Title Roundness Objective To study the methods employed in roundness measurement. Apparatus - dial dial indi indica cato toe: e: with with accu accurac racy y 0.00 0.001 1 mm(1 mm(1μm μm)) - surface plate - V-block (vee block) - Centre support - Dial indicat cator st stand and - Micrometer - Cylin ylindr dric ical al workp rkpiece iece Apparatus diagrams
Theory There are 3 conventional methods in measuring roundness i) diameter method (or 2-point method) ii) 3-point method iii) Radius method i) measuring roundness using the Diameter method (2 point method) In this approach the diameter of a round profile is measured by a micrometer at several points along the circumference of the workpiece through its diameter. Roundness is defined as the difference between the maximum diameter and minimum diameter. This approach is convenient and effective to be used in measuring mechanical component. The diameter method will not privide accurate results for certain workpiece profile. This measuring method will capture out of roundness when the number of’lobe’ is even. Nevertheless, there have 2 potential errors in this kind of analysis. ii) measuring roundness using the 3 point method The worhpiece is placed on a V-block which the supports it at 2 points. The dial indicator will touch(contact) the workpiece at the upper part of the cylinder. The workpiece will be rotated and the roundness is defined as the difference between the maximim value and minimum value of the dial indicator. The accuracy of this method relies on the angle of the V-block and form profile of the workpiece. This method may not provide accurate measurement it workpiece is in the form of an ‘ellipse’, ‘triangular’ and so forth or if the angle of the V-block is not suitable. By rotating the workpiece in the V-block, it will show imperfect roundness under the circumstance that the number of ‘lobes’ is odd. The first step is count the number of times the dial pointer lifts and falls when the workpiece is rotated 360° in the V-block. In order to measure out of roundness, it is necessary to use a V-bolck with a correct angle for the number of lobe that exists. The appropriate value of V-block angle can be obtained through the following formula.
V-block angle = 180°-360° n
where n = number of lobes
iii) measuring roundness using the radius method In this method, the workpiece is adjusted by using center supports along its centreline. The dial indicator will show the radial displacement at a particular cross-section at certain angular intervals when the workpiece is ritated. The out of roundness is determine by calvulating the difference in dial indicator readings. This method is more popular and often employedby engineer and technician. In genieral, it is more stable and is able to expose roundness imperfection in all circumstances. It could also generate error in roundness measurement. ,isalignment of the centres to support the workpiece will generate cylindicity errors. The location of centre support is critical in order ensure high accuracy measurement. On the other hand, workpiece which are bowed along their lenghh will show roundness imperfection even though it is coused by error in cylindriticity.
Dial indicator
The dial indicator is also knwon as a mechanical comparator and it is popular in many manufacturing plants. It can be used to check whether a component dimension is within specification or not. One or more dial indicator will be set up at suitable locations according to the workpiece shape to be measured. Its pointer will move to the left or right when the dimension is difference from the set nominal value. The working principal of a dial is based on the linear movement of the plunger which is converted to angular rotation of the dial pointer. It is achieved through a combination of rack, pinion and gear train. During measurement or inspection, the tip of the dial is always in contact with workpiece. Other than the ability to measure the displacement, roundness and linearity, it can also be used to examine the flatness of a surface. In this experiment, you will use the dial indicator in conjuction with other relevent apparatus to measure roundness only.
V-block
The v-block is a block fabricated with high accuracy. A normal standard v-block is usually found with the angle of 90º. However, v-block with other angle can also be found for special applications. The main tast of the v-block is to rest a cylindrical workpiece firmly. When using the v-block, it is very important to ensure that the workpiece is resting firmly on the side of the vee and not resting on the edges of the vee.
Methodology
Preparation of workpiece for measurement a) select a cylindrical workpiece with its diameter in the range of 20mm- 25mm and the lengh less than 100mm. b) Ensure that of its faces is marked with radial lines at 30° interval. c) Ensure that a marked circle is found at the middle of the workpiece. This circlew indicates the position to be measured in this experimnet. Diameter Method (2-point Method) 1) using the nicrometer, the diameter are measure of the workpiece at the 12 location by every 30°. The deviation are calculate of these diameter value from the first diameter value. 2) The workpiece are stick on a magnetic V-block and marked end facing upward. 3) The middle part are ensure to be measured is positioned higher than the top face of magnetic V-block. 4) The position of the micrometer are adjusted in the middle of the workpiece and ratchet are turn until the anvil touches the workpiece. 5) The micrometer ratchet are turn until no reading change on its display panel. Always use the ratcet only. 6) Then, the zero button are press on the micrometer so that it will indicate the value of 0.000 on the micrometer display. 7) The micrometer are release to measure the other positions. 8) The position of the micrometer are ensure for consecotive measurement with the line to be measured. 9) The micrometer ratchet are rotated as much as posible until there’s are no futher reading change on the micrometer display. 10) The readings of the display panel are recorded and these value are identify either positive or negetine. 11) Steps 7-10 are repeated. 12) The roundness deviation are ploted. 13) The value of workpiece roundness are determined. Roundness= maximum peak+ maximum valley(µm) 3 Point method 1) the same workpiece was place on the V-block. 2) The dial indicator was adjusted to stand so that it touches the middle part of the workpiece. 3) Ensure that the dialindicator touches the workpiece with minimum presure. This can be seen from the reading displayed on the dial indicator. Ensure that both the pointer of the dial indicator do not exceed the maximum limit. 4) In order to obtain reading at the maximum point on the workpiece, we need to more the plunger of the dial indicator across the workpiece several times. 5) Pay attention to the maximum reading displayed on the dial indicator. The position where the dial indicator shows itsmaximum reading is the maximum points for this workpiece. 6) The movement of the dial was stopped where it gives the maximum reading. From then on, avoid touching the dial indicator so that the maximum point is not last.
7) The reading is considered as the datum. Tou need not attempt to reset the value to 0 because it is very difficult to get the dial indicator to give 0 reading. 8) The workpiece was rotated to the next point. Do not touch the dial indicator. 9) The change that might show on the dial indicator reading was noted. These reading was copared with the datum reading. 10) Step 8-9 was repeated until 12 point. Radius Method 1) support the workpiece on centre. 2) In order to hold the workpiece, loosen the not which clamps the holder. 3) Then, the workpiece was placed in between the center support and the nots was tighten the nuts. 4) For reading the 12 points on the workpiece, refer for the steps on the (3 point method) from step 2-11. 5) The roundness deviation are ploted. 6) The value of workpiece roundness are determined. Roundness = maximum peak+ maximum valley(µm)
Result
Diameter Method (2-point Method) No Angle(º) Diameter deviation from(1)
1 0 0
2 30 -0.001
3 60 -0.004
4 90 -0.001
5 120 -0.002
6 150 -0.005
7 180 0
8 210 -0.001
9 240 -0.004
10 270 -0.001
11 300 -0.002
12 330 -0.005
Roundness = maximum peak + maximum valley = (0 + 5)µm = 5µm 3 point method No Angle(º) Diameter deviation from(1)
1 0 0
2 30 -0.002
3 60 0
4 90 0
5 120 0.005
6 150 0
7 180 0.001
8 210 0.014
9 240 0.016
7 180 0.007
8 210 0.020
9 240 0.027
10 270 0.005
11 300 0.00 9
12 330 0.0019
11 300 0.025
12 330 0.014
Roundness = maximum peak + maximum valley = (19 + 2)µm = 21µm Radius method No Angle(º) Diameter deviation from(1)
1 0 0
2 30 -0.010
3 60 -0.016
4 90 -0.017
5 120 -0.013
6 150 -0.006
Roundness = maximum peak + maximum valley = (29 + 17)µm = 46µm
10 270 0.029
Discussion
-
-
-
-
1) The important of roundness can be demontrated by considering a shaft in a bearing. If both the shaft and bearing are truely round and the fit is appropriate, the saft will rotete smoothly especially when suitable lubricant is applied. Nevertheless, if shaft is ‘oval’ or ‘lobe’. 2) From the result, the method of 2-point method the roundness is 5µm, the 3 point method is 21µm and lastly the radius method is 46µm. 3) For the roundness, which the higher value of roundness, more accurate the measurement. The lowest value of roundness is 2-point method, its because the 2-point method less accurate than others. This method only consider 2 point on the workpiece, then the measurement is micrometer. Micrometer are less accurate. The scale on the micrometer is too big,so the reading that gives is less accurate. 4) Then, the 3-point method is the second less accurate but it better than 2-point method. By this method, we use the dial indicator. We just take only 3 point on the workpiece and determine the roundness. Lastly by using the radius method, the result show that by using this method its more accurate than others. Because when we take the reading, the removable contact point did not move out from the middle part. Beside that, the measurement we use is dial indicator is more accurate than micrometer. 5) Error that exists in the experiment: Human error contribute the most in the experiment.Error that likely to occur are parallex error.When taking the measurement, the eyes of the observer is not parallel to the pointer.It will couse our reading will have uncertainty that will make our measurement to deflect.so make sure the eyes of the reader is always parallex to the pointer and device. Outside force exerted by human could couse the reading of the measurement to be extra than the actual reading.Hands on the table or where the device is being put could couse the error happen.Therefore make sure no hands or things that may exert force are put on the place the device being put to avoid it. Instrumental error also occur in the experiment.The scale on the micrometer is too big,so the reading that gives is less accurate.Less accurate will make our result to be not good than the more accurate. When do the graft, make sure that we use the right scale and point of plotting.
Conclution
-
The most accurate measurement is by using dial indicator and the best method is radius method. During the experiment, we have to be carefull with the error and avoid do the mistake for get the good result. In the method employed in roundness measurement there have many method such as 2-point method, 3-point method and radius method.
