A Lesson in Workholding Fundamentals

In this design, the blank is held between two pieces: a long arbor piece with a 32 mm diameter shaft for collet clamping at one end with an 80 mm diameter boss to hold the part, and a centering piece with a mirrored boss and clamping face, clamped by an M12 bolt through the bore.
Michael Weas, Helios Gear Products

Evolution of a Real-World Workholding Solution

We set out to demonstrate a Helios Hera 350 hobbing machine on a short timeline. Over three iterations of fixturing design, we discovered a more robust solution would be required to cut a quality part at an aggressive speed. After starting with tooling already in stock, the second iteration of the fixturing design mimicked the design of the existing pieces while providing increased pressure by decreasing the clamping surface area. After this second design failed to improve the performance of the process, the fixturing design was completely rethought. Utilizing the hydraulic draw system of the machine, the final design was able to rigidly hold the “Demo Part” for a successful application.

Initial Design

The Demo Part is a 6 DP involute spur gear with 46 teeth, 40 mm wide, and made of 8620 steel. The pitch of this part is near the maximum rating of the machine, 4.23 DP. The part was cut by a 3.5” outside diameter high-speed steel hob with one thread, fourteen straight flutes, and AlCrN coating.

We initially planned to use existing fixturing pieces to save on time and costs by fitting the gear blank onto a modular precision fixturing system that provided the concentricity and perpendicularity needed to produce a high-quality gear. In this design, the blank is held between two pieces: a long arbor piece with a 32 mm diameter shaft for collet clamping at one end with an 80 mm diameter boss to hold the part, and a centering piece with a mirrored boss and clamping face, clamped by an M12 bolt through the bore. The pilot diameter of the arbor piece is held to a tight (h4) size tolerance and 3-micron runout to the 32 mm shaft that is clamped by the 16C collet on the machine’s work spindle. The clamping faces of the arbor and center piece are also held to a 3-micron runout tolerance to keep the blank parallel to the work spindle. The assembly is shown in the machine pictured above.

Performance of Initial Design

When fully assembled, this fixturing setup measured over 340 mm in length, including the arbor piece, the gear blank, and the centering piece. This total assembly is loaded into the collet chuck by hand and clamped by the center on the machine’s tailstock. The intention was that the M12 bolt holding the assembly together, paired with the hydraulic tailstock of the machine, would provide enough clamping force to hold the part during the cut.

This setup was successful in its ability to hold the part while hobbing. However, this is not the only criterion of a successful cut. The fixturing assembly was very long relative to its width and only held to the work spindle by the collet. This combination created a great deal of vibrations during the cut that could be heard throughout the shop and felt in the floor around the machine. Even after a two-cut cycle, the finished part was not satisfactory: the surface finish was covered in chatter marks, and it is unlikely the gear would have passed an analytical inspection for even an AGMA Q6 part.

Second Design

The goal of the second design was to decrease the vibrations in the fixturing and increase the clamping pressure on the part. This new design carried over the basic principles of the original setup. There was one piece that bolted to the machine’s worktable and held the blank in a parallel and concentric position. The clamping force came from a second piece, also fitting into the bore of the blank, that was bolted to the first piece through the bore of the part. A section view of the fixturing assembly is shown in image 2. Image 2 does not show the hydraulic tailstock used during hobbing. The clamping faces of both pieces were designed so that only a 12.7 mm [0.5”] wide band would be contacting the blank. In the original design, the part’s entire face, from the bore to the outside clamping diameter, was in contact with the blank, about 31 mm. Assuming the same force clamps through the same size bolt, the decreased clamping area would provide increased pressure on the part.

Image 2

Performance of Second Design

In this second design, the solid fixturing piece was clamped to the spindle face. The gear blank was loaded onto this piece with a close sliding fit between the pilot diameter and the bore of the blank. The second piece was loaded onto the blank and clamped by tightening the bolt through the center. This setup also utilized the clamping force of the tailstock of the machine.

This second iteration improved the performance of the cut but only slightly. Interestingly, the sound from the vibration of the part was reduced drastically, but the finish was still not acceptable.

Analysis of New Design

The final design took on an entirely different form. Fundamentally, this design uses the machine’s hydraulic draw system actuator combined with the hydraulic tailstock to clamp the part instead of a bolt. This new setup includes four parts: a Base, a Rising Block, a Draw Bar, and a Clamping Plate. The Base is bolted directly to the worktable and is the same diameter. This maximizes rigidity by ensuring that the clamping and cutting forces are spread out as much as possible into the machine base. The Base is also general enough in size and shape to adapt for other parts. This allows other blanks to be fixtured without having to change every piece of the tooling. The second piece is part-specific for the Demo Part: this Rising Block fits into the Base on a tapered bore.

Then, the Draw Bar fits through the Rising Block and the bore of the part. The Draw Bar has a close sliding fit through the Rising Block and the final clamping piece, the Clamping Plate. The Draw Bar and this Clamping Plate have matching keys and keyways so that the plate can be slid over the keys of the Draw Bar, and then rotated 60 degrees. This allows the keys of the draw bar to clamp on the top side of the plate. This is shown in the isometric view in image 3.

Finally, the clamping faces of the Rising Block and the Clamping Plate also have six evenly spaced 30 mm wide groves. This helps increase the clamping pressure by further decreasing the clamping surface area. Instead of being clamped between two full bands, the part is clamped between six sectors on either side. The machine’s tailstock is still utilized in this design, by ensuring that the draw bar is in-line with the work spindle axis and providing additional clamping force by pressing down on the arbor. Not shown in image 3 is the hydraulic tailstock used during hobbing, the worktable, or any of the hardware connecting the Draw Bar to the machine.

Image 3

What Did We Learn?

Pairing the machine’s hydraulic clamping system with the tailstock is a better way to hold a part compared to a single bolt. Additionally, the larger fixture helped distribute the load evenly into the base of the machine and allowed for greater flexibility for other parts. If the part is a bore type part, a draw bar is necessary for adequate clamping. We also learned the dangers of cutting corners when working up fixturing. The time saved by using existing fixturing for the initial setup was offset by the poor quality of the cut and the time spent working on a new solution. This is a classic example of how rework is more expensive than taking the time and effort to do something correctly the first time.

Heliosgearproducts.com

About Matthew Jaster 18 Articles
Matthew Jaster, Senior Editor, has a B.A. in journalism from Columbia College Chicago and has 15+ years of writing and editing experience in automotive, manufacturing, engineering, law and arts and entertainment.