changing from UNC to UNF torque & applied force difference - SET SCREW 1

[shraska1]

Hey guys,

I'm new here, so I hope this is in the correct area. I have a question about torque of a SET SCREW as it relates to force applied. Specifically in machine tool holders (endmill holders).

It has been decided to change from course thread to fine thread set screws on the holders, but the question remains is what to torque the set screws to?

set screws:
was 3/4-10 x 3/4" long 120ftlbs torque
is 3/4-16 x 3/4" long ? torque

Is there a formula that can determine what the conversion from course thread would be? I've done a search but havent found anything yet on set screws. Everything I have found is based on Bolts (with a head) or nuts.

Thanks,
Scott

 

[KENAT]

If the thread form is the same, then the same formulas should apply for bolts or set screws, shouldn't they?

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[shraska1]

With a bolt, when you tighten it you are banking on the bolt head (assuming no nut). With a set screw, there is no stop you are banking on the bottom, and in this case the bottom is flat I'm not sure if the surface area of the set screw (flat bottom) would be equal to the surface area of a bolt head, and then the same formula's would apply? I'm not sure if that is true though...

Thanks,
Scott

 

[KENAT]

Well, you only really care about the ratios right, you want the 'load' fron your UNF thread to match that from your old UNC thread.

So the areas of the head or tip or whatever don't come into it do they?

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

 

[shraska1]

Thats true, maybe I'm over thinking this. Yes I want the load to be the same or close to what it currently is. I just need to figure out the difference in torque applied to the set screw to obtain that 'load' to avoid overtightening.


Thanks for the quick replys!
Scott

 

[dinjin]

torque for bolts fine vs coarse
list 140 and 120. I would think
the same would apply to setscrews.

 

[ornerynorsk]

Speaking from experience, course threads are not desirable in endmill holders.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[SnTMan]

I hope you change the holders as well as the set screws, otherwise your torque will be badly off

Regards,

Mike

PS - Ratio of root diameters x 120 = 140

 

[shraska1]

Disclaimer: I havent had much experience with this as I am still new to the field, so forgive me if this sounds foolish.

Are you saying it will take 140ftlbs of torque on a fine thread screw to match the holding power of 120ftlbs of a course thread screw? It just seems backwards to me, I thought fine thread screws required LESS torque to obtain the same holding power.

Thanks again,
Scott

 

[shraska1]

Just to add to that message (can you edit posts on this forum?)

I have heard you can take the Ratio of TPI x torque to get the resulting torque of the fine thread fastener. Is this an accurate statement?

Thanks again,
Scott

 

[desertfox]

Hi shraska

Have a look at this link:-

http://www.roymech.co.uk/Useful_Tables/Screws/Preloading.html#Initial_Bolt_Tension

under the sub heading "intial bolt tension" at the bottom of that section it as a formula for tightening screws taking the pitch of thread into account.

desertfox