CNC Broaching with a Driven Broach Head
A high-volume OEM for motor shafts switched its keyway broaching process to use the lathe's live tooling capabilities, improving cycle times and reaching the recommended surface feet per minute.
Because of the importance of speed in the manufacturing industry, one must consider the measurement of shop time as a key factor for profitability. How long CNC machines are tied up with one part and the time each process takes to complete is evaluated constantly and plays into the pricing of products all the way down to consumers. Saving even a tenth of a second in the production process can have a huge impact. That is how valuable shop time is. So, a shift from 20 parts per hour to 120 could be monumental.
CNC Broach Tools LLC, a manufacturer of inserted broaching tools, had a high-volume OEM client in Fort Smith, Arkansas, that had been using the company’s tooling on its CNC lathes for more than a decade. The OEM cut mainly motor shafts that were made from tough 56 HRC 4140 and the blind keyway ended into a cross-hole relief. Depth of cut per pass was 0.0006 inch and speed 180 inches per minute.
The client wanted to switch from broaching its keyways with the motion of the lathe turret to using MD Tooling’s driven broach head so it could use the lathe’s live tool capabilities. The client’s volume justified the capital investment of the live lathe tooling, and the broach head would eliminate wear on the lathe slides (which could lead to premature failures or inaccuracies). The live tool broach dramatically improved the cycle times and could reach the recommended surface feet per minute (SFM) that was not previously possible using only the turret movement.
The driven broach head changes the rotary motion of the turret drive into a linear, reciprocating motion with a drop down on the retract stroke for tool clearance. A 4-to-1 geared down ratio for extra torque provides an immediate start and stop that the turret cannot achieve.
Without the driven broach head, when only the turret is providing the motion, the programmer must allow space for the turret to accelerate to full speed before it hits the part. (CNC Broach Tools recommends 0.750 inch to 1 inch of space between the cutting edge of the insert and the part being cut.) This spacing essentially creates a longer length of stroke. Starting too close to the part forces the turret to gear up within the cut, which could cause deflection as the insert hits the part at slow speed. John Gardner of CNC Broach Tools compares the situation with chopping a tree down with an axe. “If you start too close and do not hit the tree with any force, the axe bounces off. But if you swing the axe back farther to allow you to hit the tree with speed, the cutting edge bites in,” he says.
When using the turret, the need for space applies after exiting the slot or spline as well, and the cutting edge of the insert must be programmed to stop 0.060 inch to 0.100 inch after the outside corners of the carbide broach insert exit the material. Without this relief space for deceleration, a lip or taper at the end of the keyway may result as the turret slows the broach insert within the material.
MD Tooling’s driven broach head significantly reduces the space needed to accelerate and decelerate because it can start and stop almost instantaneously. Having less space to travel means improved cycle time. This benefit is particularly evident when broaching blind keyways or splines with a limited relief area, because stopping closer to the end of the keyway in a blind key situation allows more relief space to be used by the chip.
The customer’s application had a keyway 3/16 inch and 1.9 inch long, exiting into a 1/8-inch relief groove. Using the turret, the cutting edge of the insert would need to stop 0.060 inch into the relief space, potentially pressing the 1.9-inch-long chip into the opposing wall of the relief. The MD Broach head allows stopping as little as 0.010 inch past the end of the material without fear of decelerating in the material. The additional space provides plenty of room for the chip to float free.
The redesign of CNC Broach Tools’ tool shank produced impressive results for the client. Previously, broaching the keyway using the turret motion took two to three minutes. With the MD broach head, the cycle time was reduced to 24 seconds. Previously the client was running the CNC across all three shifts. The cycle time improvement allowed the relocation of one of the operators and provided the other two the ability to run other products, taking advantage of capacity that was not available prior to the improvements.
CNC Broach Tools has released an entire line of tooling for broaching keyways and splines that fit the MD Tooling driven broach head, and the company stocks carbide inserts with two cutting edges.
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