HKUST DEPARTMENT OF MECHANICAL ENGINEERING
CNC Machine For Silver Jewelry Items Design Project: Phase II Part C: Rotary Table and Work Piece Holding Sub‐System
Group 1H: Yeung Pan, Wong Man Ka, Yu Peng 6/4/2008
Author: Yu Peng
I. Group CNC Design
I.1 Engineering Specification
I.2 Design Analysis Process
I.3 Priority Items in the Design
Individual Design II.1 Engineering Specification
II.2 Detail Design of the system
I. Group CNC Design I.1 Engineering Specification of Silver Jewelry CNC Machine
Figure 1. Assembly View of the CNC
Overall Dimension (Data in Parentheses Stands for Dimension Including Casing) Width 424 mm (450 mm) Depth 437 mm (450 mm) Height 535 mm (550 mm) Weight 50 kg (52 kg) AXIS TRAVEL X 100 mm Y 100 mm Z 150 mm A (Rotary Table on X axis) Continuously Feed Rate (X,Y,Z) From 0.25 to 1500 mm/min Feed Rate (A) 1°/ sec to 50°/sec maximum WORK AREA Table Size 200mm X 120mm Load Capacity 15Kg Minimum Spindle Nose 70mm Clearance ACCURACY
Repeatability X, Y: 0.01 mm; Z: 0.02mm, A: 0.03° Positioning X,Y,Z: 0.01 mm A: 0.03° SPINDLE & MOTOR (NSK Astro‐E 250) Brushless Drive Motor: Maximum Torque: 0.051 Nm Power: 105W Speed range: from 2,000 to 25,000 rpm Diameter: 25mm Comes with standard collet (From 2.5mm to 3mm) POWER REQUIREMENTS AC Ratings 110 ‐ 240 VAC ‐‐ Worldwide Compatibility Cycles 50 ‐ 60 Hz Amperage Maximum 10A Power Consumption 250W ACCEPTABLE MATERIALS Silver Modeling Wax Wood and other soft metal material INTERFACE USB External Controller Sensor Connector Rotary Axis & Motion Guidance Units Connector Software Pack Expansion Connector OPTIONAL PARTS Multi‐Dimension Collet Inner Ring Fixture Mounting Table and Screws for Large Work Plate Automatic Tool Changing Casing Air Gun Coolant Filter
Figure 2. Overall Dimension
Figure 3. Travel Distance
Figure 4. Working Platform Dimension
Figure 5. Rotary Table Dimension
Figure 6. Spindle
I.2 Design Analysis Process The main purpose of our design is to achieve the best performance with an appropriate cost. Based on this concept, the comparative design analysis is conducted in five main sub‐systems of the jewelry CNC. Below is a brief summary of the design analysis. Details please refer to the reference item 1. A. Z‐axis Motion System: Servo Motor and Stepping Motor As the motion in the Z‐direction is needed to be accurate, it is usually required to vary within several micrometers, so, usually, servo motor will be chosen (if price is not a major concern). Stepping motor is relatively cheap and is user‐friendly as it does not require tuning. Though stepping motor is improving its precision level, servo motor, having encoder and close‐looped feedback, is still more precise than stepping motor. Since both of them have their own advantages, a simple table is needed to find out the final solution. The table is as follows: Stepping Servo Requirements(Related to Detail Design) Motor Motor Engrave on rings (Specialized fixture with inner ring holder; Rotary table for ring manufacturing.)
Operate with a good accuracy (High precision stepping motor; Close loop control with encoder; Low friction guideway.)
Easy to use (Operation compatible with existing similar jewelry machine. No need to learn much. Simple maintenance)
Low noise (High quality bearing may be used, even if air bearing. Reduce vibration.)
Reliable (Simple design with mature products and technology in the market.)
Cost 3 Total 16.5 Therefore, stepping motor would be chosen for the specific purpose.
B. Machine Bed Material: Cast iron, Steel and Artificial Granit AG and cast iron are more suitable for manufacturing machine bed. However, after taking cost into consideration, steel becomes the most suitable material for making this small CNC. We can use a lot of existing components in the market to make it instead of design and manufacturing a new mould for casting Artificial Granite. AG is the second choice for machine bed. Once there is high requirement on anti‐vibration it may be used. The machine bed is like a support where all the other sub‐assembly will be installed.. C. Motion Guidance System: Fluid Bearing, Ball Bearing and Plastic Bearing Plastic bearing (Teflon) has the best overall performance. It has a balance in performance and economics. If there is higher requirement on friction, ball bearing guide way can be chosen. However, at the moment the cost will be several times higher. As for the drive components, lead screw is the only suitable choice for jewelry CNC machine. All types of jewelry CNC in the market use lead screw as power transmission component. With ball screw inside and proper lubrication, the overall efficiency is appropriate. And torque can be enlarge times to drive the working platform. D. Spindle System: Integrated spindle and belt transmission spindle It is easy to find that the performance of integrated spindle is much better than the belt connected type, especially in spindle speed. Due to the excellent performance of integrated spindle, most jewelry CNC in the market achieve a high rotational speed. Since the other type cannot satisfy the speed requirement, there is only one alternative left for the jewelry CNC. Integrated small size spindle should be used. E. Controller System: PC‐Card Controller and Stand Alone Controller Here the cost becomes the key factor in choice between these two alternatives.
Actually they are nearly the same in performance. Stand Alone Controller is just more stable than the PC‐Card one. So two different modules will be prepared for customers. If they are in charge of many jewelry CNCs and they are working days and nights, the stable one with higher cost is preferred. If the manufacturer is just running one machine and cost is quite sensitive to him, he may choose the cheaper one. Both controllers can support at least 5 axes so that controller won’t affect the upgrade of other parts of the machine.
I.3 Priority Items in Design The requirements of this design project are: · Engrave on rings – Specialized fixture with inner ring holder; Rotary table for ring manufacturing. · Drill holes on rings for diamond setting – Jewelry CNC should be a milling machine; High speed spindle for enough cutting rate. · Operate with a good accuracy – High precision stepping motor; Close loop control with encoder; Low friction guideway. · Easy to use – Operation compatible with existing similar jewelry machine. No need to learn much. · Low noise – High quality bearing may be used, even if air bearing. Reduce vibration. · Reliable – Simple design with mature products and technology in the market. · Safe operation – Protective casing may be selected. Additional sensors in case motion out of control. And the priority in our opinion is: Reliable>Precision>Safety>User‐Friendly>Noise
II. Individual Design II.1 Engineering Specification of the Sub‐System · Key Functions of Rotary Table The rotary table works in conjunction with a milling machine, which greatly enhance the performance of a normal mill. It provides a much better capability with round work piece. With a rotary table, a mill can complete most work of a lathe. Moreover, a numerical controlled rotary table offers a precise rotational motion. It is especially suitable for jewelry CNC which is often used for manufacturing ring shape product. 1. Accurate Repetitive Indexing Operations The stepping motor works in micro‐stepping mode which provides 400 steps per revolution. Combined with the 1: 30 worm gear on the table, 12000 step/rev
accuracy is achieved. It is able to satisfy the requirements of repetitive indexing operation in jewelry manufacturing. 2. Provide A Capability Of Variety Of Work Piece Shape A rotary table works best with a three or four jaws chuck for cylindrical work piece, which is the most difficult job for a normal mill. With a rotary table, a milling machine can automatically rotate the work piece and cutting on the whole revolution. 3. Make Certain Shape Jewelry Manufacturing Easier A lot of ring shape jewelry exists in the market. However, most operations on them are not possible on a normal mill. A rotary table with inner ring holder will perfectly solve the problem. It becomes much easier to engrave on the outer and inner surface of a ring. The process is fully automatical and greatly increase the efficiency of ring making. · Engineering Specification of Rotary Table Rotary Table Specification Overall Dimension Table Size Weight Resolution Available Stepper Torque Power Supply Maximum Speed Interface Plug Rotation Speed Backlash Compesation Software Accessories
197mm X 51mm X 130mm 120mm Diameter, 51 mm Height 6 kg Motor‐400 steps/rev Micro Stepping Mode Table‐12000 steps/rev (.03°/step ±.01°) 3 kg*cm 120‐240 VAC Input. 24 VDC 1A output Continuous Running: 10 rev/min 8‐Pin MiniDin Connector Programmable from 1°/sec to 60°/sec Programmable to be 0 Compatible with G Code Programme Standard T Stand Tilting Angle Table Riser Table
Figure 7. Rotary Table Dimension
· Key Functions of Work Piece Holding System It is used to hold work pieces. Here I use a 120mm 4‐jaw self centered chuck as the holding system. It is fit for the rotary table, which is mainly used for cylindrical or ring shape work piece. Since all jaws are moving in unison, it is convenient to clamp and locate the work piece. 1. Locate It is one of two important functions for a holding system. The four jaws of the chuck move at the same redial position which keeps the work piece in the centre of the chuck. It is also able to hold non circular shape work piece such as cube and locate it in the rotation axis. Unlike normal chuck in the market, special design should be applied to this chuck in order to hold ring shape jewelry. 2. Clamp The chuck must be able to clamp the work piece tightly and uniformly. Also, the pressure has to be controlled in a proper level in order to prevent large deformation. · Engineering Specification of Rotary Table
4‐Jaw Chuck Specification Overall Dimension Table Size
120mm X 46mm X 120mm 120mm Diameter
Weight Jaw Opening Range Inner Ring Holding Total Runout Accessories
3 kg 0mm‐45mm (Jaw Reverse 50mm‐10mm) 6mm‐40mm Inner Diameter 0.05mm M6 Wrench Riser Table
Figure 8. Chuck Dimension
II.2 Detail Design of the Sub‐System
· Part List of Rotary Table Rotary Table Part List No. Qty Name 1 1 Rotary table top Worm 2 1 Gear‐BostonGear‐d1148 3 1 Rotary table base 4 1 Worm housing
Details Worm Gear D1148 from Boston Gear.Co
1 Worm‐BostonGear‐gdvh‐1 ISO 15 ABB ‐ 178 ‐ 1 20,DE,AC,20_68 Bearing ISO 15 ABB ‐ 3912 ‐ 1 12,DE,AC,12_68 Bearing ISO 15 ABB ‐ 198 ‐ 1 10,DE,AC,10_68 Bearing 1 Stepping motor mount 1 Coupling
1 CSK243‐ATA Stepper
6 7 8
Worm GDVH 1 from Boston Gear.Co ISO 178 Ball Bearing ISO 3912 Ball Bearing ISO 198 Ball Bearing
CSK243 Stepping Motor from Oriental Motor.Co
1. Rotary table top Rotary table top is the top cover of the rotary table. It is the interface between the rotary table and the work holding devices, e.g. chuck. It has a center hole for locating the chuck and four slides for clamp. It is made of nodular cast iron which is easy to make complicated shape like this table top. The worm is installed on its back to drive it move.
2. Worm Gear‐Boston Gear‐d1148 This is the key part in the power transmission. The worm gear and worm transverse rotational axis from stepping motor to the rotary table. It is mounted on the back of table top and supported by the table base. The gear ratio of this worm gear system is 30 : 1. It is made by Boston Gear Company and available from the market. So there is no need to manufacture it. The material is brass.
3. Rotary Table Base It is the back cover of the rotary table and all the rotational parts are installed on it. It is fixed to the working platform of the CNC. The rotary table base is made of cast iron, the same with the table top in order to reduce the cost.
4. Worm housing Worm housing supports the worm and shaft. It is installed on the table base. The worm housing is also made of cast iron since the shape is not easy to mill. Casting is a more economic way to manufacture it.
5. Worm‐Boston Gear‐gdvh‐1 The worm and worm gears transmit the power from stepping motor to the rotary table top. The shaft is extended in order to be well supported by the worm housing. It is also made by Boston Gear Company and available in the market. There is no need to manufacture. The material is steel. 6. ISO 15 ABB ‐ 178 ‐ 20,DE,AC,20_68 Bearing 7. ISO 15 ABB ‐ 3912 ‐ 12,DE,AC,12_68 Bearing 8. ISO 15 ABB ‐ 198 ‐ 10,DE,AC,10_68 Bearing
These three are ISO angular contact ball bearing used for the shaft and worm gear in order to reduce the friction at these support points. All these three are standard parts so they can be purchased in the market.
9. Stepping Motor Mount The mount is the cover of worm shaft and it connects the stepping motor to the table base. It is interchangeable in order to be fit for various stepping motors. Material used is cast iron for simple manufacturing.
10. Coupling The coupling connects the shaft of stepping motor and worm. It is can be easily manufactured by turning. The material is 1035 steel.
11. CSK243‐ATA Stepper It is the stepping motor used in this rotary table made by Oriental Motor Company. All the NEMA23 size stepping motor can be used. If a larger torque or rotation speed is desired, larger motor can also be used together with a new motor mount.
· Assembly Drawing of Rotary Table
Figure 9. Rotary Table
Figure 10. Rotary Table Exploded View
Rotary table top is the connection between chuck and rotary table. The location of the center hole and four slots must be accurate in order to align the chuck. Usually the level of a rotary table is decided by the diameter of the rotary table top. The larger it is, the higher it may cost. It is the same for the load on this plate. If there is a relatively high load, the table top must be hardened and stronger motor and gears
should be used.
This is the steady part of the rotary table which supports all the other parts. It has the largest weight and volume. Center holes on it must be precise enough so that the run out of the rotational axis will be well controlled. Investment casting may be used to guarantee a good accuracy and surface finishing. The worm housing is installed on its bottom so the flatness must be under control.
This housing will hold the worm and keep a proper distance between the worm and worm gear. Its strength is very important since a large force will act on the worm. Thick cast iron is used in order to resist deformation. The bottom of the housing will be connected with rotary table stands. The parallelism should be in tolerance.
· Part List of 4‐jaw Chuck 4‐Jaw Chuck Part List No. Qty Name 1 2
1 1 1
1 1 4 1
Chuck plate Bevel Gear Bevel Gear assembly ISO ‐ Straight bevel gear 1M 98GT 12PT 20PA 12FW ‐‐‐98O90H8MD10.0N Straight Bevel Gear Tap Connected to Back Cover Back Cover Tight Screw ISO ‐ Straight bevel pinion 1M12PT 98GT 20PA 12FW ‐‐‐12O75H10MD5.0N Straight Bevel Pinion Top M6 Screw Head jaw
1. Chuck Plate Chuck plate has two important functions: guide the jaw and support the bevel gear. It is in the front of the chuck and four jaws are installed in the slot. Since the shape is complicated and there is no requirement on high strength, cast iron will be the most suitable material.
2. Bevel Gear Assembly This is the core part of a chuck which supports the jaws to clamp the work piece. It transverses the rotation of the screw bevel pinion to linear motion of jaws. It consists of two parts: The bevel gear on the back
and the flat tapped plate in front. The bevel gear can be purchased in the market and the other one is made by turning. 1045 steel is used for the front plate.
3. Back Cover Every part is installed on the back cover, which should be strong and tough. The thickness of this part is really high in order to guarantee the safety. Cast iron is still used here since it will significantly reduce the cost and simplify the manufacturing process.
4. Tight Screw(Bevel Pinion and Screw Head) This assembly consists of two parts: a bevel pinion and a M6 screw head. It is driven by the user and then rotate the large bevel gear and tight the work piece. A M6 screw head is used so that there is no need to manufacture special tight up tools. The bevel pinion can be purchased and the screw head is made by CNC. Stainless steel is used to increase the reliability.
5. Jaw Jaw is the only part contacting with the work piece in a chuck. It can be guided by the slot on the front plate. The jaw can be driven by the bevel plate in order to locate and clamp the work piece. A small rod is placed on the top of the jaw. It works as an inner ring holder. The jaw is made by stainless steel.
Powder metallurgy is used here.
· Assembly Drawing of Chuck
Figure 11. Chuck
Figure 12. Chuck Exploded View
Chuck plate of this self centered chuck will guide for jaws to clamp the work piece. The precision of the chuck mainly relies on the slots of the chuck plate. Strength is another concerned issue since clamps often sustain high pressure and deformation.
Like the bottom of a rotary table, the back cover in a chuck supports all the other parts. It must be strong enough to keep the whole chuck stable. However, the weight of it is limited by the maximum load of the rotary table. This constraint is quite important in the design of the cover. High strength and light weight should be
First, the strength must be high enough since jaw is the only part contacting the work piece. Stainless steel is used here in order to resist deformation and increase the life period. Moreover, this jaw should be corrosion resistant since it is likely to get in touch with lubricant or other liquid. One thing should be stressed is that these four
jaws are different. They have to be placed in the right slot.
Manufacturing & Assembly
III.1 Manufacturing I try to use as few components as I can to make it easy for manufacturing. Nearly half of the parts can be purchased in the market. This will reduce the manufacturing process required for these two sub‐systems. Currently, all the manufacturing processes need for making rotary table and 4‐jaw self centered chuck are casting, powder metallurgy, turning and milling. The simpler the process is, the better revenue will be obtained. The cost will be greatly reduced since the manufacturer don’t need to lease or purchase many kinds of machine. Do not need to hire many kinds of specialists. The sample work can be done by any metal workshops. In UST, DMSF can take the job except for the casting parts. PolyU IC can finish all the parts. If the number required is over ten, a workshop outside UST like Chun Fan is need in order to manufacturing more. Only if the product becomes very popular in the market, a factory will be constructed. III.2 Assembly The key issue concerned about the rotary table and chuck is the total run out of the axis. For a rotary cutting process the stability of the axis will greatly affect the precision of the manufacturing process. So the axis in both rotary table and chuck are supported by two points in front and back. Since the rotary table and chuck are not mass production products, little consideration is involved in the design for product line mass production. It is fit for technicians making in a small quantity. As said before, the screw bevel pinion consists with the M6 Hexagon screw wrench. No special tool for tight the jaw, which make the working platforms of technicians and customers a little bit cleaner.
Reference 1. CNC for silver jewelry item design report, Phase I. 2. Edward G.Hoffman, “Jig and Fixture Design”. Delmar Publishers, 1991 3. 合理化夾具設計 : 方法及附件 / 譯者吳家駒,臺北市 : 徐氏基金會 1990 4. Machine Fixture Design Handbook, National Defense Industry Press 1996 5. Machine Design Hankbook, McGraw Hill Publisher 2006