Updated: Sep 23, 2021
By Wes McCloud, Machining & Operations Leader
Whenever we look at a part print, we can tell be the way it is designed if the designer has heavy real-life manufacturing experience.
1. Do you focus on tolerances?
Many designers will submit designs with too many decimal places of precision. For every decimal place, the price to manufacture goes up exponentially. Drawings should be laid out such that it allows the machinist to easily find dimensions. Cluttered dimensions can lead to costly mistakes.
The part above did not have a standard tolerance designation block, but the dimensions on the print were all listed as four places. It was inferred that all dimensions must be to four decimal places of precision.
TOLERANCES UNLESS NOTED OTHERWISE
ANGULAR: 0 +- 2 DEGREES, 0.0 +-0.5 DEGREES
IMPERIAL: 0.0 +- 1/16, 0.00 +- 0.02,
0.000 +- 0.005
METRIC: 0 +- 0.5,
This may well be the case but more often than not, it is a specific aspect of the part that is critical i.e., a reamed hole for a bushing or dowel pin. After communicating with the customer the only critical feature of this part was the 1.7730-inch hole on the back side of the block. In this particular case the customer wanted a light press fit for a 45mm OD bronze bushing with all other dimensions having a general tolerance of 0.005”.
This simple change cut the parts cost dramatically by 50%.
Include a standard tolerance designation block and note any exceptions or key characteristic that will help the machinist quickly understand important aspects of the part. Be sure to include material type and any finish requirements.
2. Do you submit designs with a simple datum?
A datum is a plane, a straight line or a point that is used as a reference when processing a material or measuring the dimensions of a target.
If someone says reference your design by starting at a corner of the room, start there.
OK, that makes sense.
If someone says, go halfway around the circle, and start from there— you would say what are you talking about? This is the same for reviewing a print design.
For standard Bridgeport Knee Mill or ProtoTRak conversation mill projects, it is important to design from the left- or right-hand side fixed vice jaw. Understanding where the machinist will most likely setup from greatly eases the setup time and reduces the chance of human error in reverse calculation the datum point. The part stock size will usually be oversized to allow the machinist to bring to size by face and contour milling.
Many designers design from center of holes. Remember, the hole is not there as first. Designing from centers holes is fine if using a casting or if it is a secondary process where the hole can be picked up with an edge finder.
When you dimension parts, pick a datum that is at a sharp corner not at a radius – an easy way for a machinist to start from.
3. Do you design with maintenance in mind?
Many designers will submit designs without thinking about the long-term maintainability of the part. This may be as simple as putting a grease fitting at an odd angle, not providing ready access to a fastener, or a part that can be made symmetrical for dual use.
Many companies in the custom and contract machining & fabrication sector will make what the customer asked for without giving thought to the end user's needs.
Here is a tight tolerance part of +0.0000" / -0.0015 that rides inside of magnesium bronze bushing. The shaft must be a very fine slip fit to allow for precisions actuation of End of Arm Tooling (EOAT). These pieces have to be processed on both, the ROMI D600 mill & ROMI C400 lathe to create the details and tolerances required.
This shaft holds EOAT fingers and after a degree of wear or possible damage to one side, it can be taken out then rotated 180 degrees and then re-installed. The versatility of having a symmetrical part allows doubling its life span for the customer and creating a greater value add to them. We used the CNC capabilities within the tolerance requirements; and added a new feature that gives an extended life to this application.
Our CNC Romi Mill can hold tolerances of +/- .0005” with a 20-Pocket Tool Holder, that uses a variety of tools. Our travel dimensions are:
· 60" x 22" x 18" Prototrak
· 40" x 20" x 20" Prototrak
· 30" x 20" x 17" Prototrak
· 23" x 20" x 20” Romi
CNC lathes can hold tolerances of +/- .0005" with 6 varieties of tools, with travel dimensions and tail stock:
· 18” x 45” with 2.36” diameter through hole
· 17” x 40” with 2.09” diameter through hole
When you dimension parts, think about the long-term maintenance of the part. Don’t hesitate to ask the vendor for value added suggestions on how to improve life span and/or reduce costs.
4. Do you submit designs with quantities in mind?
When you order small quantities, let’s say the setup time is an hour and process time is one hour so you are paying for two hours.
And then, there is the ever-increasing steel prices. When ordering steel or whatever the material spec calls for, you want to consider the yield you can get from a standard stock length or size. One offs have to be costed on the length or size of the part. Consideration of multiples that will use an entire stock length or size can greatly reduce part costs.
Whether you have one, ten or one hundred parts, you still have setup time. If you are ordering one part a month, consider ordering twelve upfront.
We’d always recommend at least 10, 25, 50 or 100 pieces where you can take advantage of setup time incorporated into developing the tooling and drive down manufacturing costs.
5. Do you split up complex parts in your designs?
Many designers will submit designs without specifying that two parts can be welded or bolted together rather than CNC milled. This is especially true with parts that have multiple faces.
Our customer was making parts from two-inch thick stock and milling them down to one inch except for the triangular shaped block. The block is used as part of an End of Arm Tooling (EOAT) on a robotic application. Since the forces of the block are primarily in a downward direction, we recommended welding the triangular block in lieu of machining.
By splitting an individual part into multiple parts and welding them together, we saved the customer 45% on manufacturing process time and associated costs.