How can I determine the depth of cracks in a cast iron linear slide

[bigj2]

I have cast iron linear machine slide units, that have hardened rectangular box ways mounted to them, there are cracks on the slide mounting surfaces. These units are 18" wide 6" high and 30" long gray iron 40K strength material. I need to find out the depth of the cracks to determine if these machine bases can be used or if the cracks are too deep and will affect the integrity of the machine slide base. I am familiar with ultrasound and x-ray methods but I was told that method will not give me the actual depth or cross sectional view, it will only confirm that there are cracks in the material. Does anyone have a proven method that I can use to check the depth of the cracks and the integrity of the cast iron, or do I scrap these units out and start over with new castings? I have $$ thousands invested in material and labor.

 

[metman]

X-ray should do the trick. You can get a metallurgical test lab to bring in a portable unit or to recommend a vendor who can do this. I have used this type of service myself for determining depth of cracks in failure anaslysis of underground coal mining machinery.

It is also common practice for checking of subsurface weld cracks on critical welds. Therefore you might find a partable x-ray vendor via a welding materials supplier if you do not have a met lab locally.

 

[teb1]

What I would do if you called me to repair these would be to "profile drill" along the length of the crack if the geometry of the piece allowed for it.

First spray the cracked surface with dye penetrant and allow it to dry. Then drill into the crack with a 7/32" drill bit. Drill approximately 1/8" then inspect the bottom of the hole. If you can see a penetrant line across the bottom drill another 1/8" and inspect again. Do this until the crack disappears or turns. (Genarally, a crack in cast iron won't turn.)

This method will give you the most accurate means of determining the depth of the cracks in your cast iron pieces.

Hope this helps.

Tommy Breaux, Mech. Eng.
President
Engineered Casting Repair Services, Inc./METALOCK CORP.

http://www.Casting-Repairs.com

 

[Tmoose]

Generally cracks perpendicular to main load will propagate and are trouble. Cracks parallel to main load are much more benign. In some ways it hardly matters if the crack is a genuine fatigue crack or a left-over from manufacturing, since if the stresses are in the right(wrong) direction a crack is a powerful stress concentrator and is eager to grow.

This crack is parallel to the con rod I-beam which is subjected to primarily tensile and bending stress. Because it is parallel some standards would say the rod is servieable.

http://www.lambomiura.com/2005-10/cracked rod.jpg

http://materials.open.ac.uk/mem/images/images_cc/ccf11_2r.jpg