Thread as the most important part of mechanical fasteners,most of manufacturers will plate the thread for corrosion protection and decoration,galvanizing is the most common plating method, Controlling the pre-plate thread size is the fundamental guarantee of quality thread after plating. This article summarizes the methods for controlling the dimensional accuracy of the threaded products before plating.

No matter internal thread or external threads,manufacturer of threaded parts need to make suited allowance to the threads before plating, the manufacturer should know the specific plating requirements in advance. If the allowance is not controlled well, then which will leads to the threads will be too tight or too loose. The threads are important parts of threaded parts. If the threads too tight or too loose, all finished threaded parts will be rejected in the inspection process. Then much labor, time and material will be wasted, the threaded parts’ manufacturers will have great loss!

1.Effect of Electroplating on Thread Geometrical Parameters

Assuming the thickness of the coating is evenly distributed on the threads, the effect of coating on the diameter of the threads is shown in Figure 1

(External thread, the common 60 ° thread as an example), where: d is the pitch diameter before plating; d1 is the pitch diameter after plating; t is the thickness of the plating layer; Δd is the single side increment after plating .

By geometric relationship:

The total increment of thread pitch diameter after plating:

d1-d=2Δd=4t

In other words, the increase of the thread diameter is 4 times the thickness of the coating. The same token, the internal thread plating, the diameter reduction is also 4 times the thickness of the coating. In fact, the electroplating process due to prominent parts prone to excessive concentration of power lines, resulting in the so-called “tip effect” in the tooth tip easily plated thicker, as shown in Figure 2. Therefore, in the development of electroplating process must be considered.

If you are dealing with common 60 degree threads, for example, Metric thread, the ratio is 4: 1, the meaning is the plating thickness will be done 4 times than the applied amount because each flank is affected.

The NO.1 step of producing pre-plate threads is to be sure how much plating will be built up on the threads.  this is decided by two factors,   one is plating specification, the other is the thread’s place on the parts. Sometimes the threads are located in too deep place of the threaded parts, it is very hard and difficult to apply full amount of plating. We suggest when making pre-plate threads, the manufacturer should communicate well with plating suppliers, sometimes, plating company workers can give very fine suggestions.

In order to be sure the threaded parts will be quality enough to assembly and fit, we suggest that the manufacturers need to use pre-plate thread guages, by using pre-plate thread gauges, the threads can be controlled very well. although pre-plate thread gages need some cost, the investment is worth it. we suggest that the manufacturers of threaded parts send thread gauges to plating supplier, then the plating supplier can test the thread’s correctness by using the gauges.

Pre Plate Ratios for Various Threads
60° Unified thread = 4:1
29° Acme thread = 8:1
7°/ 45° Buttress thread = 4.3138:1
10° Square thread = 23:1

Based on the ratios above you will know that how much affect a specific amount of plating can be built up on a thread.

As below is the rule for determining pre plate Major diameter, pitch diameter, and minor diameters:

• Calculating pre-plate dimensions of external threaded parts. For external threads subtract the max plating thickness from the parts high limit P.D.. Then subtract the minimum plating from the parts low limit P.D. For the minor and major diameters reduce the Parts maximum diameters by half the maximum plating and reduce the parts minimum diameter by half the minimum plating.
• Calculating pre-plate dimensions of internal threaded parts.For internal threads add the max plating thickness to the low limit P.D. Then add the minimum plating thickness to the high limit P.D.. For minor and major diameters increase the minimum minor diameters by half the maximum plating and increase the maximum minor diameter by half the minimum plating.
• Calculating pre plate dimensions for thread ring gages:  For thread ring gages subtract the max plating thickness from the P.D. of the Go thread ring gage. Then subtract the minimum plating from the P.D. of the No Go thread ring gage. For the minor diameter reduce the Go ring minor diameter by half the maximum plating and reduce the No Go ring minor diameter by half the minimum plating.
• Calculating pre plate dimensions fo thread plug gages:For thread plug gages add the max plating thickness to the P.D. of the Go thread plug gage. Add the minimum plating thickness to the P.D. of the No Go thread plug gage. Increase the major diameter of the Go thread plug by half the maximum plating and increase the No Go thread plug major diameter by half the minimum plating.

Example for Thread Plug Gage:
Plating of .0002” – .0003” allowance per side multiple x 4
.0003 x 4 = .0012” Max
.0002 x 4 = .0008” Min

½ – 20 UNF 2B P/P

Basic Go P.D. = .4675” + .0012” = .4687” Go P/P pitch diameter
Basic No Go P.D. = .4731” + .0008” = .4739” No Go P/P pitch diameter

Basic Go Major Diameter = .5000” + .0006” = .5006” Go P/P major diameter
Basic No Go Major Diameter = .4948” + .0004” = .4952” No Go P/P major diameter

Example for Thread Ring gages:
Plating of .0002” – .0003” allowance per side multiple x 4

.0003 x 4 = .0012” Max
.0002 x 4 = .0008” Min

½ – 20 UNF 2A P/P

Basic Go P.D. = .4662” – .0012” = .4650” Go P/P pitch diameter
Basic No Go P.D. = .4619” – .0008” = .4611” No Go P/P pitch diameter

Basic Go Minor Diameter = .4446” – .0006” = .4440” Go minor Diameter
Basic No Go Minor Diameter = .4511” – .0004” = .4507” No Go minor Diameter

If no minimum and maximum plating thickness is given , then the given plating thickness is considered nominal or minimum plus 50% to determine maximum plating.