Need thread milling insight

With the help from Jacob at Tormach and some nice single point thread mills from Mari tool I have been thread milling quite a bit this last week.
One problem I have been having is that even though I am using the major diameters as referenced in the machinerys handbook, the final I.d threads I have been making are typically too tight and I have been needing to oversize my major diameters to be able to achieve a reasonable fit.
I have also use the drill sizes as specified for regular tapping.
Have others had this experience? Does one need to add a minor slop factor to get better fits when thread milling?

Thanks
Nathan

Probably a bit of tool flex depending on the overhang.

My experience with thread-milling is that calculation alone will almost never give you exactly the right fit, other than by dumb luck. I nearly always find it necessary to do a test piece to dial in the exact right diameter for a perfect fit on each thread size. You can add a "fudge factor" (usually something on the order of 0.005") to ensure the parts will at least screw together, but to get a perfect, close fit requires a test cut.

Regards,
Ray L.

Thanks Mike,
That is most likely one factor for the smaller thread milling I have done (#6 & #8). I have noticed that the larger sizes (9/16") have had a little less issues.

Thanks Ray. That is kind of what I have been doing to get my only successes. Just wanted to make sure there wasn't something I didn't know (or shall I say even more that I don't know)
Happy Turkey Day
Nathan

Quote:

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Originally Posted by SCzEngrgGroup

My experience with thread-milling is that calculation alone will almost never give you exactly the right fit, other than by dumb luck. I nearly always find it necessary to do a test piece to dial in the exact right diameter for a perfect fit on each thread size. You can add a "fudge factor" (usually something on the order of 0.005") to ensure the parts will at least screw together, but to get a perfect, close fit requires a test cut.

Regards,
Ray L.

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the common procedure as with threading on a lathe is to take final "spring passes". these just repeat the final pass with no changes to depth of cut in the program as the name suggests to account for flex in the tool and/or workpiece.

Good place to look for threadmilling.... .... ... . . . . http://www.cnczone.com/forums/tormac...ml#post1597008

I spoke with Frank Mari of Mari tools today. It is great when you can talk with the owner of the company and Frank has been very accessible when I have had tool questions.
Basically Frank said what a number of you have relayed - that single points due to their sharp 60 degree ends don't cut the same way taps do and need to be experimented with to find the right major diameter.
There apparently is no other formula than to just do some test cuts. If one needs tolerance then the use of GO/NO GO gauges will help guide.
Also, new to me, these thread mills can be used for metric as long as the TPI is equivalent and one just uses the same drill-size as with a regular tapping procedure.
I have dialed in the major diameters for the threads I am currently using and it is great to be able to teak them for tighter or looser fit.

You should never really expect anything to come out exactly to size - or to stay exactly the same size - so you should be adding cutter-comps to things like this. That's what they're there for.

I agree about Frank. We have talked through email a few times. I try and support them when I can!

Did this a while back with couple Maritool spt. At the time it was hard to find small ones and maritool were only ones I could find.
Like others said above it helped me get up to speed faster doing setup tests and samples.
Lets you zero in on everything. P Smidt book on cnc programming provided all the math and diagrams required to understand threading and about any other cnc op!.
Setup in cam takes a bit of messing around, still when have it done one time you can save these types of operations and use again.
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Fun stuff!