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Frequently Asked Questions

For many years Precitech customers have used our Ultracomp software with our low force probe to measure form error on parts. The data is compared to the programed shape. The resulting error map is used to create a compensated tool path that corrects for repeatable sources of rotationally symmetric AKA “2D” form error. Ultracomp is a fast and efficient tool. It is typically used on a lot sampling basis where several parts are cut with the result from measuring the first part of the lot. The method can also be used on “3D” or freeform parts. The probe and UPx routines are used to gather the data. Diffsys is then used to compare the measurement to the design and generate the corrected tool path.

There are many factors to consider when using on-machine metrology to establish compliance to a customer’s specification. Some of these factors are listed below. In general, on-machine metrology provides value when you are working with parts that:

  • Are hard to remove from the machine tool and return to the machine tool accurately.
  • The time required to remove the part, measure it offline and return it for further processing is prohibitive.

Factors to consider:

  • Measuring on a sampling basis or for every part? In either case is it economic to tie up your diamond turning system to measure parts? Non-contact probes gather data much faster than contact probes and can be considered as a way to minimize measuring time on the diamond turning system.
  • Part distortion from holding the part? Part fixtures and part holding strategies aim to minimize any distortion of the part being cut. Having the part in a truly “free state” (no stress) while cutting is hard to achieve. Stress and strain present while holding the part is released when the part is removed from the machine and the part “springs back” into a freer state. The springing back of the part results in a different form being measured on the machine vs. measured off the machine.
  • Errors in motion of the machine tool? Precitech machine tools provide among the lowest error motion of any commercially available machine tool. However, there are very low residual errors. If the same machine motions are used to measure the part the measurement error from machine motion errors is invisible to the measurement process.

Off machine measurements have many of the same factors to consider but as a purpose-built metrology platform more steps can be taken to minimize their impact on measurement error.

On machine metrology has a place as an effective tool as long as the factors surrounding its use are carefully considered.

Precitech Hydroround B axes are one of the most popular CNC axis accessories offered by Precitech. On our lathes and 5 axis freeform machines, the B axis of rotation is vertical with the B axis location on the Z axis. The work holding spindle is located opposite the B axis on the X axis of the machine.

Image of Precitech's HydroRound II Rotary B-axis

There are many special situations where a B axis brings unique value (ex. polygon mirrors) but here we will cover the advantages provided in two modes of operation, tool normal and turret operation. 

Tool Normal Operation:
Tool normal gets this name because as a tool is traversing across a curved surface of a part the B axis is rotating below the tool keeping the same point on the edge of the tool normal to, in contact with, the surface of the part. Before controlled waviness diamond tools were affordable tool normal turning was the only way to avoid form errors from tool waviness. It is the origin of the term “single point diamond turning.”

In the past setting tools for tool normal operation was tedious with the tools needing to be exactly located on center to the B axis for the method to work. With Precitech’s Virtual Center Technology setting tool normal tools for XZB cutting is as easy as setting traditional XZ cutting tools. The advantages of tool normal turning include:
  • Ability to make "undercut" surfaces that do not allow access for the tool to the surface without using B axis motion.
  • Ability to make higher efficiency diffractive parts. 
  • Lowering the cost of diamond tools by using non-controlled waviness tools.
  • Increasing tool utilization. A small rotation of B presents a fresh spot on the edge of the tool to the surface of the part. 
Turret Operation:
Precitech’s Hydroround B axis provides an ultra-precision tool turret where tools can be arranged around the periphery of the B axis. Precitech’s Hydrolock technology and UPx control can return tools to the same location within < 50 nm. Sometimes customers consider purchasing larger machines not because of the size of the parts but to get the extended X axis travel so they can set up additional tools in parallel. With a B axis you can set up additional tools on a smaller machine saving capital funds and precious floor space. Tools set up on a turret generally do not get in each other’s way when cutting large parts as the tools that are not in use are rotated away from the surface.

The advantages listed above are discussed in greater detail in the Precitech white paper: “Benefits of the Precitech Hydroround B Axis”.
A fast tool servo (FTS) is a type of control system used in precision machining processes, such as diamond turning, that allows for rapid and accurate movement of the cutting tool. The Precitech FTS system works by using a voice coil to move the cutting tool in small increments, allowing for precise control of the tool position and movement.
There are several advantages to using an FTS system in diamond turning and other precision machining processes, including:
  • Improved surface finish: FTS systems allow for precise control of the tool position and movement, which can lead to a smoother and more uniform surface finish on the machined part.
  • Reduced machining time: FTS systems can move the cutting tool more quickly than traditional mechanical systems, which can reduce machining time and increase productivity.
  • Enhanced accuracy: FTS systems can make very small movements with high accuracy, allowing for tighter tolerances and greater precision in the machining process.
  • Increased flexibility: FTS systems can be programmed to make complex tool movements, such as non-circular or spiral tool paths, that would be difficult or impossible to achieve with traditional mechanical systems.
  • Reduced tool wear: FTS systems can reduce tool wear by allowing for lighter cuts and more consistent tool engagement with the workpiece.

Overall, the use of an FTS system can lead to improved machining performance, increased productivity, and greater precision in diamond turning and other precision machining processes.

Precitech offers service maintenance to all our shipped products going back over 30 years.  There are some components that are no longer available and if one of these components goes bad the machine is no longer serviceable without an upgrade.  Contact the service department with your particular situation and it is likely that a retrofit will be available to get your old machine running again.
Many types of tools can be used on ultraprecision systems single crystal diamond tools, polycrystalline diamond tools, tungsten carbide and cubic boron nitride. When cutting to achieve an optical finish single crystal diamond tools are used. This is often referred to as “single point diamond turning”. Single crystal diamond tools can be used to machine a wide range of materials:
  • Nonferrous metals: high phosphorus electroless nickel, aluminum, brass, copper, gold, tin, silver and zinc
  • Plastics
  • IR crystalline materials: Ge, ZnS, ZnSe, CaF, Si and others
  • IR Chalcogenide glass: GASIR®, OPTIR®, Schott IRG and IG and others

Some materials chemically attack diamond resulting in premature wear of the diamond. Materials with unpaired D shell electrons have an affinity for carbon. They pull carbon out of the diamond crystal structure causing the diamond to break down. Common examples of these materials (# of unpaired electrons)** include iron, pure nickel, manganese, cobalt, titanium, chromium and tungsten. When pure nickel is alloyed with 10-14% phosphorus nickel's unpaired electrons are satisfied and the resulting high phosphorus electroless nickel becomes one of the best materials for diamond turning. Ultrasonic tool holders can slow the chemical reaction allowing for the practical turning of steel with diamond tools.

Traditional optical glasses (e.g., BK7, fused silica) are not considered good materials for cutting optical surfaces with single point diamond tools. The cutting mechanics result in a deep layer of subsurface damage that must be removed by polishing. The polishing renders the process too expensive for practical use. There are some exceptions to this rule and research goes on in advanced machining methods to diamond turn glass with minimal subsurface damage layers. Two of these methods involve ultrasonic machining and laser assisted machining.

Other aspects of materials can influence the results you can expect when diamond turning. The grain structure of a material can dominate resulting surface roughness. Grain structure can be related to alloy composition, thermal history (heat treating), even forming methods, casting vs. extruding.

Inclusions in materials can affect surface roughness and the wear rate of the diamond tool. Aluminum can harbor inclusions of silica, a very hard material. These silica inclusions chip the diamond tool and be prominent features on an otherwise smooth surface.

** Chemical aspects of tool wear in single point diamond turning
Ed Paul, Chris J. Evans, Anthony Mangamelli, Michael L. McGlauflin and Robert S. Polvani
Precision Engineering 18:4-19, 1996

A video observation system provides a camera inside the working zone with display monitor outside for the remote and magnified observation of the cutting process. A lower priced (and lower floor space) version uses a single display monitor and the user can toggle between seeing machine controls and video observation. The upgraded version uses two display monitors so the machine controls and video observation can be viewed simultaneously.
The utility requirements of diamond turning machines can vary depending on the specific model and manufacturer, but there are several common utility requirements that most machines will need. These can include:
  • Electrical power: Diamond turning machines typically require a high level of electrical power to drive the spindle and other machine components. The power requirement can vary, but it is not uncommon for machines to require 220-240 volts, 50-60 Hz, and 3-phase power. The power requirement can also vary depending on the machine's size and capabilities.
  • Compressed air: Compressed air is often used in diamond turning machines for various functions, such as cleaning the workpiece and cooling the cutting tool. The compressed air requirement can vary, but it is typically in the range of 5-10 bar.
  • Coolant: Diamond turning machines often require a coolant system to prevent overheating and maintain consistent cutting performance. The coolant requirement can vary, but it is typically a water-based coolant that is circulated through the machine during the machining process.
  • Exhaust ventilation: The machining process in diamond turning machines can produce a significant amount of dust and debris, so an exhaust ventilation system may be required to remove these particles and maintain a safe working environment.
  • Environmental control: Diamond turning machines may require specific environmental conditions to operate effectively, such as temperature and humidity control.
It is important to consult the manufacturer's specifications for a specific diamond turning machine to determine its exact utility requirements. 
Spray mist supplies a minimum quantity lubricant (MQL) to the cutting zone. Each machine is offered with two separately controlled spray mist nozzles. Each nozzle has manual controls over the lubricant and air supplies plus M code controls for turning on and off from within a part program. Optionally, two additional nozzles can be supplied to provide four spray mist nozzles.
The #1 source of form error in diamond turning is temperature variation in the machining zone. There are several process and equipment design features that will improve form in critical applications. 
  • High speed (>1000rpm) precision air bearing spindles expand over time from friction in the air film forming the air bearing. To control this growth all spindles should be water cooled. The more effective the water cooling the faster the spindle will equilibrate and less the shaft will grow at speed.
  • It is important to allow time for the spindle, fixture and component temperature to stabilize before beginning the machining process. Depending on the spindle design, size and weight of the fixture and component and material of construction this can take from 5 minutes to 30 minutes.
  • For applications where form is not critical a temperature variation of +/- 1C in the machining zone is acceptable.
  • Critical applications require additional temperature control of the machining area. This is accomplished by supplying the machine enclosure with air flow that has been controlled to a very precise temperature. A typical system will control the environment in the machine enclosure to <0.1C to 0.05C.
  • For optimum form control the entire machine can be placed in a standalone thermal enclosure with very tight temperature control. A standalone thermal enclosure controls the temperature of the machining zone, the granite and the support structure under the granite.
Below is data that shows the impact of temperature variation on component form. It both cases the component was a large aluminum mirror with a 5 hour cutting time and a room temperature variation of +/- 1C.
No Temperature Control

No temperature control
No temperature control 2
Form- 0.327 micron PV
Temperature controlled to +/- 0.1C 

Temperature controlled
Temperature controlled 2
Form- 0.046 micron PV