber.
This is a point I try to stress a lot, everything bends, flexes and does not go where you told it to, you just need to control and understand how much.
Micro-maching is way different than making wood signs for your driveway. Huge differences in what you need to do as a designer.

Mostly have fun and do not expect great results from your first try.
A B-port conversion is oh-so much easier than building your own machine but is nowhere near the experience lesson.

If you just want to make chips, do not go down this road.
It will cost you more.
Bob

 

[dkmc]

Why reinvent the wheel?
There are plenty of used and abused CNC wood router's out there that are prolly heavy enough
to mill aluminum......already come with ball screws and maybe useable motors and drives.

 

[martin_05]

BTW, part of the motivation here is to use a non-trivial project like this to teach my oldest son. There's a part of this that has nothing to do with money or the practicality of it all. And, no, buying a kit that you bolt together isn't the same thing.

Size?

I went to look at my subplate. It's been in storage for a couple of years. It's huge. 54 x 24 inches. That means the structure gets big very quickly. I can see clearing 1,000 lbs. Perhaps it's time to rethink this.

I have no problem with making it from weldments but you need to think about stress-relive or the thing twists and warps after a year or two.

Need to learn about that.

IMO, it is always a bad idea to work with motors that you have laying around or find cheap.

Well, I have a pretty good assortment. Not cheap. I do agree with you.

.... and then there is the spindle.

Yup. That's definitely not easy.

You will only learn by doing[/B][/U] so what the heck, give it a try, just start small.

After looking at my subplate I am thinking that starting small might be very, very smart (and far cheaper). I think I can get the product development work I need to do with a cutting envelope in the order of 1 x 2 ft. With that in mind everything sounds far easier.

The question is do you just want to make chips or run through the frustrating gantlet of machine design.

Make chips. With the aspect of teaching my kid aside, I simply don't have the money to spring for a VMC that isn't going to become a project in an of itself. From my current vantage point it seems I can build a nice little machine for $2K, maybe $3K and one that would be far stiffer and more accurate than the aluminum t-slot routers you could get for the same money.

Never think of your machine structure as "rigid", better to think as if the thing is made of rubber.

Agreed. That's pretty much how I view things when I design mechanical components.

Another approach I've been studying is that of a home-brewed epoxy-granite machine. I've been reading a ton about this on the CNC forum and other parts. On the surface it seems like it could produce really nice results after some practice with smaller castings.

Thanks for your input.

 

[martin_05]

Why reinvent the wheel?
There are plenty of used and abused CNC wood router's out there that are prolly heavy enough
to mill aluminum......already come with ball screws and maybe useable motors and drives.

Not sure about that. The fact that you can cut aluminum on those things doesn't mean you can cut it well. It could be an exercise in frustration in and of itself.

I probably have 2,500 lbs of various sizes of t-slot extrusions in storage along with a good pile of high quality stepping motors and, yes, ball screws (don't ask). I suppose I could throw something together and see what happens. It'll almost literally cost nothing. I have not been tempted to do it that way because, well, t-slots are like rubber bands and they don't damp vibrations worth a damn, so I really didn't even consider it.

 

[Bobw]

Never think of your machine structure as "rigid", better to think as if the thing is made of rubber.
This is a point I try to stress a lot, everything bends, flexes and does not go where you told it to, you just need to control and understand how much.

I like the way you put it.. I would have simply said "metal moves", It may put a big dent in your head, but your head moved that metal a bit...


Seriously, why reinvent the wheel. The last guy that went down the road you are about to spent 2 years in prison and is now going to have a Formula 1 team.

A look on E-bay, and you can get into a Fadal VMC 15 for 10kish. I think thats high, contact them directly and probably 6-8k. Parts are dirt cheap, support is everywhere.
Even if its beat to hell, doesn't take much to bring them back to close to new.

My first VMC was a fricken Acroloc $1800, with a 4th axis, sub plate and over 40 of their F'd up holders, and a 100 or so of the AF (accu-flex) collets they use.
I needed a new coolant pump ($9 swamp cooler pump from the hardware store) and some odd fuses. Made $180k on that machine before it was sold off and
upgraded. Fanuc 6m (General Numerics, same thing).

If your hobby is to play with stuff, converting and/or building a machine is a fantastic project. Keeps you out in the garage, keeps you out of the old ladies hair
while she is figuring out what you will cook her for dinner.

If you actually need to make stuff.... I think... I KNOW you are barking up the wrong tree.

I understand wanting to do it all yourself. You can't, prioritize and spend your money wisely. I'm sure 99% of the people here have tried to cheap out on something
and gotten F'd trying to make it theselves, I've done it, and I KNOW I will do it again.... Doing it with a machine, you could tie your capital and time up for a very
long time. If it takes me 3 hours and $20 in material to make a $50 tool, not that big of a deal... If it takes 6 months and $10k, that's a big deal.