Another finding demonstrated by Dr. Strehl’s manufacturing simulation was the reduced investment costs for a new gear factory when going the PM route. In that scenario, PM gears would be bought and heat treated, hard turned, power honed, and washed, saving floor space, heating, cleaning, and of course machinery and manning.
For a 6-speed manual, Dr. Strehl showed that sintered gears should be able to replace 3,4,5, and 6 gear stage and only 1, 2, and reverse would be kept in solid steel. This would bring the machining investments down from 27 machines to eight machines and that would bring the whole building down in size relative to the reduced number of machines. Smaller building, less people running it, less heating, cleaning, scrap handling and so on.
Dr. Strehl’s calculations are very well aligned with our internal models at Höganäs AB, but it is always reassuring when someone outside the PM world comes to the same conclusions independently.
The number of gears manufactured is important since tooling cost per gear depends on (1) how many gears one tool is able to produce and (2) the total volume of gears. There are truths that need to be challenged here. I have worked with quotations for spur gears where 10,000 gears were enough to reduce costs for customer and repay PM tooling in one year or less. For helical gears the numbers had to be higher, but 30,000 gears would do the trick. It is a case-by-case scenario and the comparisons referred to have been made together with a gear manufacturer that offered both PM and steel gears—they have given two quotations when the gears have been suitable for PM.
Also, conventional internal ring gear tooling of the reaming type is quite expensive, and the manufacturer needs several of them. In PM manufacturing one tool is needed for production and one backup die can be stored semi-finished at the tool shop, together with drop in replacement punches. Generally around 100,000 PM parts or more should be able to be produced using a carbide die, some regrinding of punches can be done and also a refurbishment of the die is sometimes possible. How many gears can one hob or reamer make before refurbishment, and how many times can the hob be refurbished? The tooling cost and the clamping tools are not that much cheaper in the long run and for the case of larger ring gears the tooling cost for PM is significantly lower.
Another aspect is the machining that could be necessary in order to create features that are not feasible to compact. This could be holes perpendicular to the press direction for lubrication or other features not possible to compact and that do not require extremely high tolerances. Instead of machining the sintered component, it is actually possible to machine in what is known as the “green state,” meaning directly after compaction and before sintering. The advantage of this is tool wear; the disadvantage is that it takes some trial and error to devise the clamping system of the part and to set machine parameters without creating burrs or flaking of small bits of the material when, for instance, the drill bit tip exits.
The consistency of the green material is like a chocolate bar, with respect to hardness, strength, and elongation (though chocolate tastes better). This should give a feel for how much clamping force and experimentation that is needed to get it to work. The tool wear will be close to none due to soft material that has lubrication mixed with it and the productivity will not be impeded by the lower strength, but the total process is more cost efficient than machining in the sintered state. That is a method to have in mind when designing parts or talking to the parts supplier.
If a more complex gear geometry is designed it can be built by two separate parts, PM-PM or PM-steel, and joined by ion beam, laser, or capacity discharge welding. This is a normal method for attaching synchronizer rings to gear bodies in manual transmissions. Another method frequently used in the U.S. is sinter brazing of planetary carriers, which are made from two PM components. The brazing is done during the sintering step so it does not add a new process to the production chain. 100% of the sinter-brazed components are normally inspected with respect to the quality of the brazing. This can be accomplished by acoustic methods or by vibratory response checking. So gears that are not feasible with a single compaction can be made as 2-piece constructions or even 3-piece constructions.
The presentation referred to was made in the University of Aachen, WZL at the “Trends in Gear Soft Manufacturing” seminar, November 28–29, 2012. Contact Mrs. Dorothèe Maffei at firstname.lastname@example.org to get information on the proceedings.