Softhoning for Hard-Turned Steel Gears

The following case study makes clear the benefits to be gained from incorporating softhoning into your gear finishing process.


Most applications reserved for flexible honing tools have restricted themselves to removing peaks, torn and folded metal, or other amorphous material created from a honing or boring process. Since most of these flexible tools resemble brushes, brush honing has been a popular name used to describe them. Brush hones work at much lower pressures than conventional rigid abrasive systems and, as such, are not designed to correct for tolerance or geometry. Presently, most material subject to brush honing has been cast iron and nonferrous metals and confined to removing surface defects above core roughness.

Purpose of Study
It is the purpose of this study to explore increased work capacity of abrasive nylon filament brush honing tools beyond their traditional indicated applications. This investigation will begin with a short physical description of different nylon abrasive filaments and abrasives.

Nylon Abrasive Filament Brush Hones (SOFTOOL*/SOFTHONING)

An important physical element of brush honing is filament beam strength. Filaments come in all different sizes and configurations. The most predominant are round, round-crimped and rectangular (see Figure 1). As a general rule, work that requires high filament flexibility and gentle cutting action, .022″ (56 mm) diameter round-crimped is the choice. For more robust work, .045″X.O90″ (1.14mmX2.29mm) rectangular filament offers the most beam strength. Of course, with the exception of the abrasive crystal itself, filament trim length and bundle widths are also factors of brush hone cutting performance.

Nylon abrasive filaments are manufactured by extruding melted nylon pellets while dispersing abrasive particles homogeneously throughout the filament. The degree to which you expose surface abrasive depends largely upon the amount of draw applied to each filament as it is manufactured. The more you stretch the nylon filament (within physical limits), the more exposed abrasive you have to perform work (Figure 2). Standard abrasive particles currently available to brush honing tools are silicon carbide, aluminum oxide, polycrystalline diamond (PCD), and cubic boron nitride (CBN). Abrasive grain sizes can vary from 60 to 3000 mesh. However, most common brush honing abrasives in use today range from 120 to 500 mesh.

Basics of Brush Honing
To borrow another common term from industry, “softhoning” will be used to describe brush honing in all its forms. Our interest here is with softhoning tools that are low pressure, low temperature, and low stock removal (in most cases, defects only, no core material removed) units that are designed to fit most common production honing machines and transfer lines. Softhoning resembles super-finishing in that both are a low pressure, low temperature processes. However, they differ in their ability to remove stock below core roughness. While softhoning stops at core roughness, superfinishing continues cutting to achieve improved work piece geometry.

Presently, softhoning has been successful as a final finishing tool after honing. They are particularly good at removing surface defects created by rigid honing abrasives plowing through metal, generating numerous peaks, metal flaps, loose burrs, and folded metal, some of which may cover a hatch valley completely. Additionally, many trapped particles from spent honing abrasives are mixed with swarf. Under pressure and heat from the honing process, these abrasive particles (and any other unwanted debris in the honing fluid) are reintroduced to parent metal. The resultant undesirable surface structure creates an amorphous layer, also know as “sheet cover formation.” Because this surface is so critical to wear resistance, it must be removed before final surface measurements are taken.

In contrast to higher pressures and temperatures of rigid honing, softhoning works at the other end of the spectrum. Because softhoning operates at low pressure, heat is not an issue. Neither is the formation of sheet cover. Softhoning acts to remove this layer by gently abrading away defects, leaving behind a vastly improved surface. This honing system will only change part dimension to the extent of the sheet cover thickness it removes. With this layer removed, you expose a true bearing area of the work piece instead of an unstable surface easily worn away. Note: Under no circumstances is it advisable to use softhoning tools to correct part geometry. These tools are not designed for that function.

The Softhoning Process
Unlike conventional honing and micro-finishing abrasives, softhoning uses nylon to bind abrasive particles. As mentioned earlier, nylon is extruded in many filament forms to suit application. Regardless of filament form, softhoning tools stay free cutting in much the same way conventional abrasives do. A self-dress process occurs when abrasive nylon filaments contact a rough or peaky surface of the work piece. Exposed abrasive particles within the nylon do most of the initial work as the nylon begins to wear (and expose new abrasive particles). If easily fractured abrasive particles are used (aluminum oxide or silicon carbide), then any given abrasive crystal may be fully consumed until released by the wearing nylon. In the case of CBN or PCD, where higher pressures are required to fracture a crystal, nylon will hold an abrasive particle in one position long enough to consume a single cutting edge before crystal evacuation occurs.

In all cases, heat is our enemy here. Generous volumes of honing fluid are necessary to carry away heat generated in the contact zone of nylon filament to work piece. In this instance, too much heat can melt the nylon, potentially cover abrasive particles, and stall abrading activity. Ample honing fluid eliminates these possibilities.

The Machine Environment
Because Softhoning tools are designed to work in the same machines as conventional hones, there is no need to build specialized equipment to deliver them. Operating pressures of 90 psi (six bar) to 180 psi (12 bar) are also well within the design criteria of most honing machines. Reciprocating and rotating forces remain the same for softhoning as it does for conventional rigid honing.

Recently there has been interest in using softhoning tools in machining centers. Because machining centers have not traditionally incorporated a honing process, the need for surface improvement at the micro level was not required. By adding a honing operation to a machining center, surface improvement beyond that which honing provides is now an issue. Here, too, softhoning tools find easy access.

I.D. Softhoning Hard Turned Steel Gears
Materials, Machine, and Part Description
The part to be softhoned is a steel pinion used in front wheel drive transmission differentials. Pinions are assembled two per shaft with two shafts and four pinions per differential. Surface compatibility between pinion I.D. and shaft O.D. is mandatory.

• Material: SAE 4615 modified fine grain steel (hardened).

Hard turning

• Material harness: Base material–60 to 62 Rockwell “C.” Side-flow @ 70+ Rockwell “C”

• Stock to be removed: Side flow and tool marks left from hard turning. Typically, this was .00008″- .00012″ (2-3 microns) Machine used for hard turning: Kasper, horizontal, two spindle boring machine

• Cycle time for hard turning: Approximately 20 seconds

• Coolant for hard turning: Dry

• Softhoning tools: 320-grit silicon carbide in .03O”X.O70″ (.76mmX1.78mm) rectangular nylon filament, 320-grit silicon carbide .040″ (1 mm) diameter round-crimped nylon filament, 120-grit silicon carbide .03O”X.O70″ (.76mmX1.78mm) rectangular nylon filament and 120-grit PCD .040″ (1 mm) diameter round nylon filament

• Machine used for softhoning trials: Sunnen 1804 horizontal spindle

• Coolant for softhoning: Sunnen MAN 845 honing oil

• Cycle time for softhoning: 12-15 seconds

• Spindle rotation speed for softhoning: 700 RPM

• Reciprocating speed: 80 strokes per minute (SPM)

• Softhoning tool pressure: Approximately 60 psi (four bar)

Effects of Hard Turning on Bore Surface

Figure 3 depicts a typical hard turned pinion bore surface. Note boring tool pattern and resultant side-flow forming at the peaks. While base material hardness is between 60 and 62 Rockwell “C,” the side-flow hardness jumps to over 70 Rockwell “C.” As witnessed from profile data, most of this surface is in peak. A bearing area curve derived from Tp data shows this surface unable to support much loading. Further, side-flow formation is sufficient to cause pinion/shaft seizure due to bits of side-flow breaking off under load. The manufacturer verified this seizure of pinion and shaft during a recent load test.

Testing Various Softhoning Brush Inserts
Step one:
The first step was to determine what part(s) of this surface had to be removed in order to create pinion/shaft material compatibility. Since dimensional change to bore I.D. is not desired, it was felt that removing side-flow only was a viable next step. Figure 4 shows two SEM photomicrographs of a pinion bore after hard turning and brushing with a 320-grit silicon carbide, .040″ (1 mm) diameter, round-crimped nylon filament softhoning insert. You will note that all side-flow has been removed but a boring tool pattern remains. Correspondingly, Tp data indicates a better bearing area for this brushed surface. Interestingly, while Ra for both as-bored and bored and brushed pinions are similar (16.5 u” [.41 urn] and 15.7 u” [.39 urn] respectively), an Rpk of 4.7 u” (.I2 urn) for the bored and brushed pinion is dramatically lower than the Rpk of 33.4 u” (64 urn) for an as-bored pinion. Results of this test indicate that softhoning did remove side-flow and once accomplished, stopped abrading at core roughness. Still, the pinion manufacturer wished to further remove tool marks left behind after hard turning.

Step two:
The next step was to change abrasive size to 120-grit silicon carbide and leave the filament at .040″ (1 mm) diameter. As expected, a coarser abrasive impacts surface more than a 320-grit. The primary change here was creation of a new surface signature (see Figure 5). Because surface structure was dramatically altered from that of 320-grit, it was not practical to compare this surface to that in Figure 4. Figure 5 more closely approximates a honed surface than that of a bored one. All that can be observed here is that bearing area did improve, as did the Rpk/Rvk ratio. Also, it appeared that a softhoning insert indeed could exceed its design criteria by altering core roughness unlike that found in Figure 4. The next step to increasing work capacity is to change filament size.

Step three:
Another softhoning brush insert was fashioned, but this time in 120-grit silicon carbide, .03O”X.O70″ (.76mmX1.78mm) rectangular nylon filament. It was thought that by increasing filament beam strength while retaining the courser 120-grit silicon carbide abrasive, more work could be done to impact core roughness and return softhoning to a freer cutting environment. Figure 6 displays SEM photographs and profile data of this test. Viewing photos, it is clear that boring tool marks are gone, but folded metal remains on cut track edges. Though surface improvements were made, long-term suitability of this brush tool was questionable. Also in question was the softhone brush tools ability to deliver consistent results in production. Compared to the as-bored pinion I.D. in Figure 3, this surface has a highly improved bearing area and marked improvement in all other profile parameter measurements. This was enough to encourage the pinion manufacturer to run a pinion to shaft compatibility test. The test was successfully suspended after 100 cycles without failure. However, this success did not come without penalty. In order to increase the softhoning tool’s ability to cut beyond core roughness (something the tool was initially not designed to do), more pressure than normal was applied. This increased pressure to utilize benefits of increased filament beam strength was sufficient to generate enough heat, even with a flood of honing fluid, to melt nylon filaments binding the silicon carbide abrasive (see Figure 7). A close look at the photo in Figure 7 shows melted nylon covering abrasive particles. Filaments in this condition are completely destroyed and will stall any further abrading activity. Even though softhoning was successful in this single test application, it was far from a production tool capable of consistently and uniformly producing thousands of parts per day. In this case, for softhoning to find practical application, a more robust, free cutting tool must be designed.

Step four:
In order to retain benefits of softhoning as described in sections 3.2 and 3.3, it was necessary to change the abrasive crystal in an attempt to increase cutting activity while decreasing insert out-feed pressure at the contact zone.

A 12O-grit PCD abrasive was chosen for its ability to cut freely under light pressures. However, using this type of abrasive limits the softhoning tool to a single cutting edge per crystal. This condition occurs because low cutting pressures and a corresponding reduction in mechanical friction normally associated with controlled crystal wear prevents these PCDs from splintering or fracturing to restore new cutting edges. With increased hardness of PCD, and the advantage of maintaining a negative cutting angle longer than that of silicon carbide, crystal cutting edge life was sufficient to match nylon filament wear.

It should be noted that, in this case, PCD abrasive was used to cut carbon-rich steel. Normally diamond crystals, when subjected to high cutting pressures at the contact zone, generates heat sufficient to chemically migrate carbon form diamond to carbon in steel. Because softhoning operates at substantially lower cutting pressures, heat is not an issue and chemical migration does not take place. For this reason, PCD retains a sharp cutting edge while honing carbon-rich steel for as long as it is bound in a nylon filament.

A fourth softhoning brush insert was made to incorporate 120-grit PCD abrasive in -040″ (1 mm) diameter round nylon filament. Rectangular filament was not used here in order to take advantage of more pronounced abrasive crystal protrusion associated with round nylon filaments (see section 3.0). Also, decreasing filament beam strength in favor of higher PCD cutting performance lowers the softhoning tools cutting zone pressure. This filament size was selected to yield the highest cutting performance at the lowest pressure possible.

This new insert was used to hone subject pinions I.D. at roughly 30 psi (two bar) for 15 seconds. The resulting surface can be viewed in Figure 8. As can be seen, practically all boring tool marks have been eliminated. Figure 8 does expose what looks like a partial valley left from boring. However, PCD brushing reduces this tool mark to just another valley in the surface. All surface defects generated through hard turning have been eliminated. What remains, are cleanly cut valleys and well-defined plateaus. To accomplish this, .00008″-.00012″ (2-3 microns) of stock had to be removed. Further, use of light cutting pressure allow nylon filaments to act and wear in accordance with their fundamentals. No melting of nylon or covering of abrasive particles was evidenced. In a longer run of over 200 pinions, the PCD softhoning insert remained free cutting and usable as a viable production alternative to other forms of micro-finishing. Although 120-grit PCD effects desirable surface changes, improvement to bearing area is indicated. Finer grit PCD inserts should help with this.

Softhoning brush inserts using PCD to micro-finish hard turned steel pinion bores affords manufacturers a low impact, cost effective tool capable of positively changing unsuitable surface topographies into usable bearing areas. As has been demonstrated through this case study, softhone brush tools offer benefits in the following areas:

• Tools that fit any existing honing equipment.

• No need for expensive specialized machines to benefit from process.

• Older machines that have lost ability to produce accuracy in rigid honing, work well to deliver softhoning tools.

• Acts to improve surface structures with least amount of stock removal.

• Utilizes lower contact zone cutting pressure than those typically found in honing or superfinishing operations.

• Low heat generation.

• Fast throughput from low cycle times.

• Gang brushing capable.

• Adaptable to many different types of work pieces.

• Will not change part geometry produced by honing or boring.

From the outset, softhone tool design criteria was always to be effective in abrading away surface defects while leaving core roughness reasonably unaffected. Using PCD in this application caused the softhoning tool to cut past core roughness. This being the case, it can no longer be assumed that the tool will stop cutting once sheet cover formation is removed and core roughness reached. For this reason, cycle time and constant cutting zone pressure should be maintained to control consistent stock removal from part to part.

Because this presentation ended with utilization of 120-grit PCD, it would be recommended that 240 and 400 grit PCD be tested. Either of these two abrasives may improve Tp values and yield even higher quality surface topographies. As with any process, constant improvement is vital to increasing overall quality.

VanSickle, Charles/Flares, Gerhard Function and Production of Diamond and CBN Honing Sticks. SME technical paper.

Auschner, Wolfgang Stone Finishing of Hard Turned Surfaces Within Process Gauaina to Size. SME technical paper.

Previous articleQ&A with Darren Martens
Next articleTerry McDonald: Site Safety
is an abrasive application specialist, and Joseph Gaser is vice president, product development, for Osborn International. Learn more at [].