Computer Aided Design 3 Assignment
Individual Design Model and Manufacture TUTOR: Tony Roberts
STUDENT NAME: Arvydas Gordejevas
STUDENT NO: 2602021
SUBMISSION DATE: 3rd May 2011
Contents
1. Parts .................................................................................................................................................. 3 1.1 Central Differential................................................................................................................. 3 1.2 Simple Gear Selection Procedure .......................................................................................... 3 1.3 Gear and Differential Selection .............................................................................................. 5 1.31 Calculations ................................................................................................................. 6
Analysing the input gear (pinion) ......................................................................... 6 Analysing the differential gear ............................................................................. 7
1.4 Central Differential Casing ..................................................................................................... 9 2. CAD Data (2D & 3D CAD Models) ..................................................................................................... 9 3. Manufacturing Process ................................................................................................................... 11
Milling Process ....................................................................................................................... 11
4. 2D Drafting Data & Design .............................................................................................................. 12
2D Draft of Exploded Assembled Differential Casing ............................................................. 12 2D Draft of Central Differential .............................................................................................. 13 2D Draft of Bottom Half Differential Casing ........................................................................... 14 2D Draft of Top Half Differential Casing ................................................................................. 15 2D Draft of Assembled Differential Casing (one side) ............................................................ 16
5. CAM Data ........................................................................................................................................ 17 Appendices ......................................................................................................................................... 19
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1. Parts 1.1 Central Differential A central differential for a four-wheel drive motor vehicle is provided on a pair of axles for right and left drive wheels. The differential has a cylindrical differential case connected to a final gear of a transmission, a pair of carriers provided in the differential case one of which is connected to the drive wheels and the other carrier is operatively connected to the other drive wheels. As mentioned in earlier report the chosen drive for transferring the motion from engine shaft to central differential will be spur gears. And before going to make differential casing we need to find our differential itself that we going to use. First of all we need to know how to select the right gear size that will transmit the motion from one shaft to another. For this I have used information source from “Mechanical Design� by Childs (2003). 1.2 Simple Gear Selection Procedure1 The Lewis formula
(where
is transmitted load (N);
is velocity factor; F is face width
(m or mm); m is module (m or mm); and Y is the Lewis form factor which can be found from Table 6), can be used in a provisional spur gear selection procedure for a given transmission power, input and output speeds. The procedure is outlined below: 1. Select the number of teeth for the pinion and the gear to give the required gear ratio (observe the guidelines presented in Table 1 for maximum gear ratios). Note that the minimum number of teeth permissible when using a pressure angle of 20° is 18 (Table 2). Use either the standard teeth numbers as listed in Table Table 1 Useful range of gear ratios 3 or as listed in a stock gear catalogue. 2. Select a material. This will be limited to those listed in the stock gear catalogues. Table 4 shows typical material matches for gears and pinions. 3. Select a module, m from Table 5 or as listed in a stock gear catalogue. 4. Calculate the pitch diameter, . 5. Calculate the pitch line velocity, . Ensure this does not exceed the guidelines given Table 2 Tooth dimension formulas for in Table 1. 6. Calculate the dynamic factor, . Table 3 Preferred standard gear teeth numbers 1
Childs, P. (2003) Mechanical Design. 2nd ed. Oxford: A Butterworth-Heinemann Title.
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7. Calculate
the transmitted load, . 8. Calculate an acceptable face width using the Lewis formula in the form, . The Lewis form factor, Y, can be obtained from Table 6.
Table 4 Typical material matches for gears and pinions
The permissible bending stress, , can be taken as ultimate tensile strength, /factor of safety, where the factor of safety is set by Table 5 Preferred values for the module m experience, but may range from 2 to 5. Alternatively use values of as listed for the appropriate material in a stock gear catalogue. Certain plastics are suitable for use as gear materials in application where low weight, low friction, high corrosion resistance, low wear and quiet operation are beneficial. The strength of plastic is usually significantly lower than that of metals. Plastics are often formed using a filler to improve strength, wear, impact resistance, temperature performance, as well as other properties and it is therefore difficult to regulate or standardize properties for plastics and these needs to be obtained instead from the manufacturer or a traceable testing laboratory. Plastics used for gears include acrylonitrile-butadiene-styrene (ABS), acetal, nylon, polycarbonate, polyester, polyurethane and styrene-acrylonitrile (SAN). Values of permissible bending stress for a few gear materials are listed in Table 7. Table 6 Values for the Lewis form factor Y defined for two different tooth standards (Mitchener and Mable, 1982)
The design procedure consists of proposing teeth numbers for the gear and pinion, selecting a suitable material, selecting a module, calculating the various parameters as listed resulting in a value for the face width. If the face width is greater than those available in the stock gear catalogue or if the pitch line velocity is too high, repeat the process for a different module. If this does not provide a sensible solution try a different material, etc. The process can be optimized taking cost and Table 7 Permissible bending stresses for various commonly other performance criteria into account if used gear materials necessary and can be programmed. 4|P age
1.3 Gear and Differential Selection My research has started from looking for a central differential with a gear drive that could be used in our 1/5th scale radio controlled car. And going through all the parts, their sizes and features I came to the stop of one particular part that made me say ‘this is it’. The title of the part is called “Central Diff Gear Box Set”, it’s sold by ‘Nutech’ company at www.nutechracing.com. It is Aluminium Diff Gear Box Set, high performance, durable & lasting is the main features. It can fill the silicon oil; also can fit almost all the 1/5 scale R/C cars (which makes no difference, as we going to build our own casing, but makes perfect knowing it’s build for 1/5th scale R/C). In Picture 1 you can see the central differential and in Picture 2 is an exploded photo of it.
Picture 1 ‘Central Differential Gear Box Set’
Picture 2 Exploded photo of ‘Central Differential Gera Box Set’
As the dimensions of this part couldn’t be found, an approach to the company needed to be done. And the response has come back with the following data:
Gear diameter – 75mm Gear pitch diameter – 72mm Gear teeth no. – 48 Casing diameter – 40mm Casing length with bearings – 68.4mm Centre diff outdrive cup (from bearing to the dog bone connection) – 12mm Bearings – 12x28x8mm
Together with this information “Nutech” company has provided with the ‘Central Differential Gear Box Set’ assembly picture (which is placed in appendix). Having a differential with a gear drive next step is to find the right pinion. And for this task “HPC Gears Ltd” is going to be used to find the solution. But before looking for the right pinion we need to calculate what module is used for our differential gear. Using formula for calculating pitch diameter (d is pitch diameter; m is module; N is number of teeth) we can find a module:
The list of gears and pinion that “HPC Gears Ltd” provides is attached to the appendix. 5|P age
1.31 Calculations Having the list of gears the selection can be started. And for that the Lewis formula and procedures that have been outlined above is going to be used. But before starting calculations, the assumption of the differential gear needed to be made. As the information we have for the central differential might be too short, we going to assume that the gear could be replaced with “HPC Gears Ltd” to any other size (just by making some adjustments in a workshop ourselves). Analysing the input gear (pinion) (taken highest RPM – 7800rpm) The first gear we going to find is pinion. And from the collaborated person Romana, who is responsible for the clutch, has given information that the bore for the pinion needs to be 10mm diameter in order to fit to the engine shaft. Plus as the team we are trying to keep the engine mounted as closer to the centre as possible (for the balancing issue). Therefore the lowest pitch diameter in 10mm bore “HPC Gears Ltd” offers d=24mm, teeth no=16. But as stated by Childs (2003): the minimum number of teeth permissible when using a pressure angle of 20° is 18 (Table 2). And in our case we are using pressure angle of 20°, so the next lowest pitch diameter will be d=27mm, teeth no=18 (Table 8).
Table 8 HPC Gears Ltd – pinion chosen size
What face width should the gears have? For a module of 1.5 “HPC Gears Ltd” offers standard 1.5 module gears with a face width of 20mm available in heavy duty steels. The Lewis formula can be used as an approximate check to determine whether the gears with a face width of 20mm manufactured from a particular material are strong enough. Calculating pitch line velocity and ensuring this does not exceed the guidelines given in Table 1.
Calculating dynamic factor
Calculating transmitted load
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From Table 6
Trying 817M40 steel knowing its tensile strength 1550MPa2 and using maximum factor of safety 5 (tensile strength taken from “Mechanical Engineer’s Data Handbook” by Carvill (1994))
Calculating an acceptable face width using the Lewis formula
This face width of 0.0373mm is less than the 20mm available, indicating that the gear will be more than strong enough. If this figure had been greater than 20mm, then another material would need to be considered. Analysing the differential gear The second gear is connected to the central differential. But as little known about this gear we assume that we have used “HPC Gears Ltd” part for our calculation. Therefore using the same pitch diameter as original the following measurements have been taken from the stock gear catalogue (Table 9).
Table 9 HPC Gears Ltd – central differential gear size
For a module of 1.5 “HPC Gears Ltd” offers standard 1.5 module gears with a face width of 12mm available in plastic and also available in steel - standard. The Lewis formula can be used as an approximate check to determine whether the gears with a face width of 12mm manufactured from a particular material are strong enough. Determining the speed of the differential gear (taken highest RPM – 7800rpm)
Where n is revolutions or rpm; N is number of teeth; and d is pitch diameter. 2
Carvill, J. (1994) Mechanical Engineer’s Data Handbook. Oxford: Butterworth Heinemann.
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This equation applies to any gear set (spur, helical, bevel or worm). For spur and parallel helical gears, the convention for direction is positive for anti-clockwise rotation.1 Therefore:
Calculating pitch line velocity and ensuring this does not exceed the guidelines given in Table 1.
Calculating dynamic factor
Calculating transmitted load
From Table 6 there’s no
therefore going to use the given numbers in between and
Trying plastic delrin knowing its tensile strength 70MPa2 and using maximum factor of safety 5 (tensile strength taken from “Mechanical Engineer’s Data Handbook” by Carvill (1994))
Calculating an acceptable face width using the Lewis formula (
)
This face width of 0.376mm is less than the 12mm available, indicating that the gear will be more than strong enough. Calculating an acceptable face width using the Lewis formula (
)
This face width of 0.37mm is less than the 12mm available, indicating that the gear will be more than strong enough. Design summary: Stage 1 (pinion): module 1.5, F = 20mm, N=18 (817M40 steel) Stage 2 (differential gear): module 1.5, F=12, N=48 (plastic delrin) 8|P age
1.4 Central Differential Casing Next question is how this central differential going to be mounted? For this part of the question the last year’s central differential casing have been looked and analysed. As going through all the designs and problem solving it looked like the best solution would be readjusting and improving last year’s casing. Also this decision has been made with the help of the braking system place, which will stay in a same position as last year – around the central differential. Improving and readjusting central differential casing:
Improve stability Readjust the sizes of the walls and holes to fit the differential Improve access to the differential for the maintenance Keep the space for the braking system
With all this in mind the casing can be reorganised and start building. The main thing for the differential casing is to be strong and stable. Therefore casing from the last year (Picture 3), which includes the braking system, will be modified to the central differential needs. Also the following have been agreed with Tayo who is responsible for the braking system: 1. Agreed to keep the same sizes of bolts and screws. 2. All the components of the braking system stay in the same position. 3. Braking disks to be bigger by 10mm in diameter and keeping them one on each side. 4. Breaking pads and disks to keep the same thickness.
Picture 3 Last year’s central diff casing including braking system
2. CAD Data (2D & 3D CAD Models) Before making the central differential casing in 3D lets start from making 3D of central differential itself. Therefore it could be sent to all the team members, with whom my part is collaborating, letting them know what size of differential I am going to be working with (Picture 4&5).
Picture 4 Central differential in Solid Edge
Picture 5 Central differential in Solid Edge
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Pictures below are the comparison of the new and old casing. Scale is used the same for both parts; therefore the difference can be seen not only in a shape, but also on a size.
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Next, casing needs to be accessible for the differential. Therefore an idea to split it in half was the first and the best solution. Picture 6 shows how the casing is divided and Picture 7 shows how the casing going to be assembled.
Picture 6 Casing divided in half for the access
Picture 7 Exploded assembled casing with the differential
3. Manufacturing Process3 Milling is the manufacturing process that has been chosen. Because of its versatility, the milling machine, particularly in its contemporary form as a CNC machine or machining centre, has became a workhorse in the parts fabricating industries. This machine performs a multitude of surface generating functions and has found utility in the toolrooms, job shops, and production facilities of a large variety of manufacturing industries. Milling machines may be classified as general-purpose, production, planer-type, and specialized machines. Milling Process3 Flat or curved surfaces, inside or outside, of almost all shapes and sizes can be machined by milling. As a rule, the workpiece is fed into or past a revolving milling cutter that has a number of teeth all taking intermittent cuts in succession. Also, the rotating cutter may be fed into the workpiece. The same kind of surface often can be milled in several ways. For in-stance, plane surfaces may be machined by slab milling, side milling, or face milling. The method for any specific job may be determined by the kind of milling machine used, the cutter, or the shape of the workpiece and position of the surface. 3
Schrader, G.F. (2000) Manufacturing Processes and Materials. 4th ed. USA: Society of Manufacturing Engineers.
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4. 2D Drafting Data & Design
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5. CAM Data 1. Before machining part with “NX CAM Express” the blank plate have been created, which will hold the part (Picture8). 2. The part is constrained into the blank plate. 3. Create new setup – “DieMold Express (metric). 4. Specifying part. 5. Specifying the position of clamps to ensure these are not damaged in the machining process. 6. Select material – 7050 ALUMINIUM 7. Setting the cutting tools 8. Setting machine operations
Picture 8 Blank plate (1)
1st cut is ‘rough cut cycle’ using a 20mm end mill (Picture 9).
Picture 9 Rough cut cycle, 20mm end mill
Picture 10 Semi-finish cut, 5mm end mill
nd
2 cut is ‘semi-finish cut’ using a 5mm end mill (Picture 10). 3rd cut is ‘finish cut’ using a 5mm ball nose cutter (Picture 11).
Picture 11 Finish cut, 5mm ball nose cutter
Picture 12 Finish cut in Y axis, 5mm ball nose cutter
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4th cut is ‘finish cut’ in Y axis to clean the bottom part, for transferring to another blank plate, using a 5mm ball nose cutter (Picture 12). After those four cuts blank plate from the back is unscrewed and a new blank plate is attached at the bottom place of the part (Picture 13 & 14).
Picture 13 Blank plate (2) Picture 14 Continue manufacturing
5th & 6th cut is ‘semi-finish cut’ in X axis using a 5mm end mill & ‘final cut’ in X axis using a 5mm ball nose cutter (Picture 15).
Picture 15 th
Picture 16
th
7 & 8 cut is ‘semi-finish cut’ in Z axis using a 5mm end mill & ‘final cut’ in Z axis using 5mm ball nose cutter (Picture 16). After all these steps of manufacturing the part, the final bit is to cut in half and drill holes for screws and insert pins.
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Appendices
Appendix 1 Central Differential Assembly
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