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How To Purchase a Milling Machine, Part 1

Written by: GRS Edited By: JMD

Purchasing a machine tool in today's market can be an overwhelming task. The modern machine tool market is quite diverse. Although the philosophy described here holds true for any machine purchase it is aimed more closely at "Modern CNC Machining Centers". Even limiting the scope there are still many machines available from the low end of technology to the high end. At one end you have work horses such as a CNC equipped knee mill, although not technically classified as a "High End CNC Machining Centers", they are in many aspects impressive, all the way up to the ultra precision high speed linear driven 5 axis machines with robotic part changers. Over the years I have been involved in many machine sales that have been, well lets just say less than perfect. There are many things that can go wrong in a sale, but there are a few issues that tend to
crop up more than others and can easily be avoided.

There are numerous considerations to take into account when pursuing a capital equipment purchase. Some are quite technical while others are financial and even human nature. The points we will be discussing in this article fall primarily into one common heading. "Do your Homework".

"DO YOUR HOMEWORK!"

The process of purchasing a machine tool is really no different in approach than any other major purchase, such as a house or vehicle. The most important thing you should do is step back; take a realistic look at what you know and more importantly, what you do not know. This 1st step is the same with any major purchase either in business or in your personal life.

There are several key areas that a large number of consumers fail to understand when heading into a machine tool purchase.

1. What Do You Really Need / "Avoiding the VEG-O-MATIC Syndrome"
2. Understand the "Buzz Words" / Hype
3. Understanding Machining Methods
4. Understanding Accuracy / "Machine Accuracy" vs. "Machining Accuracy"

The topic of the first segment of this article will be:

What Do You Really Need or Avoiding the VEG-O-MATIC Syndrome

This is the first step and probably the most difficult for most people. There are two major points where the average customer fails at the start of the machine tool purchase. The first is correctly analyzing their needs and the second is conveying them to the salesman.

1. Know Your Current Process and Your Needs!!!!
a. (not to be rude here but if you think you do, you probably do not)
2. Part Information
a. Size
b. Weight
c. Complexity [Mill / Drill to Complex Mold Cavities]
d. Part Material [Aluminum to 62 Rc Tool Steel]
e. Accuracy Requirements – Positional / Repeatable
3. Part Manufacturing Requirements
a. Work Area [ Part size or Multiple Part Set Up]
b. Spindle RPM and Horsepower
c. Max/Min Tool sizes (length and diameter)
d. Type of tool holders
e. Coolant Types
f. Axis Control 3, 3+1, 4, 3+2, 5
g. Cycle Time / Production Costs (current and desired)
h. Automation Requirements
i. Programming Requirements [Family of Parts Type or Special Routines]
4. Financial
a. Price Range
b. Service Availability
c. Support

Do not lose focus here. This is the number one area where problems are created. You must have the correct information and understand this information before continuing. Most of the points noted above are quite straightforward. Price range, you have a budget. Part size is not a complicated question. Other questions however, such as Cycle Time, Production Costs, or even Tolerances required, can be quite misleading. It has been a rather common occurrence to be told a cycle time at the start of a project only to find out later on that in reality the cycle time is 2 to 10 times the original estimate. The same goes for tolerance and finish requirements. We will delve more into these topics later.

Things become somewhat easier if the parts you plan to produce fall into a specific narrow category of requirements as in long-term production of a single or family of parts. It is when you are looking for a machine that can produce any part that comes along that the real problems begin. You must make a selection based on the majority of your needs not always the extremes. The 80/20 rule applies here. You can probably find an excellent solution for 80% of your needs. Trying to fit to 100% will force compromises that simply waste time and money over the long run.

The next step is to be able to convey these needs to the salesman. This step is not always as easy as it sounds. Difficulties will arise because the salesman may or may not be knowledgeable in your specific area of manufacturing, and quite frankly for some salesman the primary objective is to get you to buy their product and the secondary objective is satisfying your manufacturing needs. The customer often makes the mistake of thinking that the machine tool representatives are experts at producing every possible part. The truth is each industry and in some cases individual manufacturers have developed processes and knowledge about their products that the machine tool rep cannot possibly know. I once called a customer in regards to a radius that was not specified on the print. I was told that this was an industry standard and did not need to be on the print. He actually scolded me for not knowing this. I had to inform him that I was not in his industry and this was the first of this type of part that I had ever programmed. I explained that my expertise was in how best to utilize the milling machine. There will be times when the applications group does have expertise in your particular field; however you still may have preferences and requirements that they need to know. You (the customer) must have a full understanding of your needs and processes and be able to convey them to the salesman and other support staff. Over the years I have seen the wrong machine purchased for a myriad of reasons. The salesman is someone's relative, price alone (finance dept decision), purchaser not knowledgeable of part production requirements, and so on. Keep in mind that working with the machine tool company is the best approach. Trying to place unrealistic requirements on a project or not sharing current production information can make
this process a total waste of time. The more information all parties involved have, the better the resulting sale will be. Considering everything that can be done wrong, there is one scenario that I have come across way too often over the years and it is a result of a lack of knowledge and focus leading to the following:

The "Veg-O-Matic" Syndrome

The scenario plays out like this: A customer comes in looking to buy a machine to produce his widgets. The widgets are 6" square, aluminum, of complex 3D form, with a tolerance on one depth dimension to another of ±0.0002". The majority of the tools required for the part are smaller than 0.375" or 10mm. The seller shows him a small to mid size machine with a working area of about a 12" cube. This machine has a 30K rpm spindle with HSK 40 tooling, and very high axis acceleration values. Machine accuracy and thermal stability are more than adequate to produce the customer's part with a minimum amount of effort. Fine! Done! The deal should be complete. But it does not end here. The customer then says, well, I have a few parts now and then that require a
full 12" of travel in the X axis and this machine is a bit limiting for those parts. OK, the seller now shows him a bit larger machine, say 20" XY travel with 14" in Z. However the axis response is a bit less and the spindle is only 20K rpm with HSK 50 tooling. Always keep in mind, for a specific category of machine, the larger the machine, the less accurate and slower it will be (!generally!). Still, we are in the realm of ok. It will do all the parts the customer is concerned about but he will need to slow down the feed rates on some operations due to the slower spindle, work a bit harder to attain his accuracy requirements, and his cycle times will increase due to the lower axis acceleration value. A compromise yes it is, but still reasonable depending on production needs. Oh yes, it is also 20% more expensive. Fine! Again, deal should be completed. But no, we continue! Now the customer states that there is this one part that they do two of a year that requires a considerable amount of
horsepower and larger tools than this spindle can handle. The seller now scrambles to a model with a high horse power spindle that will accommodate the larger tools. This machine has much less axis acceleration, only 12K rpm, with HSK 63 tooling, and a work area of 32" * 20" * 18". Don't forget, this machine is twice the cost of
the first machine. At this point the customer walks away because the machines from this manufacturer are overly priced and too large to produce the 6" Widget, and do not have the acceleration rates to machine them in a timely manner. How did we get here? What went wrong?

"The one machine that does it all and does it all well does not exist." "Period"

What went wrong? The salesman failed because his only interest was to sell something when it should have been to provide the customer with the product best suited to the customer's real needs. It is his job to help the customer stay focused. The customer failed because he went into this with a specific, realistic list of
requirements and did not stick to them (lost focus). The customer is looking to purchase a machine that will do everything and do it all well. Oh yes, also for the lowest price. Well, for this mindset, if you open your tool box and there is only one tool in it, you must use it for everything. As it turns out it is a screwdriver! Have you ever used a screw driver for a chisel or a pry bar? How well did it work? How long did it last? Is it even usable for its original intended purpose now? If you purchase that nice new BMW would you haul plywood home in it? If you pay top dollar for a fast and accurate machine why would you abuse it with heavy hogging cuts?
Conversely if you buy a machine for brute force, don't expect to machine complex cavities to close tolerances. Be realistic in specifying you're actual requirements. This not only holds true for the sizing of the machine but also for the type of machine, horsepower, and spindle specifications. If you are looking for a machine that is very fast and accurate, you will be compromising on roughing operations or even moving them to another machine. If you insist on heavy hogging this machine will not last. If your needs are high metal removal rates, then you will compromise on the speed of complex contours and accuracy.

Pick one. You can not have both.

This however does not mean that the middle of the road machine, i.e. medium sized, with decent horse power and fair axis acceleration values is not a legitimate choice. It is. But this needs to be the objective and stay focused also.

Written by: GRS Edited By: JMD

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