How To Perform More Effective Creep Grinding

Creep feed grinding is a grinding process, but more importantly, it is not like other forms of grinding. Mr. Nelson Beaulieu, Manager of Grinding Products Department of PTJ Shop, said that in a sense, creep grinding has more technical characteristics of milling.

In the final analysis, peristaltic feed grinding is machined by means of deep cutting and large metal cutting, which is characterized by replacing the rapid reciprocating motion with a lower feed rate. Creep grinding tools, like grinding wheels, are deeply embedded in the machined materials.

Figure 1 Let a small-diameter grinding wheel be processed with a tool path like a 5-axis machining center
Figure 1 Let a small-diameter grinding wheel be processed with a tool path like a 5-axis machining center

In fact, when the diameter of this tool is reduced and close to the maximum milling cutter diameter on a typical machining center, it can show its higher efficiency. Mr. Beaulieu said that if the machine tool itself is designed like a vertical machining center, then this type of tool will show its higher practical benefits.

Mr. Beaulieu said that this idea is behind the grinder designed on the basis of VMC vertical machining center in his company. The FGC 2 “Flexible Grinding Machining Center” provided by Bridgeport (part of Hartinge) deviates from the traditional creep grinder design form, in which the direction of the spindle may represent its essential difference.

Figure 2 The surface of the above blade is processed by the creep grinding process, During grinding, the rotary axis of the machine tool is fed along the arc of the part.
Figure 2 The surface of the above blade is processed by the creep grinding process,
During grinding, the rotary axis of the machine tool is fed along the arc of the part.

Different machining methods

Mr. Beaulieu said that when the creep grinding process was developed, virtually all grinding machines used horizontal spindles. Creep grinding machining natural building is based on this platform. However, one of the basic requirements of this horizontal design is the need to use a large diameter grinding wheel. The diameter is large enough to allow the edge of the grinding wheel to reach the worktable, which facilitates the removal of the main shaft cover.

Mr. Beaulieu also said that this requirement will have certain limitations. For efficient creep grinding, the penetration force of the coolant is very critical, but the wide arc of the large-diameter grinding wheel will cause its space to shrink, affecting the penetration force of the coolant. In addition, large-diameter grinding wheels cannot be interpolated along complex machining paths. For this reason, parts often need to be clamped with complex fixtures. Due to the different characteristics of the parts being ground, when they move from one creep feed machine to another, the parts also need to be changed from one fixture to another.

On the contrary, if the spindle cover can be removed to a place outside the work area, it is beneficial to use a smaller diameter grinding wheel. Therefore, the cooling fluid can also be directly delivered to the cutting process more efficiently, so that the parts can be fed through the rotating shaft, and the small-diameter grinding wheel can be processed by the tool path like a 5-axis machining center.

In essence, the FGC machine is actually a 5-axis machining center. However, since the workpiece is replaced by milling (milling in the ordinary sense), the machine tool must have a certain degree of rigidity, that is, it must have the critical strength of a traditional horizontal platform. Unlike the typical VMC vertical machining center, its sturdy cast iron base weighs 30100lb. Not like the vertical machining center, it needs to be used in combination with a dedicated coolant system, because during the creep grinding process, the system is required to be able to deliver 1000 psi (lb / in2) pressure or 50gal / min flow Coolant.

These changes mean that the use of vertical machine tools does not require expensive initial costs. Mr. Beaulieu said that once such machines are used, they can immediately save money. The sources of savings include the following:

  • (1) Fewer fixtures and machine tools. There is no need to debug and install the card multiple times, all the grinding and machining tasks can be completed by installing the card only once;
  • (2) Shorten the time of debugging and installation of the workpiece, because no complicated tooling fixture is needed;
  • (3) The machining operation process is reduced, because the same grinding wheel can be used to change the direction of the parts in the production cycle, and different parts are processed separately;
  • (4) The machining speed is improved (the capital saving degree exceeds the money saved by the debugging and clamping of the workpiece), because the grinding wheel with a smaller diameter can work more flexibly. Mr. Beaulieu said that in this machine tool, users only need to use a lower cutting depth to get a higher metal cutting rate, because its potential feed rate is much higher than the actual speed.
Figure 3 The machine tool is used in combination with a dedicated cooling system (the system is located on the left side of the machine tool), which can transport the high-pressure and high-flow coolant required by the creep feed grinding process.

Traditional machining

If the machine tool is used in accordance with the traditional VMC vertical machining center, it can further save money. The spindle adopts the standard BT interface of the tool of the machining center, and the grinding wheel is connected to the spindle through the standard tool changer of the machining center. Therefore, the same system can be applied to a more typical machining center within the same machining axis cycle. This means that the milling cutter or drill bit can wait in the tool magazine next to the grinding wheel, ready for application at any time. If milling or drilling is required on a grinding surface, then the workpiece can still be in the same clamping state, allowing the machine to complete these additional machining tasks.

In order to make the machining platform of the VMC vertical machining center easier to apply to the creep feed machining, another method can also be used, which has a close relationship with the resources outside the work area. Because the machine tool it uses is so similar to a standard machining center, it enables the operator of the machining center to effectively program and perform creep feed machining operations, thereby improving the machining capacity of the machine tool. As Mr. Beaulieu pointed out, “There are more milling operators available than grinding operators.”

Why use creep feed grinding?

Mr. Beaulieu said: “If all the machining can be solved by milling or broaching, then we are very willing to accept it.” Creep machining is a challenge to other machining processes because there are certain problems with these other machining processes. Creep machining has the following advantages:

  • (1) Has excellent machining characteristics. This grinding process can absorb a part of the heat in machining cutting. Especially in the machining of titanium, Inconel and other materials, it is more effective, mainly because the heat generated during the machining will accelerate the wear of the milling cutter or broach;
  • (2) Maintain a good surface finish. This kind of grinding process does not need to exert particularly great pressure on the processed surface, but the ability to grind the material is very high, while still maintaining a good surface finish and perfect surface machining quality;
  • (3) No glitches will occur. No burr is needed for this grinding process;
  • (4) Can maintain inherent accuracy. Under well-designed process conditions, milling and broaching can indeed achieve high machining accuracy. However, a well-designed grinding process can achieve even higher precision.
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