What is a Dental Milling Machine? Basic Knowledge and Challenges Explained

The challenges and solutions for dental milling machines will be addressed here.
This is a must-read for those who are considering purchasing equipment and for designers of milling machine.

What is a Dental Milling Machine?

A dental milling machine is a small cutting machine that uses a CAD/CAM system.
The program automatically processes "prosthetics" such as fillings and crowns to replace missing teeth. A disk-shaped dental material called a "disc" is cut into the shape of a tooth using a tool (i.e., milling bar).
There are various types of disk materials, including non-metallic zirconia, ceramic, wax, and PMMA (acrylic resin), and metals such as titanium and cobalt. Each material has its own characteristics, such as resistance to discoloration and deterioration, and a beautiful finish, and is used according to its purpose.

Are Dental Milling Machines Increasingly Being Utilized?

Currently, there is a shortage of dental technicians and their work style.

One in three people in Japan (more than 30 million people) use dentures, and there are 20,000 dental laboratories that make and repair these prosthetics, about the same number as public elementary schools.
In the near future, Japan will inevitably see an increase in the population requiring prosthetics due to its aging population. However, it has been pointed out that the background of income and working environment of dental technicians is a factor in the shortage of technicians.

As the demand for dental care increases in an aging society, the workload of dental technicians is expected to increase as well. Milling machines, which automate the processing of prosthetics and reduce the workload of technicians, will undoubtedly be one of the equipment most in demand in the future.

This section describes the different types of milling machines and the challenges it poses.

Types of Dental Milling Machines

There are three types of milling machines: dry, wet, and combination dry/wet models.
The following is an overview of each type.

Dry type (Dry method)

This is a method that does not use water or coolant during processing.
Small-diameter tools in the 0.5mm range can be used to cut mainly soft materials (zirconia, resin, PMMA, etc.), enabling fine modeling and processing.  On the other hand, when cutting hard materials, small-diameter tools are not often used due to disadvantages such as breakage and longer machining time.

Wet type (Wet method)

This is a method in which water or coolant is applied during processing to suppress frictional heat while polishing.
It is mainly applied to process hard materials (e.g., glass-ceramic and titanium). Harder materials are increasingly in demand by patients due to their strength and aesthetic appearance.

Combination Dry/Wet method

This is a dual-use model that is compatible with both dry and wet methods.
While it has the advantage of being able to process a variety of materials with a single machine, it has the disadvantage of incurring non-productive time when switching from wet processing to dry processing, such as when cleaning and drying the machine.
Other common disadvantages generally mentioned for having both functions are inadequate processing capabilities and high initial investment.

In some cases, production efficiency is higher with dedicated machines that specialize in dry or wet processing respectively, so it cannot be generalized to say that a dual-use model is better.
It is important to use the three methods according to the purpose, such as material characteristics and frequency of use.

Challenges for Dental Milling Machines:

Issue 1. How to maintain the machining accuracy of the milling machine?

Since the bite and appearance of teeth greatly affect our daily lives, milling machines are required to have high machining accuracy.
However, the accuracy of the milling machine itself is not enough for precision processing.
Two essential prerequisites for maintaining machining accuracy are accurate "originating the tool/home positioning," and "workpiece positioning".

What is originating or homing the tool?

It refers to determining the starting point of tool machining.
Milling machines use ultra-fine tools with a diameter of 1mm or less to process hard materials, which causes wear. Machining with unexpected wear or chipping on the tool can lead directly to machining defects due to dimensional deviations in the finished product. Especially when machining continuously, it is necessary to check each time.

What is workpiece positioning?

The workpiece must be held firmly so that it will not move during machining.
If a disc is machined with a loose fixture, even with high accuracy of equipment, an error* will occur in the dimensions of the finished product, resulting in defective machining. This becomes especially important in unattended operation with a disc changer not monitored by a person.

To prevent the above risks, the tool or disc must be machined while accurately determining its position using a sensor.

Issue 2. Milling Machine Too Small to Attach a Sensor?

There is a problem of not having enough space for sensor mounting.
Many dental milling machines are small (desktop size) but designed to accommodate more milling bars, so sensor mounting space is limited. So, a compact sensor that can be mounted in a limited space is required.

Issue 3. Sensor Damaged or Malfunctions Due to Chips or Liquids

If a sensor is damaged, the equipment cannot be used until it is restored, so the sensor must also be durable.
Especially, the inside of a milling machine, whether dry or wet, is an adverse environment where fine chips and liquids scatter, and sensors with weak protection structures are at high risk of penetration into the main body and damage. Non-contact laser sensors and proximity sensors are not suitable for installation due to the high risk of failure caused by flying debris.

Solutions to Dental Milling Machine Challenges

As a solution to the "tool originating" and "workpiece positioning" problems described above, we are pleased to introduce the following METROL sensors, already used in many milling machines.

Solution 1. Tool Originating by Tool Setter

By physically contacting the tool, the tool setter sensor communicates the machining origin of the tool, which becomes the machining reference, to the machine control.
 METROL’s compact tool setter “P series” is highly precise and compact, with a repeatability of 0.5µm. Even minute dimensional errors caused by tool wear and chipping are compensated reliably to prevent machining defects.
This is indispensable for avoiding problems such as continued machining defects during auto-operation. Because of its low cost, it has been applied by many milling machine manufacturers in Japan and across Europe.

Solution 2. Workpiece Home Positioning Using an Ultra-Compact Touch Probe

The touch probe is a sensor for originating the workpiece before machining and measuring dimensions after processing.
By touching the workpiece (material) with a ball attached to the tip, it feeds back the "XYZ position coordinates, the origin of processing," to the milling machine.
The K3S series of ultra-compact touch probes can be used for highly precise originating of workpieces even in small spaces, preventing machining defects caused by loosening or misalignment of the workpiece when it is set.

Solution 3.Use of Waterproof and Dustproof Sensors

Our sensors are highly durable against water, oil, and chips.
The sensor has passed strict water and oil resistance tests and has been used in many large machine tools. They demonstrate durability against water jets and coolant from all directions.  
The sensor body is designed for protection against water and dust with IP67-68 specifications, making it ideal for milling machines.

*Coolant and dust can adhere if left unattended, leading to problems such as sensor malfunctions and tools sticking to the holder, so measures such as cleaning are necessary after use.

Solution 4. Design a Sensor Specifically for the Milling Machine

Sensors can also be designed specifically for the operating environment of the milling machine.

All METROL sensors are designed and manufactured at our own facility in Tokyo.
Sensor size, contact-probe/surface diameter, stroke, air blow (to avoid chips), contact force, and sensor protection specifications can be customized to match the intended tool, available space, and sensor mounting location.  Based on our extensive experience with milling machines, we will propose the optimum sensor for your operating conditions.

Product Summary for Dental Milling Machines

Representative Dental Milling Machine Manufacturers

The following is a partial list of dental milling machine manufacturers.
Many European milling machines are distributed in Japan, but Japanese milling machine makers have also emerged in recent years.

Japanese Milling Machine Manufacturers

• CANON ELECTRONICS INC.

Non-Japanese milling machine manufacturers

• imes icore
• Amann Girrbach AG
• Arum Europe GmbH
• Dental Machine S.r.l.
• Dyamach S.r.l
• Dental Automations
• Datron
• vhf camfacture AG

【video】Small Tool Setter - Example of Use in a Dental Milling Machine

Lastly, a case study of the installation of a small tool setter.

Inquiry about Sensors for Milling Machines

Please feel free to contact us about sensors for milling machines.
We also offer consultation for equipment-specific design.
Our experienced engineers will be more than happy to assist you.