On a threaded spindle, what are the critical features to capture?

If your finish lathe tool has a .015 radius , just put a .020-.025 chamfer on the backplate , you'll be " golden " ! :encourage:
You can check it out using a .001 piece of shim stock , or a piece of cellophane from a pack of smokes . Seat the chuck and try to pull the shim out . If it comes out , you're not seated and need a larger chamfer . Sound good ?
 
Would all threaded spindle have those locating features? I was thinking the little dividing head could use a faceplate , 1 1/2” thread . I have an old backplate that would need modifications to work
 
Consider them the datum points on threaded chucks , not taper chucks .
 
Alcap said:
Would all threaded spindle have those locating features? I was thinking the little dividing head could use a faceplate , 1 1/2” thread . I have an old backplate that would need modifications to work
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I just checked my BS-0, it seems to have the same features, although nowhere near the same dimensions. Flat face in the back is only an annular ring of about 3mm width.
 
I have just made a thread plug gage, threaded to the wire size of the spindle threads for all the lathes that I have owned, I just bore and thread the backplate to fit the gage with a free fit, and do any register by micrometer size for diameter and steel rule for depth and chamfer by eye. The register diameter should not be too tight or it may cause the spindle to gall. To my experience, it is the thread that centers the backplate, not particularly the register diameter, I have seen backplates with incredibly loose threads and register, and they still center up accurately.
 
benmychree said:
I have just made a thread plug gage, threaded to the wire size of the spindle threads for all the lathes that I have owned, I just bore and thread the backplate to fit the gage with a free fit, and do any register by micrometer size for diameter and steel rule for depth and chamfer by eye. The register diameter should not be too tight or it may cause the spindle to gall. To my experience, it is the thread that centers the backplate, not particularly the register diameter, I have seen backplates with incredibly loose threads and register, and they still center up accurately.
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^^ This.

The flat area behind the threads is only "an area with no threads..." and has nothing to do with registering a chuck. The flat vertical area of the spindle provides a reference plane for the threads to ultimately register the chuck. As long as you properly face a backplate and have reasonably accurate threads, you shouldn't have any problems with registering.

+1 on making your own thread plug gauge - it helps to prevent horsing around a heavy part in a heavy chuck to test the fit! You could also do what I do and make them two at a time. Get the thread correct on the first half, then screw it onto your spindle and bore and thread the second one. If you don't need the second one, sell it and recoup most of your costs. Doing it this way removes any variability of the chuck from the equation.
 
SLK001 said:
You could also do what I do and make them two at a time. Get the thread correct on the first half, then screw it onto your spindle and bore and thread the second one.
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Don't understand this. Are you making a M/F pair? Spindle is male. I am making a male copy of the spindle. So I don't have to deal with a heavy chuck and part, just as you said.

What are you describing? Use a few more words, since the language above is so compact, it's tough for me to decipher. TIA.
 
So we and all the others have mixed opinions on this subject . :grin:


Any and all machines I have ever built have never had the thread denoted as a datum axis as there must always be clearance on the thread to work . So I may ask , just what are the clearance specs on lathe spindle thread compared to the chuck thread ? How is the chuck ever suppose to run true when it is not a confining tolerance ? :dunno:
 
WobblyHand said:
Don't understand this. Are you making a M/F pair? Spindle is male. I am making a male copy of the spindle. So I don't have to deal with a heavy chuck and part, just as you said.

What are you describing? Use a few more words, since the language above is so compact, it's tough for me to decipher. TIA.
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Oh, I though that you were making a spindle back plate. By two at a time, I mean that two back plates face each other as mirrors. This way, you can mount the first one on your spindle once it is threaded, then thread the other. Once both are completed, you part the two apart. Rereading your initial post, the most important feature is the vertical flat area, followed by the correct form of the threads.
 
SLK001 said:
Oh, I though that you were making a spindle back plate. By two at a time, I mean that two back plates face each other as mirrors. This way, you can mount the first one on your spindle once it is threaded, then thread the other. Once both are completed, you part the two apart. Rereading your initial post, the most important feature is the vertical flat area, followed by the correct form of the threads.
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Ah, I will be making a spindle back plate as well. In any case, now I understand what you mean. Since I was planning to make a set tru back plate, it is a little thicker than a normal back plate, so I only bought enough cast iron for one piece. Maybe next time.

How do you part a 5" back plate? A 2-1/2" parting blade stick out? Hard for me to imagine.
 
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