Build Thread- Thread cutting tool form

[leeharrysouth]

I was doing some casual reading this morning and it struck me that when cutting threads (american unified standard) I have always used a shrp 'V' ground tool bit. That is I have never flattend the nose of the tool to form the proper flat crest of a american standad. In other words I have been cutting sharp V threads. Now all said, in order to cut proper american threads I would have to grind a tool for each thread pitch I am cutting and also get the proper gage to check that tool. I am sure that all taps and dies are cut for american standard and not sharp v so when I thread stock with sharp v tool I am screwing that thread into a american standard hole. I have noticed many times that my fit was a little sloppy as compard to purchased nuts and bolts. Could this be my problem? Does any one else concern themselves with this issue.

Thanks, Johnny

[mrainey]

When you use a sharp-pointed tool, the minor diameter will be smaller (external) or the major diameter will be larger (internal) than allowed by the appropriate specification. This might have a negative effect on thread performance, and could certainly result in your parts being rejected by a customer.

[Geof]

In addition to sizing and fit problems when you use a sharp pointed tool you get a sharp bottom thread and this is very, very, very bad from the point of view of strength and fatigue resistance.

[leeharrysouth]

Thanks for the input. I guess I need to get a american unified standard gage so as to properly gind the points on my tools.

Johnny

[Geof]

Originally Posted by leeharrysouth Thanks for the input. I guess I need to get a american unified standard gage so as to properly gind the points on my tools. Johnny

Use carbide inserts, they are availble in topping style for individual pitches or universal style for different pitches; much easier than grind your own and more accurate.

[leeharrysouth]

Thanks, but, I do have a question. Now knowing that each pitch requires a specific size point how can a universal insert be utilized?

Johnny

[Geof]

Originally Posted by leeharrysouth Thanks, but, I do have a question. Now knowing that each pitch requires a specific size point how can a universal insert be utilized? Johnny

They are useful for a range of pitches, you cannot use just one for everything; the nose radius is small enough for the finest pitch in the range but not too small for the largest. You turn the OD slightly smaller than what would be the case for a full profile thread and the end result is a flat topped thread. Don't ask me how much the 'slightly smaller' is, I just do it by guess.

[One of Many]

Originally Posted by leeharrysouth I have noticed many times that my fit was a little sloppy as compard to purchased nuts and bolts. Could this be my problem? Does any one else concern themselves with this issue. Thanks, Johnny

Johnny,

Since thread root and crest dimensions are basically major and minor diameter mesh clearances, they do influence to the ultimate thread strength. A loose fit is more of an issue of pitch diameter, not excessive root clearance. Provided the cutter is ground properly, aligned properly and the compound set to optimize the process, it is just a matter of measuring the pitch diameter BEFORE feeding in too far. I wouldn't trust compound or cross slide infeed thread depth calculations as a reliable method. Although, the pitch diameter should end large with a sharp pointed tool, I would rather know for sure or at least have a good threaded nut to check the fit and creep up on it.

If the thread is not under inspection demands, the best laid plan is to remove the least amount at the root to provide clearance and suitable strength without generating a sharp corner where a crack can propagate from fatigue or fail at a lower torque than expected.

Hardware store quality fasteners can have a very wide tolerance and usually the thread pitches on bolts being small and nuts being large. Obviously, if you use a hardware store grade nut to gage fit. Don't be surprised if you attain a good pitch diameter fit for that pair , then you find that pitch doesn't quite suite the fit of another nut of a slightly different pitch diameter.

For crude cutter tip radius, I usually look at the nut thread crests on the minor diameter and hone the sharp point accordingly. Perfectly functional is good enough for me.

DC

[Geof]

Originally Posted by One of Many ....For crude cutter tip radius, I usually look at the nut thread crests on the minor diameter and hone the sharp point accordingly. Perfectly functional is good enough for me. DC

Purist.

You just wait for a few years when your poor old eyes can no longer see things clearly far away or up close; then you will think that the 'bought ones' are the neatest thing since sliced bread.

[One of Many]

Originally Posted by Geof Purist. You just wait for a few years when your poor old eyes can no longer see things clearly far away or up close; then you will think that the 'bought ones' are the neatest thing since sliced bread.

I do just fine.......with my Bausch&Lomb and optivisors close by.

My Wife calls me the mad professor in his laboooratory when I am caught in the shop sporting the optivisors. I had to catch myself here a week or so ago when I was about to say.....Hey, these make your bu....!

I do use the Iscar tri-tip certs at work periodically, but the home shop hasn't come into the age where time means money, convenience or a rightoff. If I were doing production work, it would be foolish not to use the bought ones.

DC