## Posts

### Timing Diagram (Part 3 - Cycloid Cam Profile Analysis)

In previous post [Timing Diagram (Part 2 - Maximum acceleration calculation)], we calculated the maximum acceleration of the die using cycloid cam profile. We found out that this maximum acceleration of the die can be reduced if we can extend the indexing angle (Bm) or indexing time (tm) through overlap motion.

Then, let's see how we can calculate for the suitable indexing angle to reduce the acceleration of the die.

Cycloidal motion cam profile has movement equation as follows.
h = hm x [t/tm - 1/(2 x pi) x sin(2 x pi x t/tm)]

Rearrange the equation to get
h/hm = t/tm - 1/(2 x pi) x sin(2 x pi x t/tm)

The displacement profile can be plotted as shown below (dimensionless).
We can see that at the first 10% of indexing time, the movement is just only 0.65% of the total movement (stroke) and at 90% of time, the remaining movement for the die is only 0.65% of the total movement (stroke). Or we can say that, there is not much movement at the first and last 10% of indexing time.

For thi…

### Improve math skills of your kids - Learn step-by-step arithmetic from Math games

Math: Unknown - Step-by-step math calculation game for iOS.

Math: Unknown is much more than a math game. It is a step-by-step math calculation game which will teach users how to calculate in the correct order rather than just asking only the final calculated results.

The app consists of four basic arithmetic operations which are addition, subtraction, multiplication and division. In order to get started, users who are new to arithmetic can learn from animated calculation guides showing step-by-step procedures of solving each type of operation. It is also helpful for experienced users as a quick reference.

Generally, addition and subtraction may be difficult for users who just start learning math especially when questions require carrying or borrowing (also called regrouping). The app helps users to visualize the process of carrying and borrowing in the way it will be done on paper. Once users understand how these operations work, they are ready to learn multiplication and division.

For most students, division is considered as the most difficult arithmetic operation to solve. It is a common area of struggle since it requires prior knowledge of both multiplication and subtraction. To help users understand division, the app uses long division to teach all calculation procedures. Relevant multiplication table will be shown beside the question. Users will have to pick a number from the table which go into the dividend. Multiplication of selected number and divisor is automatically calculated, but the users have to do subtraction and drop down the next digit themselves. Learning whole calculation processes will make them master it in no time.

Math: Unknown is a helpful app for students who seriously want to improve arithmetic calculation skills.

### Free Solid Edge 2D Drafting ST7

Siemens provides a free copy of Solid Edge 2D Drafting ST7.
Mechanism design always starts with 2D sketches of linkages. This free software allows users to create sketches with constraints. For example: tangent constraint keeps the cam follower (roller) always touching the cam curve. We can then see how other related linkages move. Variables in the sketch can be linked to Excel. So we can automatically change values of desired variables via VBA code. It also has goal seek function that will change the desired dimension which will give the target value of other dimension. It's very interesting to explore its capability. It will be a great tool for machine designer. We will explore more in later posts.

### Engineering Plastic PEEK Polyetheretherketone

PEEK is an abbreviation for polyetheretherketone, a high performance engineering thermoplastic. It is another engineering plastic which is widely used in machine components since it has very good mechanical strength and dimensional stability, excellent thermal and chemical resistance properties and outstanding resistance to abrasion and dynamic fatigue. Its yield strength is about 92 MPa. Its density is about 1.32 g/cm3 which is 1/6 of steel.

It is a strong and stiff thermoplastic material that is often used in applications where performance at high temperatures is required. PEEK has outstanding chemical resistance as well as resistance to hot water and steam. PEEK is insoluble in all common solvents and is extremely resistant to attack by a very wide range of organic and inorganic chemicals. PEEK can be used continuously to 250°C (for short term, it can operate at 300°C) and in hot water or steam without permanent loss in physical properties. It also has high abrasion and cut through…

### Engineering Plastic PTFE Teflon

Polytetrafluoroethylene (PTFE) is another engineering plastic using in machine parts. PTFE is a non-stick substance. The trade name of the DuPont company for PTFE is Teflon. PTFE is used as a non-stick coating for pans and other cookware. It has exceptional resistance to high temperatures, chemical reaction, corrosion, and stress-cracking. Working temperature of PTFE is between -200oC and 260oC. Since it has very low friction (PTFE's coefficient of friction is 0.05 to 0.10) and good resistance to chemical reaction, it's normally used in bearings, pipes or containers of chemical substances. It's also used for sealing purpose at high temperature environment. The tapes for thread sealing in pipes are also made from PTFE. Though it can work well until 260oC, but its strength is not always high since it may expand and deform at high temperature. PTFE is easy to machine, low cost and long lasting. PTFE parts are often used in mechanisms that rotate or slide. It doesn't requi…

### Engineering Plastic PA Polyamide

PA is the abbreviation of Polyamide. The commercial name of polyamide is Nylon. Nylon is an engineering thermoplastic which is commonly used in textiles, automotive, carpet and sportswear due to its extreme durability and strength. It has tensile strength about 80 MPa which is higher than POM. Solid nylon is used for mechanical parts such as machine screws, gears, cams, bearings, guides, rollers, and other low- to medium-stress components previously cast in metal. Nylon is frequently used as a replacement for bronze, brass, aluminium, steel and other metals, as well as other plastics, wood, and rubber.

Engineering-grade nylon is processed by extrusion, casting, and injection molding. Nylon price is approximately 2 times of the price of POM (Polyacetal). Working temperature when using Nylon should be within - 50oC to 160oC. Nylons are available in various colors such as white, blue, black, etc. One important property of Nylon that should be noted when use it to make machine components …

### Engineering Plastic POM Polyacetal

Normally, we use steels to make machine components. But in some applications, engineering plastics are also commonly used in machine parts since their unique properties e.g. lighter weight, easy for machining, great quantities can be made quickly by injection, etc.

General properties of engineering plastics:
Light weight - The density of engineering plastics is usually about 1-2 g/cm3. We often use them in the portable machines that we often move them, or use in the parts that the user has to hold by hand.

No rust - We normally use engineering plastics without surface treatment to prevent rust. However, engineering plastics do not withstand the corrosion from oxygen or ultraviolet, so we need to consider this point when using plastic for long time.

Low electrical conductivity - We use plastics to prevent users from direct contact with electrical parts for safety reason.

Low thermal conductivity - Engineering plastics are good for insulation. We can prevent some heat from machine. However,…

### Bolts, Nuts, Screws, Studs

Screws and bolts are the most common types. The difference between screws and bolts is only the intended use. Screws are intended by screwing into tapped holes; bolts are intended for use with nuts. Sometimes screws are supplied with a captive washer under the screw head. They are also called SEMS. A SEMS screw is a generic term used to describe a screw pre-assembled with a free-spinning lock washer. It is a permanent assembly with the washer held in place by the major diameter of the screw thread being larger than the hole of the washer. The SEMS screw is simple, easy to use and available in many different styles making it well suited for many applications. SEMS save assembly time and eliminate the possibility that a screw will be installed without its specified washer. Normally, a bolt can also serve as a screw by using it with a tapped hole rather than a nut.
A stud is threaded on both ends. It is usually screwed permanently into a tapped hole. Threads on the two ends may or may no…

### Perpetual motion

Interesting articles from Wikipedia and other web sites...

From Wikipedia.org
Perpetual motion describes "motion that continues indefinitely without any external source of energy; impossible in practice because of friction." It can also be described as "the motion of a hypothetical machine which, once activated, would run forever unless subject to an external force or to wear". There is a scientific consensus that perpetual motion in an isolated system would violate the first and/or second law of thermodynamics.

Despite the fact that successful isolated system perpetual motion devices are physically impossible in terms of the current understanding of the laws of physics, the pursuit of perpetual motion remains popular.

There is a scientific consensus that perpetual motion in an isolated system violates either the first law of thermodynamics,the second law of thermodynamics, or both. The first law of thermodynamics is essentially a statement of conservation of energ…