A Guide for Purchasing Used CNC Machinery

When starting a business, you may not have the capital to purchase your own stock of CNC machines because the technology can be pretty expensive up-front. If you’re interested in creating products out of metal, plastic, or other materials, consider getting a used machine instead as they are cost-effective and often work just as well as a new machine.

What to Consider Before Purchasing a CNC Machine

Service

Before you purchase a used CNC machine, you should research the ins and outs of the device you’re acquiring. You’ll need to understand the difference between each CNC machine type. The following explains the 6 types of CNC machines and their functions:

• Milling: A popular type of CNC machining, milling machines use rotary cutters to manipulate materials. You can program said machines to shape necessary depth, cutting direction, and angle.
• Lathes: Lathes are useful for creating highly precise spherical and conical parts. This machine will use turning centers that shave access material while they rotate on a spindle at high speeds. Check out the guide of mini metal lathe here.
• Routers: Routers produce larger parts that are useful for cutting woods, plastics, and pieces of metal. Suitable for cutting intricate shapes.
• Grinders: Grinders use a rotary wheel that abrades the material by grinding.
• Plasma Cutters: These machines are similar to routers in setup and size, but instead, a laser flies above the table with a plasma torch. 
• Electric Discharge: EDM machines use electrical starts or releases to make shapes in specific materials. The material is removed from the material by the use of repeating electrical releases between two anodes. 

Once you know which machine you need to purchase, you’ll also need to understand how to service the device if repairs are required. As an alternative, you could buy your needs from a reputable dealer of used CNC machines who can provide expert service and guarantees.

Updates

When purchasing an older CNC machine, you’ll need to know if you need to upgrade the tools on the machine itself. Sometimes, it isn’t possible to update your machine because specific tools are discontinued or are difficult to find. You should also ask if you can upgrade your machine to use more advanced controls that are necessary to perform complex machining.

You will also need to consider CAD design. CNC machining begins with a process of creating a 2D vector or 3D solid part CAD design that the machine will process and make into a solid object. Your machine cannot simply make an item; it needs a CAD software to initiate the design process and to move the components within the CNC machine.

If you can’t use CAD software with your CNC machine, the technology is useless. Always research or ask a reputable supplier if you can use your machine for its intended purpose.

Quality

One of the positives of buying a new CNC machine is knowing it actually works – or does it? The problem with new technology is that it might not have been tested thoroughly, or the older model may have executed a process better. To ensure you purchase a high-quality machine, you’ll need to research its drawbacks and advantages first.

Some models will stand the test of time, while others will have a host of problems. I would recommend asking an expert or an experienced engineer who has worked with your model to better understand the machine you want to purchase.

Quick Inspection you can Perform

While the above guide will help you find the machine, it can’t always determine whether the machine works. You can perform an inspection, or you can ask the seller to perform this inspection themselves to determine the device’s overall quality.

Step 1: Hold your hand on all axes (X, Y, Z) and move them

Grab the axes firmly, put it in rapid mode (50 percent), and move them left, right, upwards, and downwards. See if they transition smoothly, and check if oscillation occurs when the axes stop. Grab the ball screw (if possible) at a stop position to feel if they are still moving. Listen for abnormal noises like growls or scraping because this could mean the bearings, ball screws, or other parts are loose or worn out. Listen to the servo motors for high pitch humming specifically.

Step 2: Operate the spindle

When the shop or location you’re purchasing the CNC machine from is quiet, run the spindle on multiple RPM speeds. Perform step 1 again for every single RPM.

Step 3: Put a rapid program in the machine

Ask the machine operator to put a rapid program in the machine and run full stroke in all axes. Again, listen to the machine carefully for odd noises, especially in the ball screws and bearings.

Step 4: Inspect linear guides or ways

A machine that wasn’t adequately lubricated or doesn’t contain Turcite (a self-lubricating material) will likely have pits or grooves throughout the parts and base. It can be difficult to access some areas that contain the Turcite, so you may have to remove the axis for a better look. Look for discoloration or browning on the grooves along the whole length of the linear guide.

Step 5: Troubleshoot the tool change and pallet change

While troubleshooting the tool change and pallet change, listen for abnormal noises like screeching or growling. Look at the tool change fingers for alignment problems and wear.

Step 6: Cut material with the machine

We already discussed what tool you might need for different purposes, but you also need to see the CNC machine in action to determine if it’s capable of cutting through certain materials. While the machine cuts, look at how efficient the tools work to accomplish this task. If the other steps passed your tests, there likely wouldn’t be an issue unless the machine backs up and causes the coolant to drain into the switches, motor, guides, or bearings. 

Step 7 (Lathe Machine Only): Check the holder for distortion

Check the mounting surface of the holder by running an indicator to see how straight they are. If the holder is out of alignment, you may notice a gradual decline or incline. The important thing here is that the decline/incline is gradual with no high spots and that all turret stations are the same. A distorted pocket will have difficulty keeping the drill center.