Threading is a very important skill to master if you’re intending to work with metal for any length of time. No matter how you create them, the threads that allow pieces to screw together and fasten with common nuts and bolts are needed in many projects. If your shop uses CNC tools to mill threads in parts as needed, you’re taking advantage of the most precise and easily replicable methods of creating threaded parts at volume, but how can you tell when you are on target and when you are veering out of tolerance?
Quality Standards
Threading must be precise to avoid jamming or even worse, binding so that the pieces do not seat properly and create problems for separation. Holes that are too big for their complementary male bolt threads wind up losing the effectiveness and strength they would otherwise have after being torqued down. As a result, holes and threads have to be carefully matched, and there are standard sizes.
- Metric threads M2-M12 with under hole diameters between 1.6mm and 10.3mm
- Piping threads in standard sizes between and eighth and a half inch, in two standard threads, PF and PT, with their own under hole and outer hole diameter sizes
These standards allow your threaded parts to work with standard bolts so you are not stuck custom milling the male bolts as well as the female threaded parts unless there are project specific reasons to do so. Carbide threading inserts are designed to provide you with the tool needed to create standard thread shapes, but you will need to but a new insert for each thread type you cut.
Providing True Threading
Threading true is a matter of understanding the bolt and hole diameters in the standards and cutting them to suit. The rule of thumb is that bolt holes are roughly 10% larger than the male bolts themselves. Each insert part used in CNC processes has its peak performance based on several factors:
- Insert material
- Density and hardness of the material being cut
- Depth of cut
- Type of thread
- Machine power specs and actual performance
The key to creating in-tolerance threads that just work right every time without stripping, jamming, or creating other difficulties? It’s understanding the equations governing the capabilities of your threading inserts so you can calculate the maximum safe speed for the equipment you have. Planning starts with a drill speed calculator that gives you a quick ballpark, but your skill comes in at the point where you need to dial that final program in based on the behavior of the machine in front of you. That piece of the process is as much art as it is engineering.
Equip Yourself for Any Threading Job
The key to being ready for customer orders? Keeping the equipment on hand to meet all reasonable, foreseeable requests. That means having buttress thread inserts in stock, with backup replacements for when you get a surprise order from a customer whose needs have just changed with a product redesign. That’s just one example of the kind of thing that happens often in the metalworking industry, so be sure to get yourself to tools and resources you’ll need to rise to the challenge when it comes.
Sources:
https://www.nmri.go.jp/oldpages/eng/khirata/metalwork/basic/bolt/index_e.html
