CNC Spindle Repair, Rebuild, or Replace?

Failed CNC spindles are almost always worth rebuilding. A standard rebuild runs about 30% of the cost of a new spindle, takes about three weeks, and carries a one-year warranty. Replacement makes sense only when the spindle is beyond rework or the OEM part is cheap and in stock. At MZI Precision, disassembly, inspection, failure analysis and the quote are free, so the decision is made on facts, not a guess.

Repair, rebuild, or replace: what the words mean

Repair is fixing one thing: a seal, a drawbar, a sensor. It is rare on a spindle that has failed to cut, because by the time it fails there is more than one thing wrong.

Rebuild is the standard job. The spindle is disassembled. Every part is cleaned and inspected. Bearings, seals and O-rings are replaced, and any rework of components is repaired. The spindle is assembled, run, tested and balanced. That is what “standard rebuild” means every time we use it.

Replace is a new spindle from the machine builder, or we can reverse engineer your spindle and offer a new one. For older machines it is rare for the OEM to have a replacement.

When to rebuild

• The spindle sounds different, makes more noise, runs hot, sounds rough, or vibrates.
• It crashed, or broke a tool, and has not run right since.
• It will not hold tolerance on the part you are machining.
• Runout is up and the part shows chatter.
• Tools will not seat or will not release.
• It lost orientation or the tool changer jams.

Stop running it. A spindle run after a crash does more damage to itself every hour.

When to replace

• The shaft or housing is beyond rework, and we will tell you if it is.
• A new OEM spindle is available, in stock, and the cost gap between new and a heavy-rework rebuild is small.
• You are replacing the machine.

What a rebuild costs

A standard rebuild on a typical CAT 40 milling spindle, belt or gear driven, typically runs around $7,500. That is about 30% of new.

Heavy rework raises the cost. Chrome plating and grinding a taper, a bearing journal or a housing bore adds to it. So does a drawbar rebuild or new parts. Heavy rework can take the same spindle to $12,000 to $13,000. That is 50 to 60% of new. Two things still tip the decision toward rebuilding. For older machines an OEM replacement is often not available. And a spindle can be rebuilt again and again.

Integral motor spindles typically cost more than belt or gear driven. A complex spindle may carry a minimum inspection charge, told to you before any work starts.

What a rebuild takes

Three weeks, fifteen business days, standard. A standard rebuild is disassembled within the first two or three days of arrival, in the order received. Rush service is disassembled the day it arrives and can be as fast as three to four days, for a 25% surcharge. For a production machine that is down, two weeks of your machine running properly usually pays for the surcharge.

A spindle rebuilt in four or five days is possible if all it needs is new bearings. Done properly, it takes time, and when all the details are reviewed it is ready to ship.

How we decide: failure analysis

Failure analysis is like reading DNA. When the spindle is disassembled, the cause shows in the bearings, the tool holder, the tool, and what is inside the housing: coolant, oil, chips. We document all of our findings, and you can see the detail in our quote.

Tell us everything you know about what happened. It does not change the quote, because the spindle tells the story. It gives us insight into the failure and points us to any issues with the spindle.

Most failures are not the brand. They are coolant or cutting oil reaching the bearings, a broken tool, a crash, or a spindle run long past the first warning. Many arrive long past it. We have cut bearings off shafts where the balls had deformed and welded to the steel.

The two mistakes we see

Rebuilding it in-house. A spindle looks like a shaft and some bearings. Spindles that were started in-house often arrive as a basket of parts. For example, a shaft gouged by an angle grinder from someone trying to remove a bearing adds rework that did not exist before. If you are not sure, call before you pull it.

A shortcut rebuild. Not every rebuild is equal. Some spindles carry thirty to forty different O-rings in different sizes; matching them alone takes two hours, and none are reused. Bearing preload is one of the most precise things done to a spindle, and most people have never heard of it. Bearings have to match the spindle’s speed, size and preload arrangement. Ceramic hybrid bearings, which let spindles run 25 to 30% faster and cooler, only help if they are the correct ones. A rushed rebuild shows up in the part as chatter. It shows up as a tool that will not seat, an improper drawbar, and a spindle that does not last.

Warranty

One year from delivery of the rebuilt spindle. It covers the rebuild: the bearings, the seals, the assembly, the balance. Typically, if we did something wrong mechanically, it shows up in days or weeks, and we make it right.

One thing it does not cover. A spindle that comes back with the grease washed out by coolant is an operating condition, not a rebuild fault. It is like water in a gas tank. We let customers know this up front so there is no surprise at month eleven.

No drawings, which is the point

We are an independent rebuilder. Machine builders do not send us drawings. In a given year, five or six spindles may arrive with one. The rest we rebuild on knowledge, experience and research. Experience working with precision spindles is of great value, one that takes time and is not easily found.

What to send

• The spindle, with every rotating part: couplings, pulleys, the drawbar, and actuator parts.
• A good tool holder with its retention knob (milling spindles; not needed for lathe or grinding spindles).
• The tool retention type: CAT 40, CAT 50, HSK or Capto.
• What happened, and when it started.
• The repair request form, with the spindle or scanned ahead.

Ship to 5572 Buckingham Drive, Huntington Beach, CA 92649. You get a notice when it arrives, the spindle is tracked by serial number through the shop, and disassembly is photographed and logged.

Questions shops ask

Is it cheaper to rebuild a spindle or replace it?

A standard rebuild is about 30% of new. With heavy rework it is 50 to 60% of new. Rebuilding also works when no new spindle exists for the machine.

How long does a spindle rebuild take?

Three weeks, fifteen business days. Rush service can be as fast as three to four days, for a 25% surcharge.

What does a standard rebuild include?

Disassembly, every part cleaned and inspected, bearings, seals and O-rings replaced, assembly, run, test and balance. Anything beyond that, such as chrome and grind or a drawbar rebuild, is quoted separately before it starts.

Do I pay for the inspection?

No. Disassembly, inspection, failure analysis and the quote are free until you approve the rebuild with a purchase order. A complex spindle may carry a minimum inspection charge, told to you before any work starts.