Product category:
Honing, polishing, burnishing and lapping
News Release from: Sunnen Products Company | Subject: Gear honing
Edited by the Manufacturingtalk Editorial
Team on 17 September 2007
Why honing is used widely for gear
finishing
US gear manufacturing specialist, Fred Young, explains the reasons why the company widely uses honing for gear finishing and compares it with alternative finishing processes.
Fred Young says that his company has a straightforward philosophy for acquiring and retaining business We call it Excellence without Exception
This article was originally published on Manufacturingtalk on 23 Jan 2007 at 8.00am (UK)
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It boils down to always giving the customer a higher level of quality than specified on a print - or in any other interaction with a customer.
Obviously, we want to do this without adding much cost or time to a job.
We do everything we can to distinguish our product from competitors', and we try to do it inexpensively.
Further reading
Vertical submicron CNC honing system
Sunnen's SV-1005 series vertical CNC honing system is now available with an integrated air gaging system to provide closed-loop control of tool size, along with downloadable SPC data.
Tube honing system raises barrel bore quality
Using a tube honing machine, an armanents manufacturer is realising superior accuracy and easy cleaning that result from the excellent bore geometry and surface finish produced.
Robotic honing has automatic bore sizing
A vertical CNC honing system designed for automatic precision sizing and finishing of deep bores up to 200mm diameter features robotic part loading offers variety of bore-diameter gaging systems.
On bore-type gears, we have found that automated honing is a good way to give the customer tighter control of bore size, roundness, straightness and finish.
The customer notices the difference in a smoother, quieter, more efficient drive, and unless we stumble in some other aspect of the job, we will have a repeat customer.
The trick in gear manufacturing is to add this value without adding cost for the customer.
* Fine and medium pitch custom gears - Forest City Gear's principle products are fine and medium pitch custom gears, such as internal, spline, sprocket, helical, spur and worms/worm gears.
We work to quality levels as high as AGMA 15 (DiN 2-3).
Part runs range from one to several hundred thousand.
Maximum outside diameter (OD) on most parts is 20in (say 500mm), except for worms (5in - 125mm) and worm gears (16in - 400mm).
Typical materials include 12L14, 1215, 4140, 8620, 9310 and various stainless grades, as well as aluminum, bronze, brass, Inconel, Hastelloy, titanium, plastics, wood fiber and powdered metal.
About 30% of our work is aerospace-related, 5-10% medical, 5% military, and the remainder is industrial or instrument work.
We have made parts for Boeing, Airbus, Cessna, and Beechcraft as well as the space shuttle, the space station, Martian rover vehicles and the Abrams tank.
We produce gears for several motorcycles and racecars, as well as one of the few bait casting reels made in the U.S.
Many of these applications demand high process capability where honing gives us critical advantages in control and consistency.
In addition, we invest aggressively in training and the latest technologies to make our shop one of the most modern in the world.
* Why Forest uses honing - we have used honing since the inception of our business.
Over the years, we also tried hard turning, but found it more difficult to control quality, especially for microfinishes.
ID grinding is a fine process for gears with larger (>0.75in - >19mm) bores and low L/D ratios (0.5:1), but our range of work includes smaller diameters and relatively deep bores.
When you start to reach an L/D of 2:1, honing has a real advantage in speed of material removal, and over 5:1 you might start to see deflection on a grinding spindle, exacerbating taper issues.
We still outsource parts for ID grinding, such as those with a blind hole where a counterbore leaves no relief for a honing tool.
For our own shop, however, we found that precision internal diameter (ID) grinding machines are several times more expensive than an equally capable hone.
What's important is that accuracy for the grinder is dependent on the machine's positioning capability, while accuracy is mostly tooling dependent with a hone.
Periodic checks, calibration and refurbishing are needed to ensure positioning tolerances stay tight on a grinder.
Honing tools are simple and rigid.
When they wear, you replace them.
Unlike a grinding wheel on the end of an arbor, the honing tool isn't subject to bending forces.
The tradeoff is that any given honing tool is suitable for a very limited range of diameters, so you need to have more tools in stock than you would grinding wheels.
Two other points are worth noting about ID grinding and hard turning.
If a part comes off a hone just a little too small, you can re-run it, while that's much more difficult, if not impossible, with ID grinding.
Also, neither process can produce honing's characteristic crosshatch pattern on the bore surface.
This has proven a desirable feature for maintaining an oil film for rotating gears.
* The impact of process control - when working to high Cpk requirements, the high resolution of the tool feed system and consistent nature of the process give honing a real advantage in targeting and holding a dimensional sweet spot.
This is critical because higher Cpk means the band of variability around your target values has to be reduced.
For rule-of-thumb purposes, when the target is 1.33 Cpk, we aim for about 60% of the print tolerance; at 1.67 Cpk, the target is 40% of tolerance.
A tolerance of five tenths inch (0.013mm) on bore size, thus, shrinks to three tenths (0.008mm) or less when six sigma quality requirements are imposed.
Various holemaking processes, such as boring, drilling and reaming are capable of holding good tolerances, but when a high Cpk requirement is imposed, honing has the advantage in control and consistency.
With automated honing, we can easily control tolerances to 50 millionths of an inch .
In fact, we have run capability studies where we've hit double-digit Cpk levels when honing for bore size.
* Producing a bore-type gear - one of our core products, pump gears, start as flat, washer-type blanks made on a screw machine.
These gears operate in a small, precision housing, so any perpendicularity error in a shaft-mounted gear causes wobble, loss of efficiency, noise, increased friction and possible leakage.
An adage of gear making is that a gear can be no more accurate than the blank from which you start.
On a bore-type gear, this means starting with parallel faces and a perpendicular, round bore with parallel walls, and no taper or belling.
Our minimum standard is 0.0005in 90.013mm) for parallelism and perpendicularity, and we'll work to tighter tolerances as gear quality dictates.
The pump gear blanks are double-disc ground for face parallelism and width, then rebored on an automated lathe to re-qualify the perpendicularity.
We intentionally leave some stock in the bore, because we control final ID size and finish on a hone.
* Pump gears - pump gear blanks are usually stack hobbed, grouped on an arbor in quantities based on 4x the diameter of the bore, divided by the face width of the part to determine the number of parts/load.
Lack of good parallelism and perpendicularity can introduce lead error when cutting the gear, or force a reduction in the number of blanks on the arbor, eroding your production efficiency.
Pump gears often have a standard keyway or a blind-hole keyway added that must align with a tooth.
When we cut that keyway, it throws up a tiny burr, so honing for final size allows us to clean up that burr, too.
This is where honing really shines, allowing us to control final size automatically down to a few microns.
This kind of control is a real advantage when working to high Cpk requirements.
* When to hone - depending on the requirements for a specific gear, we hone gear bores at various points in the manufacturing process, working with three different Sunnen systems.
Parts are typically honed after hobbing, but on extremely tight-tolerance gears, blanks might be honed before and after hobbing.
Fixturing on the hones allows some degree of control and correction of perpendicularity, should we need to do that.
If parts are heat treated, they are honed afterward to correct for the slight shrinkage in bore size.
And if there is a plating operation, we have found it is easier to remove a little plating from the bore using a hone than it is to mask the part for plating.
We use CBN and aluminum oxide honing tools on these machines in several different configurations, depending on whether the bore is blind, keyed, etc.
We get a tool life of about 250,000 parts, depending on the material, while typically removing 0.0020-0.0030in (0.005-0.076mm) of stock at cycle times of 15s.
In terms of finish, the hones can achieve a 16 microin or better finish.
Our newest system is a fully automated Sunnen SV-1005 vertical machine, with a rotary table and automated part handling.
Using Sunnen's Krossgrinding tools, this machine can control hole size to accuracies of 0.25 micron (0.000010in), with minimal variability.
The machine can even make corrections that are not intuitive for an operator.
Switchable control features, such as 'correct for bore shape' allow the operator to select a 'problem' bore image, for example barrel or taper, and the machine will automatically correct the part.
No matter what the part print specifies, all customers want quieter drives, smoother operation, greater efficiency and long life.
We strive to give customers a product that is noticeably better than what they would get from a competitor working to the same spec.
Honing is one way we add this value without adding significant cost.
** 'Excellence without Exception' - we use this term to describe our formula for always giving customers a good experience in every interaction with Forest City Gear.
Here are a few key ways we implement that formula.
* Invest in new equipment - we say we have the most modern gear making shop in the world, and we invest heavily to support that claim.
We want to have the very latest technology so we can do things our competitors cannot.
New equipment is arguably more productive, helping reduce lead times/costs.
It is often more accurate, too.
We invest 25-40% of our gross sales annually in new equipment, depending on our yearly performance, and we remain profitable.
Aggressive technology investments also enable us to expand our range and give us unique capabilities, making us a preferred vendor in some niche markets.
* Study other shops - we visit gear shops around the world, and always bring back at least one good idea for implementation.
I have personally been to shops in Germany, Switzerland, Sweden, France, Taiwan, New Zealand, China and Japan, to name a few.
Most shop owners are proud to show you their operations.
Our door is always open to competitors.
We have actively courted US gear makers to visit our shop, view our P and L statement, our customer list, our operations.
We have had almost no takers, which is worrisome for our industry.
* Training - we budget US$100,000-200,000 per year for training a staff of 42, and it is spread across all employees.
About 50% of our staff has completed AGMA basic courses, and our goal is to have everyone AGMA-certified.
We want all our employees to understand our products and technology, and be able to communicate intelligently with customers, even if just taking a phone message.
We are very active in AGMA, with our general manager serving as past co-chairman of the education council.
* Improvement - 'Continuous improvement' is as old as man.
It is the thread that binds all the components of 'Excellence without Exception'.
It takes more than good price, quality and delivery to distinguish a manufacturer these days.
We achieved AS-9100 certification about two years ago.
Our quality rep is past chairman of the AGMA Statistical Council and a member of the Small Business and Educational Council.
Lean manufacturing is our next goal.
Though scrap in our shop is miniscule, we still see potential for cost savings and lead-time reductions.
* About the author - Fred Young is president of Forest City Gear, based in Roscoe, Illinois, USA.
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