Milling Machine

Safety

Induction

Only those who have been through induction on the milling machine and have been signed off are authorised to operate the milling machine. To become inducted, please see Mechanical section leader who will direct you to a mentor able to induct you.

Safety Glasses

All persons in the workshop must be wearing safety glasses while the milling machine is running. Operators should warn everyone in the workshop the mill is about to be run, and should wait until everyone is wearing safety glasses, or out of the workshop. It is advised to keep glasses on at all times for general protection as well as to get used to what it is like in competition at the field and in the pits.

Safety Shield & E-stop

The milling machine includes a clear polymer shield designed to prevent broken cutters, chips, and unclamped work from hurting anyone nearby. If something unexpected happens, either hitting pause on the PC controller or the red E-stop button, though if a problem is suspected, it is best to prevent it by pausing the job so the problem can be corrected before disaster strikes. Prevention usually saves more time than clean up.

Sharp Edges

Machined parts and chips can have sharp edges. It is advised to exercise caution when handling parts, and to break all edges with a file or deburring tool immediately. Holes can be deburred (top and bottom) with a cross hole (Weldon style) countersink tool in a cordless drill run at a slow speed and high feed rate. Edges and corners should be filed so they are safe.

Deburring is usually required when indexing work for multiple operations so that the work can be held correctly.

Running the Mill

Power Up

The mill must be turned on at the power point, and a switch on the side of the machine, then the E-stop button must be rotated to release and the yellow button pressed to energise the contactors. As there is a capacitor inside the control box, it may cause the circuit breakers to trip once or twice. During induction you will be shown which breaker to turn back on.

Once the machine is powered up, turn the pendant on, and then open the "proLIGHT 1000" icon on the desktop of the LinuxCNC PC.

At the top of the screen, uncheck the red stop icon, then turn on the amber power icon to show the jog panel in LinuxCNC.

Checklist (in order):

• Powerpoint: ON.
• Side control box switch: ON.
• E-stop: ROTATE.
• Yellow start button: PRESS.
  • Open control box and flip breaker if necessary.
• Pendant: ON.
• proLIGHT 1000 icon: Double Click.
• Red stop icon: UNCHECK.
• Amber power icon: CHECK.

Homing

Homing is the act of the machine figuring out where the limits of the machine are so the machine knows where the cutter is located in relation to the table. Homing needs to be done after powering on, after an E-stop, or after a crash (which should have a mentor inspect the machine, and have the team members responsible to determine the cause).

It can take up to ~5 minutes to home, however, this time can be significantly reduced by rapidly jogging:

• Z-axis: Quill until it is nearly at the top
• X-axis: Table to the right (cutter to the left)
• Y-axis: Table to the back (cutter to the front)

Do not go too far. The table will reach a limit switch, but the momentum will mean it keeps moving, and you will have to retract the movement in AXIS by turning on the limit switch overrides. If you fail to retract, the machine will not home properly and may damage the machine. If you hear a feint click, it means you have exceeded the limit and need to retract.

Toolpaths

A toolpath is a plan for how the tool will cut. Generating a tool path is beyond the scope of this document.

All operations require a fully dimensioned drawing. This allows planning of the setup, as well as providing a means of testing the part before it is removed from the machine. If a bearing pocket is required, keep the bearing on hand for performing a test fit when the part is ready. If the bearing does not fit, ensure the pocket and bearing are free of chips, and if so, re-run the g-code for that operation to give the pocket a spring pass.

Feeds and Speeds

Most of the material we work with is aluminium. Consider using the Speeds and Feeds calculator at Little Machine Shop. For drilling, the calulators are optimised for high MRR (Material Removal Rate) rather than small chips. Consider using a G73 (peck drilling cycle with short retract) cycle if drilling more than 1D (one diameter) deep. If drilling more than 2.5D, consider a G83 (peck drilling with full retract) cycle.

Larger cutters will help get larger bore holes bored faster, but also consider the time cost of a tool change (including resetting the Z height).

The milling machine has a maximum spindle speed of ~4800RPM.

To aid in slotting and drilling, an air pump to provide a stream of air onto the cutter has been installed, use this to clear chips as they leave the cutter.

Cutting Oil

There is much debate about WD40 as a cutting oil for aluminium. TDB likes WD40 as it's good, it's cheap, and it's easy to apply. Ensure there is enough before starting work. It can be applied from a squirt bottle, spray bottle, or with a brush to the cutter or the work directly, and does not need to be applied too generously. Chips are easily stuck to the work in a pool of liquid, and often just quickly spraying the cutter quickly will be enough to prevent chips welding to the cutting edge.

Part Setup

Datums & Zero

Always have a printed drawing in front of you. In pencil, mark where and how you plan to hold the work. Consider the collet nut has a diameter of 50mm, and that you want to ensure the clamps will clear the collet nut. The drill chuck has a similar diameter, but the drill typically sticks out further.

If machining a rectangular part, always set the part datum (zero) to be in the top left corner on the upper face to be machined.

If machining a circular part, always set the part datum (zero) to be in centre on the upper face to be machined.

Axes

Refer to LinuxCNC's Machine Configurations for a diagram on how the milling machine is set up. The tool direction can be visualised with a left hand rule.

The part should be oriented such that the X-axis is the long axis, and the Y-axis is the short axis.

Reorienting Work

Some work pieces may have operations to be done on multiple faces. Extrusion for example may have holes cut on the four faces with a long X-axis. Convention is to index (roll) the part counterclockwise about the A axis (that is, if looking along the X axis from the right hand side of the machine, the part will rotate CCW). Another way to think of this is that the face you just machined should be rolled towards you.

Small production runs

A small production run is when one must create a small quantity (say 2 to 20 copies) of the same part. These runs must be optimised for high turnover, which means sometimes you may want to put two parts on the table at the same time while team members are deburring the freshly made parts and performing secondary ops. It can be valuable to optimise toolpaths to minimise tool changes, which means maybe a small 5mm cutter will be used to interpolate holes, as well as drill 5.5mm holes with a small amount of interpolation. If part changes are faster, it may be quicker to swap parts rather than to change tools.

A tool change from one slot drill or end mill to another should take at most 3 minutes (including touching off / setting the tool length offset).

In all cases, putting the new part down on the table in the same location is critical. For this there are fixture plates, as well as jigs which are outlined below.

Small production runs: Fixturing a Plate

Work can be fixtured with one of two types of fixturing plates.

1. A plate of UHMW / HDPE plastic (kitchen cutting board is good for this) with a row of reamed holes across the top and a column of reamed holes down the left.
2. A piece of aluminium tooling plate (known as a mini pallet). This will have two overlapping grids of holes. One grid will be for M8 tapped holes for clamping and the other grid will be reamed for dowel pins.

In the case of the plastic board, this can be treated as a sacrificial plate, and can be drilled / cut into, though the depth should be kept shallow as these boards are not thick.

In the case of the mini pallet, these plates are expensive to replace and so cuts should be kept smaller than 0.2mm into the aluminium.

In both cases, the fixture plate must be secured to the table. Typically two dowel pins into a T-slot will be used to ensure the plate is aligned to the table's X-axis, and strap clamps to hold the plate from moving during part changes.

The workpiece can be secured to the table with machine screws, or strap clamps into t-slots, or a combination of strap clamps and screws into tapped holes in the tooling plate. Do not over tighten the fasteners as it will strip the holes in the tooling plate. It is better to use multiple clamps and multiple fasteners than too few and cause something to slip. "Nothing too strong ever broke." -- Tom Lipton

Example use of the Mini Pallet system can be seen at oxtoolco on Blogger.

Small production runs: Fixturing a Gear (using the hex bore jig)

In the fixturing tools box, a piece of 16mm round stock has been fitted with a 0.5in hex shaft and a nylong bushing to be held in a collet. This permits the use of square or hex collet blocks in the vise for drilling and light milling ops to be performed on gears with minimal setup time.

NOTE: many Versa gears from Vex have the hex bore broached, but it is not in relation to any of the gear teeth, which means this cannot be used for sensitive timing operations. If gear timing is a critical factor, Vex gears are not suitable for purpose, and gears from an alternative supplier where the shaft / keyed feature is machined in relation to the gear teeth.

NOTE: if machining our own features into the gear, the gear can be clocked (rotated about C axis) using dowel pins in a piece of sacrificial tooling plate, and some strap clamps to hold the gear to the plate / table.

Cutter Holding

TDB mainly uses two styles of cutters: jobber drills and slot drills. A jobber drill is what is commonly referred to as a "drill bit". A slot drill is sometimes called a "2 flute end mill". This gives rise to two styles of holders:

• Collets
  • Pro: Can hold slot drills or jobber drills.
  • Pro: Range 1.5mm to 20mm
  • Pro: Very rigid as the drill can be held on the flutes close to the cutting edges.
  • Con: Longer to set up as it requires a pin to hold the quill steady and the collet wrench.
• Drill chuck
  • Pro: Very fast to set up, no tool needed.
  • Con: Can hold only jobber drills (cannot be used to hold slot drills or any form of sideways cutter!).

Currently, both styles of cutter holding require setting the Z height for each cutter after a tool change. One mentor is trialling rings for slot drills in the collets such that the Z offsets can be preserved.

Generally operations will be arranged such that all drilling with a drill chuck will be done at once for rapid cycling through drills, and then ideally only a single slot drill with the collets.

Standard Feed Rates and Spindle Speeds