I was talking with someone the other day who wanted to have a blue sky conversation about what a set of DIY CNC Mill Castings might look like.  The idea was, if you could build a set of castings for the hobby and light industrial market, what features would make those castings ideal?  This was strictly an over beers kind of a chat, and I love a blue sky conversation as much as the rest, so I told him I’d write it up on CNCCookbook so that my readers could comment on it.

Just to be clear, here are the market requirements I am targeting:

–  Suitable for hobby and light industrial market

–  Better than what you can convert.  Better than the average RF-45 CNC conversion, for example.

–  Capable of being set up for sale as a completed mill, a raw set of castings, or parts kits.

That’s a pretty short list that we can deal with.

Here’s what I came up with:

1.  Work envelope similar to Tormach or RF-45.  For light industrial, you need the mass if nothing else.  Besides, there’s lots of good smaller mills and I’m not real sure how you get an advantage over them.  A better bigger mill still has some wiggle room.

2.  Set it up for linear slides instead of using box ways.  This greatly reduces the requirement for hand scraping.  You can mill the beds for the linear slides.  Typically, you want a precision floor and one precision edge that the slides will butt against.  You’ll also need the hold down bolt holes.  Probably be a good idea to make a deal with a linear slide outfit since they all have different sizes.  If you wanted to make something generic, just machine the one wall and floor and leave it to the builder to put holes appropriately.  I think this would be popular as well as resulting in a much higher performance mill.  Done right, I don’t see why it would need to be overly expensive either.

3.  Take a careful look at the base and saddle of commercial VMCs.  They are shaped totally differently than cast hobby mills in order to support the table properly at the limits of travels.  There’s also a lot more beef on everything.  Think about adding more beef and adopting the shape of the commercial VMC castings:

Note the saddle, which is much larger than the table, and is made to really support the table well.  Also note the size of the column casting…
In terms of beef, shoot for something that was significantly more than a Tormach–300-400 lbs more.  The Tormach is already a wonderful machine, so that’s what it would take to beat it on rigidity in my estimation.  You’ll run some FEA on the CAD to decide for yourself where best to put the beef.

4.  T-Slot table is a good thing.  Needs to be machined true.

5.  Think about how to protect the linear slides and ballscrews from swarf and coolant.  Not that hard to do a set of way covers which would be a nice kit to offer.  Need to machine some mounting points in the castings.

6.  Think about home/limit switches.  How and where will these be mounted?  What type will be used?  Will need to machine some mounting points.  The style used on the Industrial Hobbies mills is straightforward and relatively contamination proof.  Make your mountings compatible with something like that, offer a home/limit kit, or offer the drawings so people can make their own.

7.  Think about the CNC motor mounts and ballscrew mounts.  This is not all that hard and need not be precision.  After working through my Industrial Hobbies conversion, I can see where you could make it amenable to a lot of flexibility.  With that said, you should offer a kit that’s ready to go.

8.  Think about leveling.  It matters hugely for setup, accuracy, and tramming the mill to squareness.  Yet, most mills in the world leave it as an afterthought.  This is an area Tormach invested more time in.  In your case, if you plan to sell a kit, you’ll want to set it up so the buyer can deal with it.

And now, for the most important part.  The heart of any mill is the spindle.

The specs for the Bridgeport spindle are about perfect in terms of rpm range for a hobby machine.  The Tormach uses a similar range for that reason, and it gives it a huge advantage over the average 1600 rpm RF-45.  You want 0-6000 rpm and some way to ensure decent torque throughout that range.  A 2 pulley step pulley system and a good vector VFD are about right in today’s world.  I would be targeting 3-5 HP, so think about motor mount on the headstock.  You’ll want to mount it to a plate atop the pulley stack.  It wants to be easy to change pulleys.

You want provision for a powered drawbar.  You can try the Tormach adaptation for R8 via TTS or something else.  You don’t have to solve the problem up front, but you need to make sure you’re not painted into a corner.  In other words, there must be room for the powered drawbar to be mounted on the motor plate.  It needs to interact with the spindle in such a way that it doesn’t load the spindle bearings.

I’d set up for a spindle cartridge like the Tormach guys use.  Source a cartridge from China to offer, but make the hole a little larger to give it a range of possibilities.  People are gonna want R8, although I personally would suggest a 30-taper option as it is a much better taper for the machine.

I’d say that’s a good overview.  I’d offer several styles:

1.  Raw castings–you do your own machining.  There will be an audience for this, so why not?

2.  Machined castings–you add all the outboard components like linear rails, ballscrews, limit switches, spindle cartridge, etc..

3.  Machined castings + matched rails.  All they need to do is add ballscrews, motors, limit switches, and spindle.

4.  A series of parts kits to go with #3:  Ballscrew kit w/ NEMA motor mounts, Spindle cartridge, Motor mount plate set up for the desired motor style (the face options are plentiful and people will appreciate choice so they can scrounge the best deals), and a spindle cartridge kit with appropriate pulleys.  Should be a 3HP and a 5HP motor option as well.  Lastly, a way cover kit will be welcome.

5.  Finished mill kit.  Pimp it out all the way.  5HP, Servos not steppers to take full advantage of those rails and get some decent speeds up, etc.

Later in the game, you can add power drawbar kit, ATC kit, and possibly a one shot oiler kit, though the rails can be greased too.

That’s how I’d do it, and depending on price, you’d wind up with a pretty sweet mill.

Now it’s your turn.  What do you like about this proposal?  What would you do differently?


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