Tips for Designers and Engineers
Turned Parts and Screw Machine Products
What encompasses turned products or screw machine products?
These are products turned on an automatic lathe, either computer controlled or cam driven. Typically, parts produced repetitively from twelve-foot bars, making part after part with a high degree of productivity and consistency lend themselves to this equipment. For more details see our turning page.
What would qualify as a "CNC turned product" versus a "screw machine product?"
Using today’s machines, often there is only a fine line between products that would be economically produced on a CNC versus a screw machine. Traditionally, the term screw machines was applied to cam operated automatics both single spindle and multiple spindle. The CNC lathe was introduced as a more flexible machine tool. It is capable of tighter tolerances, reduced costs for short runs, and handles complex products with ease. CNC’s have also been known to be considerably slower cycling than cam operated screw machine. The new CNC’s have closed the gap on this differential, but many high volume products are still best produced on screw machines.
Are "screw machines products" primarily screws?
Not at all. The term "screw machine product" is really a misnomer today. Screw machines are used for an endless variety of products turned from bar stock, far beyond the simple nuts and bolts of the past. For more information, see our products webpage.
I have a job that will run year round, wouldn’t I save money buying a machine to produce the product in-house versus letting a subcontractor make the extra profit?
Contract shops of production machining of automatic bar products typically have the following advantages in economies of scale:
Several machines of any given type
Maintenance personnel specializing in bar automatics
Process engineers and programmers
Tool makers and a well equipped tool room
Setup people specialized in each particular machine type
Machine operators trained to run multiple machines scheduled for multiple shifts.
Scheduling and procurement systems optimized for job shop and turning activities
One would assume it would be reasonable for the jobber to gain efficiency through their economies of scale and expertise that would usually allow them to be more cost effective in the overall assessment taking into account personnel and capital expenditures. Many companies have come to the conclusion that their resources are best used to focus on their product and leave the production turning to the specialist.
What consideration should be given in material selection for machined parts?
There are many grades of material specifically designed for improved machinability. Naturally, full consideration must be given to your product’s total application. However Material designed for optimum machinability often offers less weldability or less formability. The best turning steel (most economical) is AISI 12L14.
Fordsell also processes alloys, brass, bronze, copper, plastics, stainless and aluminum.
How does part configuration impact cost?
There are three principal areas where the part configuration can impact machining cost:
External considerations: Design your product to be accommodated with commonly available raw material. Allow for using hex material instead of milling wrench flat, when possible. Design the part with the tolerance of the raw material OD in mind and allow your supplier adequate part clean up, if necessary. Plastic rod is sold with tolerances on the plus side of mean, mild steel on the minus, and stainless with plus or minus tolerances. Consult with your supplier for guidelines.
Internal considerations: Avoid long deep holes. Any hole deeper than 3 ½ diameters in steel (5 diameter in brass or aluminum) is considered a deep hole drilling operation. If possible design the part to be made from tubing or maybe predrilled most of the way with larger drills. Worst case, if you do need a 1/32 hole drilled ½ deep in stainless, we can do it. But, it will be expensive!
Threads: Allow for adequate tap clearance at the bottom of a blind hole. Three and half thread pitches is good, 2 ½ minimum. On external threads, provide for an undercut or incomplete threads coming to a shoulder. Try to allow 2 ½ thread pitches or 1 ½ thread pitches the minimum.
What tolerances are achievable and practical?
This is a question that is asked all the time, but is very difficult to answer without a particular part and machine type in mind. Often someone will ask, "can you hold .002?" Naturally, the tolerance can be held, but it may change the way a part is made. It may require a longer cycle time, different tooling, a different machine or process, a more time-consuming inspection method, and likely will result in higher costs. Fordsell is eager to discuss your particular requirements and we welcome the opportunity to be involved in concurrent engineering, designing our process concurrently with the part design and assuring an ideal match. On a number of occasions, Fordsell has assisted its customers in the design to reduce manufacturing cost.
Should I request a first article?
Most suppliers are eager to supply a first article when they run a new job and it is really to everyone’s advantage to assure you are getting what you need. This should be requested when the job is quoted. If a supplier has to wait for customer approval, before running production, that adds avoidable cost to the job.
How do I assure my supplier provides the quality I need?
Look for a supplier to develop a long-term relationship with and one that can understand your needs. Discuss your requirements with the supplier and discuss any areas of special concern. If the supplier has a quality manager and prepares an inspection procedure for all products, those are good signs. More sophisticated suppliers will routinely utilize statistical methods to analyze process quality capabilities and machine limitations. Communicate with your supplier and identify critical product specifications. Discuss process limitations and how these might affect the product. With your input, your supplier can define effective process control procedures. Also, see our Quality Program webpage for more information.
Fordsell will be glad to provide customer references.
What about a cutoff burr?
Depending on the type of machine your part would be run on, it might or might not be an issue. However, in many cases allowing a cutoff nib on one end of the part will save you money.
What are the most important considerations when purchasing the product?
Work with a supplier that is committed to a long-term relationship and has the technical expertise matched with the right equipment to support your needs. Nothing will cause your delivery or quality to suffer more that selecting a supplier based solely on low bidding.
Note: Fordsell Machine extends its appreciation to Cox Manufacturing for the content on this page.