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How to dial in a threaded part?

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WesPete66

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#1
So I have this hex head bolt with a damaged threaded end that I want to salvage.. (5/8-18UNF x 3.5") It's an original bolt for my Gibson tractor & new hardware just doesn't match the old stuff.. Anyway, I don't have a die this size, but I can chuck this up and recut the damaged thread on the lathe. Long story short I got the job done, but how does one 'properly' dial in a threaded part in a 4-jaw chuck when nothing but threaded shaft is exposed?? (nothing to run a dial indicator against?)
 

pontiac428

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#2
I think a dial indicator tip with a fine offset point can trace the minor diameter of the bolt well enough to dial in. Or, get a fat, blunt tip and go off of the major diameter. At 5/8", you probably have enough tolerance to chase out those threads regardless of a thou or three.
 

David S

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#3
Was the entire thread damaged?

David
 

WesPete66

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#4
No, only the first .50" or so was smashed.
 

kd4gij

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#5
I use a disk point like the 3ard one from left

1529525708874.png
 

David S

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#6
No, only the first .50" or so was smashed.
Ok then I think I would make a slip fit sleeve with an OD larger than the bolt head if the chuck wouldn't allow for a thinner sleeve. Make one axial slit in the sleeve so it can tighten on the undamaged threads and hold it that way. That would allow you to easily rotate it to clock the threads to the threading dial.

David
 

Bob Korves

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#7
Set up your lathe for cutting a 18 TPI RH thread. Properly mount a threading tool to the compound rest, but set it just clear of the threads. Run the lathe with the half nuts engaged, shut off the power and let the lathe coast to a stop over the threaded area. Taking care not to move or bump the carriage, loosen the cutting tool, engage the cutter with the existing threads, and adjust the cutter mounting until the tool fits the thread perfectly. Tighten the tool down to the tool post or tool holder. Zero the compound. Back the tool out from the work with the compound, and, leaving the half nuts engaged, back the tool off the part. Dial the compound back to zero. Note the threading dial position for subsequent passes if you disengage the half nuts. You are now set for threading to the existing thread depth and timed to match the existing helix. If only a light cleanup is needed, thread from where you are as usual. If the work is quite rough, back off from your zero and take multiple passes at increasing depths until you reach zero. Test the nut fit, and cut slightly more if necessary to get a good thread fit. It takes longer to write it than to do it... ;)
 

WesPete66

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#8
Awesome, thanks everyone! Guess I need to add some indicator tips to my setup..
 

benmychree

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#9
A different idea: if it was me, I would use a collet die , loosen the chasers so that you can get over the damaged threads to the good threads, then tighten the chasers onto the good threads and back the die off over the damaged threads; there is also a device to do the same trick made by the Buckingham Co, of Binghampton N.Y., it is used the same way, and will adapt to most any thread diameter and pitch within its range, picture below. The tool has two serrated knives on each side, with serrations to allow them to cut, of 60 degree included angle set at an angle so that different diameters are accommodated, and they are forced against the part against a 60 degree wheel on the left side of the pic. by the knurled handle.
 

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markba633csi

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#10
You could lay a feeler gauge between the threads and the indicator (if you are asking how to get the part concentric in the chuck)
You can also eyeball it- I do
 

Tony Wells

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#11
I just lean a 1/2" X 12" flex scale against an undamaged section (if there is enough exposed), then toss an indicator to bear on the scale right at the point of contact between the thread and scale. Can't go too many revs before the scale walks away, but you can go one turn forward then one turn back. If it becomes a problem, a properly placed mag base will hold the scale.
 

P. Waller

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#12
What is often called "picking up" an existing thread for single pointing. Indicating it is easy as mentioned above by using an indicator point that will span the lead of the thread.

Picking up a thread is a last resort.

Once indicated the more difficult task of placing the tool in the correct position with the lathe gear train begins, this will be far more annoying.
 

chips&more

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#13
All good info. As far as repairing/following an existing thread. I would set the compound at 90° or parallel to the work. Move the compound until tool bit is dead center in the V of the thread. Then on each pass just dial in more cross feed…Dave
 

P. Waller

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#14
All good info. As far as repairing/following an existing thread. I would set the compound at 90° or parallel to the work. Move the compound until tool bit is dead center in the V of the thread. Then on each pass just dial in more cross feed…Dave
With a lathe that allows turning the chuck by hand after engaging the 1/2 nuts just engage them and turn the spindle by hand until you reach the thread,, crank the cross feed in then adjust the compound until the tool is in the center of the thread.

If not turn the spindle on, engage the 1/2 nuts then turn the spindle off and adjust the tool with the compound,

This is not so easy with a machine that has no gear train and can not be stopped without losing position, if you have an easy way of doing so please let me know what it is.
 

Tony Wells

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#15
I engage the half-nuts, let the lathe coast to a stop somewhere in the range of the threads, then with the threading tool loose, push it into a good thread if there is one, or pick the best spot, then snug the toolholder screws. Back out a bit and tighten the screws. Seldom is this more than a couple of thousandths out. Same basic method with internal threads; engage the nuts, stop the spindle and with the threading bar loose, push it into the thread then hold it in the toolholder until you get it locked down. Of course, on an internal thread you have to make sure you have enough bar hangout, but not excessive.

CNC is a different story.
 
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