# Thread milling



## kvom (Nov 6, 2014)

I've been interested for quite a while in trying some thread milling, and recently bought a tool for this on eBay.  Single-point thread mills can cut multiple pitches, and this one can do 13-32 TPI.  For finer pitches these tools require a relief angle on the teeth to match the ramp angle.

Since I have a good supply of 5/16" brass rod, I decided to use this for my trial.  For the g-code I used the thread-mill "wizard" in CAMBAM.  It took a number of tries to sneak up on the proper cutting depth so that a nut would fit.  Each tool needs some calibration since the tooth diameter can vary.  Here's the video:

[ame]http://www.youtube.com/watch?v=ElbDMZeL4zM&feature=youtu.be[/ame]

Finding the correct feeds and speeds is an interesting exercise.  I followed the guide posted here:  http://www.harveytool.com/secure/Content/Documents/SF_71000.pdf

Basically you find the SFM recommended for a given material, convert to RPM given the tool's diameter.  For the feed, find the chip load per tooth for tbe material, multiply by the number of teeth to find the feed rate.  Finally reduce the feed rate by a factor that depends on the threads major diameter.

Although a single thread takes as long or longer than using a die, building a fixture to hold multiple stock can mean making studs less tedious.


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## idahoan (Nov 6, 2014)

Fun stuff!

I had just finished ordering a thread mill, then saw your post. I thread milled the 1/8-27 NPT threads on the hand cocks for my Pacific engine, that sure made it nice. Milled the taper first then Master CAM lets you specify the angle you want to mill the thread.

Getting close to installing the quill drive on my mill, then I can do this type of work at home too.

Dave


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## Swifty (Nov 7, 2014)

Done some thread milling at work years ago, the biggest was about 120mm dia X 6mm pitch, machined male and female threads. Didn't have any thread software, just worked out degrees of revolution to make and final Z depth

Paul.


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## RonGinger (Nov 7, 2014)

My first thread mill was a 3" NPT- that is tapered pipe thread. It was the female thread on a throttle valve casting for a locomotive boiler for the WW&f Ry.. By the time the job was on my mill we had over $1,000 in the casting because it had to be made in a certified foundry, and Xray tested.

I had a friend make a single point tool bit  in a boring bar. The cutter was about a 1" dia when spinning. I wrote a wizard for Mach that calculated the taper helix.  I sweat bullets before I hit that GO button, but it cut a perfect thread and fit on the pipe. The boiler passed testing and is now in the locomotive.

I have never tried a commercial thread mill, they are way to expensive for me. I have wondered why you cant take a tap and grind off all but one cutting edge.  Anyone tried that?


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## kvom (Nov 7, 2014)

The thread mill I bought cost less than $40.  I have heard people grind off a tap and get it to work.

My next experiment will be to try milling an internal thread for 1/2-13.  For smaller holes I'd need a smaller tool.  If you don't have a tapping head or rigid tapping then thread milling is the way to go.

On that 3" NPT job, I hope you tried it on some scrap material first/


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## RonGinger (Nov 7, 2014)

> On that 3" NPT job, I hope you tried it on some scrap material first/



The  other end of the dry pipe was a simple flange joint- a steel disk about 8" dia x 1" thick. I did it first since it was pretty simple to make in case of a screw up. I couldn't think of anything else to make a test piece- a wood part would not have been accurate enough to get a god thread fit test, and I didn't want to waste a chunk of aluminum.

I have a tap head, and have a job Id like to do (a tooling plate with a lot of holes) but I dont know how to program it. You need to downfeed some distance- like slightly more than the needed thread depth, but how far and at what feed rate?

I used the tap head once manually- I let the program move to the right spot, then stop and I would manually feed the tap, then hit start on the pendant.  After a while I got pretty quick at feeding down, then hitting the start button, and of course eventually got to fast and hit start before finishing the retract and broke the tap.


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## Toolguy (Nov 10, 2014)

I do thread milling fairly often, but for the smaller sizes use tension-compression holders with a tap. I don't have rigid tapping on my mills so this is a good solution. I do have tapping heads for the drill presses. The T-C holders will repeat fairly well, but not close enough for a blind hole where you're going right to the bottom. They are great for through holes or blind holes with extra drill depth.


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