Surface finish from an investment casting 3

[SeasonLee]

Hi All

Hope this is the right place to post it.
Anyone has the surface finish specification from an investment casting? What is the standard on ICI (Investment Casting Institute) handbook?

Thanks for all help

Season

 

[ctopher]

What finish do you need?
A quick Google search shows: Link

Chris, CSWA
SolidWorks 14
SolidWorks Legion

 

[SeasonLee]

Thanks for the reply, Chris.

Google search tell me the standard finish from an investment casting can be reached is 125 (3.2) microfinish, but our supplier said they can reach to 250 (6.3) only, so I just want to know what is standard requirement on ICI handbook.

Season

 

[1gibson]

I would bet the supplier said they can only "guarantee" 250. They can probably get closer to 125, but don't want rejections if it is a little rougher. Capabilities and guarantees are two different subjects.

 

[tbuelna]

The answer to your question is what level of surface roughness are you willing to pay for with the investment castings you purchase? The casting suppliers know that they will not get paid for producing non-conforming castings. So when they quote your order they take factors like surface finish requirements into account.

In reality, investment castings can be supplied with almost any level of surface roughness you want. The casting surfaces can be smoothed out using various secondary processes like tumbling or hand polishing if you are willing to pay for it. Many investment castings undergo shot blasting to remove the shell material, so the general surface roughness of the delivered castings would be whatever results from the shot blast process. However, there may be local areas on the casting that are very rough, such as locations where saw cuts and hand grinding were done to remove sprues or flash. You should allow for very rough surface finish in these locations, and also allow for a fair amount of surface mismatch.

 

[geesamand]

To add onto Terry's point about subsequent finishing, we purchase steel and 300-series stainless steel investment castings. With grinding and polishing you will uncover a tiny bits of porosity and we have found that 32Ra (microinches) is the limit on subsequent operations due to these low spots. (For sand castings the limit is probably 63Ra)

125Ra is probably achievable over the bulk of the part, but the slightest imperfection will violate that limit. As a casting vendor I would be wary of any such specification that applies uniformly over the whole part. Consider using SCRATA surface comparators.