Use of Proportional Graphic Analysis to Select Appropriate CNC Lathes for Wheelchair Users in Manufacturing

Xiaoyi Ye, MID
Scott Haynes, MBME


The goal of universal design is to solve the problem of designing one machine suitable for all people. There are a number of factors that must be considered when deciding which features should be added to a machine to increase its usability by a wider range of users. This study of the CNC lathe is in the scope of universal design and seeks to use a proportional graphic analysis to select the most usable shapes of CNC lathes for standing operators and wheelchair users, based on the machine profile.


CNC lathe, wheelchair, disability, universal usability, manufacturing


This figure shows the sample of how to extract profile from CNC lathe, and illustrates the position of monitoring window and tool.Fig 1. Example of extracting profile from CNC lathe

Global competition and rapidly changing customer requirements result in great changes in contemporary manufacturing. It becomes increasingly important for manufactures to use effective and efficient tools to increase productivity [1]. Computer Numerically Controlled (CNC) machines are widely used in manufacturing industry for this purpose. However, there are reports of an increasing shortage of skilled people to utilize CNC machines [2]. In a study funded by the National Association of Manufacturers more than 80 percent of the large and small manufacturers reported a “moderate to serious” shortage of qualified job applicants [3]. At the same time, approximately 1.6 million Americans residing outside of institutions use wheelchairs, according to 1994–95 data from the National Health Interview Survey on Disability (NHIS-D). Just over one-sixth (17.4 percent) of working-age wheelchair users have jobs, the remaining 82.6 percent (489,000) are not in the labor force [4].

This figure shows all 12 categories of CNC lathe profiles, which were extracted from all type of CNC lathes from Google image, the profile including the guard, base, monitoring window and tool.Fig 2. 12 categories of CNC lathe profiles

Previous studies have shown that there is a growth trend of persons with disabilities having advanced technical education/training. They can utilize machines more effectively by enhancing job accommodations [5], but in industry, many categories of CNC lathes are not accessible to people with disabilities. However, it is not profitable for CNC manufacturers to produce completely different types of CNC for different user groups.  Fortunately the technology removes the more challenging aspects of job, leaving monitoring tasks largely to the operator [6]. In CNC lathes, the machine profile, which consists of the location of tool, the guard and a monitoring window, could affect monitoring tasks by operators. This study concentrates on selecting and improving CNC lathes to enhance universal usability based on a proportional graphic analysis (PGA) of the machine profile.


This figure shows the example of some machine features were simplified to arrive at the 12 profile shapes.Fig 3. Flexibility of the extractiong

This study utilizes data from existing research on anthropometry and manufacturing. Experiment is conducted through PGA. PGA is a common Industrial Design practice used to analyze the interaction between operator and machine by drawing the relative parts in proportion to one another. This method doesn’t mean every dimension is exactly right; the error amount can be control under acceptable range.

Through the Google image web search and the product catalog, information on 226 different shapes of the CNC lathes were identified. All of them are summarized into 12 different profile shapes. Fig 1 illustrates the structure of a CNC lathe example and how to extract the profile from it. Fig 2 shows all 12 categories of CNC lathe profiles, which were extracted from all types of CNC lathes from Google image. Some machine features were simplified to arrive at the 12 profile shapes.  An example of this simplification is shown in Fig3.


Different profiles of the CNC lathes affect operators’ ability to monitor the working tool process in two situations – standing up or sitting in a wheelchair. These result in ability to monitor clearly, difficult to monitor, and can not monitor.

Definitions of the independent and dependent variables

Independent Variable A: 12 categories of the CNC lathes (base on their profiles).

Independent Variable B: a -- Work standing up; b -- Work with wheelchair.

Dependent Variable: a -- Monitor clearly; b -- Difficult to monitor; c -- Can not monitor

“Can not monitor” means the worker can not observe the tool through the window of the lathe. “Monitor clearly” means the worker can observe the tool through the center area of the window, where extending from the center point of the window, the range is two-third of the distance between the center point of the window and the top (or bottom). Otherwise it is “Difficult to monitor”.

The controls for extraneous variables

In industry, CNC machines can be extremely large, or small enough to be put on the table. This research focuses on the mid-range of CNC lathes. Regular or average data are utilized. Height of 20 random samples of CNC lathe is listed in Tab 1. Average height of all these 20 lathes is 75”, stand deviation σ=7.9.

Tab 1. CNC lathe height of 20 random samples (Inch)

Sample ID





















CNC Lathe Height





















Fig 4 shows dimension of the CNC lathe: overall height is 75”, window width is 24” and height of the tool center point is 34”, the distance between the guard and tool center point is 14”.

Fig 4. Dimension of the CNC lathe

Optimal height for sitting adjustable work surfaces is 22” to 33”; optimal height for standing adjustable work surfaces is 35” to 42”; [8]. The optimal fixed height for sitting and stand work surfaces is 34”, so the height of the tool that operators work with will be 34”. The depth of the tool is calculated from 4 random samples ranging from 13” to 15.7”, where 14” is the average [9]. According to the height of the CNC machine (75”), the tool maximum cutting diameter is 24”, so the optimal size of the monitoring window is 24”. Window position has to follow the shape of the guard, if there is possibility that window can be in any position, it is assumed in the middle of that part. Dimension of the CNC lathe is shown in Fig 3.

Fig 5 shows anthropometric measurements for the average US males and US wheelchair mobile males, standing eye height is 61.7” and wheelchair user’s eye height is 47.1”, and knee height is 24.9”.Fig 5 Anthropometric measurements for the US males

Anthropometric measurements for the US males (average) [10]:

Stand eye height: 61.7”

Anthropometric measurements for the US wheelchair mobile males (average) [11, 12]:

Eye height: 47.1”

Knee height (Floor to anterior surface of the thigh): 24.9”

Measurements are shown in Fig 5.


There are 24 treatment conditions in this study. 01a, 02a, 03a…12a are scenarios for operators without disability; 01b, 02b, 03b…12b are scenarios for operators who work from a wheelchair. Fig 6 illustrates the proportional graphics.

Fig 6 shows proportional graphics of 24 treatment conditions while operator works with CNC lathe. 01a, 02a, 03a…12a are scenarios for operators without disability; 01b, 02b, 03b…12b are scenarios for operators who work from a wheelchair.Fig 6. 24 Treatment conditions while operator works with CNC lathe

 The operators have to keep close to the CNC lathe, so they can work with it and monitor the process. In the standing situation, the distant between eyes and tool would be better within 32”-36”, it’s the range between safety and monitor clearly. In the wheelchair situation, operator’s foot pad reaches the base of the CNC lathe. If both of them can monitor the tool working clearly, this category of CNC lathe has high level of universal usability. “The operator monitors the tool working clearly” means: when monitoring the tool working process, the operator’s line of sight can go through the a range of the monitor window, where extending from the center point of the window, the range is two-third of the distance between the center point of the window and the top (or bottom) of the window. If the operator’s line of sight goes out of the monitor window, it means the operator can’t monitor the tool. Otherwise, it means it is difficult for operator to monitor the tool working. The analysis results are shown in Tab 2, where both operators working with wheelchair and operators without disability working standing up can monitor the tool working clearly through the monitor window of CNC lathes in three categories (01, 06, and 09). In other words, 01, 06, 09 CNC lathes have high level of universal usability in terms of monitoring the tool working process.

Tab 2. Analysis the ability of monitoring CNC lathe tool working

CNC lathe   category
















MC = Monitor Clearly, DM = Difficult to Monitor and CNM = Can Not Monitor


Fig7 shows three categories of CNC lathe that operator work with wheelchair or standing up can monitor clearly have been selected.Fig 7. Three categories of CNC lathe where operators work with wheelchair or standing up can monitor clearly

In the experiment, three categories of CNC lathe where operator work with wheelchair or standing up can monitor clearly have been selected (Fig 7). But the operators do not monitor the tool working only. Sometimes they need to open the guard to change the tool or move the work piece. As shown in Fig 6, 06, 09 CNC lathe has indent base compared with 01, so the operators sitting on the wheelchair can get closer to the machine, reach the guard, tools, and work pieces easier.

 As a result, 06, 09 are the best CNC lathe based on profile to accommodate wheelchair users in manufacturing. In other words, profile 06, 09 match with the universal usability needs well in CNC lathe configuration design.


In this study, the existing research data are utilized and the experiment is performed through PGA. The advantage of PGA is that it provides a quick focus on certain types of machine shapes in a large number of machines. The PGA method, is easy to operate in any condition and demonstrate benefits clearly because is uses only average dimensions without focusing on individual machine details. The disadvantage is that the analysis is a little “ideal”, eliminating many special situation, such as extra large or small size machine; operator with hyperopia or myopia; special size wheelchair. This Method Not only for the lathe. But also for the CNC milling, CNC turing center, that have a large number of alternative shape (profile). This PGA method could help to select appropriate categories of them for the universal demand.


This research was conducted through RERC on Workplace Accommodations, which is supported by Grant H133E020720 of the National Institute on Disability and Rehabilitation Research of the U.S. Department of Education. The opinions contained in this publication are those of the grantee and do not necessarily reflect those of the U.S. Department of Education.


  1. Shuzi Yang, “Trend in the development of advanced manufacturing technology”, Chinese Journal of Mechanical Engineering, USA, 2003.10, pp, 73-77.
  2. Job opportunities related to CNC. %20cnc.htm.
  3. Eisen, P. (2003). Keeping America Competitive –How a Talent Shortage Threatens U. S. Manufacturing (White Paper). Washington, D.C.: National Association of Manufacturers, The Manufacturing Institute, and Deloitte & Touche.
  4. H. Stephen Kaye,Taewoon Kang, and Mitchell P. LaPlante, Wheelchair Use in the United States, May 2002 / Number 23.
  5. Phillips, C.; Giasolli, M. Assistive technology enhancement using human factors engineering. Biomediacal Engineering Conference, 1997, pp, 26 - 29
  6. Wall, T.D., & Davids. K.(1992). Shopfloor work organization and advanced manufacturing technology. In C. L Cooper &I. T. Robertson (Eds.), International Review of Industrial and Organizational Psychology (Vol. 7). Chichester, Enland: John Wiley & Sons, Inc.
  7. V.Ryan, CNC work – an introduction,
  8. Work surface height: Recommended work surface heights
  10. Harry C. Sweere, Ergonomic factors involved inoptimum computer workstation design,
  11. Das, B., Kozey, J., 1999. Structural anthropometric measurements for wheelchair mobile adults. Applied Ergonomics 30 (5), 385–390.
  12. Victor Paquet , David Feathers, An anthropometric study of manual and powered wheelchair users, International Journal of Industrial Ergonomics 33 (2004) 191–204.

Author Contact Information:

Xiaoyi Ye, MID
Center for Assistive Technology and Environmental Access (CATEA)
Georgia Institute of Technology
490 10th Street, NW
Atlanta, GA  30332-0156
Phone: voice/tty (404) 894-9156
Email: xye3@


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