Moulding Knife Breakage

by Russ Rego

This article will provide some safety tips, suggestions, causes and solutions on how to by-pass moulding knife damage and or breakage on industrial style moulders.

Industrial moulding knife damage occurs daily in our business, it is an unfortunate situation but it is inevitable; this is something that will materialize with all moulding machine operators eventually.

Everybody’s main concern should be the safety of the moulding machine operator and others around the moulding machinery; this reiterates that obligation, all moulding machine guards, hoods, and other safety devices should always be in there proper place and accurately functioning throughout the service life of the moulding machinery.

Below is an image of a damaged moulding knife we received for sharpening. It is obvious these crown-moulding knives hit multiple foreign objects in the lumber while in the milling process. We will refrain from speculating on how these moulding knives were broken, thankfully no one was injured.

Damaged & Broken Moulding Knives

We have compiled a list below of possible causes and solutions of items that might contribute to possible moulding knife fracture and breakage. Your safety and health are our primary concern, it is your duty and responsibility to abide by all safety practices set forth by your employer and the manufacturer of your moulder machinery.


  • Cause: Moulding knife encountered the bedplate, chip breakers and or fences while in operation.
  • Solution: Rotate moulding knives slowly by hand in reverse of normal rotation during setup. This procedure will confirm moulding knives will clear all obstacles they may encounter during milling operation. Perform a crash test on all chip breakers, chip deflectors, and pressure elements with a 2x4 during set up, adjust cutting circle if necessary.
  • Cause: Knots in the lumber may dislodge while milling, they became projectiles striking the moulding knives, which causes fracturing, and or breakage.
  • Solution: Possessing a small rubber or hard plastic mallet would allow the moulder operator to lightly tap the knot or knots to assure oneself that these knots in the lumber would proceed thru moulder milling and remain intact throughout the process.
  • Cause: Moulder operator halted the moulding spindle motors prior to the moulding machine’s feed system. Motor spindle brakes stop the moulding spindles very quickly, if moulder spindle brakes activate while the feed system is in operation it could be the origin of moulding knife breakage, some moulders have safety limits in place to avoid this action from occurring.
  • Solution: Incorporate systematical procedures for starting and stopping of the moulder spindles and feed systems, all moulder operators are required to obtain extensive training in these guidelines.
  • Cause: Moulding operator neglected to remove the lash or slop in the adjusting screws threaded rod from moulding spindles during set up procedures. The probability exists that when running a moulding profile where the moulding knives are close to the bedplate, while operating, slop or lash in top spindle adjusting screw threads will allow moulding knives to encounter bedplates resulting in breakage of the knives.
  • Solution: While setting up moulder, all final movements of moulder spindles should be toward the lumber except the top spindles, this action will assure the moulder operator that all lash in adjusting screw threads was eliminated. On top spindle, set knives horizontally during set up the top spindle cutter head, this procedure will eliminate the possibility of the moulding knives encountering bedplates when removing lash from top spindle adjusting screw.
  • Cause: Moulder operator started moulding machine with woodwork piece already in moulder, overlooking manual hand spinning heads in reverse of normal rotation prior to exclude any tension that might occur on work piece before starting.
  • Solution: If moulder was shut down with lumber still in machine, while in the off position, manually release spindle breaks if applicable, check spindle rotation by hand, in reverse of normal rotation to assure cutter head is capable to spin freely when spindle motor is activated.
  • Cause: The Moulder operator placed the moulding machine under workload before the machine had a chance to reach full rpm’s causing stress on moulding knives even breakage.
  • Solution: Most new style moulders that utilize flat belts on spindles and motor pulleys have soft start motors. Soft start is a two-stage start up procedure to achieve full rpms and keep the belts from burning or breaking during startup. Soft start motor stages are controlled by timers in the electrical cabinet, give the moulder about 15-20 seconds after starting, this time allows for the second timer to engage giving full rpm’s power to the motor, eliminating the opportunity for moulding knife breakage.
  • Cause: Improper moulding machine set up techniques; fences, chip breakers, and guides tightly adjusted, feed system not properly fine-tuned for thickness of lumber, this triggers feeding issues. Whacking the ends of boards against one another to get the lumber to advance through moulder feed system will increase probability of breaking moulding knives quickly.
  • Solution: A firm setup is fine, the tighter the setup the more friction and increased pressure on the feed system, the feed system should be set at the finish thickness size. Proper training will eliminate most feeding problems for moulding machine operators.
  • Cause: Improper cooling of moulding knife steel after cutting and while grinding can cause fracturing of the tool leading to breaking upon use of moulding knife. Please refer to our article on aligning moulder knives.
  • Solution: Do not submerge moulding knives in cold water after cutting to length; let moulding knife blanks cool at room temperature, the sudden temperature change can cause the knife to fracture without you even realizing it. Use enough coolant from the face (smooth side) of the moulding knife while profile knife grinding. Burn marks in the knife steel indicate burning of the knife steel, which makes the moulding knife fragile, the more brittle the knife steel the easier it is to break.
  • Cause: Dull moulding knives, the spindle motor will generally always make the knife cut, but if the knife is dull it will build up heat, initiate finish issues, and eventually fracture leading to moulding machine damage.
  • Solution: This is an easy fix; keep your moulding knives sharp!
  • Cause: Improper steel size, we always supply you with the correct steel sizes on all our moulding profile cad drawings, if you routinely use a bigger size steel to make it last longer your only increasing the size of your cutting circle, this can lead to knife breakage.
  • Solution: Please refer to our article on cut steel size for moulding knife blanks this will detour the possibility of moulding knife breakage.
Casing Moulding Knives

We have provided you with some very important information about moulding knives and some reasons for failure during moulding manufacturing. Although we may not have covered all the reasons for moulding knife failure, these solutions will solve many issues. A highly skilled, experienced moulder operator with proper formal training can significantly reduce the possibility of the concerns expressed in this article. We also recommend reading our article on production tips for moulder operators to eliminate concerns not discussed in this article.

If we can be of any further support, concerning moulding knives or any other topic in wood mouldings please, contact us.