I would like to find out what others are doing about plating parts with tapped holes. Many times we send out parts for plating that have had the internal threads inspected and find that after the parts are plated, the Go Gage no longer enters. If we try to chase the threads, we often wind up with the No Go member going in. In the past we have tried tapping oversized, but this has not produced acceptable results either. I would like to know what others are doing because this is adding to our cost of production without adding to the value.

Doug

I would like to find out what others are doing about plating parts with tapped holes. Many times we send out parts for plating that have had the internal threads inspected and find that after the parts are plated, the Go Gage no longer enters. If we try to chase the threads, we often wind up with the No Go member going in. In the past we have tried tapping oversized, but this has not produced acceptable results either. I would like to know what others are doing because this is adding to our cost of production without adding to the value.

Doug

If the parts are rack plated, you might consider plugging the holes prior to plating. If they're barrel plated, it's almost impossible to predict how plating will build in holes, and if chasing is necessary, you may want to consider chasing with a screw rather than using a tap (if that's what you're doing). Depending on a number of factors not in evidence here, sometimes it's best to tap after plating.

When I was QM at a plater I became aware of the "Four Times Rule" when coating threads (internal and external). The rule is as follows and may help resolve your fit issue when specifying thickness.

"Since the thickness of a plating deposited on the flanks of the threads results in a flank diametral displacement equal to 4 times the plating thickness (add thickness to diameter in four directions-top, bottom, left and right sides), it seems that the thickness that could be added is limited to a maximum of 1/4 the allowance. However, the coating thickness must also allow for a working tolerance for the plater to insure that the maximum thickness does not exceed the limit."

When I was QM at a plater I became aware of the "Four Times Rule" when coating threads (internal and external). The rule is as follows and may help resolve your fit issue when specifying thickness.

"Since the thickness of a plating deposited on the flanks of the threads results in a flank diametral displacement equal to 4 times the plating thickness (add thickness to diameter in four directions-top, bottom, left and right sides), it seems that the thickness that could be added is limited to a maximum of 1/4 the allowance. However, the coating thickness must also allow for a working tolerance for the plater to insure that the maximum thickness does not exceed the limit."

The 4x rule comes into play most often on external threads. The problem with internal threads is usually more a matter of buildup at the top, around the edges of the entrance to the hole (a high current-density area), and maybe the first thread or two. Running a tap into a hole in that condition is likely to achieve the clearing of the entrance, but also might enlarge deeper threads where deposition thickness is relatively small or nonexistant. The goal in chasing threaded holes that are overplated should be to begin by clearing the first thread or two and not go deeper if it's not necessary. Again, using a screw (rather than a tap) is less likely to result in removal of material beyond the deposition thickness.

And one more suggestion: sometimes a deburring tool (http://www.deburringtools.com/deburringtoolswithhandles.html) (see the attachment) may be used to clear the plating buildup from the top of the hole without affecting the threads or base metal.

Hi silentrunning,

how do you calculate the internal thread dimensions before plating?

Hi silentrunning,

how du you calculate the internal thread dimensions before plating?Thanks for your interest in this thread.
Do you mean
"How does ANYONE calculate the internal thread dimensions?"
OR
"How does silentrunning and his organization do it?" (because you suspect a clue in the way silentrunning performs the process may lead to an easy answer?)

"How does silentrunning and his organization do it?"

I would like to find out what others are doing about plating parts with tapped holes. Many times we send out parts for plating that have had the internal threads inspected and find that after the parts are plated, the Go Gage no longer enters. If we try to chase the threads, we often wind up with the No Go member going in. In the past we have tried tapping oversized, but this has not produced acceptable results either. I would like to know what others are doing because this is adding to our cost of production without adding to the value.

Doug

Hello Silentrunning,

I have been into your shoes earlier. I am manufacturing sheet metal components for filters, and have faced similar problem with the baffle plate/top plate (of spin-on filters) having internal threads. I tried to control the plating, since I have in house plating facility, but it did'nt work. The reason is that the plating deposit on internal threads has very wide variation. I found that the deposit at the apex of the thread is double than the deposit on the plate, and yet the bottom of the groove remains uncoated! I had to increase the plating deposit from 8 to 12 microns to cover the groove of the thread. The 'GO' of the TPG does not enter now.

Chasing the thread with a standard tap at this stage always permit the 'NO GO' of TPG to pass through. I tried chasing the thread with a one step down tap and yet came out with the same result. Since the plated material is soft, even the 'rub' of the tap easily removes the deposit.

After a lot of trial and error, I finally embarked on an idea which may not go down well with the most, but is working well in my unit. I am using a step down tap for chasing the plated threads, where the tap is held in the collet of machine head and the job is held in (gloved) hand. To increse the rate of production, I have welded a rod to the tap stem, the hand held job is chased on the tap and dropped on the rod, to accomodate next piece. After chasing 10-12 pieces in a row, they are dimounted to start the next lot. Thogh the idea sounds crude in today's technologically advanced word, I am successfully using it, day-in and day-out. You may give it a try.

Umang :D

Nota-bene: By holding the job in hand, the axis of the job becomes flexible w.r.t. the axis of the tap, which makes the difference.

Screw Thread Plating and Coating

Minimum and maximum plating thicknesses are specified.

The computation of the corrections for thread diameters can take place arithmetically or statistically.

The most often used method is the arithmetic method, sometimes called the maximum - minimum calculation method. This method guarantees full assembly and working interchangeability of components. However, due to the demand of higher accuracy of the closed component, it results in too limited tolerances of partial components and therefore high manufacturing costs. The arithmetic method is therefore suitable for calculating dimensional circuits with a small number of components or in case that broader tolerance of the resulting dimension is acceptable. It is most often used in piece or small-lot production.

Statistical methods of calculation of dimensional chains are based on the calculus of probability. These methods assume that in a random selection of components during assembly, the limit values of deviations only rarely occur with more partial components simultaneously, as is the case of combined probability. The probability of the occurrence of limit value of deviations in manufacturing individual dimensions on one component will be similarly small. With a certain, pre-selected risk of rejection of some components, the tolerances of partial components in the dimensional chain can be increased.

The statistical method guarantees only partial assembly interchangeability, with a low percentage of unfavourable cases . With respect to larger tolerances of partial dimensions, however, it results in a decrease in manufacturing costs. It is mainly used in mass and large-lot production, where savings in manufacturing costs outbalance increased assembly and operating costs resulting from incomplete assembly interchangeability of components.

See attached example.

What class of thread gauge are you using?

I work with painted parts, pre-paint we use a class 6H thread gauge, post paint we use a class 6G thread gauge.

The paint thickness is typically 20 to 30 microns, so there is an allowance in the thread gauge class to ensure that we tap the thread to the correct size to give a good condition thread after paint.

It sounds from your post that you are trying to use the same gauge for pre and post plated parts.

Are you using oversize taps designed for this problem to start with?

Are you using oversize taps designed for this problem to start with?

Here (http://www.eng-tips.com/viewthread.cfm?qid=170366&page=8) is a discussion about this topic.

Stijloor.

Over here in the 'old country' we use 'pre-plate taps' in conjunction with 'pre-plating' thread gauges. After plating (zinc and yellow) all mating parts assembly ok.

peterh

At last!! A thread where this dummy might actually contribute a bit of useful information!!

Two questions, Silentrunning. First- Are you cut tapping or roll tapping/cold forming? There is a siginificant difference in the thread form between the two.

Second, and more important - what sort of plating are we talking about? In my work I deal with things that are called 'plating' that very from chem film on aluminum with zero build-up, to cadmium plating that results in a few tenths, to heavy nickel film on precipitation hardened steels and high temp alloys up to .003 on a surface. Through the school of hard knocks, I've learned a few tricks. Please specify a bit more about your process and maybe, for once, this idiot can actually be helpful.

Also- I mostly agree with the '4x' rule, but have found that it does not always apply with all coating processes. I'll give one example: magnesium oxide.

The problem with internal threads is usually more a matter of buildup at the top, around the edges of the entrance to the hole (a high current-density area), and maybe the first thread or two.

Yes, ID threads have some of the worst variation due to the range of high to low current densities. Perhaps another option to help you out may be increased chamfers at the ends of the ID to allow for more build-up. Trying to predict finished dimensions, and balancing OD to ID depositions, is always tough - especially with heavy or hard plating.

Thread chasing can also chip brittle plating, which can create corrosion issues, too, if anyone is noticing.

I hate it when you have to plate or heat treat to your final dimensions!:cool:

The statistical method guarantees only partial assembly interchangeability, with a low percentage of unfavorable cases . With respect to larger tolerances of partial dimensions, however, it results in a decrease in manufacturing costs. It is mainly used in mass and large-lot production, where savings in manufacturing costs outbalance increased assembly and operating costs resulting from incomplete assembly interchangeability of components.


First, you have to have a plater that is serious about control. I did find one, once. Rack plating may be your best bet at that. But, the current density variation across the rack can create a good deal of havoc on the statistics. You almost have to plot each rack location individually - as you should do with each head of a CNC or each spindle of a screw machine, etc. The chemical variation of the bath over time, part handling, part variation and so forth, all make it hard to utilize the predictive capabilities of SPC.

Silentrunning-

Oops- one more thought and one more question- is your organization the design activity or are you working to your customer(s) engineering documents? If you are the design activity and have the authority to change the design, I can turn you on to some newly developed surface treatments that are in some applications much superior to conventional plating processes in terms of both wear resistisence and corrosion resistance. Depends on the material and the application.

Hi Prototyper,

I work with painted parts, pre-paint we use a class 6H thread gauge, post paint we use a class 6G thread gauge.

You should use pre-paint a class 6G thread gauge, post paint a class 6H.

The max. thickness (build up) of the plating is then (Allowance G) / 4.

I'm not exactly sure if this is off topic, but I DO want to make folks aware not ALL plating creates this problem. Back in the early 90's I discovered electroless nickel plating - deposited chemically versus electrically or mechanically.

Since we were dealing in very close tolerance work, the anomalous consequences of uneven current distribution at sharp edges ("dogboning") would make grown men cry.

We were able to successfully switch many customers requesting protective or cosmetic finishes over to electroless nickel. We were elated to discover there was even a version with teflon which reduced friction dramatically.
(The "killer" demonstration was performed with two metal "bricks" - one finished with normally deposited electrolytic nickel plating and the other with electroless nickel-teflon, BOTH equally shiny.

We were directed to try to lift each brick by holding a sheet of ordinary typing paper to pick it up (like using paper to pick bagels out of a bin.)

There was no problem picking up the electrolytic nickel one, but the electroless nickel-teflon one defied even the strongest grip!) [you could pick it up with bare hand or rubber glove, but not a cotton or nylon inspection glove]

The beauty of electroless nickel plating is uniform deposit on edges and points and in nooks and crannies. It cost a bit more, but was paid for many times over by its advantages.

The beauty of electroless nickel plating is uniform deposit on edges and points and in nooks and crannies. It cost a bit more, but was paid for many times over by its advantages.

Excellent point! Sure do not have high or low current density plating variation with no current! But, even with electroless, the plater has to pay attention to their bath conditions to ensure repeatable results. You also have to make sure that the solution reaches all of the areas equally. Some "dead zones" or parts stuck together can cause unexpected variation. And....the customer has to want that finish, too!:cool: