holding +/- .001" after Zinc Plating and Heat Treat

Trying to figure out how much to oversize a 1.136 +/- .001 thru holes in 3/4" 4140 steel plate

part has to be hardened and zinc plated

Material: 4140 Steel
Thickness: 3/4"
Harden to Rc 40-45 & Zinc Plate (PLATING FE/ZN 12 - TYPE II YELLOW)

thank you for any help on this

We did a lot of yellow zinc on small electronics components that had tight hole tolerances and tiny threads. We would send a small sample lot with a few sizes and then measure when they got back to see how it went and determine our hole size from that.

The other option is very slightly oversize the hole and then ream after plating. Theoretically it should leave some plating there if the hole was big to begin with, but I never liked the practice myself. Not something I'd try on that big of a hole, that thick.

4140 will move around in heat treat so at less than one inch thick I would suggest getting your stock hardened and then cutting it in the hard state. 4140 still cuts great at 40-45HRc but start with decarb-free. I don't see a flatness or parallelism call on there so I assume you are just getting flame cut blanks and then punching the two holes and cutting the one end?

Ask your plater for guidance. Make sure that a clear plan is written up for any product to be plated, and confirm with each batch delivery to the plater. This may include coupons, or guarantees of in-process measurement.

BTW, are these parts under stress? Presuming they are, you should also consider hydrogen embrittlement risks due to the plating. A proper baking of plated parts is usually done to reduce the chances of this being an issue.

Hydrogen embrittlement - Wikipedia

The zinc plate will not be your issue- most zinc plate is around .0002" thick. Your big issue is how accurate your magic decoder ring is regarding the heat treat to 42 Rc and how much those holes are going to shrink.
As Rick mentioned above, you may want to plan on milling/boring the holes to size after heat treat and before plating. 42 is not that hard and can be machined after heat treat. As mentioned above you will want to specify a post plating bake to prevent hydrogen embrittlement. Your plater should be able to to this- the parts don't need to go back to the heat treater for this.

Dan from Oakland said:

As mentioned above you will want to specify a post plating bake to prevent hydrogen embrittlement. Your plater should be able to to this- the parts don't need to go back to the heat treater for this.

Click to expand...

If the plater can't do the bake, find another plater. It should be done ASAP after H/T to drive out the entrapped hydrogen as effectively as possible.