Tips for Designers and Engineers
Turned Parts and Screw Machine Products
Question: What encompasses turned products or screw machine
Answer: 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
Question: What would qualify as a "CNC turned product"
versus a "screw machine product?"
Answer: 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.
Question: Are "screw machines products" primarily screws?
Answer: 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
Question: 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?
Answer: 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
Scheduling and procurement systems optimized for job shop and
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.
Question: What consideration should be given in material selection for
Answer: 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
Question: How does part configuration impact cost?
Answer: 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.
Question: What tolerances are achievable and practical?
Answer: 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
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.
Question: Should I request a first article?
Answer: 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.
Question: How do I assure my supplier provides the quality I need?
Answer: 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
Program webpage for more information.
Fordsell will be glad to provide customer references.
Question: What about a cutoff burr?
Answer: 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.
Question: What are the most important considerations when purchasing
Answer: 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
Fordsell Machine Products Company ©2013