What is honing?
The honing process generates a cylindrical bore with precise control for roundness, straightness, size and surface finish by rotating, reciprocating and gradually expanding a cylinder of abrasives. This cylinder of abrasives progressively removes material from the bore at a controlled rate. The cutting forces cannot be applied in any way but support from the bore being honed. Therefore, the smallest portions of the bore are the first to be cut and stock removal progresses to a true cylindrical shape.
Why do we hone?
The honing process is capable of generating cylindrical tolerances of .0003mm, size consistency of .001mm and a controlled, base material, surface finish. The surface generated by the cylinder hone process is free of amorphous material and surface tension.
During the honing cycle, the honing tool enters the bore, a rapid expand occurs, spindle rotation starts and fine feed begins. There is a brief period at the start of each cycle in which there is no functional contact or possession of the bore by the honing tool. Bore possession occurs only when the fine feed expands the honing stone to the bore diameter. During this period the honing tool is bouncing around in a part that is free to float so there is a considerable amount of impact and pounding going on. It is essential that this damaging period of the cycle be kept to a minimum or permanent damage to the abrasive can result.
The rotating and reciprocating motion as described above creates a cross-hatch pattern in the bore. The angle of the cross-hatch pattern indicates the relationship between rotation speed and reciprocation rate. Historically the cross-hatch angle that is most effective is 26-32 degrees. 20-45 is acceptable based on individual applications.
Abrasive Dressing: When the motion of the tool is properly applied to the bore and a controlled cross-hatch is obtained, this action will dress the honing stones straight while generating a straight bore.
Abrasive Length & Stroke:
To accomplish this generated straight bore, an abrasive length that is equal to the length of the bore to be honed is generally applied for a through-bore application. This abrasive is reciprocated in the bore so as to allow roughly 1/4 to 1/3 of the abrasive length to exit the bore at each end of the stroke. The stroke length should be central to the centerline of the bore and the float centerline of the fixture.
The amount of abrasive that travels out of each end of the bore translates into bore shape. If the abrasive is coming out too far, bellmouth will occur. If the abrasive is coming out more at the top than it is at the bottom, the bore will be larger at the top and taper will be induced into the bore and the abrasive. If the honing machine is consistently running parts larger at the top than at the bottom, the stroke should be shortened at the top and lengthened at the bottom. This will cause correction in the bore geometry and regeneration of the abrasive honing cylinder. The full effect of this change will not be seen for at least 7 or 8 parts so don’t make further adjustments until the full effect is realized. This correction is reversed for taper at the opposite end of the bore. In any case, all effort should be made to correct taper while maintaining stroke length. The stroke length should generally stay the same. Only the position of the stroke should be changed. If the stroke is lengthened excessively the result can be either bellmouth or barrel shape. Generally, stroke adjustments of 1mm should be used.
The combination of the feed system and the cone angle make it possible to exert several thousand pounds per square inch (psi) of force between the abrasive and the bore. When part of the abrasive is out of the bore this force increases due to having less abrasive area to distribute the force. If only 50% of the abrasive is still in the bore, the psi exerted on the abrasive now doubles. The abrasive can be permanently damaged under these conditions or the abrasive will dress and cut differently and cause geometry issues. It is critical that the stroke and feed rate are controlled to avoid this situation.
Surface Feet Per Minute:
Honing tool rotational speed (rpm) is dictated by bore size, hardness of material, bore configuration, number of abrasives, symmetry of bore, required surface finish and part configuration. Honing tool reciprocation rate is dictated by desired cross-hatch angle. If the rpms are changed the reciprocation rate must change to retain the proper cross-hatch angle. The rpms and reciprocation rates translate into sfpm (surface feet per minute). A symmetrical, easily balanced, hard part (60-62 Rc) should run from 60-100 sfpm.
The cylinder hone process requires a few seconds of spark out time at the end of the cycle to dissipate the cutting forces. Spark out is accomplished by stopping the fine feed rate and holding it for a given length of time and then the feed system is retracted allowing the abrasives to retract. Honing tools should not be withdrawn from the bore while under fine feed. This action will damage the abrasives and bellmouth the top of the bore.
The honing stone process requires a gymbal type, floating fixture to properly present the part to the tool. A floating fixture allows the part to follow honing tools and generate a cylindrical bore without changing location. To facilitate the honing process the fixture must provide balance, angular & radial float, torque resistance and thrust resistance. The float centerlines of the bore must coincide with the float centerlines of the fixture. The best machine built with the tooling available will not generate accuracy without proper fixturing. Fixtures are as critical as honing tools and must be maintained and kept clean.
There are basically two types of honing tools used throughout the industry and their uses are dictated by the type of bore being honed. The most common is the single honing stone tool. This tool consists of two high concentration diamond wear guides that are soldered to the tool body at approximately the 4:00 o’clock and 7:00 o’clock positions. These guides are accurately ground to the diameter being honed. The expanding abrasive is fit into a slot at the 12:00 o’clock position. The abrasive is also accurately ground to the diameter being honed. The advantage that this tool has over other designs is better bore geometry correction.
There are some limitations to these honing tools:
The other option is a multi stone tool that contains four or more expanding abrasives. All abrasives are accurately ground to the diameter being honed. All abrasives are expanded at the same rate. The advantages that this tool has over the single stone are no eccentric forces applied to the bore being honed, range of stock removal is greater, effective abrasive cutting area is greater for improved stock removal rate and there is no burnishing effect on the surface of the bore.
The common tool arrangement for honing is single honing stone tools for the rough & semi-finish passes and a 4 stone tool for the finish pass. If the incoming material is straight and round with good geometry then a 4 stone tool will work fine. If the incoming material has geometry problems such as bow, out-of-roundness and taper, a single honing stone tool will do a better job.
There are many variables involved in the cylinder hone process and while it is a very capable process, care must be taken to control the stock removal and condition of the bore prior to honing.
Contact us about hour honing tools and the cylinder hone process!
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