“Here is some good info for anyone new to the profession, or in need of a refresher.”, offers Eng-Tips Forums member ctopher. He posted a link in an Eng-Tips Forums thread to a document titled “Tips on Designing Cost Effective Machined Parts,” written by Joe Osborn, from OMW Corporation.
Drawings and Prints
Make things easy for the machinist. Technical drawings and other print documents are often your only communication with them.
First off, clearly telling the machinist how you intend to use the part will help them to understand what it is you will need to provide context for their task.
After that, the documentation supplied to the machinist should be as clear and specific as possible:
1. Ensure that the text is easy to read, that lines are dark enough to see clearly and that you’ve avoided using any color (such as light grey, blue and especially yellow) that is difficult to read and doesn’t scan or fax well.
On the left, an appropriate print includes dimensions, revisions, contact information and is easy to read. On the right, an illegible print. Do not provide this quality of print to a machine shop. (Image courtesy of Joe Osborn, OMW Corporation.)
2. Give all parts unique names to reduce the likelihood of mix ups. Make sure to use the full title block on all drawings, including any revisions, the material requested and any required finishes. It is also a good idea to include contact information on each drawing so that any issues can be remedied quickly.
3. Even a hand drawn diagram is acceptable. It doesn’t need to be exactly to scale, but the dimensions and tolerances must be conveyed clearly. Remember to always dimension from the same point to avoid tolerance buildup.
4. Watch your decimals, and avoid using high spec tolerances unless absolutely necessary, since this will make for a very expensive part. The same goes for thread tolerances.
5. Do not specify the tap drill size unless absolutely necessary. In this area, the machinist often knows best.
Design for Machining
Next Osborn offered suggestions on how to design with an eye toward the machining process. Consciously thinking about the way your part will be machined makes the process easier for both the machinist and for your budget.
7. Design to avoid end mill chatter. End mills work best when they are rigid, and the deeper a pocket is, the more likely the end mill will chatter. This results in a poor surface finish.
8. Avoid designing parts that have corners the same size as a typical end mill diameter. Adding .20 or .30 to any corner with a standard diameter will lessen the surface area coming into contact with the part.
The surface area of the end mill in contact with the corner of the part is decreased when the corner diameter is increased. This helps to prevent chatter and maintain a good surface finish on the part. (Image courtesy of Joe Osborn, OMW Corporation.)
9. No matter how small the end mill, any pocket corners will remain slightly round. If a mating part has sharp corners, the inside corners of the pocket can be cut away to avoid jeopardizing alignment.
10. Toolholder clearance can be an issue when machining a pocket next to a high wall. To avoid this issue, specify multiple parts designed to be bolted together after machining.
11. For maximum strength, a hole only needs to be tapped to a depth of 1-3 times the diameter. Any deeper is unnecessary and costly. If a through hole is necessary, have it drilled through and specify threads as needed.
12. Limit risk factors for the machinist. Machining multiple simple parts is less costly than machining one complex part. This is due to the risk factor involved, and most shops will charge a premium for more complicated parts. This is because if a mistake occurs it is costlier to redo a complex part than it is to redo a simple one.
Setup and Fixturing
Proper workholding can reduce the chance for error and ensure that parts are machined to the proper specifications. Osborn shared several considerations when planning a workholding method:
13. The most common and least expensive method of workholding is vise fixturing. This method does require the part to have two parallel sides that can be gripped by vise jaws.
14. More complex parts designed without parallel edges are machined using softjaw fixturing. This is when vise jaws are cut away in the same shape as the part to be machined. Softjaw fixturing does require an extra machining step, but is still fairly cost effective.
15. Double sided tape works well for parts that don’t require flood cooling during machining but it does require large surface area for the tape to hold and the adhesive is sometimes difficult to remove after machining.
16. For larger, plate-like parts that cannot be held by a vise, through holes are used to bolt the part down. Sometimes existing holes can be used, but it can save the machinist time if the specification documents include a note stating that holes may be cut for this purpose if necessary.
17. Try to avoid multiple setups. The most cost effective parts are those without features on the side or back. This is because the machine doesn’t have to be reset and the part re-fixed.
Other Ways to Save
The graph displays the dramatic decrease in cost per part when increasing the number of parts in the order. (Image courtesy of Joe Osborn, OMW Corporation.)
A small increase in the quantity of parts when only machining a few can make a huge difference in cost per unit. This is because the cost of setup and programming is spread out. Ordering similar parts together can save on reprogramming and fixturing, as well.
For Osborn’s last piece of advice, he said to always treat your machine shop professionally. Communicate regularly and promptly, consult on design issues and pay your bills on time. Avoid using them as a free quoting service or negotiating on part costs, and instead, ask if they have any advice on ways to decrease the cost.
The Eng-Tips Forums community response to the provided resource was overwhelmingly positive. “As a former toolmaker who now works in design, the author has covered the topic very well,” posted one forum member who goes by the username “thixoguy”.
This is the advantage of sharing in a community forum with other like-minded professionals: these resources can fill in the gaps between what was taught in school and how the professional engineering environment works, from a real-world perspective.
To discuss the resource and this article, join the conversation in the Eng-Tips Forums.