Form tap to fix anodized undersized holes in Ti?

[Matthew Hajicek - Singularity]

I asked this on PM but figured I'd ask here too.  

I have some fairly high dollar (for us) Ti6Al4V-ELI parts that came back from hard-coat anodize with some threadmilled 6-32 holes ever so slightly undersized such that the go gauge will almost but not quite pass through. Holes are through a .075" thick wall. Is it safe to use a greased form tap to resize them?

Also, my plater claims that the anodization doesn't change the dimensions at all. Is that bull? I need to now if I need a pre-plate thread gauge to ensure this doesn't recur. Maybe I should get a go gauge that's a few tenths big, and use the standard no-go?

Thanks!

[JParis]

Typical regular anodize, doesn't change but a very little.....0001/.0002, if that much.....hard coat, that's a different story...

I have seen threads come back from anodize a mess because of crud that gets into the holes....I have only chased those by hand with cutting taps...

Chansing hardcoat is going to be problematic....typical build-up is .0005/.0007" per surface, that quadrupled on threads...you can find thicker calls of anodize...determined really by time in tank

[gcode]

You should be using a pre plate thead gage if the threads are getting hard anodized.

If you are tapping or roll tapping, , you'll need oversized taps as well.

If you're threadmilling it's simpler as you have precise control over pitch diameter.

I hate jobs requiring hardcoat. No matter how careful you are, it can all go south at the plating house.

I don't have any idea how to fix your parts. The hard coat is hard and abrasive and will kill your taps.

You may be able to strip them, chase the threads and replate, but that is risky and your customer may

not permit it.

 

[Newbeeee™]

Matt,

A screw will fit no problems. I guess it's not an assy, and the surgeon has to fit them?
If it was an in-house assy then no problem, but if it's a surgeon I can see why in-house inspection would fail them!

If it's that near on going in, it maybe worth trying another gauge...

Can you make a dummy plate (same mtl/thickness) and tap a few holes and get the platers to process. Then try a tap on this?

Hardcoat on aluminium is 50/50 into the material/build up. For titanium, it looks like it's very different to aluminium thickness and only a couple of tenths, opposed to a couple of thou. Hence you're unlucky and the gauge is *nearly going in*

https://www.titaniumfinishing.com/capabilities/titanium-anodize-functional/

[Matthew Hajicek - Singularity]

16 hours ago, gcode said:

You should be using a pre plate thead gage if the threads are getting hard anodized.

 

13 hours ago, Newbeeee™ said:

Matt,

A screw will fit no problems. I guess it's not an assy, and the surgeon has to fit them?
If it was an in-house assy then no problem, but if it's a surgeon I can see why in-house inspection would fail them!

I've made several batches of these plates before, and they've come out good.  I'm thinking of getting a pre-plate gauge, but the plater claims zero dimensional change.  I don't believe them.  I'm thinking of bringing the pitch diameter tolerance in .0005" on each side for pre-plate to allow for some random change at the plater for future batches.  Yes, the screw goes in in the field.

Thanks

 

[gcode]

Matt,

draw up an accurate 2D model of a 60° thread and set up some points to measure the pitch diameter

then offset the thread lines the nominal amount of build up and measure the pitch diameter again

because the pitch diameter is driven by 4 faces, all being built up radially by the hard anodize, you will find

the pitch diameter has changed approximately  4X the plating thickness

[Newbeeee™]

34 minutes ago, Matthew Hajicek™ - Conventus said:

 

I've made several batches of these plates before, and they've come out good.  I'm thinking of getting a pre-plate gauge, but the plater claims zero dimensional change.  I don't believe them.  I'm thinking of bringing the pitch diameter tolerance in .0005" on each side for pre-plate to allow for some random change at the plater for future batches.  Yes, the screw goes in in the field.

Thanks

 

When you say "in", you mean "out"... as in make it bigger not smaller.

Are the holes blind, because you'd probably only be getting build up at the top of the hole anyway.

If through, you could make so the no-go starts to go in and then back off a tad - so it's a "sloppy fit" on the go.

Hard to get a pre-plate gauge if you don't know the plating thickness though. That's the number 1 thing to establish I guess.

[Matthew Hajicek - Singularity]

4 minutes ago, Newbeeee™ said:

When you say "in", you mean "out"... as in make it bigger not smaller.

Are the holes blind, because you'd probably only be getting build up at the top of the hole anyway.

If through, you could make so the no-go starts to go in and then back off a tad - so it's a "sloppy fit" on the go.

Hard to get a pre-plate gauge if you don't know the plating thickness though. That's the number 1 thing to establish I guess.

When I say "in", I mean I'm going to make the go gauge bigger and the no-go gauge smaller, to tighten the tolerance on both sides before plating.  That way I'm covered whether the holes grow, shrink, or stay the same.  Holes are thru.

 

Thanks

[gcode]

how are you threading these holes

 

[Leon82]

Clear anodized maybe will put a tenth on a hole.

 

But with black anodized on aluminum we have to go up about four or five tenths on a hole for them to plate intolerance

 

Most of our titanium parts get tiodize but i don't remember ever having to reach chase the threads

[Newbeeee™]

12 hours ago, Leon82 said:

Clear anodized maybe will put a tenth on a hole.    (This is Chromic)

 

But with black anodized on aluminum we have to go up about four or five tenths on a hole for them to plate intolerance    (This is Sulphuric)

 

Most of our titanium parts get tiodize but i don't remember ever having to reach chase the threads

^My adds in brackets

Hardcoat for ally is a couple of thou (at 50/50 into mtl and buildup on material). This can be a nightmare to control because the tolerance on the plating thickness, can often be more than the finished tolerance on say a bore....

For internal threads, allsorts come into play - not just the theoretical 4x build up, but size of thread and through hole or blind.

The platers bath can have an influence too - how packed the parts are in, and where the parts are jigged from (creating a "dead spot"). And if the parts have blind holes and are jigged upside down, air stays in the hole so no plating at all...

 

[MATT@CNC]

I WOULD TRY THE NEXT "H" SIZE UP. MOST TIME IVE ROLLED TAPPED I WAS PUTTING A HELICOIL IN THE HOLE SO PLATING WAS NOT REALLY AN ISSUE. BUT IF YOU ARE CLOSE TO SIZE I THINK EACH  "H" NUMBER YOU GO UP, OPENS THE THREAD .0005 TOTAL .00025 EACH SIDE. THAT SHOULD GET YOU THERE BUYING CUSTOM GAUGE UNLESS ITS A PRODUCTION RUN (HIGH QUANTITY) MIGHT BE PRICEY .

[4VRGJ03]

is it a requirement to have the threaded hole hard anodized? Usually threaded holes do not have a hard anodize requirement just the regular anodize. I highly doubt a 6-32 hole requires hard anodize. If it is a requirement then this would be the first time I have ever seen that on a thread. Usually hard anodize is used for frictional surfaces for wear resistance....but what do I know?