Different Tool for Each Pitch?

[SteveHGraham]

Let me open a giant can of worms. Because that's what I do.

I want to put internal threads on my garlic press thing, and I read about Mesa Tool's threading tools. I decided to order one. It has a carbide bit on each end. One is for external threading, and the other is for internal threading. The Mesa site says I can use them for 11 TPI and up. It will work on any bore over .710". That seems fine to me, since I can get taps for internal threads on smaller things.

A long time back, I was flabbergasted and traumatized to find out that REAL machinists made a different tool for each pitch. I haven't forgotten that, but dang, here I am with this neat new tool, and I do want to use it. The thing I'm threading is stainless, so I didn't want to fool around, trying to make an HSS tool that might bust on me. I figured it was less aggravation to get carbide and get it over with.

I guess what I want to know is, how much of the stuff about one tool per pitch is for real, and how much is just perfectionists imposing unnecessarily high standards on lazy people like me?

The tool looks nice, and they got it here REALLY fast.

[ctwo]

Everything has a tolerance. The more money you want to save, the looser the tolerance. The Chinese are expert on this...

I look at it like a statistical thing. The example of turning motorcycle shafts was a good illustration and one area where I might not want to play my odds, especially if it was for someone else (any way you look at it though, there is potential to affect someone else).

I'm sure with enough details of the thread you are cutting, what tool you are using, the material, and the stresses involved, a good engineer can run some calcs and let you know the scoop. I would not be surprised, if under the right conditions, that the one-size-fits-all would yield a very high likelihood of failure...those conditions may be at the extreme and the reality is that, in general, the one-size-fits-all will only marginally affect reliability and performance. That's your gamble barring due diligence...

Without the specific details, it's all just wild speculation... I wouldn't put my money on it either.

P.S., I just read your post again and realize this is for your garlic press. I'd say, don't sweat the petty stuff...what's the worst that could happen?

[Chuck K]

"P.S., I just read your post again and realize this is for your garlic press. I'd say, don't sweat the petty stuff...what's the worst that could happen?"

Apparently you have never experienced catastrophic failure of a garlic press....Scary!

[JTiers]

For a garlic press, file it like they did in 1425 AD........

The different cutter is for real, though, the thread specs call for a specific flat width etc and the point of the tool would be different for each pitch. Mostly, "we" either use sharp V threads, or use the same cutter for several pitches.

if you want to be totally correct, a separate cutter with provision for "topping" or cutting the crest as well as the flanks and minor diameter is needed.

For to-print work, yes indeed..... for a garlic press, not so much.

[warmstrong1955]

For stuff where it really can make a difference, yes, I have different tools with different flats. In can be important, along with radii. Things that see bending, do not like sharp vee's.

For a lotta stuff....probably most stuff.... it doesn't matter. Garden tractor and wood chipper parts, or garlic presses.... it really won't make a hill of chips.

I think I have the same Mesa bar you bought. Mine is 1/2" x 4-1/2". I like it. I've used mine on 304, 316, ad 17-4PH, but mostly 1045. I have found though, that if I don't knock a little flat, (I try and go for a rad) on the tip of the insert, that it will chip....quickly and easily.

Bill

[Harold_V]

I have found though, that if I don't knock a little flat, (I try and go for a rad) on the tip of the insert, that it will chip....quickly and easily.

The first pass of a thread is generally when tip failure occurs, so it pays to take a rather shallow cut or two, and to not plunge cut the threads, so chip isn't formed on both flanks of the tool. Also, HSS will generally tolerate the cut better than carbide, and while it doesn't have the wear resistance of carbide, it has greater tensile strength, so the tip isn't as easily destroyed.

For those of you who may wish to learn more about proper thread profiles and features, I'd direct you to the H-28 manuals published by The U.S. Department of Commerce, National Bureau of Standards. They make terrible reading, but are intense with technical data.

Harold

[BadDog]

For an OD thread, beyond correct form, it's mainly just about the point radius, so you can "get away" with lots of range if you (can) fudge tolerances a bit. In all honesty, for steel OD threads (most of what I do) I now almost always use my Top Notch with either larger or small "universal" threading insert. The 2 combined are situated so that the spec tolerances don't get blown too badly over there range, and if you DO need inspection passing threads, they have full and partial form inserts for those needs as well. They do such a fine job it's just become the tool I reach for.

But ID changes that. If we assume the same leeway of root/point radius as needed for the same range in OD threads, and the bore is sufficiently large, we can still get some range. But the smaller the bore the more you run into problems. The larger the thread relative to bore also makes things more problematic, so fine would be more forgiving than standard for working with a wider range on one tool. The gist of the problem is that working in a bore you are limited by simple physical interference plus required clearance as to how much support you can leave under the edge, plus as you cut deeper your bit is going down inside a developing helical valley, so on small ID coarse (faster lead) threads you loose almost all support on the leading edge. You can fight back some of that support by making the support ridge fit nicely down in the valley of the developing thread, but particularly with deeper/coarser threads, that means actually grinding in the helix angle below the point, with a little added clearance to keep from rubbing on the thread flanks.

That's why I have only a very few bits for OD threads, mostly varying slightly on point radius (no inspections here, but still try to be reasonably faithful as those specs weren't created just to be frustrating). But I have at least 6, maybe more, tiny little stubby threading tools ground in 1/8 and 3/16 bits used for ID threading. And for them it's more about the clearance sufficient for the bore combined with the helix angle necessary for the lead. Thankfully I have taps for most anything below about 5/8" and spotty coverage above up over 1". I really do my best to avoid single point threading below 5/8" diameter. In those sizes, if it's not an odd ball special thread (guns, metrology gear, etc to name a few) where they are both pricey and special order (and I don't happen to have scored one), it makes much more sense (monetarily, time, frustration, success ratio, etc) to use taps.

[SteveHGraham]

So it sounds like it matters, except when it doesn't.