1. ## multi start thread cutting on 10TF fanuc

I need to program a thread on fanuc control:

3.375-.0833P - 0.3333L - ACME 2G
NOTE: Start root of thread on indicated centerline
equally spaced at 90 degrees

Would this be considered a 4 start thread?
Would it also be a 12 pitch ACME thread?
Can you provide sample G-code for this?

2. As best as I can tell, you've identified the thread correctly. It is a 4 start thread @ 12 TPI. So now that we know what it is how do we cut it? I assume that you're cutting this on a lathe. The first thread is easy. It's just a standard G76 line(G32 or G92 are also threading cycles). I know to cut four threads, however, you would have to "index" the part to move the starting point of the thread 90 degrees. The first question I would ask is do you have a C-axis on your spindle? If so than that would be easy. Otherwise...I'm not sure how to do it. You may be best to refer to your control documentation and see if they have a section on cutting multiple start threads. BTW, I'm speaking on a strictly theoretical basis on how to cut this thread on a CNC. I've only cut one acme thread in my life and it was on a manual lathe....

3. I looked up G76 in one of my machine manuals. It is for Haas but I think the basic idea will be the same on any machine.

" ... to now create a multiple start thread. To calculate the additional start points the feed is dived by the number of start points..."

I think you already have the numbers that would come out of that calculation:

0.3333/4 = 0.0833

"... This value is then added to the initial start point in order to calculate the next start point. Add the same amount again to the previous start point to calculate the next start point... etc."

So if your first start was Z0.5, the code sequence would be:

X something Z0.5
G76 blah blah
X something Z(0.5 + 0.0833)
G76 blah blah
X something Z(0.5 + 0.0833 + 0.0833)
G76 blah blah
X something Z(0.5 + 0.0833 + 0.0833 + 0.0833)
G76 blah blah

The blah blah is all the stuff in your G76 for lead, diameter and depth of cut information.

4. I think thah the info given by GEOF is correct. I have the same thing but my sounds easyer than yours. I'm cutting a 3/8-20 duble lead and someone fax me info that sounds a lot like the above. Makes me feel real good about it.

• ## You Are Correct Again Sir

Originally Posted by JOHN CNC
I think thah the info given by GEOF is correct. I have the same thing but my sounds easyer than yours. I'm cutting a 3/8-20 duble lead and someone fax me info that sounds a lot like the above. Makes me feel real good about it.

Yes we cut many multi start threads when I was in the ball screw bis you just change your start point. Divide the lead by the number of threads and you got it. .250 lead 2 start thread start your first threat at Z.5 then after its done start the next one at Z.625 no timing or anything like that to be concernd with. But make sure you start your first lead far enough away to get a good ramp on so you get a good smooth transition from lead to lead

Bluesman

• Thank you for the info. A few have given me info on this subject and I thank you all. I'm new to cnc zone but think it's a cool place full of some cool people that don't mind helping. Thanks again.

John