Did you know that the tolerances on your drawings could be wasting a ton of money each year? They’re just a few numbers on a drawing sheet, but it could result in you paying 45 times more for each part.
The simple fact is that tight tolerances will cost you a lot of money. To better understand this idea, we put together this piece to talk all about tight tolerances, what’s wrong with them, and how much extra they’re costing you.
What Is a Tight Tolerance?
The definition of “tight tolerance” is going to vary a little bit. The general rule of thumb is that anything that calls out a tolerance less than 2 mil (0.002”) is a tight tolerance.
This is only the case for a CNC-machined part. If you’re hand-cutting something with an axe, a “tight tolerance” might be anything within half an inch.
As the machinery gets more precise, the tolerance can become tighter.
The Common Issue with Designing Tolerances
A common issue with designed tolerances is an engineering department that picks an arbitrary value and sticks with it.
This isn’t an issue for wide-open tolerances, but it’s a big problem if they default to a tight tolerance. Simply saying every single part needs to be within 2 mils could add a ton of money to the budget each year.
Instead, each part should be closely examined. Maybe only one or two key dimensions need a tight tolerance. A tolerance might not seem like a big deal to a designer, but it makes a lot of difference in the quote.
Understanding How Tight Tolerances Change the Cost
The biggest issue with tight tolerances is due to the limitations of your machine. As we mentioned earlier, using a crude axe to cut a piece of wood comes with a very different set of tolerances than using a precision mill.
As tolerances get too tight for a CNC mill to attain on its own, you’ll have to implement secondary and tertiary production steps. This means more time, more inspection, more expertise, and more equipment required.
All of these differences result in a bigger quote to fabricate the part for you.
Why Your Price Goes Up as Tolerances Go Down
Look at it this way: a part with wide-open tolerances can be quickly thrown on a CNC, briefly inspected, and sent out the door. The inspection might be as simple as running a pair of calipers around the part and checking the drawing.
The machine can also run at faster speeds and take out bigger chunks of material with each pass. Tool chatter and vibration will add into the machinist’s achieved tolerance.
As you narrow the tolerance down to 1 mil (0.001”), then things get more complicated. Machine speeds have to slow down, more passes need to be made, and different equipment needs to be introduced.
For example, a standard vertical mill probably won’t achieve the hole precision you need, so the part will need to be transferred to a lathe to be bored. As dimensions get tighter, it might need to go to a grinder or lapper as well.
During inspection, precision tools will need to be used to measure the part. In some cases, jigs will need to be created to correctly position the part so different gauges can measure dimensions.
If the part is outside of the specified tolerance, it needs to be thrown out and re-made.
What specific things cause the price to go up when the tolerance goes down?
– Equipment used. Not all shops have the precision machinery required, you’ll have to pay premium prices.
– Expertise required. With more training and experience in the machinist comes a bigger quote price.
– Additional time for fabrication. Hourly costs associated with machinists.
– Added inspection steps and potential jigs. Material costs, manhours, and lead time for fabrication.
– Time setting up and using different equipment. Extra manhours means a bigger quote.
– Higher potential for scrapped units. Units outside of tolerance need to be thrown out. This wastes time and money.
The best way to describe the price discrepancy is to look at a chart made by N.E. Woldman, Machinability and Machining of Metals, the “Approximate Relative Cost Of Progressively Tighter Dimensional Tolerances”.
It presents the following data:
– Rough Machining (+/- 30 mil): Standard cost (100%), 1x cost
– Standard Machining (+/- 5 mil): 200%, 2x cost
– Fine Machining (+/- 1 mil): 440%, 4.4x cost
– Very Fine Machining (+/- 0.5 mil): 720%, 7.2x cost
– Fine Grinding (+/- 0.2 mil): 1,400%, 14x cost
– Very Fine Grinding (+/- 0.1 mil): 2,400%, 24x cost
– Polishing/ Lapping (+/- 0.05 mil): 4,500%, 45x cost
For example, let’s say you want a standard cube of steel to be machined. The goal is to make a 2” x 2” x 2” cube.
If you open the tolerance and allow for rough machining, let’s say the machine shop quote comes out to $250.
The same exact piece made via lapping and high-precision machining will cost you $11,250. Yes, you read that right.
That’s the difference that a tolerance of 30 mil makes compared to 0.05 mil.
It’s time to stop overpaying for your parts. Reach out to our experts at Rapid Axis and let us tell you what tolerance is right for your project. Our precision shop can fabricate competitively-priced tight tolerance parts when you need it.