Used CNC routers for sale: how to read a machine before you wire money

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  Key takeaways
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  What a router actually is, mechanically
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  Why spindle hours matter more than the build year
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  Backlash, squareness and the tests you can run on site
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  The controller decides whether the machine still has a future
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  As-is versus rebuilt: where each one makes sense
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  Which used router fits which job

A CNC router is not a small machining center. On most gantry routers the work stays still and the bridge moves, so the mass in motion is the gantry, not the table. That single fact explains most of what you feel when you push the head by hand: flex shows up at the gantry legs, not at the part.

The cutting loop is short. Ballscrew or rack turns rotary motion into linear travel, the linear guide constrains it, the spindle holds runout to a few microns, and the controller closes position feedback. Every one of those four elements wears at a different rate.

Rack-and-pinion drives dominate on long-travel routers because a ballscrew over 3,000 mm whips at speed. Racks wear at the pinion, not along the tooth flank, so wear is usually uneven and shows as localized pitch error.

Ballscrews on smaller routers hold better accuracy but load the thrust bearings. If a machine has been run hard in one pocket of the table, the screw is worn there and backlash will read clean everywhere else. Test the middle, not the corner.

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  Moving massGantry weight sets the acceleration ceiling you can actually use.
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  Drive typeRack for long travel, ballscrew for short and tight.
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  Spindle interfaceHSK, ISO 30 or ER collet changes what tooling you can inherit.
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  FeedbackEncoders on servos, or scales on the axis. Scales age better.

Spindle bearings have a finite life, and that life is consumed by speed and load, not by the calendar. A machine built in 2013 that ran light wood panels at 12,000 rpm may still hold ±0.02 mm. A 2020 machine that ran aluminum at 24,000 rpm for three shifts may already be at the end of its bearing preload.

Ask for the hour meter and the maintenance log together. If the log shows spindle replacement, the hour count since replacement is what matters. If there is no log, treat the spindle as due for rebuild and price it in.

Runout is the number to trust. Indicate the taper with a dial test indicator at 50 mm from the gauge line. On a healthy router spindle you should see under 0.010 mm. Over 0.020 mm and finish quality will drift regardless of how well the machine moves.

Thermal growth is the other half. Run the spindle at working speed for 30 minutes and re-check. A spindle that moves 0.03 mm when hot will not hold a tolerance on a long cycle, and no amount of tool compensation fixes a moving zero.

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  Under 0.010 mmHealthy taper. Plan normal tooling changes.
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  0.010–0.020 mmUsable for roughing and wood. Budget a rebuild.
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  Over 0.020 mmRebuild or replace before you quote tight work.

Backlash is the gap between commanded and actual motion when the axis reverses. It hides in climb versus conventional cuts and in corners that come out rounded. The test is simple: mount a dial indicator against the spindle nose, jog the axis 0.10 mm in one direction, then reverse in 0.01 mm steps and record where the needle moves.

Repeat that at three positions per axis: near each end and in the middle. A router with 0.05 mm backlash in the middle and 0.01 mm at the ends has a worn screw or a worn rack section, and that is a different repair than a loose thrust bearing.

Squareness matters more than most buyers expect. Cut a 300 mm square and measure the diagonals. A difference over 0.10 mm means the gantry is racked, and that error scales with part size. It cannot be tuned out with software offset.

Check the rails for brinelling, which looks like a line of dents where the balls sat under load. It happens when a heavy gantry parks in one spot for months. Brinelled rails feel notchy under slow hand feed and will never give a smooth finish.

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  Backlash sweepThree points per axis. Look for variation, not just the number.
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  Diagonal test300 mm square, diagonals within 0.10 mm.
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  Rail surfaceA bright worn stripe means the block is riding off center.
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  Table flatnessCheck with a straight edge and feeler gauge across the full bed.

Iron can be scraped and resquared. A dead controller with no post-processor support cannot. Before you buy, identify the exact control model and check that the OEM still sells boards, drives and parameters. Call the distributor and ask for a spare drive lead time.

Then check the CAM side. Your toolpaths must post to that control. If the post exists and is maintained, you are fine. If the only post is a decade-old file that nobody updates, every new feature in your CAM software becomes a manual edit.

Parameter backups are worth more than they look. Ask for a full parameter and PLC archive on a USB stick. Without it, a battery failure or a drive swap can turn a working router into a multi-week recovery project.

Network and file transfer also age. Some controllers still rely on floppy or serial transfer. That is workable, but it slows setup and adds a failure point. Factor it into your cycle time, not just your convenience.

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  Spare drive lead timeUnder 2 weeks is workable. Over 8 weeks is a risk.
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  CAM postConfirm your software vendor still maintains it.
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  Parameter archiveGet it on media you can read today.
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  License transferCheck whether the control software license moves with the machine.

As-is suits a shop that knows exactly what it will cut. If your work is nested panels, sign letters or foam tooling at ±0.1 mm, a used router with tired rails and a serviceable spindle will earn its keep for years. You are buying travel and table area, not precision.

A rebuilt machine is the right call when the part tolerance drives the purchase. Rebuilding means new rails or reconditioned blocks, a reground or replaced ballscrew or rack, spindle rebuild, and a laser alignment of the gantry. That work is measurable and repeatable.

The arithmetic is straightforward. Add the purchase price, freight, rigging, spindle rebuild, rail replacement and alignment. If that total lands under 60 percent of a comparable new machine and the control still has support, the rebuild usually wins. Above that, the gap closes fast.

One more boundary: if your parts need more than three interpolated axes, a used three-axis router will never become a five-axis machine. Do not buy travel and hope to buy capability later.

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  Buy as-isLoose tolerance, large table, wood and plastics.
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  Buy to rebuildTight tolerance, known geometry, supported control.
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  Walk awayDead control, brinelled rails, no parameter archive.