Jeff Albro: CNC Production on Mini Mills

CNC Production on Mini Mills

I recently purchased a CNC mini mill for my business. I do small runs of different parts that are hard to farm out.

I've run into some issues with the lack of production features of the mill. These are all things that revolve around increasing the time the spindle is cutting metal. Some have to do directly with production, and some have to do with maintenance. You can think of this as a list of "things I wish I knew a few months ago." While this list may be obvious to those of you who work with a $40,000-plus VMC, I hope this list will help other small and hobby shops figure out what they need to make alot of something on their CNC machine.

Table Size and Suitability for Workholding

Everyone looks at table size, and work envelope. If you are using vises to hold your work, then the question becomes how many of what size vise can you use? I wouldn't recommend going smaller than the 3" vises from Littlemachineshop.com. When I do alot of smaller parts, I plan on building pallets that fit into the vises and then using some sort of Mitee-Bite clamps.

Recently, I've been looking at the Syil X3, and am thinking I would use two 4" vises. The 4" ones are nicer in that there are a variety of off the shelf jaws that will fit.

Limit and Homing Switches

Homing switches mean you don't have to touch off on your vise with an edge finder every morning. Also, you can keep a table of work offsets for the corners of your vises for making multiple parts.

On small mills it is common to have one set of switches and use them for both homing and limit switches. You can also use soft limits in the software to stop from exceeding travel limits.

They will also help you track down problems with missing steps after a program.

Spindle Taper and Toolchanging

Most people know that there are more tools and toolholders available for R8 spindles than for MT2 or MT3. If you want a quick change or repeatable Z height toolholder, there are way more systems available for R8.

There are only two systems I know of that work with MT systems, and they are either very light weight or not quick change without a power draw bar. Some small mills (i.e. the Emco F1) are available with 30 Taper spindles, and often these change tools very quickly.

Backlash Adjustment

Backlash is the sworn enemy of CNC. You can remove alot of the backlash using software, but that only works if the backlash doesn't change overtime, and lower backlash will allow for a smoother, more accurate cut.

Backlash can come from two main sources:

The fit of the ball or acme screw to the nut.
The connection of the screw to the thrust bearings to the table.

If you have adjustable backlash acme screws it can be hard to tell if your backlash comes from the thrust bearings or the nut. Also, acme screws wear their nuts in quickly, requiring frequent adjustment of the backlash. Ball screws make a CNC mill *much* easier to work with. I wouldn't buy a machine without them. Check to see if you can adjust the thrust bearing pre-load without removing the screw from the table.

To reduce backlash of ball screws, you can either load high tolerance balls into the nut, reducing the backlash to .002" or so, or have dual nuts that are spring loaded or adjustable.

Check to see if you have to take the table off or flip the mill over to adjust the backlash. Check how well the nuts hold their backlash adjustment.

Gib Adjustments

How hard is it to adjust the gibs? Can you reach the adjustments from the front of the mill? Do you have to get behind the mill?

This can be very important when adjusting the anti-backlash system. To gauge the amount of friction added by the backlash system, you may need to loosen the gibs.

Many Chinese mini-mills use multiple set screw/lock nut combinations to adjust the gib pressure. These can be made to work better by lapping the gib surface smooth. They are more easily adjusted if you replace the set screws with socket head cap screws that accept a larger hex key.

Spindle and Coolant Control by G-Code

Having control of the spindle and coolant does four things:

Speeds up work loading by removing four switch throws.
Give you something to listen for to know when a job is done and it is time to load the next batch of work.
Reduces the overall time of the spindle running, saving energy and reducing bearing wear.
If the spindle speed is also controlled by G code it makes reproducing results much easier.

Lubrication System

Lubricating the ways and screws of a mill with an oil can means putting oil in 9 to 15 different, hard to reach locations. You need to do this every day you use the machine.

Many machines don't even have oil ports. You can oil next the the ends of the saddle, but the oil may not get where it needs to go, and excess oil will drip into your coolant system causing problems in your coolant.

A one-shot lubrication system, or even a gravity feed oil manifold makes things MUCH easier.

Coolant and Swarf Protection and Collection

A CNC system can make a ton of chips, and the speed at which it can make them is greatly improved by using flood or mist coolant. This makes one hell of a mess.

You need to protect the machine from the chips and coolant. Make sure the machine comes with bellows or that you can easily add your own. You also need to protect the motors, wiring, and other electronics from the chips and coolant.

Unless you want to spend your days vacuuming chips you will need to contain the chips. Unlike large VMCs, most small mills do not come with enclosures. You can build one as part of the support table, or you may want to look at making an enclosure that fits on the table and moves with the table to capture the chips and coolant.

On my next mill, I will probably combine them. I will put a sheet metal drip tray under the vises to drain the coolant away from the table and saddle. This will help keep the chips from getting stuck in the T-slots, and the coolant from getting into the saddle.

If you do have a flood coolant system, make sure you have a better way to strain out the chips than the little mesh basket that comes with the coolant tank.