Having received several requests for clarification of what I was talking about where I left off last week, please refer to the following diagrams while I go over the specific points raised.
Starting on the left half of the drawing, showing the Machined Shaft, in the upper right corner is an isometric view of the part. Immediately to the left is a view that is normal to (perpendicular to) the large cross-drilled hole. Right below that is the actual front view of the Shaft showing both slots milled straight through. To the right of that is a section view (Section A-A).
This was the original configuration of the part as machined from bar stock. When the process was changed to investment cast, (which was a very good and money-saving idea), the exact same drawing was used, resulting in the exact same part, only now investment cast rather than machined. This is what I referred to as “lazy engineering.”
Now, look at the right side of the drawing showing the Investment Cast Shaft which, though seemingly equivalent to the Machined Shaft, has some small feature changes that add up to big functional differences. In place of the large cross-drilled hole, I made a square hole. This has a very big difference in the operation of the machine. First, some background.
The Shaft is mounted on bearings that are retained by snap rings that fit into the 2 grooves on the lower half of the Shaft. The shaft of the motor, located under the Shaft retained by the bearings and snap rings, drives the Shaft by means of a pin in a cross-drilled hole in the motor shaft. This pin rides in the lower slot of the Shaft. Now, in use a rotor is placed on the top of the shaft and has a pin in a cross-drilled hole that is turned by the edge of the large, cross-drilled hole in the Shaft. The circular arc on the top-side of the shaft keeps the rotor from riding up and possibly off the Shaft, which would be a catastrophic failure.
Now, due to production tolerances of the various components, the location of the pin in the rotor relative to the large cross-drilled hole can vary over a distance. If the pin happens to engage the hole in the Shaft exactly at Point A, there is only a rotary motion imparted to the rotor. However, if contact is made above or below Point A, there will be an axial component of the force (along the centerline of the Shaft), that puts an additional thrust load on the bearings. This is not a desirable situation. So, with the square hole in place of the round hole, there is no possibility of an axial thrust component of the force, because due to the geometry, there can only be a rotary force applied. The small lip at the top of the Shaft acts as a backup that precludes the rotor from lifting off the Shaft.
Looking now at the small slot in the bottom of the Shaft, the Machined Shaft has it as straight-through milled slot. In the Investment Cast Shaft design not only was the .150” wide slot given a fillet of .075”, but the whole profile of the cut (on both the inside and outside surfaces) were additionally filleted with an .015” radius. This results in a stronger Shaft that is much less likely to fracture (no sharp-cornered stress raisers) and is easier to produce. Putting these features on a machined part would be quite expensive and time consuming. But, on an Investment Cast part they are free!
Looking at the top of these parts, notice that on the Machined Shaft you have a sharp point where the large cross-drilled hole meets the top surface. Not only is this a potential safety hazard, but for use in the European Union this configuration was not even allowed. With the square hole, you end up with a more blunt “point” that is acceptable. Notice also that all of the edges on the top of the Investment Cast Shaft have a .015” fillet radius. Again, in a machined part, putting in these features is very expensive and time consuming, while on the investment cast part, you get them for free.
One last point that will surely and thoroughly beat this to death, notice in the isometric view of the Machined Shaft is a call-out “Datum Surface.” All this means is that the dimensions measured along the shaft centerline are measured from this surface. In practice, the length of the Machined Shaft, overall, is obtained by removing whatever needs to be cut off from the bottom of the shaft. This results in a nice, machined, shiny surface buried inside the machine, while an “as-cast” surface is an in-your-face appearance surface right on top. All that was needed to was to change the location of the Datum Surface to the bottom of the part and create all the dimensions from it. Having done this, the overall length is obtained by taking a cut off the top of the shaft resulting in a nice and shiny appearance surface.
I know it’s a small point, but it demonstrates how I “sweat the details” so you don’t have to.
With the years of experience I have had in many industries being applied to your specific situation, you end up with products and equipment that not only work the first time, but that are very robust and can be produced and assembled quickly and efficiently. All of this results in a favorable impact on your bottom line.
I very much appreciate hearing back from those of you who decided to comment on what I have written. If you have any questions, just fire away and I’ll do my best to answer you.
Again, as things are starting to pick up for me, please call right away if you need some assistance with any of your product design or re-design issues. Tooling assistance, in the form of design of fixtures, gauges, assembly aids or complete machines is provided too. “Big or small, I do it all.”
(Originally published on 5-20-10)