Estimating jobs

A few weeks ago I made some comments on Mike’s Models blog about machining job estimation. The key things I was taught to look at are “design for manufacture”, setting up time and numbers of operations. These three are all linked.

Designing for manufacture is not always something you can do anything about as a model engineer as the design has already been done. However, if you are designing your own components, castings or tools then you can think about how the part will be machined and how you can make it easier. Common things are the “keep it simple” approach, including additional mounting points to allow easier setup or work holding and chosing part sizes close to stock material sizes to reduce machining time.

The number of operations is something I would use to estimate the duration of a job. Basically the idea is to break a complex job into it’s individual steps and then get a rough estimate for each of those steps. For example, facing off a rod would be one operation, turning a diameter one operation, boring a hole would be two operations on my lathe; drilling an initial hole and then boring it to size.
My estimate for Mike’s job was:

“There’s approx 10 operations to make that handle, between 30mins and an hour a job? So between 5 and 10 hours”

Mike’s actual time (he didn’t time things accurately) was between 8 and 12 hours so I was not too far out.

This weekend I was given a job to drill some holes in two steel bars. I decided to do a time and motion study to see how long it took. Here’s my results, 2 hours to drill 20 holes.

Cleaning material, Marking out and centre punch 35 mins
Setup vice on drilling table 15 mins
Centre drill 20 mins
Drill sharpening 5 mins
Drilling 20mm deep hole x 10 25 mins
Drilling 10mm deep hole x 10 15 mins
Clean up 10 mins
Total 125 mins

Design for manufacture.
http://www.engineersedge.com/design_guidelines.htm

Setup Time
http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.50.2164
http://findarticles.com/p/articles/mi_m3101/is_n5_v68/ai_17450812

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