Why de-burring works

[mwolson]

When you de-burr a block, you get rid of sharp inner and outer corners, replacing them with smoother inner and outer radiuses. The reason you do that is because metal stresses tend to collect at points with sharp inner or outer corners.

I have a cheap Harbor Freight 12 ton hydraulic press I use in the garage. I was trying to press a bushing out of a Honda front lower control arm, and this is what happened to the arbor plate.

Notice that the main break runs from the large right angle sharp inner cutout to the small right angle sharp inner cutout. Notice that neither of the round cutouts failed, but the sharp outer corner at the edge of the larger round cutout is where the second break occurred.

This is a graphic example of why deburring a block is a good idea:

 

[b dub]

Did you deburr your block Mark?

 

[98 N/A 4V]

Mark, would the break in the middle mean that it broke there because thats the section that has the smallest cross section in relation to where the force was being applied to? I'm assuming that the press was pushing straight down right in the middle of the piece?

 

[mwolson]

Mark, would the break in the middle mean that it broke there because thats the section that has the smallest cross section in relation to where the force was being applied to? I'm assuming that the press was pushing straight down right in the middle of the piece?

Disclaimer: I am an electrical engineer, not a mechanical engineer, so an ME can probably shed even more light on this than I can.

But I think the primary break happened both due to the small cross section and the sharp inner corners. Both contributed.

But it is interesting to note that the secondary break didn't happen at the smallest cross section. If it had, it would have broken at the center of the half circle. Instead, it went to the sharp outside corner near the edge of the half circle.

And Boone, I did deburr my block, and my son's Honda block.

That said, I haven't seen too many Teksid blocks fail, even when they haven't been deburred. But I think it is good practice when you are building a higher HP engine.

 

[PSUCOBRA96]

stop being HE-MAN and breaking your tools and blaming it on physics, you know your just too strong

 

[mwolson]

stop being HE-MAN and breaking your tools and blaming it on physics, you know your just too strong

You sound like my dad. He used to tell me, "Only a poor workman blames his tools."

Upon closer inspection, I found air bubbles in the metal where the breaks join. Could have also had something to do with it...

 

[98diablo]

Gotta love cheap Chinese porous cast press plates!!!
I have the same press at work and broke almost the same exact way.

 

[DOHCscreamer]

as a mechanical engineer i would like to clarify:
1) what you are referring to is not really de-burring. deburring is the removal of unwanted material that becomes present after machining process. you are referring to a chamfer or fillet.
2) 98 N/A 4V is correct. the plate broke first at the smallest cross-section. it then broke at the next smallest cross section.
3) adding a fillet or chamfer to round a discontinous piece of material is to avoid crack propagation due to fatigue cycles. it would not have helped you here.

 

[mwolson]

Thank you for chiming in. All these years I was laboring under a misconception.

This is what is great about these forums. :beer:

 

[b dub]

And Boone, I did deburr my block, and my son's Honda block.

Would love to see pics if you have them.

 

[mwolson]

I put a couple of pictures up at the bottom of this page: http://www.classictiger.com/mustang/TeksidTerminator/BlockWork/BlockWork.htm

I did a lot more work all over the block, but I figured these pictures were representative of what I did.

 

[CJK440]

Probably the best analogy for a stress riser in the real world is if you ever scored and broke glass or plexiglass. The score is the stress riser. When the glass is flexed the stress concentrates at the scored area instead of spread equally across the piece. The glass would snap with less tension than a thinner piece of glass without a score mark.