Expertise, agility, and the right mix of machines ensure one creepfeed-grinding operation stays on top.
Creepfeed grinding produces parts in a single pass, making it an attractive alternative to milling or broaching.
A turbine airfoil CF ground by Abrasive-Form.
These tiny terminal chips were CF ground from an aerospace alloy in batches of 120,000. Part dimensions are 0.098 0.062 in.
Creepfeed (CF) grinding offers an attractive alternative to other machining processes such as milling and broaching. But it also entails more variables. For an operation such as Abrasive-Form Inc., Bloomington, Ill., understanding the nuances of the process is key to exploiting CF's competitive advantages. It also helps to have the right machines in place on the shop floor.
In CF grinding, a formed grinding wheel plunges full-depth into the workpiece, producing a finished part in a single pass. This contrasts to surface grinding, where the grinding wheel rapidly reciprocates as it gradually lowers to the final depth-of-cut.
CF variables include machine rigidity, stability, and repeatability; wheel materials and performance; dressing systems; coolant types and application; fixturing; and gaging. Once these are managed, the process makes for fast cycle times, high repeatability, and close tolerances. In addition, the process easily handles superalloys and fully hardened materials, giving superior surface finishes with burr-free edges.
Abrasive-Form CF-grinds aluminum, cold-rolled steel, high-nickel alloys, hardened materials, carbide, tool steels, and ceramics, with volumes from a single part/day up to 50,000/day. Applications include aerospace engine components, landbased turbines, automotive engine and drivetrain components, tiny medical parts, and parts for the military.
"There's very little we can't do," says Ken Kummer, Abrasive-Form CEO, "including slots for hydraulic van pumps and aerospace turbine blades — jobs in difficult-to-grind materials and with close tolerances. We do linear operations of all sorts as well as contours. We can do 3D grinding and have machines with up to 5-axis control. With a combination of CBN and diamond wheels, plus continuous dressing, we have a lot in our arsenal."
Typical tolerances run ±0.0005 or ±0.0002 in., with CpKs of 1.33.
Controlling CF variables
Kummer says creepfeed grinding requires powerful and rigid machines designed for the process. His company uses 30 Mgerle and Blohm machines, from United Grinding Technologies (UGT), Miamisburg, Ohio.
He explains, "Engineering talent in Germany and Switzerland in CF machines is unparalleled. The Blohm and Mgerle machines have extraordinary rigidity and thermal stability. You can idle them, and come back, and immediately make a part right-to-size. And for us, to guarantee the highest quality levels — which used to be a competitive advantage but today is a routine expectation — we must rely absolutely on our technology."
Fixturing is also critical according to Kummer. "Anybody can go out and buy a 50 or 65-hp Blohm or Mgerle, but the question is, what are they going to do with it and how. Crucial to answering this question is fixturing." Kummer says a lot of issues tie into fixturing:
- Whether the entire wheelwidth is usable.
- If several operations can be performed in one pass — or does the application require orienting on different sides or angles.
- If all the forms can go on the wheel at once, eliminating operations.
- If all grinding operations can be performed on one machine, in one pass.
Kummer continues, "Our company has a division dedicated to designing our fixturing with topflight toolmakers and engineers. Also crucial are good machine tool suppliers that not only provide high-quality creepfeed machines and service but also are an involved source of engineering input — especially in the make-orbreak details such as the latest in wheels and new processes."
Gaging is also critical. Kummer says, "Shops offering CF-grinding services must know how to gage the part and — just as important — they have to know how the customer intends to gage it. This must be precisely correlated, so both parties are locating on the same datum points."
Because CF is typically seen as a niche process, Abrasive-Form often needs to educate its customers about it for the best results. For example, the CF house may be able to tell if it can do a certain part. But cooperation with the customer lets it optimize the process. For this to happen, the customer needs to share qualified locating surfaces or give provisions for clamping surfaces that could be machined away later.
"That's why it's important to sit down with customers early on and brainstorm projects," Kummer continues. Shops don't know all of a CFgrindingcompany's capabilities, and the company can't assess all the customer needs from blueprints alone. For instance, prints don't show functionality and product-reject issues or past manufacturing problems.
In some cases, working together in the early stages significantly improves jobs. For instance, it lets grinding processes be concurrently designed before part castings are poured to compress leadtime. Because all products have limited life cycles — usually about two to five years — saving six to eight months time-to-market pays for everyone.
Asking, then acting
Kummer claims there is no substitute for meeting face-to-face. "Asking questions combats the common misconception that CF is an isolated discipline. Instead, it should be seen as an integral part of the manufacturing process and the product lifecycle."
For example, one Abrasive-Form customer commented that worn-out parts had to be replaced on costly service calls. So Kummer's team suggested using a harder material, both solving the customer's wear problem and enhancing quality. The CF cost for grinding either material was the same.
In another application, a customer performed a deburring operation after grinding slots. Here, Abrasive-Form changed the grinding wheel form so that the wheel chamfers the slot at the same time it is ground.
Kummer believes CF-grinding companies should have three, four, or even five backup machines in a given work envelope or horsepower range. "That way, they never hold up customers' jobs if scheduling changes. Instead, they just move jobs to other machines."
Backup capability is necessary because often part quantities at the start of projects are rarely the same as when jobs are in production. Difficulties arise, for example, when a shop buys a machine based on an anticipated quantity of 10,000 parts and then sees that quantity fall to 5,000 or even less. Making things worse, this quantity could leap to 50,000 the following year.