How to quote Swiss machining work

The part is a stepped shaft for Swiss machining, 0.375 inch diameter and 2.8 inches long, with 4 diameters, a cross hole, a thread and a groove. It is 316 stainless, the print calls a 0.0005 inch tolerance on 2 of the diameters, and the volume is 60,000 a year.

An estimator who prices this like chucker work will be somewhere between 20 percent high and 30 percent low, and which one depends on details that do not appear on the print. Every figure below is illustrative and one shaft carries through. The build-up is the one a machined part quote uses, with 3 lines behaving differently.

Why Swiss machining cost behaves differently

A Swiss lathe supports the bar right at the cutting tool, which changes what is possible rather than just what is fast. The guide bushing means a long slender part does not deflect, so the 0.375 diameter at 2.8 inches long holds tolerance in one operation instead of needing a steady rest and a second setup.

That single fact drives most of the cost difference. Work that would take 2 or 3 operations on a chucker often takes 1 on a Swiss, and every operation you remove takes its setup, its handling, its work in process and its inspection with it.

The second structural difference is that the machine is fed from bar stock continuously. Nobody loads parts. That decouples labour from cycle time in a way no chucker does, and it is where the estimating mistakes start.

Bar feeding, and what it does to the material line

Swiss work is quoted from bar, so the material line is a length calculation. The shaft is 2.8 inches long and needs an illustrative 0.08 inches for parting, so each part consumes 2.88 inches of bar.

A 12 foot bar is 144 inches, which yields 50 parts. The remnant matters more here than on other processes, because a Swiss needs a minimum remnant length to stay in the guide bushing, and an illustrative 10 inch remnant is unusable. So 144 inches becomes 134 usable, which is 46 parts rather than 50.

That is an 8 percent material penalty that never appears if you divide the bar by the part length. On 316 stainless at 60,000 a year, 8 percent of the material line is real money and it is the single most common Swiss quoting error.

Divide by the usable bar.

Bar quality also matters more than on other work. Swiss machines run tight guide bushing clearances, so bar that is out of round or oversized either does not feed or wears the bushing. Centreless ground bar costs more than mill finish and sometimes the process requires it, which is a material decision made by the machine rather than by the print.

Long unattended runs and the labour assumption

The labour line on Swiss work is an attention percentage, not an operator. Once the machine is running with a full bar magazine, one person can watch several machines, and the labour cost per part is a fraction of an operator rather than a whole one.

That is why the same work center rate logic used for a manned machine gives the wrong answer. A Swiss cell running 3 machines to 1 operator carries a third of that operator in each machine’s rate, and the burden of the machine itself dominates.

The nuance is what unattended actually means on your floor. Running lights out overnight assumes the bar magazine holds enough for the shift, that the tools last that long, and that a crash does not go undetected for 6 hours. Where those are not true, the attention percentage climbs and so does the rate.

Tool life is the constraint that decides it. If an insert lasts 400 pieces and the magazine holds 900 pieces of bar, the machine stops for a person partway through, and the unattended assumption in the quote was optimistic by half.

Unattended is an assumption, not a fact.

Material choice pushes on the same lever. The 316 stainless on this shaft work hardens and is unkind to tools, so tool life is shorter than it would be in a free machining brass, and the attention percentage climbs with it. Two identical prints in 2 materials are 2 different labour lines.

The sub-spindle and the second operation it removes

A sub-spindle catches the part and machines the back end while the main spindle keeps working. On the shaft, that means the cross hole and the back face happen without a second setup, a second fixture or a second inspection.

Costed against a chucker route, that removes an entire operation with its own setup time, its own queue and its own handling. On an illustrative comparison, the chucker route needs 2 operations at 45 minutes of setup each, where the Swiss route needs 1 setup of 90 minutes.

The setup is longer and there is only one of them, which is the trade at the heart of Swiss quoting. Ninety minutes against 90 minutes looks like a wash until you count the work in process, the second inspection and the second queue that the chucker route also carries.

The removed operation also removes a source of variation. Every time a part is unclamped and reclamped, concentricity between features machined in different setups depends on the fixture rather than on the spindle. On a part where 2 diameters have to run true to each other, doing both in one setup is a quality argument as much as a cost one, which is the same reasoning an assembly quote uses when deciding what to machine before joining and what to machine after.

Tooling positions as a constraint on the part

A Swiss machine has a fixed number of tool positions and the part has to fit inside them. The shaft needs turning tools for 4 diameters, a threading tool, a grooving tool, a cross drilling tool and a cutoff, which is 8 positions before anything goes wrong.

If the part needs 11 tools and the machine holds 9, the job either does not run on that machine or runs as 2 operations, which removes the advantage the Swiss was bought for. This is a quoting question and it is answered by counting tools against the specific machine, not the category.

The estimating habit worth building is to write the tool list before pricing the cycle. It takes 10 minutes, it settles whether the part is a 1 operation job, and it catches the parts that look like easy Swiss work and are not.

Count the tools before the seconds.

Live tooling is the version of this that catches people out. A cross hole needs a driven tool, and driven positions are fewer than static ones on most machines. A part with a cross hole, a flat and a slot may need 3 driven positions where the machine offers 2, and that is a routing problem discovered at the quote or discovered on the floor.

Tolerance sits alongside it. The 0.0005 inch callout on 2 diameters means an in-process gauging step or a slower finishing pass, and on a part where the whole cycle is under a minute, adding 6 seconds to hold a tolerance is a 10 percent cycle penalty that has to be in the number.

Where the volume crossover actually sits

The crossover is a setup absorption question and you can calculate it rather than assert it. Take the setup difference between the 2 routes, divide by the per piece cost difference, and the answer is the volume where they meet.

On the illustrative shaft, the Swiss route carries 45 minutes more setup and saves an illustrative $0.38 a piece across cycle, handling and the removed second inspection. At a loaded rate of $95 an hour, 45 minutes is $71, and $71 divided by $0.38 is 187 pieces.

Above roughly 190 pieces in a lot, the Swiss route is cheaper on this part. Below it, the chucker wins on setup alone. At 60,000 a year in lots of 5,000 the answer is not close, which is why the arithmetic is worth doing once and then trusting.

The same calculation is what settles most process choices, including the machined part routing decisions that look like preference and are usually arithmetic nobody ran.