Looking for sound advice

[PixelsnPrint]

I'm totally new to all this machine design and concept of building a CNC router but here's my thoughts and concepts, not to mention questions. First the media I plan to cut/route typically would be MDF, UDF, Acrylic, Sintra board; basically your typical sign building components. Working envelope either 48"x48"x6" or 48"x96"x6", I'm designing and budgeting for the larger one but will resize if figures get away from me. Here are a couple key questions, One given material options for constructing the gantry and it's support structure from should I use MDF or Corian both are wood working tool compatible and require no special skills I do not already have. attached is a pdf of my concepts granted x-y-z cars are not complicated designs but should suffice for my cutting needs. Please advise as to thoughts and suggestions.

Thanks in advance,
Richard Morton

[mekanik]

Hi PixelsnPrint
Welcome to the forum
My only knowledge of CNC construction/building has come from following the threads and reading the build logs, but if funds allow i would forget about using wood or plastics as construction material, Ally profile is an option but this can get expensive and you will find that steel box section is your cheapest route. For the sort of duty you are looking @ you could get away with round supported rail. Have a look through the build logs and you will get a better idea of the way ahead, and if possible post a sketch up detail drg and the lads will put you on the right track.
Regards
Mike

[Neale]

Richard - I have built and use a router constructed in MDF. Can't remember the exact cutting size but it's less than 1000x600. It bends when it gets warm, it bends when it gets cold, when it's dry, when it's humid, when it starts cutting and when you look at it. It also warps when you are not looking at it... Cold-rolled cow dung is not a sound structural medium. Probably not so bad if you paint or varnish it well and keep it in a warm dry environment but I'm about to start on a steel-framed replacement. Don't know about Corian although it's probably a lot more stable but I doubt if it is really fit for a structure of the size you are proposing. For sign-writing type use you probably don't need enormous structural rigidity but if you can't keep the bed flat, you are not going to get uniform depth lettering.

[PixelsnPrint]

LOL I hear ya. If I stick with the 3/4" and 1" MDF as per my drawings or even move to the Corian I just happen to have some off cuts laying around I'll be building a torsion box bed. My alternative is to go box steel as you have mentioned but for my first go I am trying to keep as much of the fabrication in-house as I can.

[Ger21]

The constructions you show work OK for a small 2'x2' machine, but will result in a very poor 4x4 machine. If you want to build from MDF, everything needs to be torsion boxes, not just the table. The gantry sides should be 3" thick torsion boxes. The gantry beam should also be a torsion box, at least 4"thick x 8" tall.

Your Z axis as shown will not be nearly strong enough. I suspect it will be very loose, or will break very quickly. Using the skate bearing on pipe method requires that the bearings be forced very tight to the pipes. You have no way to do that with the Z axis.

Most of the members here build only in aluminum or steel, and wouldn't even consider MDF. My personal belief is that it's very possible to build a good machine from wood. However, it takes a lot of knwledge, skill, and techniques that add considerable time over metal builds.

I believe that the design you've shown will not work very well at all. My suggestion is to build a small 24" x 36" machine, and you'll see where the shortcomings are. You'll gain valuable experience that you'll really need in order to build a much larger machine.

And don't even think about using corian. It's not flat, it's brittle, it doesn't hold screws well, it's not very rigid. It's just not a good choice for building a CNC machine.

[PixelsnPrint]

LOL I hear ya. If I stick with the 3/4" and 1" MDF as per my drawings or even move to the Corian I just happen to have some off cuts laying around I'll be building a torsion box bed. My alternative is to go box steel as you have mentioned but for my first go I am trying to keep as much of the fabrication in-house as I can. Are you familiar at all with the Mec-Mate design of CNC router I also have plans for that build but was concerned about costs. Now with your information it may be a better direction to go.

[Ger21]

How serious are you? Is this for light hobby use, or do you plan on using it a lot?
A mechmate would be about 100 times better than what you've drawn. Yes, it will cost more, but it will be a LOT better.

[PixelsnPrint]

I have a large format printing shop where we do everything from cards and letterhead to backlit signs and billboards. I'm getting into dimensional signage and am hoping to utilize the machine for that purpose. I guess my dream of building a robust yet low cost machine is dieing a slow and painful death. LOL

[Ger21]

Yes. If you want to make money with it, you'll need to spend some money. If you need a machine that big, then a Mechmate is a good choice.

[Neale]

Again, speaking as someone who has built a router from MDF (and, despite what I've said, it has turned out some useful work, but very slowly and with a lot of care needed to retune and adjust each time it's used), the real costs are in ballscrews, guide rails, motors and electronics. Your design using skate bearings and supported bearing surfaces for them will be cheaper, obviously, but at the cost of time and effort to build, adjust, and readjust. As Gerry said, it's easy to underestimate just how much force you need on those bearings to keep them in contact with the rails, which then need to be supported to stop them flexing, which makes it difficult to build the necessary box structure around them. The actual material cost for the structure is not that big a part of the overall build. The structure for my MDF machine probably cost around £100. The steel for a somewhat larger machine will cost, maybe, £300. The motors and electronics cost, maybe, £500, and ballscrews, profiled rails, etc, about £1200 (based on prices from China - local suppliers would cost a bit more). Decent water-cooled spindle around £300. So, the difference in going from MDF to steel adds around 10% to the overall cost, and will give a far superior machine.
OK, your numbers may be different depending on choice of leadscrews (trapezoidal are cheaper but have disadvantages) and guide rails (supported rails are cheaper than profiled rails, for example) but do the sums for the overall solution and compare the different approaches. If this is going to be a machine for a commercial operation, then factor in the general messing-about time for MDF, and costs associated with slower machining speeds.
And if you do still go with MDF or whatever, look at how you can incorporate things like steel angle along the edges of panels and steel box section reinforcing components to stiffen structures. Even bolting these in will improve the machine (mine now has a couple of steel box sections supporting the bed which has reduced sag enormously). I would still worry about the gantry for a 4' cutting width all the same.