Cad Pre-Plate Thread Dimensions 1

[Inspector3]

Howdy All,

First post, really looking forward to learning and helping others when I can.

Have been a Machine Shop Inspector for a number of years, just recently changed jobs and at my new Job the Inspection Department has been tasked to set the pre-plate machining dimensions for threads, and other dimensional properties.
The type of plating being done is a Cad Plating, with their tolerances being .0003-.00049 thickness. Also, part of the problem is finding something that the Machine Operators can use to verify they are running the parts correctly and consistantly.

Will be interested to hear how and what ya'll use to check and verify this type of thing.

Thanks and Take care,

Craig

 

[CoryPad]

You will need thread gauges (try Johnson or Greenslade). Then, Machinery's Handbook has the basics:

Coated 60-deg. Threads.—Although the Standard does not make recommendations for thicknesses of, or specify limits for coatings, it does outline certain principles that will aid mechanical interchangeability if followed whenever conditions permit.

To keep finished threads within the limits of size established in the Standard, external threads should not exceed basic size after plating and internal threads should not be below basic size after plating. This recommendation does not apply to threads coated by certain commonly used processes such as hot-dip galvanizing where it may not be required to maintain these limits.
Class 2A provides both a tolerance and an allowance. Many thread requirements call for coatings such as those deposited by electro-plating processes and, in general, the 2A allowance provides adequate undercut for such coatings. There may be variations in thickness and symmetry of coating resulting from commercial processes but after plating the threads should be accepted by a basic Class 3A size GO gage and a Class 2A gage as a NOT-GO
gage. Class 1A provides an allowance which is maintained for both coated and uncoated product, i.e., it is not available for coating.

Class 3A does not include an allowance so it is suggested that the limits of size before plating be reduced by the amount of the 2A allowance whenever that allowance is adequate. No provision is made for overcutting internal threads as coatings on such threads are not generally required. Further, it is very difficult to deposit a significant thickness of coating on the flanks of internal threads. Where a specific thickness of coating is required on an
internal thread, it is suggested that the thread be overcut so that the thread as coated will be accepted by a GO thread plug gage of basic size.
This Standard ASME/ANSI B1.1-1989 specifies limits of size that pertain whether threads are coated or uncoated. Only in Class 2A threads is an allowance available to accommodate coatings. Thus, in all classes of internal threads and in all Class 1A, 2AG, and 3A external threads, limits of size must be adjusted to provide suitable provision for the desired coating.
For further information concerning dimensional accommodation of coating or plating for 60-degree threads, see Section 7, ASME/ANSI B1.1-1989.

 

[Inspector3]

Howdy Cory,

Thanks very much for your reply and help. I was kind of weary last night when I posted that and probably should have included more info, like it's a 3A/3B thread.

One of the main problems we have, is to gently say that our consistancy to machine the PD accurately enough on all pieces so as to be able to request .ooo4 of plating from the Vendor and have that produce acceptable results on all the threaded parts using Certified Thread Gages(Go & Not Go)

I have in the past worked at Shops that used the Not-Gage very snugly was the acceptance medium before plating, ensuring that when the minimum coating allowance was put on then the part was acceptable. This technique is so feel oriented that even then we have "mistakes" <G>. Of course then there seemed to be more Machinist/General Machinists in the Workforce so this was not a critical as today.

I have tried to see about getting actual ring gages set to the pre-plate dimensions, but at this time it's kind of cost prohibitive.

I also calculated the min/max post plate thicknesses out, and found that it was really restricting the amount of deviation by taking the -- PD tolerance--away from the Machine Shop, and giving more leeway to the Cad Plating House.

Thanks again, appreciate your insight and reference material.

Take care,

Craig

 

[DennisP]

We run parts at pre-plate dimensions all the time. For external threads, the pitch diameter low limit is decreased by 4X the low plating limit, and the pitch diameter hi limit is decreased by 4 x the hi plating limit. This decreases the as-machined pitch diameter limits and requires a tri-roller or a special thread gage to check. Then you rely on the plater to meet the limits called out in the plating spec. I fI recall correctly, we do not gage threads after Cad plating as the plating smears and fouls the gages.

 

[Inspector3]

Howdy Dennis,

Thanks so much for your reply, that is exactly what we've come up with, cutting down the PD Tolerance is one of the things that has concerned us as it's been difficult enough to keep running in tolerance, and just as you've said you need something very accurate to check it with.

We do check threads after cad, since our Customer does, and if we don't we would get a lot of parts returned, which is one of the main reasons to try and get better accuracy before sending them to the Plater.

Thanks again,

Craig

 

[DennisP]

Craig,

Glad I can be of assistance. It's unfortunate that there is so much variation in the plating thickness, especially for class 3 threads. Could a plating that is more controllable in thickness be substituted for the Cad?

 

[Inspector3]

Howdy Dennis,

Thanks again, I agree about the plating thickness, but with the Specification callout of .0003 to .00049 for most of the Threads we do.

I keep trying to figure out a method to put in our planning so that we can tune it if we need to and everyone will have the same numbers to work to.

Thanks Again, Take Care,

Craig

 

[israelkk]

The tolerance on the pitch diameter of machined external and internal threads is in the range of ~0.040" therefore, with current CNC machines the thread can be machined to much more accurate tolerances. Therefore, the easiest is to use the 3rd lower part of the tolerance range (closer to the minimum for external threads and to maximum for internal threads) and this will give more than enough space for the accumulated 4X maximum coating thickness. For tapped holes you need to purchase oversized taps. There are standard grades of oversized taps for different coating thickness. To be on the safe side you will need GO-NOGO special gages for the oversized threads. Then after coating the standard GO-NOGO should be used.

One other practice is to machine the thread to the standard dimensions, then while on the machine check with GO-NOGO. If it is OK than repeat the thread machining to remove just enough material to accomodate the accumulation of the maximun coating thickness. Repeat the GO-NOGO check after coating.