17 Tips for Designing Cost Effective Machined Parts
17 Tips for Designing Cost Effective Machined Parts
2
haileyk (Materials)
(OP)
FYI, we just published an article from a thread of discussion at Eng-Tips Forums at ENGINEERING.com. Hope you enjoy it.
17 Tips for Designing Cost Effective Machined Parts
17 Tips for Designing Cost Effective Machined Parts





RE: 17 Tips for Designing Cost Effective Machined Parts
RE: 17 Tips for Designing Cost Effective Machined Parts
STF
RE: 17 Tips for Designing Cost Effective Machined Parts
"Even,if you are a minority of one, truth is the truth."
Mahatma Gandhi.
RE: 17 Tips for Designing Cost Effective Machined Parts
One of the most impressive examples of mass production engineering I can think of is the stamped steel rocker arm used on millions of GM OHV engines. It was invented by a manufacturing engineer at Pontiac named Clayton Leach. The management at Pontiac did not think Leach's idea was worth pursuing at the time, so Leach worked to perfect it in his own garage, with his own money. His simple stamped steel rocker arm design likely resulted in many $millions of profit for GM over the years.
RE: 17 Tips for Designing Cost Effective Machined Parts
If I just dimension what matters, then I will have a rough dimension to one side of the slot, and a very fine dimension across the slot. One less accurate dimension to make, and it's a much easier one because it's that accuracy over a shorter distance, and it can have twice the tolerance size because the error to the datum in one side of the slot no longer makes a difference.
The only issue with this philosophy in tolerancing, that I have run into before, is sometimes, the tolerance scheme that leads to the theoretically easiest part to machine from a machine capability point of view, ends up being very unclear to the machinist. And if the tolerances are all quite easy to achieve anyway, it doesn't add much value, especially for short runs where programming is a large part of part cost.
RE: 17 Tips for Designing Cost Effective Machined Parts
As an engineer who spent most of his former life in my father's mold shop making parts, I'm more concerned with the aspect ratio of the cut than the chosen corner radius.
In other words, the component's internal corner radius and the height of that wall dictate the tool diameter required to achieve a favorable arc of engagement. What you really want to understand at that point is... whether or not standard tooling (length/diameter) can create it or if the design necessitates a more costly solution. Most likely... the more costly machining solution could be avoided all together if we follow tbuelna's advice and do a little consultation.
But if you're designing machined components, you should have some basic understanding of the standard/off-the-shelf tooling your shop employs.
RE: 17 Tips for Designing Cost Effective Machined Parts
The flip side to designing-for-manufacturing is that you can sometimes take it too far and make solutions to problems that don't exist. You really have to have a good foundation in the particular manufacturing process to be able to make wise decisions. More preferable to the 'oddball radius size' for non-critical features, in my opinion, is to simply specify the maximum allowable radius, or a min-max radius range, if you don't want the corners too-tight, depending on application. That'll give us much leeway as possible.
Overall it's a really good point that will make green designers put themselves in the manufacturer's shoes, so it's a good tip overall. Just felt like adding another layer to the onion, I suppose.
RE: 17 Tips for Designing Cost Effective Machined Parts
From a tolerancing point of view I agree with your idea of loose tolerances on <edit> internal <edit> corner radii to give the machine shop flexibility. For corners where the radii can't functionally be too small (usually stress concerns drive min internal radii on parts I've designed) then I'll often use what appears to be an appropriate nominal tool radius (allowing for 5X max cut depth rule of thumb or similar) then add .02 to .03 and make that the max radii.
(Can get a bit messy on models feeding into CAM depending what dimension/tolerance designation flexibility your CAD has but hey, you can't always address everything.)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: 17 Tips for Designing Cost Effective Machined Parts
or, as in my workplace, we see them more as "guidelines" than rules, like the Pirate Code. ;)
edit-to-add (nor do we claim compliance with ASME standards in any completeness)
RE: 17 Tips for Designing Cost Effective Machined Parts
It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.
RE: 17 Tips for Designing Cost Effective Machined Parts
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: 17 Tips for Designing Cost Effective Machined Parts
If you design (if feasible) the corner such that even a non-radius 'male' could fit, then you've left control of the fit to the length/width of the pocket (unilateral + tolerance) vs. the length/width of the component (unilateral - tolerance) occupying it. Works really well for male components that have--or must have--really small corner radii.
Back to machining, this leaves a corner that lends itself well to high-speed machining practices, where the goal is to maintain x% tool engagement throughout the process or rest machining, where a much larger tool could rough the pocket, leaving only the undercut portion for the smaller tool. And of course, all of this is out the window if corner clearance needs to be tightly controlled... and in some work, it does.
Fit is easily checked with hard gaging, like many shops still do. If the gage fits, the component should too, with no corner radii interference issues.
RE: 17 Tips for Designing Cost Effective Machined Parts
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?