Cast Iron Dilemma 1

[maniacle]

I have a "1 off" class 40 casting that has a partial drilled hole (17/32) in the wrong location. The correct location only moves the hole about 100 thousands from the incorrect location, and I need the entire depth of the hole to be threaded. The partial drill is at most .25” deep and the final depth will be 1.25”. What would be the best method to fill this partial hole so that a new one may be drilled and completely threaded? The suggestions I’ve received are brazing in new cast iron with some special brazing rod we’ve obtained, or mechanically plugging the hole with a tight press fit. I was hoping someone in here might be able to shed light on some other ideas, or tell me if the two options I’ve got wont cut it. There are many concerns about the cast iron becoming too brittle if we were to braze in new material, and concerns that a plug, even if only a partial depth, will not support the loads. Replacing the casting is not an option at this point. Can anyone help?

Thanks

 

[MechEng13]

maniacle,

For us to be able to help you on the decision we will need to know some more information, what type of loads will the threaded area see, will it be cyclic or repeat loading?

I'm not very familiar with cast iron, but I know that you have not given enough info for a good answer. My personal thought would be to go with the press fit as it shouldn't effect the mechanical properties of the cast iron. However don't rule out brazing. In my last company we brazed a great deal of steel hubs to steel stampings and the break away loads were in the 10-23k lb range.



Alan M. Etzkorn
Product Engineer
Nixon Tool Co.

http://www.nixontool.com

 

[metengr]

Maniacle;
Based on your situation you can attempt to plug weld the mis-drilled hole based on its size and depth. I would not braze, I would use either a cast iron weld rod or a high nickel electrode (Ni-Rod 55). Was the casting exposed to oil that would require a high temperature bake out?

Extreme care must be used to weld cast iron. From what I have seen, you need to work with small beads and avoid excessive penetration of the weld rod into the cast iron base metal. The reason for this is to mininze dilution so the formation of high carbon martensite (brittle material) in the heat affected zone of the casting is minimized.

Preheat of 600 to 800 deg F is benefical for gray iron castings. However, to avoid generating thermal stresses that could potentially crack the casting, the entire casting should be preheated in this temperature range. If this is not possible, do not preheat and use a low interpass temperature (less than 300 deg F temperature) between weld passes. As I stated above use small beads to avoid cracking the adjacent base metal. Take your time.

Your second option of using an inteference fit plug would not work because in the process of re-drilling the new hole, you would be relieving the residual stresses that hold the plug in place (based on your new hole location).

You might have a third option without the need for weld repair. If appearance is not critical, you might want to consider drilling the new 17/32" diameter hole at the correct location and machine a tapered shoulder around the entire circumference of the hole to the depth (0.250") and width that consumes the mis-drilled hole. Basically, you would have a counter-sunk arrangement of the new bolt hole with a 0.250" reduction in wall thickness of the casting along the base of the shoulder. If you decide this is feasible, make sure you radius the corner of the shoulder to reduce stress concentration.

 

[maniacle]

Sorry about that, got caught up in the frustrations of the morning. The hole is 17/32 and will be tapped to a 5/8-11 UNC. It's a through hole, so at 1.25 deep its poking out of the casting. The studs going into the holes are grade 8, and only are supporting 1500 lbs at most. This load is steady while the machine is running, and is just the preload while it is off. Unfortunately i am not able to tell you what the preload will be on the studs, but it could very well be in the range of 10000 lb.

hope this helps

 

[Carburize]

To avoid welding on the cast iron which, while possible is not ideal, can you make one special stud. Machine out the hole over-size thread and then fit perhaps a shouldered stud with an offset extension to line up with the originally planned stud location? If this is not acceptable how about drilling out a bigger hole and threading that then inserting a big plug and drilling a threading the new big plug in the correct location? The big plug could be a piece of structural steel with the appropriate strength to match the casting and if necessary coulb be bonded into place or pinned>

 

[TVP]

I like the suggestion from Carburize. Making a strong mechanical fastener would be favorable vs. the welding option.

 

[strokersix]

I agree with Carburize's suggestion and offer a refinement: Use NPT threads for the plug rather than straight threads. The tapered threads will seat firmly in place.

 

[Metalguy]

One more thing. CI may be very poor in tension, but it actually isn't too bad in shear, the way female threads are loaded. Keep this in mind if you attempt to calculate the allowable stresses.

 

[maniacle]

Thanks for all you responses. We ended up doing a few more hand calcs and discovered the specified thread length was more generous than what was required, so were going to mill out the bad starter holes and drill again. Many on the team were adament we'd have to fill them, and one guy even went as far as investigating some epoxys. Thankfully the solution was right in front of us...all we had to do was calm down and see it.

thanks