Ever wonder what press fit tolerances are or where to get a press fit calculator to make it easy for you to machine press fits? Perhaps you need this information for a press fit bearing, for example.
Look no further–we offer a free press fit calculator. In fact, it figures out all sorts of ISO tolerances such as Loose Fit, Sliding Fit, Slight Interference Fit, Force Fit, and more. It looks like this:
If you already have G-Wizard, you can find it under “Geometry” and then “Fits/Tols”, which is short for Fits and Tolerances.
Like so many things, there are standards governing the exact press fit tolerances allowable. In this case they are ISO 286, ANSI B4.2, and DIN 7172.
To find the tolerances, you can start from either a hole diameter or a shaft diameter. It’s easier to make a shaft of a particular size on a lathe than a hole on mill (at least for many machinists without digging out a boring head), so I prefer to go from the hole basis.
For example, let’s suppose we want to know the press fit tolerances for a 1″ diameter hole. You’d set that up in G-Wizard like this:
In practice, you’d enter the measured diameter of the hole which you can determine with a telescoping gauge or dial bore gauge.
As you can see, the hole can range from a minimum of 1″ to a maximum of 1.0008″. You’ll need to hold a fairly tight tolerance to hit that range, which why I suggest just measuring the hole and holding the tolerance on the shaft if you can. For a press fit bearing, you have that luxury for the shaft but not for the bearing pocket.
The shaft, can range from 1.0009″ to 1.0015″. So, we’re going to have at least 0.0001″ (if the hole is 1.0008″ and the shaft is 1.0009″) of material interfering with the fit. That’s why it’ll have to be pressed together.
No Press? How About a Shrink Fit With Hot and Cold Temperatures?
It’ll take a press to force the two parts together, or will it? It just so happens that another free calculator that’s built into G-Wizard is a thermal expansion calculator. I wonder what temperature difference is needed to expand the hole enough to clear the 1″ shaft if the shaft is the worst case 1.0015″?
In other words, we need to expand the hole, which is treated as a ring in G-Wizard, to 1.0016″ or more. Here’s that problem all set up for you in G-Wizard:
Note that I selected a ring, I’m using a standard reference temperature, and my result tells me the change in the radius. Since I want to hit a diameter of 1.0016″, I need a change in radius of 0.0008″. That’s pretty easy. Assuming I measured the original hole diameter at the reference temp (put in whatever temp you measured it at if not), I need to heat it up to 180 degrees and I should be able to carefully slide the shaft in.
I can make things even easier by putting the shaft in the freezer so it shrinks.
Doing this kind of shrink fit in the shop is easy if the parts are small enough to use a fridge and an oven to heat and/or freeze them. I do this all the time to put a drill chuck onto an arbor before using it.
So These Calculators are Free?
Yes indeed. We do charge for G-Wizard Calculator, but it’s only for the Feeds and Speeds Calculations. All of the many other useful calculators are yours for free when you sign up for the Free Trial. When the trial ends, Feeds and Speeds will stop working (unless you purchase), but you can go right on using all the free calculators like Fits and Tolerances or the Thermal Expansion Calculator. Pretty cool eh?
Sign up here:
When to Use Press Fits in Your Designs
Avoid Press Fits in Plastics (Cold Creep)
First, try to avoid press fits for plastic parts. The trouble is they won’t last due to Cold Creep. If constant force is applied to plastic, it will flow until it has deformed enough that you no longer have a press fit. At that point the components come apart and you (or your customers) are left unhappy.
Check the Forces Involved
Using a temperature-based shrink fit reduces this problem, but with a press you still need to keep in mind assembly forces. They can be very large. This makes press fits not the greatest thing for ease of assembly. Plus, you’re introducing relatively tight machining tolerances to create the parts to start with.
Press fits seem simple, but you want to make sure that this simple design isn’t making assembly too hard for your part.
Use One Press Fit and One Sliding Fit Alignment Pin
Here’s another important tip–use only one press fit pin and make the second pin a sliding fit for alignment only. Trying to line up two press fits is again painful for assembly.
Keep to the Same Materials
Press Fitting two different materials together can be problematic because they have different rates of thermal expansion. If your parts have to deal with much temperature variation at all, you’ll want them to be made of the same material so the rate of thermal expansion is similar.
Use GD&T to Specify Your Tolerances
If your design uses Press Fit Tolerances, you should specify them using GD&T where possible.
The Ideal Case for Press Fits
The ideal case is when the materials for both parts are going to be the same and both parts are made to close tolerances and require close tolerance alignment. If that’s not the case, you should consider alternatives.
Press Fit Alternatives
For metals, align with sliding fit dowel pins and lock it down with bolts. For plastics, you can still use locating pins with a sliding fit and snap together construction for assembly.
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