In the welding table manufacturing industry, the most common “standard” for tooling isn’t a standard at all. You’ll see tables that claim to have 16 mm boreholes (about 0.629″), and you’ll see tools labeled as 16 mm, or 5/8″. In fact, everyone just throws out the 16mm number everywhere!
But if you think about it, if everything is 16mm… well that’s technically impossible: a hole that is exactly 16 mm in diameter, and a tool’s pin that is also exactly 16 mm, won’t fit together! There must be some clearance.
Overlapping Tolerances: A Real-World Example
Let’s say:
- Manufacturer A makes welding tables. They advertise “16 mm holes,” but they actually target a nominal 0.629″ (15.98 mm) hole with a tolerance of ±0.002″.
- So, their hole diameters can range from 0.627″ to 0.631″ (about 15.92 to 16.03 mm).
- Manufacturer B makes “16 mm” clamps. They aim for a nominal 0.630″ (16.00 mm) pin with a tolerance of ±0.001″.
- So, their pin diameters can range from 0.629″ to 0.631″ (about 15.98 to 16.03 mm).
At first glance, both say “16 mm”—yet check out what happens at the extreme ends of these tolerances:
- The smallest hole from Manufacturer A could be 0.627″ (15.92 mm).
- The largest pin from Manufacturer B could be 0.631″ (16.03 mm).
Clearly, a 0.631″ part will not fit into 0.627″ hole, so you’ve got a mismatch right out of the gate. Even if Manufacturer A’s hole is on the larger side, if you line that up with a larger-than-average pin from Manufacturer B, the fit might still be too tight or simply fail to go in.
This is why you sometimes hear about people buying a table and clamps from different brands, only to find out—too late—that they don’t fit together at all.
Why Does This Happen?
There’s no true, widely accepted “16 mm standard” that locks in both the nominal size and acceptable tolerance. Each company picks a number around 16 mm or 5/8″ and then assigns a tolerance to it. The problem is, when you stack these variations on top of each other, the parts can overlap or completely miss each other.
Our Standards
In the past we’ve adjusted our table hole sizes once before in order to better accommodate tooling from all manufacturers. Unfortunately, we’ve found that some of them do not even hold the same tolerances over time among their own product lines. And we can’t keep making holes larger and larger because it creates too much play in the tooling.
So, in order to guarantee a proper fit for our tables and tools, these are the standards Texas Metal Works is adopting going forward:
- Pin Diameter: 0.6275″ ± 0.001″ (15.93 ± 0.03 mm)
- Range: 0.6265″ to 0.6285″ (about 15.90 to 15.96 mm)
- Hole Diameter: 0.63″ ± 0.001″ (16.00 ± 0.03 mm)
- Range: 0.629″ to 0.631″ (about 15.97 to 16.03 mm)
- Pin Length: Between 19 mm and 20 mm (roughly 0.748″–0.787″)
In all fairness, these are exceptionally tight tolerances and many manufacturers can not (and also need not) meet them. So in cases where it is cost prohibitive we will allow for modification as follows:
- Tolerance range on hole diameter may be adjusted with the absolute smallest size being .629″ (~15.97 mm).
- Tolerance range on pin diameter may be adjusted with the absolute largest size being 0.6285″ (~15.96 mm).
In the worst-case scenario (largest pin vs. smallest hole):
- Largest pin: 0.6285″ (~15.96 mm)
- Smallest hole: 0.629″ (~15.97 mm)
That still leaves us with 0.0005″ (~0.01 mm) of clearance, ensuring the pin always fits into the hole. Our goal is to eliminate the dreaded overlap where parts are supposed to be “16 mm” but don’t actually fit together.
Theoretically, these specification should still be backwards compatible with older tables we’ve made in the past, and MOST of the tools sold in the past. As for other manufacturers, their tools should fit these sizes, and our tools should fit their holes. But again, we’ve seen some pretty terrible tolerances in the past, especially with products from certain countries. So we can’t obviously guarantee other manufacturers’ complete compatibility.
The Bottom Line
Ask for exact numbers and tolerances of tools and tables you’re evaluating, and don’t just accept “16 mm.” A fraction of an inch—or even a fraction of a millimeter—can mean the difference between a clean, consistent fit and tools that refuse to go in.
Alternatively, stick to the tools and tables from a single manufacturer so you can at least have recourse if for some reason things don’t fit properly.