New Approach to Chucking by Soft Jaws

Considering the descriptions of the parts and castings, it sounds to me that you are doing some rugged cutting, therefore I would not recomend plastic jaws, not only will they not hold on to the part well enough, they will probably not hold concentricity either.

It would take some effort to setup, but you could design several sets of jaws that would be precision pinned to the base jaws in the chuck, or make a set of adapter jaws which would allow for repeatable exchanges from one set of jaws to the other. The main reason you re-cut soft jaws is because when you take them out and put them back again you lose concentricity. Take the extra trouble to make the exchanges from one set of jaws to the other repeatable in location and concentricity and the problem is solved. Remember also that you will not accomplish this with old beat up lathe chucks, but will require a high quality precision chuck.

There may already be a solution to this with off the shelf tooling that I am not aware of yet. I don't know how soon you need to accomplish this project, but if you have time you should visit one of the machine tool trade shows such as Eastec, or in September you could go th IMTS in Chicago. It is too late for WESTEC, as it was in March.

I'm not sure I fully described the situation. Currently we remove and replace our machined soft jaws (top jaws) from the base (Schunk ROTA THWB 315). Concentricity is currently fine, and we only cut new soft jaws when the old ones wear out. The issue is a lean consideration: we have 30 or so soft jaws, and the setup times are slow. We run 1-2 piece orders at times, and we sometimes spend more time changing setups than actually machining.

The jaws I am looking at making will be pendulum jaws, purchased off the shelf, and I plan on replacing the hard, serrated inserts with a polymer--with a Shore D hardness in the 80-90 range. Too hard and it will be slick, too soft and the part will come out when the tool starts to turn or face the part.

This idea accomplishes my lean goal of setup reduction by utilizing the chuck's ability to swap base jaws quickly. I don't want to buy base jaws for 30 or so soft jaws, but rather for 3 more flexible jaws.

I've seen some polymers used to hold parts: in our plant, in order to hold various shafts, D1.0-4.5, we use a 6-jaw chuck with a glass-fibre phenolic material as an insert that is turned to fit. Also, I've seen in a Schunk catalogue, a type of machinable fiberglass soft jaws, that claim to have 3 times the friction of ordinary steel soft jaws. So it is possible to use a polymer to work as an insert, but I've yet to select which polymer, and I haven't decided on the geometry of the inserts, either. I am trying to find someone who has actually tried this, or something similar, and what their results were.

It sounds like you are already on the right track. You have a tough situation, lean prototyping is not easy. Are these 1-2 piece orders repeat jobs? I know that this idea goes against lean manufacturing theroy, but I have found that at times it is easier to run several orders worth of a particular part and put it on the shelf than to incurr the set up cost over multiple orders. I understand that in this area you are going back and forth over a fine line. Whether or not this works depends on the cost of the materials or in your case the castings vs the time and cost of the set up, and the cost of storage and inventory. Your available machine time and work load are also factors. For me this is a daily challenge. One of my customers orders as much as a years worth of parts in advance and is constantly adjusting the delivery schedule. Each week I have to decide if I should make the whole order or just what I need this month.

I like the sound of the phenolic jaws, but I have not tried them. I wish you good luck, and please share what you learn with the rest of us when you get it worked out.

The problem with large orders and storage time is that it often costs us scrap. We make custom electric motors, so if we make too much of a part it will sit on the shelf so long it will often end up obsolete before it gets used. It's very hard to quantify how much overproduction costs, but we try to limit it as well as setups.

If you sizes, (that you have to cut the soft jaws too) are repeating, you might want to look at removing the jaw altogether and going to a collet system. Quick to change and requires no cutting to set up.

You can get creative and make some collets that have the carbide gripper bits in them.

Map out the turret so your not taking tools in and out, what a waste of time. You can re-wright the programs to call out the tools that are in the turret rather then swap them out.

I've been successful in reducing change over on all kinds of machinery, the thinks you read about in the trades really do work, you have to believe in them.

Not a user of cnc machines but i do use an older lathe ... i wont go as far as to say often .. but enough to have made a mistake or 2 in secureing the material properly. It wasnt to expensive the first time .. not so the second. That was a machine bout 15,000 and change. I wouldnt want to repeat that on anything like what your talkin bout. Do as you will but proceed slowly and cautiosly.