Single point threading - What size to turn to?

[Christianstark]

Need to cut M16 x 2 threads, and the first several I had to cut well beyond a triangle thread to get it to fit a M16 nut.

Where can I find what dimensionsI should be starting at for imperial and metric threads on the lathe.

For reference I was starting at 16mm.

Thanks!

 

[WobblyHand]

I use info from https://www.engineersedge.com/hardware/metric-external-thread-sizes1.htm
From there you can find imperial sizes and dimensions for inner threads. I shoot for nominal major diameter. For M16x2, I get 15.822 OD.
I also found an app called UN Threads. It can give you all the same info, plus a little more.

 

[GT-6 Racer]

Grab a copy of Machinery’s Handbook. A must have for anyone working metal. It’s all in there. usually easy to find on ebay or used from Amazon at reasonable cost.

 

[WobblyHand]

Grab a copy of Machinery’s Handbook. A must have for anyone working metal. It’s all in there. usually easy to find on ebay or used from Amazon at reasonable cost.

Very correct. The guide. I have a copy from my father. Relatively easy to pick up a used one at a decent cost.

 

[benmychree]

I always start with the nominal diameter for external threads, and usually do a bit of filing to deburr the peaks of the threads, so the finish diameter ends up being a small amount smaller than nominal size. For ordinary work, we can get overly technical about things, going to Machinery's Handbook can be a rabbit hole, timewise, if the nut or thread gage says that it fits, it is good to go. If I am making a thread plug gage, I go to MH and use the thread mike or wires. If single pointing internal threads, the tap drill diameter will do the job in most instances.

 

[Christianstark]

Awesome! Thanks all! I found a rule log thumb online that worked pretty well as well. Major Diameter - 10% of thread pitch.

M16 x 2. = 16mm - (2mm * .1) = 15.8mm

This worked well. great fit on a nut I had for testing.

 

[Bi11Hudson]

Most modern threads are to the "Unified Thread" form, even metric. Such a thread is of a "V Sharp" form with rounded off valleys (root) and peaks (OD). Such roundness is ostensibly to allow for tool wear in a production environment. For the "casual" user doing a single point thread on a single use object, this involves cutting a "V Sharp" thread and then smoothing the peaks of the thread form a little. Usually 8-10% of the thread size is sufficient.

Casual machinists will often cut down the OD of the device to make this adjustment. To do so has become a common practice. It is not a true reduction, just a "git by". While it does work, it takes an extra pass (or two) before threading. If the application is not a "spec" job, like an aircraft, threading full size "V Sharp" and touching up with a file or other abrasive will suffice.

.

 

[Christianstark]

Even cutting a M16 OD starting at 15.8mm required some file work on the threads to fit the nylon lock nut I had, and even then it required a wrench just to get it to the nylon. After the first 3-4 turns I could tell it was a tight fit laterally and in and out. No slop. Was threading H13 tool steel

 

[SLK001]

I use this online METRIC thread calculator. Their IMPERIAL thread calculator is fantastic.

 

[Chewy]

You can find online versions of Machinery Handbook for free. Get a later number like 28 or 29 to make sure you have Metric. It is the bible. I printed out the pages and put in separate notebook that is in shop for Metric and Imperial. I use a number a couple of thousand below the max 3A fit for most of my stuff.