“We don’t believe it!” – that’s how experienced machinists at Waltz Brothers Inc., a precision grinding and machining operation producing flight-critical hydraulic pumps, reacted to their first encounter with the Sunnen SV-1015, a honing systems that had produced nine bores per part with only 0.000020 in. (0.5 µm) variation, all day long, with essentially no operator attention. That’s 2.21 Cpk process capability.
It’s hard to hit this tolerance window honing one bore per part, much less nine, as the shop had been doing with a standard machine. With nine opportunities to make a micron-sized mistake, it takes great skill and many time-consuming machine/measure iterations to make these parts on a standard machine. The new hone also records its final air gaging measurements to track with each serialized part. One of the keys is a smart tooling technology that servo controls the force on a honing machine’s tool feed system.
Waltz had been using standard horizontal honing technology from Sunnen, where the operator would hone one bore, clean the part, air gage it at three different levels and then at 90° bottom/middle/top to see if there’s any issue, such as taper at the bottom of the bore which would have to be feathered out. After honing, the company confirms CMM inspections on each part and bore. Honing and inspection could take two hours per part.
“Our challenge to Sunnen was to automate everything, including part indexing, air gaging, and recording of gage readings,” said Larry Waltz, president of Waltz Brothers Inc. “We achieved all this with the Sunnen SV-1015, but it was one of the standard features in this machine – multi-feed honing – that has played a role in taking our honing process capability to a new level.”
Multi-feed honing gives users a choice of tool-feed modes to achieve the shortest cycle times, lowest part cost, and longest abrasive life. Multi-feed combines Sunnen’s controlled-force tool-feed with its existing controlled-rate feed system. The two different modes allow the user to select the better option to suit the workpiece geometry, material and tool type/size.
Typically, a production honing process is set up to use an abrasive tool with a combination of grit size and bond optimized for specific part conditions. Tool expansion and final size is programmed based on rate of time. However, when a batch of parts comes in with a different heat treatment, distortion or a size variation, the operator must intervene or the tool may expand too quickly and be damaged. In the opposite case with a softer-than-normal or oversize workpiece, the tool will still expand at its programmed rate, when it might have been able to expand faster to reduce cycle time. Expansion at too slow a rate may also result in glazing of the honing stones, which won’t self-dress if the cutting force is too low. By controlling the force in the tool feed system, the machine can sense and compensate for these variables. The controlled-force feature, which works in concert with the machine’s standard rate-feed system, functions like cruise control to maintain the optimum cutting load on the honing abrasive throughout a cycle, irrespective of the incoming part’s hardness, geometry or size variation.
Waltz’s SV-1015 vertical CNC honing machine has a single 5-hp AC spindle with 10-hp servo stroking system and X-Y air bearing table. The operator loads the workpiece in a dedicated fixture that uses the part’s kidney slot as a locating feature. The table indexes 90° after each bore to an Etamic air gaging station where the necessary readings are taken and stored. If the bore passes inspection, the table moves the part to a robot that indexes it for the next bore, duplicating this routine eight times to complete a part. A fixed position master set ring inside the machine’s work envelope ensures correct gage calibration for current environmental conditions.
“On a recent project with an allowable bore tolerance of 0.000240 in. (0.006 mm), we easily held a tolerance range of 0.000060 in. (0.001524 mm) – that’s 25% of the total allowable tolerance, and we tripled the previous production rate, while reducing the labor component by 80% so the operator can do other work in the cell,” said Waltz. “We know from re‑inspecting the parts on our CMMs that the results correlate well. The process capability and data reporting features in the machine have been a great advantage to us and our customers, allowing that data to be downloaded to a spread sheet or SPC software.”
Sunnen Products Company