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There are several factors that must be considered when designing a finger joint knife to produce a specific length joint.
The biggest factor affecting joint length is runout. Runout is defined as the amount of procession in the knife tips as they cut through the wood. The path cut by the knife tips will be the determining factor in joint length. Any deficiencies in the spindle, spindle bearings, hydraulic sleeve, and head perpendicularity will have an effect on joint length. Prior to designing the knife, the runout must be known, so it can be incorporated into the design. If the runout exceeds the amount designed into the knife, the joint will be shorter than desired. If the runout is less than designed into the knife, the joint will be longer than desired. It is impractical to design the “perfect” knife for every machine. Aceco’s engineering team designs finger joint knives with “normal” runout taken into consideration.
Each of the below items will have an effect on runout, thereby changing the joint length.
Machine spindle diameter:
The spindle must be measured with a micrometer for size and roundness. Measure the top, middle and bottom of each spindle at 90-degree intervals in the area where the head will be installed. The spindle should be round within 0.0005” to 0.0010” of nominal diameter.
Spindle bearing play:
If the spindle is remotely mounted, remove any drive belts before attempting to measure bearing play.
Install a dial indicator (fingerstyle indicator is preferable) to the highest point on the spindle. Zero the indicator with the spindle in the relaxed position. Gently push the spindle away from the indicator and note indicator reading, then gently pull the spindle toward the indicator and note the reading. Rotate the spindle 90 degrees, and repeat the process. Re-position the indicator at the flange on the lower part of the spindle. Move the spindle to check for play and note the reading. The variance should not exceed 0.0010”.
Finger Joint head sleeve:
Measure the internal bore of the hydraulic sleeve with a dial bore gauge. The middle of the bore and both ends should be measured at 90-degree intervals for roundness and diameter. The sleeve should be round within 0.0005” of an inch and the bore within 0.0010” of nominal diameter. This measurement should be executed with the relief valve open.
Finger Joint head & knives:
When measuring for runout, the head (with knives installed and posted to the correct position) is mounted on the spindle and pressurized to 450 bars. This measurement will be all-inclusive of the spindle, spindle bearings, head-sleeve perpendicularity, and knife stack accumulation. A fingerstyle indicator is positioned to measure one knife stack as close to the tip of the knife as possible. This will reflect the maximum indicator reading, as the tip of the knife produces the root of the joint. The head is then rotated under the indicator in a direction that will allow the indicator finger to “walk-up” onto the bevel and read the highest point at the cutting face of the knife. Record each indicator reading. Typically, the readings will slowly increase or decrease from zero to either a low or high point and slowly return back to zero at the starting point. If one or two knife stacks spike the reading – excessively low or high – the individual stack is the problem. A reading of less than .001″ is desirable on joints less than 1/4″.
NOTE:
The finger joint head & knives cannot be accurately measured if the spindle, spindle bearings and head sleeve are not in good condition. Typically, any out of range measurements of the spindle, spindle bearings and head sleeve are the result of wear from years of usage and will continue to deteriorate as use of the tooling continues. Any measurements outside of the parameters outlined above must be corrected prior to measuring the head and knives to achieve an accurate measurement.
When measuring spindle, spindle bearings, head sleeve, and head runout, please take into consideration all possible combinations. This will be necessary to maintain joint length consistency among all head and machine combinations.
Designing a knife to compensate for worn components will only provide a temporary solution until additional wear occurs, then joint problems will arise.
Burrs on the knife faces from the grinding operation can contribute to a shorter than desired joint length after sharpening. Although a .001″ burr on either side of the face may seem small, it will increase the root of the joint by .002″ until the burr wears off. Forcefully removing the burr will most likely dull the knives. The object in the grinding operation is to not generate a burr. Best practices call for keeping the grinding wheel clean, using conservative feed rates and spark out passes.
Joints 1/4″ (6.35mm) and shorter are more sensitive to runout than longer joints.
A COMPARISON OF RUNOUT VERSUS JOINT LENGTH VERSUS GLUE AREA:
| Part # | Runout | Joint Length (inch) | Joint Length (mm) | % Length Reduction | Effective Glue Area | % Glue Reduction |
| C16-048 | .0010″ | 0.378 | 9.60 | Baseline | 4.943 | Baseline |
| .0020″ | 0.368 | 9.35 | 2.72% | 4.803 | (2.91%) | |
| .0030″ | 0.357 | 9.07 | 5.88% | 4.663 | (6.00%) | |
| .0040″ | 0.346 | 8.79 | 9.25% | 4.524 | (9.26%) | |
| Part # | Runout | Joint Length (inch) | Joint Length (mm) | % Length Reduction | Effective Glue Area | % Glue Reduction |
| C16-104 | .0010″ | 0.252 | 6.40 | Baseline | 4.303 | Baseline |
| .0020″ | 0.240 | 6.10 | 5.00% | 4.097 | (5.03%) | |
| .0030″ | 0.228 | 5.79 | 10.53% | 3.891 | (10.59%) | |
| .0040″ | 0.216 | 5.49 | 16.67% | 3.686 | (16.74%) | |
| Part # | Runout | Joint Length (inch) | Joint Length (mm) | % Length Reduction | Effective Glue Area | % Glue Reduction |
| C16-249 | .0010″ | 0.184 | 4.67 | Baseline | 3.501 | Baseline |
| .0020″ | 0.172 | 4.37 | 6.98% | 3.271 | (7.03%) | |
| .0030″ | 0.160 | 4.06 | 15.00% | 3.042 | (15.09%) | |
| .0040″ | 0.148 | 3.76 | 24.32% | 2.813 | (24.46%) | |
The comparison illustrates that as runout increases, joint length and glue area decreases. The percentage of joint length and glue area lost to runout dramatically increases in the shorter joints.