Plasma/Router CNC

For the uninitiated, CNC stands for computer numeric control.  In short, you draw a part on the computer and it is the computer then controls the movement of the plasma torch or router in order to cut the part from metal or wood.

Now that we are dedicated to building a sailboat we are ready to spring for some better shop tools to get the job done faster and easier.  Since this sailboat will have a shop in the cargo hold for a shop, we also plan to take the Plasma/Route CNC table with us.

We two rolls for the CNC; we want to cut aluminum parts that will be assembled into items for the boat such as hinges, hatches, toe rails, tanks and such. And we want to use it to cut foam into 3 dimensional shapes that will be coated with resin and used to construct decorative walls, ceilings and fixtures for the boat. Hence the CNC table will take a plasma torch for cutting metal and a router for carving the foam.

Plasma

Since we will be working primarily with 3/8 inch aluminum on a 65ft boat, we selected a Hypertherm Powermax1000 with a 50 ft umbilical between the machine and the hand torch; part number 083179.  The price was just over $2,400 with shipping from www.bakersgas.com in early 2008.  We don't need 50 ft of umbilical for the CNC table, but it will come in very handy when working out on the boat.

Plasma torches require lots of dry air, or they will not cut cleanly and they will burn up the expensive consumables faster.  We already had 5 HP compressor capable the required of 6.7 cfm at 90 psi, however we added a low cost "3-in-1 Refrigerated Air Dryer" unit from Harbor Freight for $300. 

Hypertherm, T60 Consumables

The Electrode and Nozzle are the most frequently replaced.  As the nozzle wears the plasma arc will get wider, more dross will be produced, and the cutting kerf will get wider.  Check the hole in the nozzle and replace it when the hole is oval or has a notch in it. Even if the hole looks good, you can try a new nozzle to see if the cut quality improves.  The electrode has copper surrounding a hafnium core that is slowly burned away.  Once the hafnium erodes back into the tip the arc will start burning the copper.  The arc will have a greenish color and the plasma stream will be wider, which will quickly eat up the nozzle.  When replacing the electrode is a good bet that the nozzle needs to be replace too.

The Swirl ring only needs to be replace if it has noticeably damaged or if you have replace the nozzle and electrode and the cut quality is still poor.  When you do change the Swirl ring, use silicon grease to lubricate the rubber o-ring.

The Shield gets replace only when the hole is no longer round and the Retaining cap is only replaced when physically damaged.

The T60 torch has a separate set of consumables for gouging which is cutting the surface without cutting all of the way through the metal and fine cut for accurately cutting thin sheet.  For more details go to www.hypertherm.com and fine the Service and Operator manuals under the Library tab.

Tips for extending consumable life

• The pierce height of the should be as far above the plate as possible without causing a misfire.  A misfire is when the touch is too far from the plate to start an arc.

• Always start and stop the cut on the plate.  Do not run the arc off the edge.

• Make sure the ground is good, especially on thin sheet.

Cut Quality

• If the cut is not 90 degrees to the surface check the height of the torch.  The shape of the cut will match the shape of the arc.

• With a normal clockwise Swirl ring the right hand side of the arch as you cut away from you will be more vertical and cut a straighter edge. So Right is Wight.

• A little dross or slag is normal on the top surface. Dross on the bottom is a sign that the cut rate is too fast or too slow, or the consumables are worn.

• Arc voltage, feed rate and torch height are all inter related. When cutting faster than what is recommended in the cut charts you need to lower to voltage, and when cutting slower that recommended you need to increase the voltage.

• It is best to use Nitrogen as a plasma gas when cutting aluminum and it will increase the consumable life however air will work fine if the highest quality cuts are not necessary.

• Routinely test the pre-flow and cut-flow gas pressures.  Not have the proper flow will quickly destroy the consumables.

Hypertherm Cut Charts

CNC

To build or to buy, the eternal question.  Hands down the best site to start researching CNC is www.cnczone.com. It is also the most overwhelming.  This might be the best place to start: www.cnczone.com/forums/forumdisplay.php?f=251&order=desc

Bottom line is capabilities of a CNC should be dictated by "form follows function".  It would be nice to have a CNC that could cut large plates for boat building, but having build the sub, I know that 90% of complex the parts are less than 2 sq feet, so a 4x4 table is ample.  I also know few things I build needed to be very accurate. Most of my cuts were only within 1/32 inch, however the few parts that do need to be accurate, like gears of winches and rudders are very expensive to buy.  I am not going to mass produce parts so speed is not an issue, and 45 to 140 ipm is the optimum cutting speed for the Hypertherm when working on plate that is 3/8 to 1/8 inch thick.

The requirement for a router is two fold.  I'd like to do relief carvings in acrylic and foam that will then be epoxy coated in order to produce decorative works, and then I'd like to be able to foam carvings that will be used for lost foam casting.  Toward that end I'd like a Z-axis of travel of at least 5 inches.

I also will only use this machine one or two days a month so it needs to be able to be broken down and stored without too much effort and its cost should be in keeping with it output, which is to say; cheap.  It needs to tolerate being at sea too, since this is one tool we plan to take with us. So aluminum is our natural choice.
 
So, to build or to buy. If I buy this machine will cost about $8,000.  If I build, my best guess at this point is that it will cost about $3000 to $4000.  Time wise I think I can build it next winter, while it is still too cold to start the sailboat.  So my plan is to plan on building and if that does not work out, then buy or abandon the idea for a while.
 

CNC Router for 3D Relief Carving

Good Explanation: www.shopbottools.com/3-d_work_v2.htm

Steps
1.  3D CAD Model
2.  3D CAM Cut Path Software generates a  Part File - PartWorks 3D $795 - Cut3D $299 www.vectric.com
or  ArtCAM Pro  www.artcampro.com -- Interprets 3D from grayscale.

Portable CNC

A portable CNC is a great idea for how we plan to use our machine. It can be setup in the cargo hold or on deck or moved ashore.  During the build, it can be moved into the shop during bad weather or carried out to the steel under the gantries so that moving the steel is not so difficult a task.

SteelTailor and it's knock offs are the design that I think will work best for our needs which is plasma cutting larger steel plate parts for a boat and routing foam.  Routing wood in not something we plan to do and that's good because I doubt the arm could support much of a router.

The problem with the StealTailor type of design is that it is not going to support the tork needed for a router to work on acrylic. However routing foam would not be an issue. 

I like that the material does not need to fit into the dimensions of the table.  You could easily set the unit down on top of a 1/4 plate of steel that is on a welding table or even already part of the boat and have it cut a perfect hatch opening.

I have not yet seen it done, but I can see any reason that a 3x3 CNC table can't be built without legs or a bottom. That would again give it an advantages of portability and being able to work on a sheet of material that is far too large for even the large tables and far too large to move into the shop.

CNC Parts

Drivers and Motors

www.hobbycnc.com -- Small machine, Low Cost, 3-Axis Kits
www.xylotex.com  -- Small machine Moderate Cost, Plug and Play Packages
www.CandCNC.com -- High end. Larger machines  Good Support
 

Torch Height Control (THC)

Heavy plate steel when plasma cut will just lay there, but thin steel will flex and bend.  The THC automatically raises and lowers the torch over the bumps by monitoring the voltage of the arc.  This prevents the torch from crashing into the metal and keeping it the correct height for cutting.  www.CandCNC.com sells THC and control packages.

Motors: Stepper or Servo

Servo motors are more susceptible line noise and there rpm is much higher so they need to be geared down. Stepper motors are slow but more common, especially on router tables. Count on about 500 rpm from a stepper motor.  A router needs slower speeds and a plasma needs to do 100 ipm on 3/16" steel.

Rails and Drive Options

All thread.  - Cheap, Best for small 3 x 3 machines.  Problems keeping them clean.
Acme lead screws.  - Better than all-tread. Travel is smoother. Come in standard and high quality grades.
Ballscrews - Expensive but very efficient.  Still prone to problems with grit.
Cog or Synchronous Belt - Low cost. Problem with stretching with higher speeds. Wear is a problem
Chain - Subject to stretching
Rack and Pinion - Low wear.

www.vxb.com - Ball Bearings and Rails
www.qbcbearings.com V-wheels and Track - online pricing
www.superiorbearing.com V-wheels and Track
www.stdsteel.com - Rack and Pinions
 

3D Laser Scanner

www.NextEngine.com  $2500
www.david-laserscanner.com $250 Scan Software + $20 Green Line Laser + $80 1224x768 USB Camera
http://meshlab.sourceforge.net Stitch Software (free)
 

Links

solsylva.com - Plans for router tables and good explanations.
www.cnczone.com/forums/showthread.php?p=395992#post395992 - Good Intro by David Moore www.dmoore.com
buildyourcnc.com Good Step-by-Step with videos for an easy DIY CNC router.
www.cnczone.com/forums/showthread.php?t=5017  Moondog - Big heavy belt drive reduction plasma table for cutting boat kits .
www.torchmate.com  Kits and Complete Machines
www.mechmate.com  Free plans for big heavy router table.