TOLERANCES TOLERANCES, does anyone know why there are tolerences on a drawing?
The reason is because you can't make parts on a repetition basis dead to size EVERY TIME.
Tool wear, machine wear, and other factors come into play the moment you start to make more than one item.
I once had a mate who had a set to with a foreman who insisted that the parts be made to top tolerence.
As well as that he was expected to do the job in the same time.
It appeared that the mating parts were all oversize, (bore sizes).
Now I ask you, how can you make a part dead to size, +/- nothing and still do it in the production time of a tolerenced item, which is what he was being told to do by working to the top limit only.
Only after a bitter argument was the part retolerenced to the new over sized dimension.
Even the gauges used in production have a tolerence when set to top and bottom limits.
Incidently, production is carried out by semi skilled tradesmen, and women, hence the need for tolerences.
A skilled tradesman wouldn't lower himself to rub shoulders with those who are only capable of operating machinery set up by others.
That is why the pay reflects the effort put into producing the parts.
There is no such thing as a skilled operator.
A skilled operator is a costly item, so semi skilled operators are the answer to making the job pay.
In the old days the semi skilled operators worked the capstans, turrets and mills, whereas today you have the CNC set-up where the operator just loads the parts and initiates the cycle.
As unpallatable as it may seem there is a definate need for a person who can work all day long just winding handles and pushing buttons, and in the process produce parts that will pass QC in the allotted time.
Production makes money, jobbing shops rarely do.
The secret is to deskill the job, so that anyone off the street can produce accurate work with the minimal tuition, and for the least wage.
By applying a piece work situation the operator will be his/her own task master.
The scrap rate is the price to pay, but with the scrap rate built into the price of the job, it's money in the bank.
A scrap rate of 10% is not a too unrealistic quantity to bear.
How many times have I seen a foreman working after hours trying to reclaim scrap parts, when they should be written off as par for the course.
It cost more to rework a part than make it from raw material.
Ian.