Thread recommendation for cast Iron part 1

[Pravinb1234]

Hi dear friends and resp. seniors,

My topic is about to have ur valuable suggestions regarding thread type to be used in cast iron parts as per the application.

So basically I am working on design modification of cotton Ginning machine in which all the bolts were given earlier in BSW threads. We used it for more than 4 decades. But due to the shortage in availability and market trend of switching from BSW to metric bolts, we had also switched from BSW to metric threads and bolts. But as time goes on we faced some problems of getting thread failure in some casting parts like made up of FG200 grade. Then we analyzed the difference in both the threads of similar diameter and found that there is lot of variation in the thread height and pitch. We come to conclude that thread height and pitch in BSW threads is more as compared to same dimension Metric thread. Due to more thread height, the gripping between male and female thread is higher in BSW thread and due to more pitch, the quantity of material in two consecutive threads in more which results in more stripping strength to the threads.

I want your valuable suggestions after looking into the fact in above paragraph that which type of threads/bolts will be technically more beneficial to use specially in casting parts, after neglecting the issue of availability.

Thanks.

 

[3DDave]

To make a proper comparison, the same fastener location in parts cast from the same batch of material, some with BSW and some with metric threads would need to be used. The fasteners must be of the same material, same heat treatment, same plating, and installed with the same tension (installation torque should not be the same because the pitch, pitch diameter, and thread form angle is not the same.)

There must be photos available showing the assembly and the failed fasteners.

 

[tbuelna]

Pravinb1234-

If you are producing the internal threads in your casting by tapping, one thing you might consider is using a roll forming type tap rather than a conventional cutting type tap. Roll forming taps are available in standard metric thread sizes up to about M18. Roll formed tapped threads typically provide better fatigue performance than cut threads in most materials, but I don't know about your FG200 cast iron. To use a roll forming tap, the only adjustment you might need to make is the drilled hole size.

http://balax.com/browse-products?category=7

Hope that helps.

 

[MikeHalloran]

If you going to screw around with different taps, try a Spiralock(r) tap.

Mike Halloran
Pembroke Pines, FL, USA

 

[desertfox]

Hi pravinb1243

From your original post I see that a comparison of the bolt threads was carried out and differences noted.
What I would do is work out the female shear areas of the metric and BSW threads and divide the areas into the bolt preload, that way you will obtain the shear stress the internal threads are seeing on assembly and you can compare these stresses with the allowable shear stress for the cast iron material.

I suspect that the failures are cyclic failures due to fatigue if they are failing after a period of time, maybe a fatigue analysis might help if you have data but for the moment I would calculate the above stresses and see what the stress differences are.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[hydtools]

You might consider round root inch-series threads. Or go back to BSW.

Ted

 

[Screwman1]

I saw that tbuelna suggested a forming tap- that won't work in cast iron because of the low ductility in the material. You need to use a cutting tapping cast iron.

 

[juergenwt]

Instead of going to M12 (from 1/2" = 12.7) can you go to M14? Seems to be your logical choice.
Tap drill 12mm.

 

[tbuelna]

Screwman1-

Good catch. I checked the material requirements published by a manufacturer for using their roll form taps, and they recommended an elongation of >5%. Grey cast irons (like the FG 200 grade noted in the OP) have very low elongation characteristics and do not meet this requirement. But many types of ductile cast iron do have sufficient elongation to use roll form taps.

 

[Pravinb1234]

Thanks everyone for ur valuable discussion and suggestions.