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Screw Machine Shops - Common Misconceptions

By Edited Sep 21, 2015 0 0

Don't be fooled into thinking that a machine shop can't compete with a screw machine shop for small threaded parts. Your required volumes dictate which kind of shop will be most competitive for your needs. The setup time required for screw machines is typically longer than what is required for gang tooled or turret lathes.

Cost - The cost of the setup is amortized across the volume of parts you need. You can expect the cost of the setup to be between $35 to $250 for most machine shops. The setup cost for screw machines can take up to 12 hours for older machines. With a shop rate of $110/hr. you will pay dearly for some setups. Low volumes, between 10 and 20,000 pieces can usually be more economical for the buyer who sends his requests for quotation to production machine shops instead of dedicated screw machine shop companies.

Volume - For volumes over 35,000 pieces multi-spindle screw machines become more competitive. However, even this can be deceiving. For example, consider the fixed costs associated with a screw machine shop using newer equipment. New machines can run up to $300,000 each. Shop rates can run up to $150/hour. On the other hand, consider the small shop using simple machines, such as gang tooled types. Machine costs can be as low as $30,000. Used machines, of relatively good quality, can be purchased for as little as $12,000. Untended shop rates for this type of shop can run as low as $10/hour in the USA. So, where is the advantage in running state-of-the-art multi-spindles for hundreds of thousands of parts compared to running 6 single spindle machines on the same part? Part complexity is usually king. Finding the right type of shop for your type of parts is much more important that trying to decide which type of machine will run your parts most economically.

Where To Find a Machine Shop - Try posting your job online on a manufacturers platform where vendors and buyers can post and bid on manufacturing jobs. Another option is to search directories and lists and try to contact manufacturers individually, although this can be time consuming.

Machine Types - Screw machines come in two basic types, single-spindle and multi-spindle. Single spindle, often Swiss type, are fast and versatile. Multi-spindles have the machining operations divided between the different spindles. In other words, each part is moved from one spindle to the next as it undergoes the cutting or forming operation set up at the different spindle positions. Once the part visits each location it is parted off and the cycle begins again. In theory, a 6 spindle multi can be six times faster than a single spindle screw machine. Newer multi-spindles are CNC controlled and much simpler to setup versus the older cam types. Even so, setting up a multi-spindle is no task for a novice. Setups are an order of magnitude more difficult for a multi-spindle. Bear in mind that multi-spindle machines are nowhere near as versatile as single spindle screw machines. Part simplicity, as well as volume, dictate which machine type is best for you.

Quality - A common mistake in sourcing small screw type parts is searching for the highest quality shop available. It is not politically correct to state that quality costs money - but it does, and it costs dearly. If your part tolerances are wide open, and most are, the quality issues you need to concern yourself with can be as simple as requiring your vendor to do part sampling and submitting that data along with each part shipment. Why pay for more quality than you need. As untended machining gains popularity, untended inspection has not kept pace. Hence the high cost for overkill inspection programs that are designed to lull most buyers into a false sense of security regarding the quality of their parts.

Materials - Screw machine shops can't exist without raw materials designed to permit long untended operation. The obvious materials best suited for screw machines are free cutting brass, aluminum screw machine stock (2011), free cutting steels (12L14 or the 11XX) types, 303 austenitic stainless steel. While other materials can be used, these are the backbone of the screw machine material list. The correct choice of materials is critical to keeping the manufacturing costs low. It is very common for customers to request 304 stainless for very precise parts, this is all but impossible.

Don't be afraid to talk with your machining vendor aboutrelative costs from material grade to grade. He usually knows them better than any design engineer who doesn't deal with them on a day-to-day basis. Certainly, the design engineer must require some of part attributes, but often, several materials with do. Once the design requirements are met don't be afraid to explore the options for easy machining materials. This is a powerful component of your final part price. Requests for Quotation - Most buyers rely on a handful of local shops located within driving distance. This can be a costly way of doing business. With UPS and FedEx covering the US within 5 working days, and the power of the internet to reach out to every shop in the country, why limit your search? There are several online locations to help with finding the lowest prices which meet your quality needs. One of the newest sites is http://www.contractauction.com. The service is free and incorporates a rating system to instill the trust you need in your newfound vendors.



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