Day 15 

Intro to Engineering

Allen Sam

7-3-18

Activities 7.3 and 7.4

7.3 Tolerances

1.      Study the drawings below to identify specified tolerances.

a.      Highlight each dimension that has a tolerance associated with it using something like a highlight marker.

b.      Identify the type of tolerance in each highlighted example by labeling each tolerance dimension with one of the following: limit dimensions, unilateral tolerance, or bilateral tolerance.

c.      Label each identified tolerance with a separate letter, A through Z.

d.      Beginning on a new page in your engineering notebook for each part, record the letter of each tolerance identified on that part drawing, the type of tolerance, a short written phase that describes the dimensional variation allowed for that dimension, the tolerance (a number representing the total allowed dimensional variation), and an explanation as to why that particular dimension requires a tolerance. You may wish to duplicate the following table in your notebook to organize your notes.

Part Name: __________adjustable rocker arm_________________

Letter	Tolerance Type	Written Explanation	Tolerance	Why?
A	bilateral	can vary in size either bigger or smaller	+- .003	The size can vary but the variation is small because the hole needs to fit something well.
B	unilateral	the tolerance only goes one direction, either bigger or smaller than the ideal dimension, but not both/	+.004 -0.00	The tolerance can only get bigger because another piece needs to be able to move around freely in the hole.

Letter	Tolerance Type	Written Explanation	Tolerance	Why?
C	limiting	gives the ranges of what the dimension can be	.005	The hole needs to be extremely fit with the insert piece
D	bilateral	can vary in size either bigger or smaller	.1	the srcews need to be collinear when placed

Letter	Tolerance Type	Written Explanation	Tolerance	Why?
E	limiting	gives the ranges of what the dimension can be	.005	The diameter of the circle need to fit with another circle, so it needs to be fir but it needs to move around.
F	limiting	gives the ranges of what the dimension can be	.004	The diameter of the circle need to fit with another circle, so it needs to be fir but it needs to move around.

Letter	Tolerance Type	Written Explanation	Tolerance	Why?
G	limiting	gives the ranges of what the dimension can be	.01	There is an interlocking piece, but the shape of the piece makes it so the dimensions can be more free and still lock

2.      Consider the adjustable rocker arm represented in the drawing above. An axle is to pass through the smallest hole. The manufacturer is considering buying bar stock for the axle that is manufactured according to the following drawing using the same general tolerances as those specified on the Rocker Arm Drawing. Answer the following questions and SHOW YOUR WORK.

a.      What is the tolerance (the acceptable amount of dimensional variation) for the diameter of the rod?

 .010 + .010= .020 inches

b.      What is the tolerance for the rod length?

 .03 + .03= .06 inches

c.      Is the fit between the rod and the hole a clearance fit, interference fit, or transition fit? Explain your answer.

A clearance fit must have the part touching, an interference fit needs to have the external dimension be bigger than the internal dimension, and a transition fit has to have a specific tolerance.

d.      What is the allowance between the rod and the smallest hole?

.0025 +.0025=.005 inches

e.      If the design of the assembled machine requires that the rod is sized such that the actual clearance between the rod and the hole is never greater than 0.005 inches, will the current rod design meet the requirement? Explain.

no, because both pieces can have  total tolerance of more than .005 inches.

f.        The machine specification requires that the rod is no longer than 3.025 inches and no shorter than 2.955 in.

·   Let L represent the actual length of the rod. Write the length constraint as a compound inequality.

2.955≤ L≤ 3.025

·   Will the stock rods always meet the length constraint? Explain.

yes, because the limiting tolerances will fit the inequality of the variation.

g.      If the stock rods do not always meet the machine specifications for rod length, how can the manufacturer alter the rod part drawing such that the rod length will always conform to the specifications?

The rod makers can lower the tolerance of the rod length.

3.      Design a rod that will have an interference fit so that the allowance (the maximum interference) is 0.010 with the center hole in the Rocker Arm and the tolerance on the rod diameter is .002. Sketch a section view of your rod design below. Be sure to specify the tolerance in the diameter dimension of your design.

4.      Consider the axle pin and the body of the Automoblox vehicle (shown below). The axle is designed to remain inserted into the body of the vehicle so that it cannot be removed by a child.

a.      What type of fit would you recommend for the axle and body? Why?

I would have an extremely small interference fit because you want the small piece to fit as tight as possible to the hole so the child can't take it out.

b.      Assume an interference fit is used and that the ideal interference between the axle and the hole in the body of the vehicle is 0.005 inches. However, the interference can increase to as much as .010 without damage to the parts. With the addition of glue to help adhere the two pieces, an interference of .002 can be used successfully. Assume that the specified diameter of the axle is 0.300 inches as shown on the pictorial above. On the images above indicate your recommendation for:

·   the specific tolerance to the axle diameter dimension\
.300 +_ .001

·   the diameter and associated tolerance for the hole in the bed
.3005+_ .002

c.      Calculate the following for the exterior axle pin diameter using your suggested dimension tolerances. SHOW YOUR WORK.

Specified dimension:  0.300 inches

Upper Limit:  ___.300+.001=.301inches_____________

Lower Limit:  ______.300-.001=.299inches_________

Tolerance: _______.002inches__________

d.      Calculate the following for the diameter of the hole in the body using your dimensioned part drawing. SHOW YOUR WORK.

Specified Dimension:  ______.3005__________ (if applicable)

Upper Limit:  ________.3005+.002=.3025inches________

Lower Limit:  ____.3005-.002=.2085inches___________

Tolerance: ____.004inches_____________

e.      Calculate the allowance between the hole and the axle pin diameter based on your recommended dimensions and tolerances. SHOW YOUR WORK.

.004+.006=.006iches

5.      Open the drill block drawing file that you created in Activity 7.2 Sectional Views. The drill block drawing is shown below.

6.      Consider the drill block from the question above. 

a.      If the detail drawing used chain dimensioning to locate the holes along the 5 in. dimension as shown in the image below (and therefore the tolerances are additive), what would be the upper and lower limit of the dimension from the left edge to the center of the 0.25 inch diameter hole on the right (see below) assuming general tolerances apply?

.75+_.05 = 1+_.05 + 1.25+_.05 + 1.25+_.05= 4.55 inches to 4.95 inches

b.      What is the upper and lower limit of the dimension from the left edge to the center of the 0.25 inch diameter hole in the drawing you created (using datum dimensioning) assuming general tolerances apply?

4.75+_.05= 4.7 and 4.8 inches

Conclusion

1.      Why do engineers place tolerances on dimensions?

We place tolerances in dimensions because manufacturing is never perfect and there needs to be a limit for how variation a dimension can have when making a part.

2.      What are the three types of tolerances that appear on dimensioned drawings?

There are bilateral tolerances, unilateral tolerances, and limiting tolerances.

3.      What is the difference between a general and a specific tolerance, and how can you tell the difference on a drawing?

A general tolerance is one that will apply if no specification is applied, and a specific tolerance is assigned for only a certain dimension. A general tolerance will be located in the general tolerance  chart while specific dimensions are at the specific annotation for the dimension.

Activity 7.4

Conclusion

1.      What is an offset and how is it used?

 It is a type of constrain where you may not want something to be completely flush with the other chosen part of an object.

2.      What is the difference between a mate and flush constraint?

 A mate flush connects the two faces together while the flush constrain aligns the two faces so that they are collinear

3.      What constraint would you use to place a pin inside a hole? Which degrees of freedom does this constraint remove? Use a sketch to define the degrees of freedom.

 I would use a mate constraint because it connects the inside of the hole with the outside of the pin. This removes horizontal and vertical degrees of movement and only the z axis is still allowed. 

4.      Describe a situation in which you would use a tangent constraint.  Which degrees of freedom does a tangent constraint remove?  Use a sketch to define the degrees of freedom.

 You can use a tangent constraint to make a face touch with another point you choose. The tangent constraint removes  the vertical movement of the part.

5.      What is a subassembly? How is it useful in the assembly of a complex product that involves multiple parts?

A subassembly is a part that is a part of a bigger assembly. It is useful to have completed sub assemblies because you can rotate them correctly.