Squelching stamping noise.
Any attempts at reducing noise in a press shop start with the obvious source of noise--the press. But there are several secondary noise sources that either alone or cumulatively can contribute considerably to the noise problem and to its ultimate solution.
Start with tool design
The tool designer's contribution to noise abatement can be substantial and highly effective. It is well established that sound intensity is directly related to die impact intensity. The time element is very important. Noise levels are highest when high press forces act over a very short time period.
Consequently, the well known and widely practiced methods for decreasing cutting forces in dies by performing progressive, successive cutting operations (instead of all in one hit) are excellent ways to decrease noise.
Here are two basic examples: provide simple or double shear inclinations on the cutting edges, Figure 1, and stagger the respective cutting heights of multiple cutting operations, Figure 2. In both cases, the cutting action is extended over a longer time period while reducing the maximum cutting forces. The result is smooth, progressive cutting that yields considerably less noise than impact with straight cutting edges that are acting simultaneously.
By inclining the shearing cutting edges at about stock thickness, the noise decrease will be about 5 dB to 10 dB. With staggered punching, the noise decrease is usually less drastic.
Additionally, anything that makes the shearing action easier, will make the operation less noisy. Examples are completely tapered die-plate openings (as in Figures 1 and 2), large punch-to-die clearances, and limited punch penetration into the stock. These tactics lessen the shock and can reduce noise up to 6 dB. (According to some, the fineblanking process is much less noisy than the conventional blank-through process.)
In delicate cutting and forming tools, it is customary to provide bumper blocks for positively limiting the shut height of the dies. To eliminate the resultant impact noise, put a resilient shock-absorbing plastic ring or disc on the bumper blocks. Thin layers work best, about 1/16 thick.
Drastic noise reductions can be achieved this way. Bumper blocks are also very effective in controlling punch penetration in pressworking operations. By holding punch penetration to a minimum to keep noise intensity within reasonable limits, you also get as fringe benefits better overall performance and longer die life.
Damping rings or discs are sometimes put between tool members, Figure 3a, where the die shut height is governed only by the damping ring. It is much better to leave a small metal-to-metal matching design, Figure 3b, in case the elastic material should fail. Such arrangements can reduce noise by as much as 5 dB to 10 dB.
In coining operations, try to minimize the contract area, using only the essential points or surfaces.
The ejection or removal of parts and scrap may be a considerable source of noise. Although the basic problem of ejection belongs under material handling, it is the tool designer who must decide how the part, slugs, and scrap will be removed from the die and disposed of. Thus, this detail should be incorporated into die design.
For stamping ejection, our first preference should always be given to pushthrough evacuation. The stampings should simply fall onto the press bed or bolster plate and be pushed out mechanically, unless the press is inclined or the press bed and bolster plate have sufficiently large openings for direct evacuation.
Use direct air ejection only as a last resort. Unfortunately, because compressed air is so inexpensive and realily available, it is very popular, particularly with small stampings. However, keep in mind that air jets cause excessive and unpleasant noise levels, and ejection is usually erratic and unreliable.
Where you must use air-jet part of scrap removal, you can minimize noise levels by using a combination of these recommendations:
Reduce air pressure and velocity by in creasing piping ID. (Air flow increase with the fifth to eighth power of velocity, while work done by it increases ?? to the second power. So reduce air speed and pressure, and increase the acting area. At 20 psi, air-jet noise is less than ?? percent of what it is at 100 psi.)
Make the air cycle as short as possible.
Use specially designed, commercially available air nozzles with built-in muffling action.
Select the smallest nozzle possible. For larger stampings, use several small-diameter nozzles instead of one large one.
Use only streamlined nozzles with no sharp corners, edges, or shoulders that increase noise unnecessarily.
Bring the nozzle point as near as possible to the part to make air travel very short. Any obstacle in the air flow causes turbulences and increases noise. A ject directed at an edge, Figure 4, produces high noise, so streamline the tool edge to obtain a smoother flow.
Try to make the nozzle part of the die set, or even better, the die itself (using quick-acting air couplings). Better yet, eliminate the nozzle with a few strategically drilled holes connected with a common conduit, Figure 5.
Material handling can be noisy. Noise pollution is usually greatly affected by the way stampings and scrap are handled after they are ejected from the die. This can produce more noise and higher intensities than the stamping process itself. Here are some recommendations.'
In either automatic or manual part removal, noise from parts striking hard chutes, tote boxes, etc can be substantially reduced by padding, lining, or coating the chutes, hoppers, boxes, trays, tumbling barrels, etc both inside and outside with vibration-damping materials such as neoprene, fabric, carpet, or other shock-absorbing, nonmetallic materials. According to reliable studies, such linings can effect up to a 70-percent reduction in noise levels.
Replace metal chutes with wooden or plastic ones.
Use tote boxes made from wire; they're practically noiseless.
Regardless of the part-ejection means, keep the free travel to a minimum. Parts will strike with lower impact and noise. Deflectors can be used to reduce impact.
To avoid the transmission of press vibration, don't fasten chutes rigidly to the body of the press.
Use small conveyors (with rubber belts) wherever possible for stampings or scrap.
Cover conveyor rollers, stock-straighteners, reels, etc with neoprene or fabric to reduce the noise produced by these accessories.
In addition to damping methods for the dies themselves, here are some other ways to achieve noise damping:
One of the simplest procedures is to insert an elastic sheet between the die shoe and press bed or bolster plate. Up to 9 dB noise reductions can be realized.
Some feel a heavy coating of oil in the die region provides a certain amount of damping and cushioning.
Wherever possible, give preference to hot stamping processes over cold ones because hot working produces less noise.
Strict press preventative maintenance is highly recommended. Maintenance of machinery, related equipment, and the entire press installation (checking air lines for leaks, for example) should be detailed and up to the minute. Worn, sloppy machines (dynamically unbalanced, improperly adjusted) are noisy and will not perform properly. Automatic feeds, clutches and brakes, bearings and gears, etc should be checked regularly.
Keep machines properly lubricated and adjusted. Keep fasteners tightly locked. (Loose screws, bolts, or nuts mean loose machine members and unnecessary sources of noise.) Keep movable press components in good repair, adjusted, and dynamically balanced. Replace worn parts immediately. Eliminate noise amplifiers such as loose guards and sheet-metal covers.
Prior attention should also be given to die maintenance. Especially important is the keenness of cutting edges. The sharper the edge, the less the cutting effort, the less the resultant noise level.
Many small practical hints of limited value can be combined to cumulatively produce quite handsome results. Here are a few worth considering:
Special low-noise electrical motors are now being marketed that can produce up to 10 dB noise reductions.
To avoid metal-to-metal contract, make the couplings between driving motors and shafts with intermediary nonmetallic discs.
Streamline press air lines. Any change of direction should have a mean radius of at least three to five times pipe diameter. Cover noisy hydraulic piping with acoustical pipe wrap.
Cover air exhausts of pneumatic devices with commercial or homemade mufflers. Use pipes or hoses to lead exhausts away to isolated portions of the building or outside the plant, not into pressure chambers.
Avoid safety guards or covers made from thin, solid sheet metal that can vibrate and become secondary sources of noise. Use instead open mesh, woven wire, sheet plastic, or perforated metal.
Fence off, enclose, and soundproof the air compressor.
Remove from the shop floor any empty drums, tanks, or other large containers. Or at least turn them upside down. This will reduce reflections, reverberations, or resonances of sound energy.
Strictly forbid the use of air guns for personal cleaning, chip removal, or the blowing out of dies unless the air pressure is 30 psi or lower.
Photo: 1. Inclination of the punch edges extends shearing action.
Photo: 2. Staggering punch heights distributes cutting forces.
Photo: 3. Damping rings should not rely solely on the rings' elasticity, a. It is better to leave a small amount of metal-to-metal contract, b.
Photo: 4 Air jets on die edges can produce noise, a. Instead, streamline the die edge to get smoother air-jet flow and less noise, b.
Photo: 5. Die holes, connected by conduit, can yield noiseless part ejection. Use this approach with an inclined press.
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|Title Annotation:||reducing noises in a press shop|
|Publication:||Tooling & Production|
|Date:||Oct 1, 1984|
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