Cordless Handtool Report

7/29/2019 Cordless Handtool Report

1/27

IMPERIAL COLLEGE LONDON

Cordless Hand tool

Assignment

Ali Siddiq

22/03/2012

Department of Mechanical Engineering


2/27

i

Executive Summary

A market niche was exploited for portable reciprocating saws. There is currently no reciprocating

saw in the affordable DIY products price range that is truly portable. Firstly, no saw in this price

range of up to 60 that allows users to change the blade without using any tools. In addition, the

battery life of competitors products was about 15 minutes. This concept product can achieve

twice as much battery life.

The product is going to be made using a lot of existing successful product parts of Bosch for which

this saw is being developed for. Handle, switch assembly, battery and motor were used from a very

successful hedge trimmer name Asb. This was due to the fact it was already in the market and

these parts have been tried and tested and have proven to be successful.

To achieve the specifications, a lot of designs were made of possible mechanisms, blade clamps,

gearbox and motor orientation. It was decided to go with a wobble plate design to convert rotary

to linear motion as its compact and lightweight compared to other considered mechanisms. In

addition, the blade was attached to the reciprocating arm using a clamp that involved a very easy

cam rotation to lock and unlock the blade. Furthermore, the gearbox was designed so that it takes

the least amount of space in the housing with the motor placed axially. This allowed achieving acompact design.

The specification involved a lot of features that would make this product very hard to beat in the

market. For example, compact design, tool less blade change, light weight and most importantly

ergonomics while sawing. These specifications were successfully achieved and the costing analysis

also shows that the product will make a lot of profit if made into an actual selling product.


3/27

ii

Table of Contents

1 MARKET RESEARCH .................................................................................................................................. 1

2 PRODUCT DESIGN SPECIFICATION ...................................................................................................... 4

2.1 QFD1...................................................................................................................................................................... 4

2.2 QFD2...................................................................................................................................................................... 5

2.3 SPECIFICATION....................................................................................................................................................... 6

3 CONCEPTUAL DESIGNS AND FINAL DESIGN ...................................................................................... 73.1 DECIDING WHICH ASB OR ISIO PARTS TO USE .................................................................................................. 7

3.2 DESIGN FOR ROTARY TO RECIPROCATING MECHANISM .................................................................................. 8

3.3 GEARBOX DESIGN FOR THE CHOSEN MECHANISM ......................................................................................... 11

3.4 DESIGN FOR BLADE CLAMP ............................................................................................................................... 12

3.5 POSSIBLE BODY DESIGNS ................................................................................................................................... 14

4 DETAILED DESIGN AND DRAWINGS ..................................................................................................16

4.1 MOTOR SELECTION............................................................................................................................................. 16

4.2 CALCULATING BATTERY LIFE............................................................................................................................ 16

4.3 GEAR SELECTION ................................................................................................................................................ 17

4.4 CALCULATING STRESSES IN GEARS .................................................................................................................. 18

4.5 CALCULATING CONTACT STRESS ON WOBBLE PLATE................................................................................... 184.6 CAD MODEL ........................................................................................................................................................ 19

5 DESIGN FOR MANUFACTURE AND PROCESS SELECTION CONSIDERATIONS ......................21

5.1 BODY..................................................................................................................................................................... 21

5.2 GEAR HOUSING ................................................................................................................................................... 21

5.3 BLADE CLAMP ASSEMBLY .................................................................................................................................. 21

5.4 WOBBLE PLATE .................................................................................................................................................. 21

5.5 OTHER PARTS...................................................................................................................................................... 21

6 COSTING ANALYSIS ..................................................................................................................................22

7 DISCUSSION ................................................................................................................................................23

8 CONCLUSION ..............................................................................................................................................24

9 REFERENCES ..............................................................................................................................................24

10 APPENDICES ...............................................................................................................................................24


4/27

1

1 Market ResearchThere is a market niche in affordable good quality cordless reciprocating saws. A lot of

the competitors products cost from 50 to 200. Existing products in this range dont

have the facility of tool less blade change. In addition, they are not aesthetically

pleasing. Furthermore, they suffer from short battery life and lack in performance, for

example the maximum thickness of wood they can easily cut. In this price range, theheavy duty ones which can perform big cuts quickly are all chorded. Some of the

products have been analysed below:

DeWalt DW304PK :

Figure 1: Dewalt reciprocating saw

For:

4 position blade clamp Key less blade change Variable speed Variable stroke length Fast, 5000 strokes per minute

Against:

Big size Corded Expensive, 100-130 Heavy at 3.5 kg

Ryobi LRS180:

Figure 2: Ryobi Reciprocating saw

For:

Key less blade change Variable speed Aesthetically pleasing Costs 60-70 Fast, 3000 strokes per minute Adjustable shoe

Against:

Big size Heavy at 1.8 kg Doesnt come with battery or charger


5/27

2

Bosch GSA:

For:

Key less blade change Variable speed Compact Lightweight at 1.2 kg Fast, 3000 stroke per minute Variable speed

Against:

Expensive, 170-200 Aimed at professionals Not aesthetically pleasing

Black & Decker CHS6000:

Figure 4: Black and Decker reciprocating saw

For:

Compact Lightweight at 1.3 kg Aimed at DIY market Affordable at 40-50

Against:

Very low duty, less than 2000 strokes per minute Not aesthetically pleasing Very low battery life, 15 minutes of continuous use Blade change using key

Figure 3: Bosch Professional reciprocating saw


6/27

3

From the products above, it is certain that in the DIY market, no one produces a good

quality and affordable reciprocating saw. The Black and Decker shown in figure 4, is

affordable, however cant really be used if cutting for one whole DIY session due to its

low battery life. It cant also perform cuts on thick woods. The other options are good,

but expensive.

The best cordless and portable saw is produced by Bosch as shown in figure 3. It has all

the great features that any DIY customer will wish for, however it is very expensive. It isalso seen that all the cordless saws perform at around 3000 strokes per minute. In

addition, they weigh at around 1-2kg.

The shorter stroke length is ideal for better control in tight areas, while the longer

stroke length delivers more aggressive cuts for heavy demolition[1]. Most of the cordless

reciprocating saws have stroke lengths ranging from 10 to 25mm.


7/27

4

2 Product Design Specification2.1 QFD 1

Figure 5: QFD 1


8/27

5

2.2 QFD 2

Figure 6: QFD 2


9/27

6

2.3 SpecificationAfter the market research, it was found that there was a niche in portable DIY reciprocating

saws. Therefore it is decided that a reciprocating saw will be designed Bosch. This will be an

inclusion to the other vast range of products they already offer to their customers. From

figures 5 and 6, it can be seen that it is quite important that the proposed design should have

the following features for the product to meet the customer demands and hence be

successful:

Should weigh around 1-1.5 kg Should be compact Should cost 50-60 Should have a speed of 2000 to 3000 spm Should do accurate cuts quickly and easily, hence a stroke length of 20mm is good as

small stroke length results in accurate cuts but slow speed and big stroke length

results in inaccuracy but high speed, therefore a middle value is chosen between

10mm to 30mm

Should have a tool less blade change Should be able to accommodate second hand if needed while cutting Should last 30 minutes of continuous use

Furthermore, the product should use existing Bosch parts in order to reduce the cost of the

design process. These parts are already developed and tested; therefore this will not only

reduce costs but also offer the customers a high quality product. The product should use the

following parts from Boschs already successful ASB product.

Handle Switch module Motor 10.8V Battery

Charger


10/27

7

3 Conceptual designs and final design3.1 Deciding which Asb or Isio parts to use

From the specification, it was decided to use existing Bosch handle, battery and switch

module of the Asb product as these are already developed and don't effect the performance

of the developed product if used. Furthermore, the motor of this product was also suitable

(as explained in the detailed design) for the required cutting task.

The shoe attachment was taken from Bosch's GSV reciprocating saw (shown in figure 3)

mainly because its compact and already developed. The functional head had to be changed

as Asb is a hedge trimmer and its head shape has been developed accordingly. This shape is

not ergonomic for a reciprocating saw, as it requires two-hand operation for accurate cuts.

This is shown in figure 7. Parts of Isio were not suitable for this product as it requires the

large 10.8V battery for longer battery life (as explained in detail design).

Figure 7: Showing hand position and ergonomics of the Asb handle


11/27

8

3.2 Design for rotary to reciprocating mechanismBefore coming up with any design, it was vital to consider what mechanism to use for the

reciprocating motion. Four different mechanisms were considered for this purpose as

shown below:

Figure 8: Rotary to linear mechanisms

For compact design, it is best to place the motor and reciprocating arm in an axial

arrangement. This is achieved using mechanism 4 which is a wobble plate. Mechanisms 1,2

and 3 could also be used with axial arrangement as shown in figure 9, however they will

require bevel gears which will increase the cost.

Mechanism 4, although it has more components is very compact and conforms to the

required design specification. In addition, it is cheaper than Scotch Yoke mechanisms (1 and

2) if cost of bevel gears is also considered. Furthermore, the Scotch Yoke mechanisms have a

certain delay after every revolution. This could have an impact in the performance of the

saw. Mechanism 3 is very expensive to manufacture and wouldnt last as long as the other 3due to more friction experienced by reciprocating arm and the stress concentrations in the

gear.

Mechanisms 1 and 2 can also be used by placing the motor in vertical orientation as shown

in figure 10 and 11; however, this increases the size of the product and goes against the

product specification. The purpose is to keep the product as compact as possible hence

mechanism 4 is chosen as shown in the IBIS map in figure 12.

1

2

3

4


12/27

9

Figure 9: Possible design for mechanism 1,2 and 3

Figure 10: Possible design for mechanism 1,2 and 3 with different orientation


13/27

10

Figure 11: Gearbox design for mechanism 1,2 and 3

Figure 12: IBIS map for choosing the mechanism


14/27

11

3.3 Gearbox design for the chosen mechanismGearbox orientation and design was very important as its main part of the product being

developed and it also has some effect in the overall look of the finished product. Couple of

designs were considered for the gearbox as shown below

Figure 13: Gearbox designs

Figure 14: IBIS map for gearbox selection

1

2

3


15/27

12

As shown in figure 14, Gearbox 3 was chosen as it takes as much less space as possible and is

a complete unit unlike gearbox 2 which although take less space but has wobble plate

outside the housing. Furthermore, gearbox 3, is much more user-friendly as it allows easy

dismantle.

3.4 Design for blade clampAt the price range of 50-60, there is a niche in the market for reciprocating saws with toolless blade change mechanism as explained in the market research. Hence user-friendliness

for changing blades was kept in mind while designing this part. The following designs were

considered.

Figure 15: Idea 1 for connecting blade



16/27

13


Figure 18: IBIS map for connecting blade

As shown in figure 18, idea 3 met the specification, hence was chosen. It was not as compact

as idea 2, however the ease of blade change was a major priority which was fulfilled using

idea 3


17/27

14

3.5 Possible body designs

Figure 19: Idea 1

Figure 20: Idea 2


18/27

15

Figure 21: Idea 3, Final idea

Figure 22: IBIS map for choosing design

All of the above designs use the finalized parts as explained before. Idea 3 best suits to the

design specification hence it was chosen.


19/27

16

4 Detailed design and drawings4.1 Motor selection

From the specification, it was decided that the asb motor would be used for this product.

Following calculations were performed in order see if this motor selection was appropriate

or not.

First, the force required to cut wood was calculated. using equation 1

(1)

Where is the shear stress of wood and A is the cross section area of blade

Assuming a blunt blade cross section of 0.05mm, the surface area would be

The shear stress of most woods is about 10.5 MPa[2]

This gives the force required at steady state conditions to cut the wood as

Using this value, Torque can be calculated using equation 2:

(2)Where r is the radius of the wobble plate where the bearings connect.

Assuming r to be 10mm

The Asb motor has specifications of 17500rpm at 71W of output power[3]

Using the equation 3, the Torque produced by the motor can be calculated

This gives the output torque as 0.039Nm, hence the motor was appropriate for the requiredtask as the output torque is greater than the torque exerted by load.

4.2 Calculating battery lifeAssuming that the Asb performs at 30W at steady conditions i.e. constant cutting of wood,

the current used by the motor can be calcuated at this setting using equation 4

Where V is the voltage provided by the battery i.e. 10.8V


20/27

17

The current was calculated to be 2.6A

The battery is rated at 1.3Ah

Using this value and equation 5, battery life was calculated in minutes

By plugging in the values the battery life was calculated to be 30.08 minutes which is really

good considering the competetion at this cost range.

4.3 Gear selectionThe specification requires a motor sreciprocation speed of 3000 spm. This converts to 3000

rpm for the motor speed as at one full revolution, the reciprocating arm performs one

stroke.

The motor needs to be stepped down from 17500rpm to 3000rpm. It was also arbitrarilydecided that the pinion connected to the motor would have 10 teeth. As shoen in equation 6

lower number of teeth in pinion would result in low number of teeth for the gear.

Where N is the rotational speed and n is the number of teeth.

The number of teeth for the gear were calculated to be 58.

To calculate the module of the gear, a pitch circle diameter of 40mm of chosen arbitrarily forthe gear. its a good value as its not that big and allows for a compact design and not that

small to avoid high stresses as shown later on in this section

Using the PCD and n value, the module of the gear was found using equation 7

m was found to be 0.7

Using this value, gears were selected from the hpc catalogue, the corresponding values are

given in the table A1


21/27

18

4.4 Calculating stresses in gearsThe contact bending stresses in th gears were calculated using the Lewis formula described

below

Where is the transmitted load on gears, F the face width, m the module and Y the Lewis

form factor. The transmitted load and velocity factor can be found using the followingequations.

Where P is the transmitted power and V is the pitch line velocity and can be calculated as

follows

Where PCD is the pitch circle diameter in mm and n is the speed in rpm.

After that safety factors were found using steel's yield stress of 250MPa

The results are shown in the table below. Table A2 in appendix shows how the

corresponding values for calculating the safety factors

Bending

Stress,

(MPa)

Safety

factor

Pinion 23.5 10.6

Gear 13.4 18.7

Table 2: shwoing the safety factors

As can be seen from table 2, the safety factors are very high and steel is a good

material for the gears.

4.5 Calculating contact stress on Wobble plateAt the end of the wobble plate, the bearing only transmits the load on it from its eadge. Letsassume that the cross section of this edge is 1x1mm and the force transmitted is the same as

calculated in section 4.1 i.e. 2.62N, the stress acting at the end of the stroke will be 2.62MPa.

If the plate is made out of plastic i.e. Tufnol the safety saftey factor is 26 using the Ultimate

tensil strength of 68MPa for Tufnol[5]. This is substantially high, hence Tufnol was chosen for

the wobble plate material.


22/27

19

4.6 CAD model

Figure 23: CAD illustration of parts arrangement

Figure 24: Blade Assembly

Bosch Blade,

fits easily in theblade assembly

Lever to easilyremove blade

Asb switchassembly and

handle

Asb battery and

circuit board

Compact

gearbox

Groove made tomake it easy toaccess the lever

handle

Locks the blade

easily

Cam pivotsaround the

housing

extrusion


23/27

20

Figure 25: Back view of the blade assembly

Figure 26: Detailed look at the gearbox

Reciprocatingfixed at this end

Bearingssupporting the

shaft

Easy todismantlehousing

Wobble plate

out of Tufnol

Bearing to

increase the lifeof the roduct


24/27

21

5 Design for manufacture and process selection considerationsGear At the design stage, it was important to consider how a certain design will be

manufactured. This process was used on each of the designed parts as explained below.

5.1 BodyThe body needs to be manufactured the same way as the Asb handle as this was used in the

design to save costs of development. The Asb handle is manufactured using injection

moulding out of ABS thermoplastic. The designers of the handle had to design it accordingly

i.e. make sure that the thickness remained within 5mm. This was also made sure while

designing the body. Tight quality control has to be in place as this part of the product should

satisfy the customer with their purchase. Any blemishes on the surface will leave a bad

impression and the company will lose that customer

5.2 Gear HousingThe housing was made out of die casted steel as its cheap and readily available. Other

options were also considered for example sand casting and aluminium dies casting. Sand

casting is really cheap, however not suitable for mass manufacture as mould has to be recast

after every batch. Aluminium die-casting is more expensive, however has more advantagesthan steel. For example, its lightweight and conducts heat well and does not rust either. For

the given specification steel was best choice as its very cheap compared aluminium and does

not require much heat conduction or rust protection. Furthermore, the advantage of being

lightweight does not outweigh the savings as shown in the qfd1.

5.3 Blade clamp assemblyThis was also made using injection moulding out of ABS plastic. Care had to be taken to

make the lever pivot accurately so that the hand of the assembly has a nice and tight hold of

the blade once locked and doesnt touch the blade when opened.

5.4 Wobble plateThis will be cut out of cylindrical Tufnol sheet readily available to buy. It will be cut from

high accuracy machines. Quality control will have to be placed to make sure right-angled

cuts are made for consistent performance of the saw.

5.5 Other partsOther parts such as switch assembly, battery and motor will be used from the existing Asb

product and their manufacturing process has already been tried and tested. Gears and

bearing will be bought from whole sale suppliers such hpc gears or Rs catalogue


25/27

22

6 Costing AnalysisFor the costing analysis, three main costs were considered i.e. overhead costs, labour costs

and material costs. A lot of estimation has been made in this analysis.

Design Costs:

These costs were calculated assuming the company hired, one full time designer being paid

at 20 an hour, 1 part time design reviewer being paid the same wage. In addition, two

prototype manufactures being paid 12 an hour. Assuming the whole design process takes 4

months, the following costs can be calculated. The analysis assumes that the full time

designer works 36hrs per week and the part time designer works 12 hours per week. In

addition the prototype technicians start their job after 3 months of design process working

full time i.e. 36 hours per week

Employee Time worked (hours) Cost ()

Full time designer 576 11520

Part time designer 192 3840

Technician 1 144 1728

Technician 2 144 1728

Total 18816

Manufacturing:

These costs were calculated by estimating how much it would cost for the manufacturingbuying and assembling the product parts. Cost of the parts was estimated at buying quantity

of 50000 with 80% discount. The costs involve labour costs in manufacturing

Part Buy/ make Cost of 50000()

Wobble Plate Make 50000

Bearings Buy 80000

Gears Buy 160000

Body Make 20000


26/27

23

7 DiscussionThere was a market niche in a good quality reciprocating saw. At the moment, there is no

reciprocating saw in the price range of 50-60 in the DIY market that has good battery life

and ease of use i.e. tool less blade change. Hence it was decided to exploit this market. The

design specification was created specifically with the user friendliness in mind. The saw will

be made for Bosch, hence it had to have really good quality design to be able to stand out

from already successful Bosch products.

Firstly it was decided to use Boschs existing successful hedge trimmer (Asb) parts in the

design as much as possible as it is already developed and proven to be successful in the

already competitive DIY market.

Therefore Asb handle, switch assembly, battery and motor were used in the design. The

head had to be changed as sawing is a very different process to trimming and has different

ergonomics requirements. The specification involved a lot of features that would make this

product very hard to beat in the market. For example, compact design, tool less blade

change, light weight and most importantly ergonomics while sawing. These specifications

were successfully achieved and the costing analysis also shows that the product will make alot of profit if made into an actual selling product.

To achieve these specifications, a lot of designs were made of possible mechanisms, blade

clamps, gearbox and motor orientation. It was decided to go with a wobble plate design to

convert rotary to linear motion as its compact and lightweight compared to other

considered mechanisms. In addition, the blade was attached to the reciprocating arm using a

clamp that involved a very easy cam rotation to lock and unlock the blade. Furthermore, the

gearbox was designed so that it takes the least amount of space in the housing with the

motor placed axially. This allowed achieving a compact design.

During the design, a lot of time was spent on considering the best ways to manufacture the

product. It was decided that gear housing will be made using die casted steel as its an easyprocess and suitable for mass production. Steel was chosen as its readily available and cheap

compared to other option like Aluminium. The body has to be manufactured using injection

moulding as the Asbs handle is manufactured in the same way. The wobble plate will be

manufactured by cutting a tufnol tube into the required shape

Using the calculations, it was decided to use two gears for reducing the motor speed to the

required speed. In addition, stress calculations suggested that the gears should be made out

of steel and the wobble plate made out tufnol.


27/27

8 ConclusionA market niche was exploited for portable reciprocating saws. There is currently no

reciprocating saw in the affordable DIY products price range that is truly portable. Firstly, no

saw in this price range of up to 60 that allows users to change the blade without using any

tools. In addition, the battery life of competitors products was about 15 minutes. This

concept product can achieve twice as much battery life.

The product is going to be made using a lot of existing successful product parts of Bosch for

which this saw is being developed for. Handle, switch assembly, battery and motor were

used from a very successful hedge trimmer name Asb. This was due to the fact it was already

in the market and these parts have been tried and tested and have proven to be successful.

To achieve the specifications, a lot of designs were made of possible mechanisms, blade

clamps, gearbox and motor orientation. It was decided to go with a wobble plate design to

convert rotary to linear motion as its compact and lightweight compared to other

considered mechanisms. In addition, the blade was attached to the reciprocating arm using a

clamp that involved a very easy cam rotation to lock and unlock the blade. Furthermore, thegearbox was designed so that it takes the least amount of space in the housing with the

motor placed axially. This allowed achieving a compact design.

The specification involved a lot of features that would make this product very hard to beat in

the market. For example, compact design, tool less blade change, light weight and most

importantly ergonomics while sawing. These specifications were successfully achieved and

the costing analysis also shows that the product will make a lot of profit if made into an

actual selling product.

9

References1. (http://www.handyguyspodcast.com/1359/choosing-a-sawzall-reciprocating-saw/)2. http://www.woodworkweb.com/woodwork-topics/wood/146-wood-strengths.html3. http://moodlepilot.imperial.ac.uk/mod/forum/discuss.php?d=16184. http://www.roymech.co.uk/Useful_Tables/Drive/Gears.html5. http://www.tufnol.com/tufnol/default.asp?id=26

10AppendicesModel PCD

(mm)Mod, m(mm)

Number ofteeth

Face width,F (mm)

OuterDiameter, OD

(mm)Bore(mm)

Weight(kg)

Pinion G0.7-58 7.7 0.7 10 6 9.1 4 0.003

Gear G0.7-10 40.6 0.7 58 6 42 6 0.062

Table A1 showing gear data

Pitch line

velocity,

V (ms-1)

Veloci

ty

factor,

(sm-1)

Transmitt

ed load

(N)

Lewis

form

factor,

Y[4]

Bending

Stress,

(MPa)

Safety

factor

Pinion 7.055 0.46 10.06 0.2196 23.5 10.6

Gear 6.377 0.49 11.13 0.4047 13.4 18.7Table A2 showing calculations for safety factors

Cordless Handtool Report

Documents

Transcript of Cordless Handtool Report