In this Series of discussions, we will explore some past projects that involved some trouble shooting and some machining tips that were learned along the way. This might help you set your mind when certain problems arise in your next project.
NYLONS: Case Study Continued
In this study we are going to talk about several different grades of Nylons that we explored for this case study, a brief discussion on each so you understand a little about material choice. For general purpose wear and structural parts Nylon is a good choice, but which one?
Note: Nylons can absorb up to 7% (by weight) water under high humidity or submerged in water. This can result in dimensional changes up to 2% and a corresponding reduction of physical properties. Proper design techniques can frequently compensate for this factor.
Nylon 6/6 (30% Glass-Reinforced): For applications requiring higher compressive strength and rigidity, improved load capacity and or improved frictional characteristics, 30% glass reinforced Nylon 6/6 is also available. It is stocked in diameters ranging from or .394” to 5.910”.
Common Applications for Nylon 6/6 (30% Glass-reinforced):
Brake Fluid Reservoirs
Nylon 101: Of all the unmodified Nylons, Nylon 101 is the strongest, most rigid and one of the highest melting points. It is stocked in both natural and black. Other colors are available on a custom basis. Nylon 101 natural is FDA, USDA, NSF, and 3A-Dairy compliant. Great for small parts and screw machined Nylon Type 6/6 components.
Common Applications for Nylon 101:
Electrical Insulator Components
Food Contact Components (Conveyor Wheels and Rollers)
Gears, Cams, Pulleys, Sheaves and Sprockets
Nylon MC® 901: This heat stabilized nylon offers long-term thermal stability to 260°F. It is blue in color and used in a variety of bearings, structural applications and custom parts. This is a great replacement for large cast Type 6 Nylon parts.
Common Applications for Nylon MC® 901:
Rack and Pinions
Nylon MC® 907: Unmodified Type 6 Nylon offering the highest strength and hardness of the Nylon 6 grades. MC 907 natural is FDA, USDA and 3A Dairy compliant. It is off-white in color and primarily used for food contact parts.
Common Applications for Nylon MC® 907:
Food Contact Components
Nylatron® GS: Let's move on now to Nylatron® Grades that we have used and tested in this project. This is for improved load bearing capabilities. Let's start with (GS Nylon) this grade is an improvement for load bearing capability over a standard Nylon, this grade is filled with Molybdenum disulphide (MoS2) which improves strength and rigidity. This grade has a lower coefficiency of linear thermal expansion than a Nylon 101 and maintains better clearances and fit with fewer tendencies to seize as bearings.
Nylatron® GSM: For best wear resistance and lowest coefficient of friction applications. Nylatron GSM contains finely divided particles of molybdenum disulphide (MoS2) to enhance its load bearing capabilities while maintaining the impact resistance inherent to nylon. It is the most commonly used grade for custom parts and is grey black in color.
Common Applications for Nylatron GSM:
Nylatron® GSM Blue: The first cast nylon to combine both molybdenum disulphide (MoS2) and oil for the load capability of Nylatron GSM nylon, plus improved frictional characteristics. It excels in higher pressures, and at low speeds–up to 40 fpm. It offers 20% lower coefficient of friction, 50% greater limiting PV, and a lower “k” factor than Nylatron GSM, and the lowest “slip-stick” of any nylon product. Nylatron GSM Blue should be considered for any oil-filled nylon application and is dark blue in color.
Common Applications for Nylatron GSM Blue:
Nylatron® NSM: Best bearing and wear nylon product available today. Proprietary type 6 nylon formulation produced using Quadrant’s Monocast® process. Solid lubricant additives impart self-lubricating, high pressure / velocity and superior wear resistance characteristics. Nylatron NSM was developed specifically for demanding applications where larger size parts are required. In wear applications, Nylatron NSM lasts up to 10 times longer than standard Type 6 nylon.
Common Applications for Nylatron NSM:
Coolants are generally not required for most machining operations (not including drilling and parting off). However, for optimum surface finishes and close tolerances, non-aromatic, water soluble coolants are suggested. Spray mists and pressurized air are very effective means of cooling the cutting interface. General purpose petroleum based cutting fluids, although suitable for many metals and plastics, may contribute to stress cracking of amorphous plastics.
Sawing Tip: Rip and combination blades with a 0° tooth rake and 3° to 10° tooth set are best for general sawing in order to reduce frictional heat. Use hollow ground circular saw blades without set will yield smooth cuts up to 3/4” thickness. Tungsten carbide blades wear well and provide optimum surface finishes.
Drilling Tip: Coolants are strongly suggested during drilling operations, especially with notch sensitive materials and glass or carbon reinforced products. The insulating characteristics of plastics require consideration during drilling operations, especially when hole depths are greater than twice the diameter.
Milling Tip: Sufficient fixturing allows fast table travel and high spindle speeds when end milling plastics. When face milling, use positive geometry cutter bodies. Climb milling is recommended over conventional milling. To ensure finished part flatness, always machine a plate flat to start. Do not force a plate flat with a vice or vacuum.
Threading and Tapping Tip: Threading should be done by single point using a carbide insert and taking four to five 0.001” passes at the end. Coolant usage is suggested. For tapping, use the specified drill with a two-flute coated tap. Remember to keep the tap clean of chip build-up. Use of a coolant during tapping is also suggested. Use of a coated tap will create radii at the root of the threads resulting in a stronger and tougher thread which is less prone to cracking from over-torquing.
Turning Tip: Turning operations require inserts with positive geometries and ground peripheries. Ground peripheries and polished top surfaces generally reduce material build-up on the insert, improving the attainable surface finish. Fine grained C-2 carbide is generally best for turning operations.
Post Machining Annealing: In our experience and parts we have produced this step was avoidable with a roughing in machine stage and leaving parts unclamped for 1 to 2 days. This worked great for critical tolerance work. Here is some information on the annealing process that we feel is important to at least mention.
Case Study: Wire Rope Excessive Wear: Various Nylons
It is rather difficult to describe a machine assembly in words, the complexity and blend of different components of all kinds of materials is lost without the use of pictures and specifications. This again is a proprietary machine and project; I can talk about this client's machine troubles but can't show any of the machine images or any of the plastic components we machined and tested.
A client reached out to us to see if we can help repair or restore a very old machine of the 1900s era that is still in use. As you may know those where the days of heavy thick-walled Cast Iron and steels. Well made without a doubt and has survived the test of time. This large machine performed some type of mechanical action that use to be done by hand, like the old 1900s machinery used in Woolen Mills.
The machine consisted of steel shafts, cast iron gears and sprockets attached to timing cams. Large steel sheaves with steel cables that attached to large heavy cast iron turntables that connected to heavy counterweights, that I assume is used to ease the rotation load. These turntables rotated on Babbitt bearings and rested on spring loaded bronze wear pads. Large flat belts with 45 - 90° twists, casted metering blocks, casted filter mounting cap and large valves. On the lighter side of things there where long rack and pinion sets all over the place, a few smooth steel rollers, casted electrical housings in various places and yes tons of grease.
Challenge: Replace broken and worn cast iron gears and sprockets that attached to the timing cams. Address the excessive wear on the steel cables that control the turntables rotation. Replace the Babbitt bearings, resurface spring loaded bronze wear pads, and replace any worn rack and pinion sets. Resurface or replace steel rollers that are badly pitted and corroded, replace all broken electrical housing covers, repair casted filter mounting cap, and yes clean up the grease!
What a laundry list of things to review, as you can image this machine is not replaceable to this client.
Solutions: So where did we start in this lengthy multi-material project and why Nylon. First thing ruled out was that this is not a machine that came into food contact, so that rules out any FDA compliant materials from the list. Second thing we are dealing with gears, pulleys, sprockets, rotational wear situations and heavy loads. Third thing is the corrosion on parts that should be smooth from areas that are not shielded to the environment.
It was decided to start with the least accessible parts first and work our way out isolating the main sub-assemblies. The first sub-assembly is the turntables that consist of bearings, sheaves, wired steel rope, counterweights and spring-loaded wear pads. Turntables had a lot of bearing play and needed to be replaced, original material was Babbitt. Since this was a hard to get to area, we decided to use Nylatron® NSM for these parts large size and the solid lubricant additives impart self-lubricating, high pressure / velocity and superior wear resistance characteristics of this material. Next are the steel sheaves that attach to the underside of the turn table shafts. The client expressed a high wear condition with the steel wire cable that is connected to these sheaves. We replaced the all the steel sheaves with Nylatron® GSM cast nylon to increase the wire rope life, this also reduces rotating weight, eliminates corrosion over its former steel part. Before attaching the turntables, we must resurface the wear pads, after reviewing the current bronze pads. It was decided to replace the thin bronze pads with Nylatron® GSM Blue because of its lowest “slip-stick” quality of any nylon product.
One quick side note here, while we assembled these new components on this sub-assemble. The customer expressed annoying thumps and bangs when the counterweights were moving and suddenly stopped. After reviewing this we suggested adding Nylon 6/6 guide wearing strips to control the shaking of the counterweights.
Moving on to the corroded steel rollers, these rollers are on 6" to 12" long and attach to several arms that move up and down on a timing cam. These rollers are also exposed to heat and moisture which explains the rust and pitting all over them. Now we did measure the pressure that these rollers produce when they reach there down stroke and concluded to keep the steel rollers but turn the diameters down and sleeve them with Nylon MC® 901. This heat stabilized nylon offers long-term thermal stability to 260°F.
We now reach the gears and sprockets that are in several locations throughout the sub-assemblies that connect to the main assemble. We replaced all bronze, Babbitt bearings with Nylatron® GS since this grade has a lower coefficiency of linear thermal expansion than a Nylon 101 and maintains better clearances and fit with fewer tendencies to seize as bearings. All worn gears and sprockets were replaced with Nylatron® GSM for its excellent wear resistance. Along the machine are broken and worn rack and pinion sets that are exposed to a heated platen, we decided again to use Nylon MC® 901 to resist the long-term heat exposure from the heated environment.
Lastly are the odd ends of the machine that are broken or missing. Let's go to the casted filter mount cap, this looks like a took a whack and was cracked. We replaced it with a standard black nylon which will work fine with the low-pressure water that runs through it. There were several electrical boxes or junctions throughout the machine that were missing or badly fractured from tampering. We used Nylon 101 and created new covers for the various housings, we chose this material because it is a good electrical insulating properties and strength. Finale parts where the metering blocks that were in good shape, however there was damaged oil reservoirs that were cracked and missing. We could have used a standard Nylon here but chose Nylon 6/6 (30% Glass-Reinforced) for its strength and rigidity, since we did keep a fairly thin wall to slightly see the fluid level.
There you have it; I know it is much more complicated than this article can describe in this case study. This should however give you a thought process behind some of the choices here. Although the customer could have machined many of the parts in their original materials like bronze and even the steel rollers. There are better materials now than the early 1900s had to offer, some of the large cast iron gears however could not be reproduced at a reasonable price, the quantity didn't constitute the cost of creating a sand casting. We must mention that we had to outsource many of the gears, sprockets, sheaves and bearings due to their size and the fact that we don't cut gears here, the rollers, metering reservoirs, filter mount cap, wear pads and electrical housing covers we did fully design and machine.
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