The incredible versatility of Swiss-type screw machines has opened a whole new range of part possibilities for medical design engineers. Unlike other kinds of screw machines, Swiss-type machines cut part features by moving the stock and the cutting tool at the same time.
- Cutting with Swiss-type machines works well for smaller, high-tolerance parts
- Think about machinability when designing
- Common design pitfalls
The incredible versatility of Swiss-type screw machines has opened a whole new range of part possibilities for medical design engineers. Unlike other kinds of screw machines, Swiss-type machines cut part features by moving the stock and the cutting tool at the same time. The advent of multi-axis machines equipped with cross drilling, milling, and tapping spindles, sub-spindles with full C-axis capability, and automatic bar loaders, let designers create configurations and tolerances that would not have been possible in the past. Over the last 10 years, medical components have gotten more sophisticated and smaller, while at the same time often becoming less expensive. But there are hidden traps in this new world, and it is often the machinist on the shop floor who gets snared.
Thus, while CNC Swiss screw machines can do some amazing tasks, designers should be aware of potential pitfalls that machinists frequently see on medical component drawings. It is common for a machinist on the shop floor to spend hours struggling with a feature, only to find out later that it was not important to the engineer.
With that said, a machinist’s 10 worst nightmares, in no particular order are:
1. Restrictive material specifications or call-outs for unavailable grades. When choosing a material, design engineers should ensure that the material is available in the form required and that it is a machinable grade. We often see grades that are meant to be formed, not machined. We also see plastics specified that are meant to be molded and aren’t even available in rod form.
Medical component designers are fond of 316 and 304 stainless steel, and for good reason. Often, though, these difficult to machine grades are not needed. Free-machining 303 stainless should be substituted whenever possible. When specifying an odd material, the engineer should provide a source to the machining vendor.
2. Close tolerances or over tolerancing. We often see tolerances that are unnecessarily tight. Sometimes the CNC Swiss vendor is called upon to make up all of the tolerance stack-up that was given to vendors of mating parts. It is common for us to machine to ± 0.0001 in., but this kind of tolerance comes at a very high cost. Adding just 0.0001 in. to the tolerance (for a total of 0.0003 in.) can often mean a savings of 20 or 30% of the total part cost.
Also, unnecessarily tight tolerances may result in the need for expensive secondary operations, such as centerless grinding, cylindrical grinding, or honing. We have seen drawings with tolerances of ± 0. While machinists may appear to have superhero abilities, we really are still human.
3. Holes with high length-to-diameter ratios. Medical devices are often delivering fluids, guiding wires, or used to view into places that are not easily accessible. These devices often require long tubes or deep holes. But there is a limit to how deep we can drill on a CNC Swiss with conventional drills. Deep holes add cost, especially if they have close tolerances or fine finishes.
New tooling technology, though, has definitely stretched the possibilities. Coolant-fed drills, taps, and reamers now let us drill deep holes we wouldn’t have attempted 10 or 15 years ago. However, even coolant-fed drills have limitations to how deep they can drill, usually 10 times the drill diameter. These drills are great in some of the tougher materials, but are expensive. They don’t usually come in sizes smaller than 1 mm (0.0394 in.).
The use of tubing often gets us around the issue of length-to-diameter (L/D) ratios, but tubing usually comes in relatively thin wall sizes and tougher-to-machine materials.
When specifying hole sizes and tolerances, remember that drills come in standard sizes. It is much easier for the machinist to alter a turned diameter than to adjust a hole size.
When designing a medical component with a deep hole, think about whether you need the same diameter for the entire depth. Often, by drilling first with a larger drill, the L/D ratio for a smaller diameter can be greatly reduced.
4. Over-use of geometric tolerancing. Geometric tolerancing has become much more common, and rightfully so. It lets designers communicate part requirements much more clearly. However, the use of CAD software often makes it too easy to plop a geometric symbol on a drawing where it may not be needed. We see drawings of relatively simple parts with multiple geometric specifications on almost every dimension. The medical component designer needs to think, “How will the machinist on the shop floor measure this?”
Go easy with the geometric tolerances. It is simple to measure a diameter or a length on the shop floor, but add in true position or runout and you are adding a lot of time and effort for the machinist and the inspector. This is not to say that these dimensions shouldn’t be used, but make sure that you actually need them and that they make sense.
5. Proprietary or custom specifications. We often see drawings with specifications that are unique to particular companies. In reality, there’s not much that hasn’t been documented before, so try to stick to industry standards rather than creating your own.
When using industry standards though, be sure that they are still active. If you do specify a custom specification, then send it with the drawing when you request a quotation.
6. Insufficient thread clearance. This is a big issue on the shop floor. When tapping holes, we need some clearance at the bottom of the hole for the lead at the front of the tap. A plug-style tap has a tip chamfer equal to four to six thread pitches, so if you ask for a 10-32 tapped hole, then let us drill at least 3/16 beyond the requested thread depth (6 x 1/32). If it is absolutely necessary, we can use a bottoming tap or a thread mill, but we still need at least two thread pitches for clearance.
When specifying drill and tap depths, always specify the minimum depth only. This lets us drill and tap deep enough to meet your specification. Another issue we encounter on medical device threads is insufficient undercut where an external thread needs to go to the shoulder.
7. Unnecessary surface finishes. Many of the materials commonly used in medical components, such as 300 series stainless, 17-4 PH stainless steel, and titanium, will usually yield good surface finishes when machined on a CNC Swiss. A 16 micro-inch finish should not cause any issues on a turned surface. Once you go below this, though, you are adding cost. We have seen drawings with a 2 micro-inch finish. Except for some very special circumstances, it is unlikely a finish this fine would ever be required.
If you need a good finish, then design your part for electropolishing, grinding, lapping, or polishing. For internals surfaces needing good finishes, design medical components for boring or honing.
8. Straightness (runout). One reason why many medical components are made on Swiss-type screw machines is the capability to hold close tolerance on parts with high length-to-diameter ratios. When making heavy cuts on long parts though, stresses are introduced that will cause parts to bow or distort, so don’t ask for tight straightness unless you can afford post-machining straightening.
Another opportunity for bowing on long thin parts is introduced when parts are heat-treated after machining. 17-4 stainless is commonly used in medical components. At Swissturn, we usually heat treat the raw material, thus avoiding the opportunity for distortion and discoloration from post-machining heat treating.
9. Insufficient tolerance for plating. Electro-plating inherently has great variability. Depending upon the type of plating and the thickness required, tolerances of 0.0005 in. or more may be needed by the plater. Since the plating tolerance is applied to the coating thickness, it has to be doubled over a diameter. It is not uncommon for the entire tolerance to be taken up by the plater.
If you need a finished part tolerance of ± 0.0005 in., then don’t specify an extremely thick plating process. CNC Swiss screw machines are the best turning machines available for consistently holding close tolerances, but no matter how close a tolerance is held in the machining process, it can all be for naught if the parts are ruined in the plating process.
When creating drawings on a CAD system, include the part surface area, volume, and weight. This will make it much easier to estimate plating costs, particularly when precious metal plating is needed.
10. Unreadable drawings. CAD software is great, but we often see drawings with extremely small fonts. Make the dimensions as large and as bold as practical. Remember that the machinist on the shop floor may end up with a drawing that has been printed, scanned, and reprinted, so small dimensions will fade and introduce the opportunity for error.
CNC Swiss screw machines have opened new opportunities for sophisticated medical components, but the design engineer should always keep in mind the machinist on the shop floor who has to make the finished product.
Send your drawings to your vendor early in the design process so that the issues mentioned can be identified before the design is finished. Invariably, by the time the machinist receives the drawing, there is a sense of urgency to have the parts delivered. This is not the time to find out that a design is going to cause problems in the machining process.
In the end, it’s a matter of communication. The medical design engineer needs to communicate the important features of the component and the machinist needs to be able to give direct feedback to the engineer.
Written by: Kenneth J. Mandile President Swissturn/USA Inc Oxford, MA | Medical Design