View Full Version : Thread depth

02-28-2011, 02:39 PM
When single-pointing threads, is there a way to take the tool radius into account, so as to know exactly how much infeed will be enough? Calculating the theoretical infeed with a perfect V-tool is easy, whether plunge cutting or flank cutting. But to get a perfect fit with a real, radiused tool, I've always had to cut and try. Is there a better way?

02-28-2011, 02:47 PM
Whether the tip of the cutter has a radius or sharp V will not change the pitch diameter of the thread and it's the pitch diameter your going for, not the depth of the tip of the cutter.

I use a chart to tell me how much the compound is fed in but I still have to mike the pitch diameter or use a test thread to get it right. I also use a sharp V threading tool. Sometimes I may stone a small flat on the tip but that's all.

02-28-2011, 04:17 PM
But to get a perfect fit with a real, radiused tool, I've always had to cut and try. Is there a better way?

absolutely. You wouldn't cut and try a shaft for a bushing diameter, right, there's no need to do so for threads either. At the risk of opening a bbs can of worms (there was a brutally distracted and prolonged thread on the subject).....thread wires. You can test with its mate if convenient, but thread wires is the way to get the correct fit and it's the convenient way when the work is tailstock supported.

there's also thread mics, but the good ones are expensive. I picked up a Mit one recently, its nice, but maybe out of habit i still keep grabbing for the thread wires :o :)

Al Messer
02-28-2011, 08:53 PM
What percentage of thread engagement are you shooting for between the male and female parts?

02-28-2011, 10:33 PM
I got a pair of thread mics, 0~1" and 1~2" for $20 each on ebay, they seem to work very well, came with a set of anvils for all the diffrent TPI's.

One thing that did trip me up however, Is I once made a test thread in aluminum.. it fit well after being a little stiff initialy.

Next I made the real thread in steel.. It sorta binded, but was free moving after binding.. I assumed burrs, but decided to put some dye on it and check.

Turns out, the square crests of my threads where interfearing with the rounded vallys of the female thread. I measured the root of the female thread exactly... Too exactly. Should of cut my OD down a few mils due to the square crests of my threads. Cutting the thread deeper.. Would'nt of helped untill the thread became way undersized. Thankfuly jaming it in and pulling it out basicly rounded the crests for me, and did'nt get stuck.