Types of tools used with CNC and VMC machines..

There are various types of tools which are used with CNC machines. These tools are differentiated by their method of material removal. Generally there are classified as below,

1. END MILLS: 

• Flat nose mills are used for milling 2D contours and pockets. Ball nose mills are used for 3D milling. Bull noseend mills have a radius corner. They are used to create a fillet on the bottom of a wall. Because they are sturdier than an end mill they are also sometimes used for roughing operations. Chamfer mills have an angled nose used to create a chamfer or to de-burr parts.

Types Of End Mills

• Centre Cutting End Mills : Machine tools are generally centre cutting or non centre cutting. The advantage with centre cutting tools is that they can plung directly into the work-piece where as non centre cutting end mills cannot. In case of centre cutting end mills the flutes of cutting tool are extended upto the centre of the tool as shown in figure below but in non centre cutting end mills the flutes advances to a centre hole.

Centre Cutting & Non Center Cutting End Mills

• No of Flutes: Milling tools usually have either two or four cutting flutes. Two flute cutters provide more chip clearance when milling in close areas. Four flute mills are more rigid, can be fed faster, and are preferred when greater chip clearance is not required, such as when milling an outside contour.

   2. FACE MILLS:

• The face mills consists of "cutting inserts" which can be replaced when worn out. These cutting inserts are mounted on a  central boss. There are used for first roughing operation so as to produce a smooth surface for following machining operations. These may have upto 8 or more cutting edges (inserts) hence making material removal process fast.

Face Mills

   3. SLOT MILLS:

• These types of cutters are used to cutting T-slots or dowtail type of shapes. T slots are very popular in case of machine beds for mounting fixtures or die etc. 

Slot Cutiing Mills

   4. HOLE MAKING TOOLS:

• SPOT / CENTER DRILL - These are used for creating a conic spot on the face of the workpiece so as to facilitate easy entrance of the twist dill at that point. The are short and rigid and provide precise location for a hole. The conic section created by the spot drill avoids wobbling and eccentric rotation of twist drills.

Center Drills

• COUNTERSINK DRILLS - These drills are used to create countersink type of hole profile so as to accommodate countersink screws. Drills having both spot drilling and countersink profiles are also available so as to carry out both operations by using single tool.

• TWIST DRILLS - These comes in various standard sizes according to metric and English standards. Used for drilling standard holes for various purposes. These are made up of High Speed Steel (HSS), carbide or Titanium Nitride (TiN) coated for long life and strength.

Twist Drill Terms

• TAPS & REAMERS - Taps are used to create threads in a pre-drilled hole. These form the materials by shearing away unwanted material. Taps require a hole drilled to the correct size to ensure the thread is formed properly. Most CNC Machines support rigid tapping, which means the tap can be held in a rigid holder. The tap is advanced at a feed rate that matches the thread lead into the hole. The spindle then stops, reverses, and backs out of the hole. Reamers are used to make hole bore of a precise size and excellent surface finish. Reamers give precise holes within 0.0001 mm. 

Images Courtesy : CNC Handbook HSMWorks

Machine and Tool Offsets..

As we know it is difficult to place a vise in the exact same position on the machine each time, the distance from Home to the WCS is usually not known until the vise is set and aligned with the machine. 

Machine set up is best done after the program is completely written, because it is expensive to keep a CNC machine idle waiting for the CNC programming to be done. To complicate matters further, different tools extend out from the machine spindle different lengths, also a value difficult to determine in advance. For example, a Ø20 Ramping Cutter extends further from the spindle face than a stub length a drill or any other milling cutter. 

If the tool wears or breaks and must be replaced, it is almost impossible to set it the exact length out of the tool holder each time. Therefore, there must be some way to relate the Machine C oordinate system to the part WCS and take into account varying tool lengths. This is done using Machine Tool and Fixture Offsets. 

There are many offsets available on CNC machines. Understanding how they work and to correctly use them together is essential for successful CNC machining.

• Fixture Offset XY:

Fixture offsets provide a way for the CNC control to know the distance from the machine home position and the part WCS. In conjunction with Tool Offsets, Fixture Offsets allow programs to be written in relation to the WCS instead of the Machine Coordinates. They make setups easier because the exact location of the part in the machine envelop does not need to be known before the CNC program is written.

As long as the part is positioned where the tool can reach all machining operations it can be located anywhere in the machine envelope. Once the Fixture Offset values are found, entered into the control, and activated by the CNC program, the CNC control works behind the scene to translate program coordinates to WCS coordinates.

Notice in Figure below how Fixture Offsets (+X, -Y) are used to shift the centerline of the machine spindle directly over the WCS.

Fixture Offset

• Tool Length Offset (TLO):

The tools loaded in the ATC (automatic tool changer) unit are of varying length. The length of each tool from the spindle varies according to requirement. Also due to tool wear the length of tool varies hence it becomes nearly impossible to set the tool in same position for each operation. Hence in CNC programming we use Tool Length Offset. It can be found out using following methods.

1. Generally the the TLO can be found out by directly touching the tool tip on the workpiece surface as shown in 1st method in figure below. Here the tool is made to travel from machine home Z-position to the part Z-zero position. In CNC machine controller settings the TLO settings are provided. This value of tool travel length is registered for each tool in that controller register. But during this method as we have to touch the workpiece with tool tip material removal takes place there. Hence this method is not preferable
2. In second method a sample block of some exact standard dimensions is made called as 1-2-3 block whose height is maintained precisely to a predefined value. Here the tool tip is made to touch the block surface and then TLO is calculated..
3. In 3rd method we simply use a Touch Probe which is connected to the machine controller. Here as soon as the tool tip touches the probe it send tool offset distance to the machine controller and the controller automatically registers the corresponding tool offset values.

Ways To Set Tool Offset

So in this module we have seen how to consider fixture offset and tool offsets so as to get precise machining. In next module we will discuss about the types of tools used with CNC machines. 

Images Courtesy : CNC Handbook HSMWorks

CNC MACHINE COORDIANTES

The CNC Machine Coordinate System is illustrated in Figure below. The control point for the Machine Coordinate System is defined as the center-face of the machine spindle.

The Origin point for the machine coordinate system is called Machine Home. This is the postion of the center-face of the machine spindle when the Z-axis is fully retracted and the table is moved to its limits near the back-left corner.

VMC Machine Coordinate System (At Home Position)

As shown in Figure above, when working with a CNC, always think, work, and write CNC programs in terms of tool motion, not table motion. 

• For example, increasing +X coordinate values move the tool right in relation to the table (though the table actually moves left). 
• Likewise, increasing +Y coordinate values move the tool towards the back of the machine (the table moves towards the operator).
• Increasing +Z commands move the tool up (away from the table). 

CNC Motion Control:

Most CNC machines can position each axis within .0002 inches or less over the entire machining envelope. This accuracy is achieved in part by the use of a closed-loop servo mechanism, illustrated in Figure below. 

The machine control sends a motion signal, via a controller board, to a servomotor attached to each machine axis. This causes the servomotor to rotate a ball screw attached to the table or column, causing it to move. The actual position of the axis is continuously monitored and compared to the commanded position with feedback from a servo transmitter attached to the ball screw.

Ball screws have almost no backlash, so when the servo reverses direction there is almost no lag between a commanded reversing motion and corresponding change in table direction. CNC controls employ electronic compensation to adjust for any minor backlash that may exist. 

CNC Motion Control

Work Coordinate System :

Obviously it would be difficult to write a CNC program in relation to Machine Coordinates. The home position is far away from the table, so values in the CNC program would be large and have no easily recognized relation to the part model. To make programming and setting up the CNC easier, a Work Coordinate System (WCS) is established for each CNC program.

The WCS is a point selected by the CNC programmer on the part, stock or fixture. While the WCS can be the same as the part origin in CAD, it does not have to be. While it can be located anywhere in the machine envelope, its selection requires careful consideration.

The WCS location must be able to be found by mechanical means such as an edge finder, coaxial indicator or part probe.

• It must be located with high precision: typically plus or minus .001 inches or less.
• It must be repeatable: parts must be placed in exactly the same position every time.
• It should take into account how the part will be rotated and moved as different sides of the part are machined.

For example, Figure below shows a part gripped in a vise. The outside dimensions of the part have already been milled to size on a manual machine before being set on the CNC machine.
The CNC is used to make the holes, pockets, and slot in this part. The WCS is located in the upper-left corner of the block. This corner is easily found using an Edge Finder or Probe.

Work Coordinate System

A typical example of WCS is show below with a WorkNC environment.

WCS Shown in WORKNC

Now as we became familiar with the various coordinate systems related to CNC machine we can get started with the WORK NC tutorials.. 

In next post we will learn about  setting a tool and work piece offsets so as to start machining with cam software.

Images Courtesy : CNC Handbook HSMWorks

Before Starting.. Introduction To Machine/Work Co-Ordinate Systems And Machining Units..

Cartesian Coordinate System

CNC motion is based on a 3D Cartesian coordinate system.

Number Line: 

The basis of this system is the number line marked at equal intervals. The axis is labeled (X, Y or Z). One point on the line is designated as the Origin. Numbers on one side of the line are marked as positive and those to the other side marked negative.

X-AXIS Number Line

3D Cartesian Coordinate System:

The Cartesian coordinate system consists of three number lines, labeled X, Y and Z, set at 90 degree angles to each other as shown in Figure below. The origin, or Datum, is where the three axes cross each other. 

The labels, orientations, and directions of the Cartesian coordinate system in Figure below are typical of most Vertical Machining Center (VMC).

3D Cartesian Coordinate System

Quadrants:

Any two axes form a plane. Planes are named by the axes that define them. For example, Figure below shows the XY plane, which is the primary work plane for machining on a VMC. A plane can be divided into four quadrants, labeled I, II, III and IV with axes designations as shown in the illustration below.

Quadrants

Units:

CNC Programs can be written in either Inch or Metric units. The machine can be switched with a single code to accept either. 

In India mostly we use metric units. In the United States, most programming is using inch units because most tooling is in inches and machinists are more familiar with the inch measurement system. Even if the part is designed in metric, it is usually converted to inch units for machining and metric tools are used only when no inch equivalent is available (for example when creating metric tapped holes).

Table below shows the units and maximum precision for inch and metric data used by CNC machines.

Units And Precision

So this was basic introduction to co-ordinate systems used by the CNC Machines. In the next post we will discuss CNC co-ordinate systems and work co-ordinate systems..

Introduction To WorkNC V21..

Hello Again..

Lets start with the introduction of WorkNC. Lets see what we can do by utilizing WorkNC software. Basically it is CAM software. 

WorkNC can be used by any CNC operated machines such as VMC(VERTICAL MACHINING CENTRE), HMC (HORIZONTAL MACHINING CENTRE), CNC Routers etc. I am currently using this software for programming of HARTFORD VMC 1600.

We can do following type of machining by using this software.

WorkNC CAM main functions include:

• Automatic geometry and machining zone detection and management
• Specific fluid and progressive toolpaths designed for High Speed Machining
• Full user stock definition (block, CAD, STL)
• Dynamic 3 and 3+2 stock Management (Real time toolpath updated)
• Complete tool and holder collision check with automatic stock update
• Powerful toolpath editor
• Virtual 3D machine representation and machining simulation (dynamic editing of points and vectors)
• Powerful tool and holder library (holder components managed)
• Automatic HTML workshop documentation.
• Fore-casted calculation and machining times can be exported to WorkPLAN, the ERP software from Sescoi
• User predefined machining sequences for automatic machining
• Machining from STL files and point clouds
• Batch mode calculations
• Comprehensive postprocessor generator (NURBS, cycles, circular interpolation...)

Roughing Toolpaths :

• Global Roughing and Re-roughing toolpath designed and optimized for HSM (high speed machining)
• A range of specific toolpaths with trochoidal, spiral or plunging movement, including HVR (high volume roughing)
• Roughing strategies use the Tool and Holder collision check with an automatic update of the stock
• Automatic calculation and machining of rest areas based on dynamic stock
• Re-machining toolpaths enable automatic rest material machining with increasingly smaller tools

Finishing Toolpaths :

• A wide range of Finishing toolpaths optimized for HSM machining
• Z Level finishing, Planar finishing, Flat surface finishing, Contour finishing, Edge finishing
• Automatic Rest-material finishing with a sequence of progressively smaller tools
• 3D Display of rest-material areas
• Automatic 5 Axis conversion possible

2 and 2.5 Axis Toolpaths :

• Pocketing, Contouring, Curve machining, Engraving, Rib machining, Facing, Drilling, Tapping ...
• Automatic Drilling Module
• Automatic feature detection and recognition, Pre-defined drilling sequence selection, Automatically generated drilling operations, Deep hole and intersecting hole drilling management
• Customized Postprocessor

5 Axis Toolpaths :

• Automatic 3 to 5-axis conversion with WorkNC Auto 5 
• Wide range of Simultaneous 4 and 5 Axis toolpaths
• 5 Axis Rolling, Planar finishing, Spiral Blade, Impeller, Tube, Laser ...
• Collisions detection and machine limits management.

Courtesy: Sescoi India.