Micro 100 threading tool

[dpb]

I need to cut a 22mm x .5 internal thread, 1.1 inches long, in 6061 aluminum.
.5mm pitch works out to approximately 50.8 tpi.
Looking at the Micro 100 catalog, I find product IT-2901250. This is a right hand threading bar, maximum bore depth 1.25”, minimum bore diameter .290, nose radius .002”. So far, so good. Thread range is listed at 14-40 tpi. Why 40? The nose radius is small enough for what I need, they have other tools with the same radius, listed for up to 76tpi, but they aren’t long enough. What other dimension should I be looking at?

 

[Tozguy]

Interesting question and I will be following what gets posted.
Maybe it is a question of relief angles or rigidity. To cut a 76 tpi thread you need a very precise, rigid and clean cutting tool.

 

[JRaut]

Very good question.

I recently had to buy a Micro 100 internal threading bar for the opposite reason --- the thread I wanted to cut was too coarse for any of the indexible insert threading tools I have that'd fit inside the bore.

I took a brief look at the catalog page you're talking about. Seems to me the 'fineness' limit on any of those boring bars is associated only with the nose radius because they're all 'sharp'. The thread form is just a 60* groove, for cryin' out loud.

If it were me, I'd buy the tool you mention. Hard for me to believe it won't work.

Here's the longer version (IT-2901750) of the one you mentioned, listed on eBay for about half-price:

1 Micro 100 Boring Bar IT-2901750 Carbide Threading Bar | eBay

Find many great new & used options and get the best deals for 1 Micro 100 Boring Bar IT-2901750 Carbide Threading Bar at the best online prices at eBay! Free shipping for many products!

www.ebay.com

 

[benmychree]

No need for carbide here, you can buy a form relieved internal threading tool from the Bokum Tool Co., that will give you many years of use, they arwe sharpened on top only and do not lose their form when sharpened

 

[Bi11Hudson]

I don't know the exact numbers but fastener thread forms are designated as "truncated" triangles specificaly to allow for tool "dulling". Effectively taking the sharp point off a "V-Sharp" thread form. Measuring a threaded fastener from the hardware store, such as a 1/4-28, will actually measure less than 250 thou. Only by a few thou, but enough to see. This applies to both male and female thread forms.

I do a bit of threading at 80 TPI. Not very far, just a few pitches. And use hand ground HSS cutters to get a "V-Sharp" shape. So I can't say how much "nose radius" is there. All I can say is that my fasteners fit a tapped hole. Size Nr 0-80 is only 0.060 full diameter. The truncated thread form is a little under that so I have difficulty determining the difference between an imperial Nr0 and a metric 1.5mm. I have to use a tapped sizer plate to make that call for the slightly different pitch.

What I'm speculating on is that you should cut the thread in a waste piece and try it. If it fits for a few pitches, you are a go. If it doesn't fit, you need to make another pass or use a sharper cutter. No big deal. After all, this is a hobby and the tested parts are waste. Just do it and see what happens. If it doesn't work, well, it's just waste.

.

 

[benmychree]

The Bokum cutters that I referred to are of the sharp V form.

 

[Mitch Alsup]

I need to cut a 22mm x .5 internal thread, 1.1 inches long, in 6061 aluminum.
.5mm pitch works out to approximately 50.8 tpi.
Looking at the Micro 100 catalog, I find product IT-2901250. This is a right hand threading bar, maximum bore depth 1.25”, minimum bore diameter .290, nose radius .002”. So far, so good. Thread range is listed at 14-40 tpi. Why 40? The nose radius is small enough for what I need, they have other tools with the same radius, listed for up to 76tpi, but they aren’t long enough. What other dimension should I be looking at?

The distinction is the angle at which the thread is cut. Finer pitches are closer to 90º and the tip needs clearance on both sides of the point. The right/left is which flank need more relief.

On the other hand, only the first 5-6 threads do any holding, the rest are simply there to make problems for the threads doing the holding {unloess the threads are for something like a micrometer thread and are for measurement not for holding}.

In my estimation, unless you are cutting class 3 threads, either will do.

 

[mikey]

The distinction is the angle at which the thread is cut. Finer pitches are closer to 90º and the tip needs clearance on both sides of the point. The right/left is which flank need more relief.

Mitch, clarify this for me, please. I do not understand what you mean by finer pitches being closer to 90 degrees.

 

[JRaut]

Mitch, clarify this for me, please. I do not understand what you mean by finer pitches being closer to 90 degrees.

I think @Mitch Alsup is talking about the so-called Helix Angle of the thread.

A (for example) 4 TPI thread looks something like this: \ \ \ \ \ \ \ \ \ \
Compared to a 50 TPI thread, which looks more like this: |||||||||||||||||||||||||

Not only are the threads spaced closer together on a 50 TPI thread, but they are also more 'upright' (i.e., closer to 90*), if that makes sense.

I still need to think about the relief angle thing though.... seems if you're using the method where you infeed using a compound set at 30-ish degrees, the relief angle on the 'back side' of the tool is immaterial.

 

[bakrch]

I don't know the exact numbers but fastener thread forms are designated as "truncated" triangles specificaly to allow for tool "dulling". Effectively taking the sharp point off a "V-Sharp" thread form. Measuring a threaded fastener from the hardware store, such as a 1/4-28, will actually measure less than 250 thou. Only by a few thou, but enough to see. This applies to both male and female thread forms.

I do a bit of threading at 80 TPI. Not very far, just a few pitches. And use hand ground HSS cutters to get a "V-Sharp" shape. So I can't say how much "nose radius" is there. All I can say is that my fasteners fit a tapped hole. Size Nr 0-80 is only 0.060 full diameter. The truncated thread form is a little under that so I have difficulty determining the difference between an imperial Nr0 and a metric 1.5mm. I have to use a tapped sizer plate to make that call for the slightly different pitch.

What I'm speculating on is that you should cut the thread in a waste piece and try it. If it fits for a few pitches, you are a go. If it doesn't fit, you need to make another pass or use a sharper cutter. No big deal. After all, this is a hobby and the tested parts are waste. Just do it and see what happens. If it doesn't work, well, it's just waste.

.

That is my guess. Technically the tool is within spec (in a perfect world), but essentially no wear allowance before things start to go awry. Perhaps their tolerances in making the tool's .002 root exceed the thread specs by too much.