Here's what I have so far...
After building a couple of desktop machines and trying to beef them up, milling steel effectively is still out of my reach. I'm designing a CNC router specifically for milling metals and wanted some suggestions. It will have a workable area of 15x36x?Z
Design: I want to make an all steel welded structure, I want it to look like the design in autocad, but I will have to use what ever I find at the metal recycling yard. On the Y axis I will use the chinese SBR12 linear rails from my current machine butted together, and with double bearing blocks, in a moving table design. As shown in the pictures I will use a pair of 41'' rollon linear rails I scored on ebay for $30 each, they have absolutely no play, and weigh about 15lbs each. The rollons will be used on the X axis, bolted to the fixed gantry. I need help with the z axis design, I could use the sbr 12 linear rails from my previous router, but I don't think they are heavy duty enough. Or I could go for a CRS skate bearing setup, if I could find some rectangular linear rails on ebay for under 50$ I will get those for sure.
Currently I have some 3/8-12 precision acme leadscrews on my machine, but I will need to choose some multi-start acme probably 2-4 effective tpi.
Propulsion: I am currently using the tb6560 board and haven't had problems other than slightly low motor rpm than gecko's. I will reuse the 300 oz in Nema 23 stepper motors from my previous machine, but I don't know if their torque will be sufficient to drive the axis's.
Spindle: I have a refurbished 2.5hp porter cable router I am wanting to use for milling. The same router was used by InventIt in his diy cnc that milled steel so I'm thinking it is a good choice.
Coolant: I know by experience (sparks flying from my endmill)that I'm going to need some way to cool the metal being cut, even with fancy endmill coatings. I want to employ a full time pressurized air nozzle for woods and aluminum, but I'm still unsure about how flood coolant systems work for steel.
Background: I'm currently a freshman in a pre-engineering highschool, I'm mechanically talented I would say. I'm also the captain of the schools solar car team (partly why I'm building this to help them build the car), and a member of the schools diy engineers club. I would really appreciate some advise from the pro's, I'm going to need it taking on this monumental challenge.
Here's what I have so far...
My best guess is if you want to mill steel effectively, a router is NOT the way to go. Even with a SuperPID you can reach 5000RPM, but evevn that may be too fast. InventIt's CNC is a pretty heavy machine, and although he used a PC router it's not ideal; at the very least you'll kill it pretty quick. Plus wood router + flood coolant + metal frame = not a good mix! You COULD use a smaller router as a spindle with timing belt and pulleys 3:1 reduction to drive a Sherline spindle set up for 10KRPM.
I've routed some mild steel brackets on my CNC, but it's not the ideal setup. They actually came out a lot better than I expected. I just put some Ashburn TD Foamy on the steel and cut it fine.
To note, I use ROLLON TLV and ULV 43 rails on my X and Y axis, and soon to be rebuilt Z axis. I use a SuperPID with a Bosch 1617EVS. I have 425in*oz steppers pwered by a G540 and 48V.
You don't need a lot of speed to cut steel, but it's preferable for aluminum. I don't know if your driver and PSU will be enough for the size machine. Remember that with the moving table the weight of your work will have an impact on your machine's performance as well. My overall thoughts: I don't think the TB6560 runs a high enough voltage to get your steppers near the performance you'll need.
That all sounds pretty good, except for the PC router. The kind of spindle you want for steel goes much slower, and with more torque. Just because someone said he'd milled some amount of steel with one of these doesn't mean it did a good job or kept on working for very long. For aluminum, it might work better, but for steel you'd be better off with a big old motor, a ball-bearing arbor threaded for toolholders, and enough pulleys to slow it down considerably.
And since your bridge isn't moving, beef it up with some diagonal bracing. Flood cooling works well for steel; just mix in enough of the concentrated coolant to keep it from rusting your machine, and provide a sump and enclosure to capture and recycle it.
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I bought my Taig spindle from: http://www.A2Zcnc.com
link for a Sherline spindle: http://www.a2zcorp.us/store/ProductD...line:Accessory
Last edited by DonFrambach; 03-31-2012 at 08:27 PM. Reason: added links
Have you ever used a Bridgeport? It sounds like exactly what you need. There's probably a machine shop nearby that would donate one.
Nothing beats hundreds of pounds of cast iron when the task at hand is shoving a half inch endmill into a hunk of steel bar. Except maybe several tons of cast iron. A dozen tons is even better.
My main machine: Multicam MG series (MG101) with original Extratech H971 controller, Minarik servo motors, Electro-Craft BRU-series drives, 4KW Colombo. Let's talk Multicam!
This design approach is generally not suitable for steel - I haven't seen a single machine of this type being able to mill steel reasonably. The long travel of 36 inches makes it even worse. You'll get far too much flexing and vibration - with steel the cutting forces are way above aluminium.
As Boltz said before: you'll need hundreds of pounds if not even tons of cast iron... try to get a hand on an old toolmaking mill and convert it to CNC. You'll find some decent ones from the 1960s to 1980s - and they have everything you need: a heavy, stiff frame, a suitable spindle - often with a bucketload of tools and are nearly indestructibly as long as they get some oil from time to time.
CAM Software and open source CNC controller: www.estlcam.com
Here's a link to a page that puts some numbers on the problem. As you can see you're never going to get a router motor to spin that slowly with any amount of cutting torque. With higher speeds you'll just be burning bits 'like there was no tomorrow'. You would be better off optimizing your machine for wood and aluminum and forgoing the steel thing altogether (in the router format).
Last edited by OCNC; 04-01-2012 at 12:22 PM.