Grit blasting and effects on dimensions
I know this is not welding related, but does any1 know if grit blasting can make minor changes to dimensions.
This is the situation an OD with a range of 21.493 to 21.497 the machinist measured it @ 21.494 and I measured it @ 21.493.
The mating part has a spec of 21.500 to 21.505 the operator measured it @ 21.503 and I got 21.504.
Both parts were grit blasted and painted. The assemblers sanded the paint off the parts but they still won't fit.
I went back to verify the dimensions and found the readings for the od are now 21.497 in one direction and 21.499 in another.
I cannot verify the mating part.
Any1 who takes measurements down to the thousandths knows .oo5 is a huge number, when I set the verns to my original measurement they don't pass thru (without force)
Can grit blast cause high spots that could change the readings?
MDK
Grit blasting sort of peens the surface being struck by the grit or shot. Much like striking a flat plate with a ball pein hammer and then examining the dent. You will note a depression in the center and slightly raised surfaces along the perimeter of the dent. This doesn't matter if it was shot or grit therer will be some upsetting, or displacement, of the metal. Now how much displacement you can expect, I would not dare to say.
Larger shot/grit makes bigger "dents" but I would not expect large blast media to be used on machined parts with tolerances of thousandths.
In the steel fabrication world, we protect machined surfaced from being blasted so as not to have the problem in the first place.
this has happened before i asked the eng to rethink the tolerances, but it takes and act of congress to channge things, i agree that a machined surface with such a tolerance should not be blasted and painted but i need to go by the print.
I agree that there is a certain amount of peening going on, but like you i dont know how much
i could run a test and i will when i find a test piece.
Thank you for confirming my thoughts.
MDK
Hello Mikeqc1, to add just a bit to what Chet has already included, the short answer is yes it can definitely change dimensions and the shape of items and for all of the reasons that Chet has included. Just by the progression or method that the blasting tech used it can influence these characteristics. I would compare blasting of some parts to heat cambering, the more you apply it in certain areas the more effect it will have. You are "stretching" the surfaces of the material to varying degrees by the amount of, pressure, size of medium, angle of medium blast, and any number of other variables. If you really want to see how it can cause distortions, take a piece of 10 GA. sheet a couple feet square and blast the center of it for a while, you will see some serious oil-canning of it's surface. Just a bit more for you to consider. Best regards, aevald
yes i have seen that when doing bend tests for themal spray the small strips bend all over the place. i put this question out to get your opinions on this cuz im going to make a serious issue when the nxt piece is made.
The fix is to remachine the part then assemble it, but they do not repaint it so i will ask why paint in the first place.
thx chet,aevald
MDk
This is quite possibly a constant for your operations anyway, but have you considered temperature changes and the thickness variations of the two parts. When working on heavy equipment in the NW we ran into that often. Exact machining to tight tolerances, measured different times of the day if the shop is not temp controlled can amount to several thou difference at another time of the day. And the two parts may not change at the same rate if they are different thicknesses or materials.
Have a great day, Brent
Perhaps you can change media, such as to walnut shells or glass beads. If grit blast is specified, perhaps pressure controls are off. Many blasters simply burn the parts by running the highest volume and pressure that their equipment allows.
I often see products blasted for painting that are supposed to have a 1 to 2 mill profile that come out 3.8 to 4.6 mils!!!
By G.S.Crisi 
Date 10-09-2008 20:56
Edited 10-09-2008 21:01
If I've understood well, you're talking about a male end (possibly a shaft end) that must enter a female end (possibly a coupling).
Although there's a minimum clearance of 4 mils on the diameter (21.503 minus 21.499 is equal to 4 mils) which means 2 mils on the radius, the male end wouldn't enter into the female end.
Have you considered heating up by a few degrees the female end in order to expand it and then try to fit the male end into it?
Giovanni S. Crisi
Sao Paulo - Brazil
