# Thread Repair of a stripped out Master Jaw



## RobinHood (Mar 25, 2021)

A new to me 16” 4J chuck has Master and Top jaws. The chuck is a Pratt, Steel Body, made in England sometime between 1962 and 2002 (based on the serial number). It is in overall very good condition with little to no wear. The metal surfaces have some staining, which could be polished out.

There is one big problem: on the #4 Master Jaw, the inner hold down thread is stripped. The 3/4-10 SHCS has no more threads to grab onto and slides in and out with little to no resistance.






The stripped thread





What it should look like (still need to clean the threads better)





I consulted with a member here and discussed some options:

a) keep doing what previous users have done - turn the master jaw around and use the good (outside) thread to hold the top jaw. The stripped thread will be outside now and since it is not under tension, use the chuck that way. Very sketchy using only one of the two hold down bolts.

b) fill the hole with weld, rebore and thread 3/4-10. The heat from welding could distort the jaw.

c) over bore the thread using the same 10 TPI. This will require two custom bolts - one long one and one short one depending which way around  the top jaw is mounted to the master. Can be done with an oversized SHCS turned down and threaded 10 tpi to fit the oversized master thread. Would be fiddly as there is no real accurate way to measure the new internal thread.

Option c) got me thinking: if I am going to over bore and thread, why not use an insert after to bring the size back down to nominal 3/4-10? That’s when I decided on the HeliCoil approach.






The jaws are hardened. So not sure if the HeliCoil tap will work as it is “only“ HSS.

To have the best chance of success, I am using the lathe. The mill would work if I were certain that the tap will cut the new threads - I am not.

Work holding was the next challenge. I have a 8” 4J for the 13” lathes. So I tried it. Too sketchy as the bottom of the master jaw sits only on one, turned around, jaw pad. Also the 4J only grips the bottom 1/4” of the master jaw.






So I got out the 12” face plate and “blocked” the master in using strap clamps, the top jaw (also as counter balance), and some cold rolled stock.











There is a short counter bore above the original threads. I am using it to indicate the center of the stripped threads.






The next step is to use carbide tooling to bore the hole to 25/32” (0.7813”) for the HeliCoil tap. Then I plan on using an internal threading bar to cut the 10 tpi threads. I will use the included tap as a thread gage - and to clean up the threads before inserting the HeliCoil.

To be continued....


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## YYCHM (Mar 25, 2021)

What are you hanging a 16" chuck on?


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## RobinHood (Mar 25, 2021)

A “Bella Macchina” made in Italy with a 4” spindle bore...

I will introduce her to the forum when I get a little more work done on her.


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## Dabbler (Mar 25, 2021)

Very nice save on that chuck...  Great idea on the HeliCoil


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## Tom O (Mar 26, 2021)

Nice a 4” chuck is to drool over hmm I should take a look I believe I have a plug for a Modern tool spindle it may be 4 1/8 though.


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## PeterT (Mar 26, 2021)

Wow that's a big boy chuck. I hope it works. I don't have a lot of experience with helicoil. One thing I've always wondered because they say not to use Loktite to help prevent insert rotation - is the re-tapped hole always intended to be blind so the helicoil has something to be bottomed into? Or is the tap itself kind of a tapered affair so insert starts to basically interfere & tighten up? I've heard conflicting reports of success but of course behind every repair is 10 factors that go unmentioned - blind vs through hole, over torquing the fastener, heat/environment factors....


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## RobinHood (Mar 26, 2021)

I have successfully used HeliCoil in both blind and through holes. The insert coil is 304 SS, 1.25” long. The instructions say to tap as deep as physically possible. In this case, the blind hole is about 1.5” deep. The tap looks like it has parallel threads.

Yes, there is a possibility of insert rotation after installation - especially since in this application the bolt will be removed and re-inserted any time the top jaw (TJ) is flipped around. I plan on lubricating the SHCS with SS anti-seize to prevent galling the insert and hopefully extend its life a bit that way. If there are any signs of binding, I will have to replace the coil.

Bolt torque will be critical for sure.

I think one of two things have lead to stripping the thread out in the first place:

1) over torquing the 3/4-10 SHCS (but then all the other master jaws (MJ) are fine - so maybe they learned or that was not the cause)
2) bottoming out a bolt before the TJ was tight - this is the most likely scenario as there are two different length bolts used to secure the TJ to the MJ. If you switch them around, the one side of the TJ is almost snug, possibly leading to “just a little more torque to get it tight”. That will strip the thread right away as the SHCS is tougher than the MJ thread.


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## Tom O (Mar 26, 2021)

At the Calgary Herald we used helicoils on some of the bolts that clamped down the knife blocks on the folder they last a long time and are easy to remove if they become worn. 
They were subjected to centrifugal force as well as vibration from cutting forces.


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## RobinHood (Mar 26, 2021)

Thanks for that info Tom. Gives me some more confidence that once installed, the HeliCoil may work well.

That brings me to today...

Was able to successfully bore the old mashed threads out. The carbide insert held up well and left a decent finish.






I was not so lucky with the counterbore. It is needed to give the tap (and the helicoil) clearance to get into the hole. That area of the MJ is really hard. You can see how the material is chipping off. The insert took a beating as well.






So I switched to the tool post grinder to get the last 15 thou on the diameter. That worked well.






The plan was to pre-thread the bore with the internal threading bar and finish with the tap. Well, the threading bar is too large for the bore. So I tried with just the tap. It is cutting, but the material is not cutting well - it seems to just break off in bits.






So I stopped there for the day. Will have to make a threading tool next...


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## Brent H (Mar 26, 2021)

@RobinHood: what type tap were you using?  I have a couple that are rated for cutting threads in extra hard steel.  I use a 1/2 - 13 for modifying some of the metric m12 threads to 1/2-13 in my 30 tapers - seems to cut a decent thread in the hard material but I usually add a lot of tapping sauce to get things done and keep things from sticking up in the hole.  Nice use of the tool post grinder


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## RobinHood (Mar 26, 2021)

It is the special tap that comes with the thread repair kit. The major diameter is about 0.885 with 10 TPI. It is HSS. Lots of cutting fluid. The MJ material is just hard and brittle from the heat treat process.


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## RobinHood (Mar 27, 2021)

Success! Job done.

Thanks to @Dabbler for letting me borrow his 5/8” Kennametal threading bar. It took about 45 one thou passes to get the threads roughed out. The insert held up perfectly.






Then the tap did the final clean up.






Inserted the HeliCoil, and we have a good as new thread again. The coil is about 1/2 turn too deep in the hole - I was fiddling around to break off the tang and managed to seat it deeper than I wanted.






Need to get all new SHCS for the chuck as right now there are 4 different lengths, two different hex sizes and some with less than ideal threads.

Here it is assembled with the old bolts.






On to the next project...


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## johnnielsen (Mar 27, 2021)

Beautiful.


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## Dabbler (Mar 27, 2021)

Nice!!  That chuck looks great!


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## RobinHood (Mar 29, 2021)

Yes, it did clean up very well. Everything slides really smoothly as well. Just need a D1-8 backing plate and a chuck key.


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## YYCHM (Mar 29, 2021)

What does that thing weigh?


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## RobinHood (Mar 29, 2021)

About 190 lbs without the backing plate. Depending which plate I get, it will add another 30 lbs or so.


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## David_R8 (Mar 29, 2021)

Holy moly, that's a beast. Makes my 8 in 4-jaw look like something for a watchmakers lathe!


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## RobinHood (Mar 29, 2021)

It is beefy all right. Lots of inertia once that thing starts turning.


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## PeterT (Mar 29, 2021)

Good job. Nice chuck. 

I'm curious about your TPG. Is it a collet system holding the grinding stone shank?


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## RobinHood (Mar 29, 2021)

Yes, the TPG came with an adapter that takes standard 1/8” shank mounted stones. It gives great versatility when it comes to selecting cutting tools/stones.

Here are the types of stones I can use. L to R: regular 3/8” bore grinding wheels directly mounted on the spindle; 1/4” bore stones with the adapter/extension; and the 1/8” mounted stones with the 1/8” collet adapter.

The spindle is capable of 36600 rpm, so even the little 1/4” stones work well.


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## Mcgyver (Apr 2, 2021)

impressive, I would have been very leering of threading the hardened jaws, but your got it done!


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## RobinHood (Apr 2, 2021)

I was hesitant as well. When you see Pratt’s prices for accessories, a set of MJs would have set us back $1K+, assuming you can find one. So I had to try it at least, as it was not safely usable.

Option ”B” would have been to weld up the hole and re-thread it to 3/4”-10 in the hope that the weld fill is not as hard as the jaw. Heat distortion becomes a big factor with welding.


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## Mcgyver (Apr 2, 2021)

When I saw thread I thought grind and loctite in a sleeve (at 3000 psi on steel should be more than enough)...but its moot as you challenged the machining gods and won!


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## RobinHood (Apr 2, 2021)

Yes, I was lucky with this repair.


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