What to Know Before Buying a Used CNC Machine

In the manufacturing industry, a Computer Numerical Control (CNC) machine is extensively used because of its speed, accuracy, and flexibility in cutting or shaping a desired product. However, these machines are expensive, that’s why it is more convenient for first-time users or small business owners to purchase a used one. With this, their initial equipment investment cost will be reduced while their productivity and profitability will increase. Although purchasing a used CNC machine over a new one is a wise decision, it also takes a lot of research and analysis in terms of its working condition, maintenance, one’s production needs, etc. To elaborate on these, here are some of the things you need to consider before buying a used CNC machine:

• A list of requirements for your production needs – It is very important to know your production needs first before purchasing a used CNC machine. Make a list of requirements with these considerations:
  • What specific CNC machine do you need based on the purpose you want to use it for?
  • Is this machine capable of producing the output that you need in a certain timeframe?
  • Is fast production speed crucial in your business?
  • What is the size or what should be the size of your workspace?
  • How much money are you willing to spend for this machine?

  You should also be aware of the differences between several models and their technical specs. Once you have a clear idea of what you really need, you may proceed on buying the machine which is capable of meeting those needs.

• Condition of the machine – Finding out the condition of the machine on your own is not an easy task. You might want to seek help from a machine inspector or an experienced engineer about the models that are long-lasting and suitable for your machine shop. Try to get a hold on as much documentation as you can. Ask the sellers for the cleaning and maintenance so that you will know how well the previous owner took care of it. Ask for the any proof of services performed on the machine, how long it has been used since its purchase and any damages it may have obtained. Try to get the original bills, receipts, manuals, and repair history from the previous owner. Look also for the data sheet on the back panel of the machine as it contains an overview of the basic functions and options which the machine came with. One notable information you can find on the data sheet is the serial number which you can use to contact the original equipment manufacturer (OEM) for more information regarding the machine.

  When it comes to the axes of the machine, ensure that the transition around these axes are smooth and there is no oscillation after putting the engine to a stop. If there is still a movement around the ballscrew at a stopped position or any abnormal noise when the machine is operated at a high revolution per minute (rpm), it usually indicates a damaged ballscrew, bearing, worn ways, or a damaged turkite in some cases. High pitch humming of the servo motors indicates some problems as well.

  Another way to check the condition of the machine is to examine the full length of the groove for discoloration and pits browning. When you sense a pitted groove, it is better to remove the entire axis to have a better look at the turkite. Pitted linear guides and grooves can be caused by lack of lubrication, and if not fixed or maintained properly, this may lead to the complete damage of the turkite.

  Conduct a mechanical inspection if possible, so you can also check the spindle movement, tool alignment, abnormal noises, guides, and others.

• Machine hours – Another factor to be considered in purchasing a used CNC machine is its machine hours. However, not all logged hours have the same impact on the machine so ask the seller about the spindle time, cutting time, and power-on hours. The spindle time is used to determine how long the spindle has been doing a similar job for the previous owner of the machine.

  While the cutting time is the number of hours the machine spent working on materials to determine what work each part has gone through or if there are parts that should be replaced. Finally, the power-on hours determine how long the machine has been turned on, including the time when it was not used for cutting or for both pre and post production. It would be better to ask the machine inspector or engineer if the power-on hours have impacted the machine’s longevity.

  Be vigilant about sellers who can alter or reset the machine hours and to address this, take a closer look at the machine and compare it to the spindle/machine time you are seeing.

• Tolerance of the machine – The tolerance of the machine refers to the acceptable deviation of the physical dimensions of a manufactured piece. Deviation is a part of the manufacturing process caused by external factors which can easily affect the CNC machine such as temperature, vibration, and humidity. Although it is normal and cannot be entirely avoided in the manufacturing process, too much deviation makes the pieces unusable.

  It is essential to be familiar regarding measurement when talking about tolerance. The unit of measurement used for it is thousandths of an inch, known as ‘thou’, which is expressed in numerical as 0.001in (ex. two thou = 0.002in). The most accepted standard for the tolerance level of a CNC machine is usually around a deviation of 0.001in-0.002in which means that the closer the tolerance is to the ideal measurement, the tighter or usable the parts are. At some point, all used CNC machines will require additional maintenance but it is best to maintain an accurate tolerance level at all times.

• Machining Center: Vertical or Horizontal – When purchasing a used CNC machining center, there are two types to consider and these are the Vertical Machining Center (VMC) and Horizontal Machining Center (HMC). To compare these two, here are some of the factors to remember:

  For the price, the VMC is cheaper compared to HMC which is why most small and mid-sized businesses prefer VMC. On average, VMC costs $115,000 while HMC costs around $375,000.

  For the productivity, HMC is more advantageous than VMC because HMC use its 4th axis 85% of the time whereas VMC only uses its 4th axis 25% of the time. In other words, HMC does the job 1/3 of the time than VMC.

  For the workspace, VMC takes less space than HMC.

  For the availability, VMCs are more common in the market which makes it easier to find operators, repairers, spare tools, and parts of VMCs than HMCs. However, HMCs don’t require spare tools so it saves more money in this matter.

  Whatever choice you make, evaluate first which is suited for your production needs.

• Machine updates – The update of the machine is also essential in buying a used CNC machine to be able to find out if it has been upgraded to suit your needs or if it can be upgraded at all. It would be better if you can find a trusted supplier where you can get high quality replacement components and upgrade parts whenever you need.

  Like any other machines or things in general, a used CNC machine can last for many years when properly cared for. So when you are planning to buy one, make sure to not only buy a well-maintained machine but also take care of it for maximum productivity and profitability.

While investing in a used CNC machine can be risky, being well informed about what to look for can help ease your decision making. If you’re ready to purchase a used CNC machine, make us at S&M Machinery your first call when buying surplus CNC Machinery! S&M Machinery is the leading independent machinery dealer of late-model surplus used CNC industrial machine tools in the United States With over 300 machine tools.