cutting thread tips off for 70% thread - is there formula? 1

[USAeng]

When cutting threads on a lathe... is there a formula somewhere in the machinist handbook if you want to have only 70% engagment to help prevent galling or do you just take major dia minus minor dia (for internal and external) and multiply by .7 and use that number to figure it out? just taking the tips off to help prevent galling

thanks

 

[KENAT]

The amount you're playing around with UN threads based on your various posts, I think you're past the point of relying just on machinist hand book and need to get a copy of ASME B1.1.

While I'm not sure if it covers your specific case it has a bunch of info on modified/special/different UN forms that may be of use to you. It may be that there's a standard modified thread form you could use.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Compositepro]

I believe you would reduce the outside diameter of what you are threading before you start cutting threads. If you turn down the diameter after threading you will have burrs on the working faces of the thread.

If you are talking about reducing the pitch diameter of the thread then I think you need to talk about the class of fit rather than percent engagement.

 

[USAeng]

yeah thats what I mean... just cutting down the bar or making the hole bigger before machining

and yes its for those 1 1/4-8 UN threads... I will look at b1.1 I have copy somewhere around the office

 

[Compositepro]

What you are suggesting will not help with galling. Once you take even a little of the thread tip off, the tips and roots cannot touch. Taking more off the tip does not change the clearances in the thread.It only reduces the thread bearing area which might actually increase galling.

 

[USAeng]

hmmm... I was just going to try it because I thought I remember reading somewhere that it would help. Maybe I am mistaken

 

[CorBlimeyLimey]

For loads of information on galling and how to prevent it;

http://www.eng-tips.com/search.cfm?...lling&t=&h=&searchSite=2&searchArea=1&d=1&s=1

 

[Tmoose]

I think the common charts show percent theoretical strength at various tap drill sizes. Bigger tap drills reduce power required for tapping, with a reduction in strength. It would seem lowered strength might imply greater loading/higher stress, although not necessarily "contact" stress I suppose.

 

[tbuelna]

USAeng,

I would agree with Compositepro. Reducing the OD of the external thread body will not reduce galling, since most thread spec dimensions are established to prevent interference between the external tips and internal roots.

In the absence of preload, the most common cause of thread galling is interference at the engaged thread flanks. This can be caused by pitch error accumulation, pitch diameter runout, or other such profile errors. You can solve the galling issue by reducing the engaged thread length, reducing pitch/profile errors, or by increasing the clearance between the mating threads.

Using a reduced tolerance class (ie. cl.2 vs cl.3) will result in a "looser" thread, but you will also need to make allowances for this less accurate fit in your loads analysis.

Good luck.
Terry

 

[USAeng]

thanks a lot... should be good now