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attempting a project need help

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jbalp

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#1
so this will be something I've never tried before. need help with some dimensions . I don't have my machinist handbook yet. it's on the way. I bought a 4 jaw chuck and the back plate is 2-1/8" x 8tpi .. I need 2-1/4" 8tpi . so bore it out and re-thread it. ok this might be beyond my skill level. maybe someone can talk me through it .
 

Ulma Doctor

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#2
what tools/machines do you have available?

to redo your backplate you may need a lathe for best results
if you have a faceplate, you can affix the new back plate to the faceplate, and indicate the bore true.
you may be able to chase the existing threads to new depth but it may be time consuming to set up.
for best results make a plug gauge for the intended fit beforehand, if possible.
but you can do it the hard way by removing the backplate and testing the fit as the work is performed
 

terrywerm

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#3
I would do as Mike mentioned, and mount the back plate to a faceplate, get it centered up, then bore to your required ID.

Next, you would need to set up a threading tool and get it properly 'registered' with the existing thread so that you can cut it deeper to achieve the new, larger thread.

According to Machinery's Handbook, you would want an ID of 2.115" to 2.1297" (class 3B thead) then cut your thread deeper until you reach the thread major diameter of 2.250". Since measuring the major diameter of an internal thread can be a bit difficult, I believe you could safely cut the thread deeper until it fits properly on the spindle thread, or plug gauge if you go to the trouble of making one.
 

Asm109

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#4
Step 1. Make a male thread plug gage that matches the spindle thread. By that I mean same OD and measurement over 3 wires.
Step 2. Mount to face plate and indicate bore. Bore ID to spec for thread.
Setep 3. Register tool bit to follow existing thread and cut deeper. Check frequently with your plug gage.
 

markba633csi

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#5
Fortunately this is for a 4-jaw chuck so a slight bit of error (runout) would be tolerable, but of course the thread itself needs to get close
Mark
 

rock_breaker

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#7
If I were doing this I would turn a male external test piece to the 2.25 TPI to check the new internal threads. If you can arrange to check the male test piece in an existing chuck that would be best. This test piece in my opinion should fit the chuck threads snugly but preferably not wrench tight. Set your cross feed to the left at 29.5`

Hopefully the machinists will be along soon. I have done this once. If you have or can get another 4 jaw chuck to fit your machine mounting the back plate in it would be a plus. Obviously you will have to dial the 2-1/8 threads in to zero run out. Here is where the experienced machinists may have different approaches. Knowing the thread pitch is 0.12500" the thread depth calculates to 0.0625/tan 30 or 0.116". This doubled and subtracted from 2.250"= 2.028" which would be the ID of the internal diameter of the 2.25" thread. Now is the time to sketch this out and check the numbers. The internal diameter of the 2.125" thread is just that; 2.125" so there will probably be thread grooves remaining.

To take advantage of those grooves, you will need to set the controls of your lathe accordingly. My first adjustment would be to get the 60` cutting bit snugly into the old thread. I would be using a boring bar and cutting towards the head stock. Set your quick change gear box or install the gears to cut 8 threads per inch. Close your half nuts on the lead screw; this will probably require manipulating the cross feed and compound slide. You may have to turn the lead screw to get your threading dial to the closest number which you will use when cutting the new threads (or leave the half nuts engaged). Adjust the cross slide and compound feed as required to keep the cutting tool snug in the old grooves with the thread dial on the chosen number, gets a little tricky but take your time to get it ready to cut new threads. Set your cross feed and compoun slide dials to "0".

Run the compound slide in until it just touches the 2.028" diameter the move the carriage to the right. Start the threading operation, when the thread dial gets to the proper number engage the half nuts to run a "dummy pass". Doing this will let you check for any problems without serious damage to the new bore.

Hopefully I haven't left any thing out and you will get your chuck in operation soon.

Have a good day
Ray
 
Last edited:

benmychree

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#8
I have made plug gages to fit all the lathes that I have owned except one, a 34" swing American High Duty, did not make one for that one because we never removed the 30" four jaw chuck. Sizing the thread on the plug gage is best done with a thread micrometer, but it is possible with thread wires; the thread should be as close as possible to the exact size of the spindle threads and as smooth as possible, tapping fluid helps with finish.
When cutting the thread in the backplate, ideally the fit attained should not be overly tight with respect to the gage; I have seen chucks that fit quite loose and the register fit also loose, yet the chucks ran true, the thread angle seemingly doing the centering. If the threads and register are overly tight there is the risk of galling, I have seen it happen.
 

jbalp

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#9
ok you guys are right there would have been old threads remaining .. I've already turned most of the old threads out of it and it's at 2.140 and still not cleaned up so I guess I'll just buy a semi machined one and fit it to the chuck ... it would have been a good learning exercise for me but it's not going to work .. I was kinda on the fence about using this old back plate anyway as the OD is only 2.654 it wouldn't have left much thanks for all the replies I knew it wasn't just a bore and thread project ..
 

chips&more

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#10
ok you guys are right there would have been old threads remaining .. I've already turned most of the old threads out of it and it's at 2.140 and still not cleaned up so I guess I'll just buy a semi machined one and fit it to the chuck ... it would have been a good learning exercise for me but it's not going to work .. I was kinda on the fence about using this old back plate anyway as the OD is only 2.654 it wouldn't have left much thanks for all the replies I knew it wasn't just a bore and thread project ..
I’m confused? It should have worked. Just making a bigger internal thread with the same pitch is not a problem. What am I missing?
 

Suzuki4evr

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#11
I would do as Mike mentioned, and mount the back plate to a faceplate, get it centered up, then bore to your required ID.

Next, you would need to set up a threading tool and get it properly 'registered' with the existing thread so that you can cut it deeper to achieve the new, larger thread.

According to Machinery's Handbook, you would want an ID of 2.115" to 2.1297" (class 3B thead) then cut your thread deeper until you reach the thread major diameter of 2.250". Since measuring the major diameter of an internal thread can be a bit difficult, I believe you could safely cut the thread deeper until it fits properly on the spindle thread, or plug gauge if you go to the trouble of making one.
What he said.
 

Asm109

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#12
You need to clock your work so the thread cutting tool exactly follows the helical path of the existing threads. Then it is a simple matter of cutting them deeper to the new size.
 

jbalp

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#13
You need to clock your work so the thread cutting tool exactly follows the helical path of the existing threads. Then it is a simple matter of cutting them deeper to the new size.

I guess I wouldn't know how to do that..
 
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