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Calculating Pre Plate Screw Threads & Gages


It is important to understand how various plating affects manufactured parts featuring internal and external threads. All too often component parts are manufactured without taking into account the correct plating requirements. This results in rejection in final and incoming inspection or even in the field resulting in a recall. Either way proves to be very costly and time consuming.

Manufacturers of threaded components should understand the specific plating specifications and make specific allowances to the threads prior to the plating process. If this is not done or done incorrectly there is high probability that the thread will be too tight or too loose. This pre- plating allowance must be considered for both of the mating parts.

When working with the common 60 degree thread, the ratio is 4:1, meaning the plating thickness will build up 4 times the applied amount as each flank on both sides of the thread is affected.

The first part of making pre plate threads is to determine how much plating build up will be applied to the thread. This is determined by the plating specification as well as where the screw thread is located on the part. There may be cases where an internal thread is located deep down in a bored cavity and it may be difficult to apply the full amount of plating. It may be a good idea to discuss this with a qualified plating company.

To ensure the parts will assemble and fit correctly, the manufacturer should utilize pre plate thread gages so that the thread can be controlled correctly. This investment will pay dividends assuming the correct amount of plating is applied. This can be confirmed with standard or after plate thread gages. It may be a good idea to supply the plating facility with these gages so they can confirm that their plating process is correct and deliver back parts that are in tolerance.

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 can see how much affect a specific amount of plating can build up on a thread. There are several screw thread engineering programs that will handle the calculations for pre plating , including ThreadTech v2.24.

The rule for determining pre plate pitch diameters, major diameters, and minor diameters of parts and gages is as follows:

To determine pre plating dimensions for 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.

To determine pre plating dimensions for 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.

To determine pre plating 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.

To determine pre plating dimensions for 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

- 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

- 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.