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BigBoy1
05-19-2009, 07:52 PM
The spindle on the lathe is 1-7/8" by 8 tpi. The 4-jaw chuck that came with the lathe has a backplate with a 2-1/4" by 8 tpi thread mounting backplate.

My quandry is which way to go.

Do I make a whole new backplate for the chuck so that it will mount on the lathe? or

Do I cut out the 2-1/4" by 8 tpi threads on the current backplate, weld a solid plug in the hole and then redrill the hole and thread for the 1-7/8" by 8 tpi thread?

My current thinking is to go the cut out the threads, bore and rethread the existing backplate route as that seems to be less work than making a whole new backplate. Also, the current backplate already fits the chuck and fine fitting would not be necessary. Comments and suggestions.

JCHannum
05-19-2009, 08:18 PM
There is a good chance the backplate is cast iron, which adds a degree of difficulty to the bore and plug scenario. In addition, when fitting a backplate, the best approach is to thread the blank backplate and then machine it to fit the chuck without removing it from the spindle. This produces the most accurate mounting.

franco
05-19-2009, 08:48 PM
Maybe turn out the backplate to accept say a 2 3/8-16 thread and make an adapter plug threaded 2 3/8-16 on the outside and 1 7/8-8 on the inside. Use loctite when assembling. This avoids possible heat distortion of the backplate. I have a couple of backplates which have been modified this way, and have never had any problems at all with them.

This may not be satisfactory if doing heavy turning with the spindle running in reverse, but usually with threaded spindles this does not happen.

franco

lakeside53
05-19-2009, 09:38 PM
I'm not sure how you would keep the existing concentricity by plugging.. I'd make a new backplate (the cast iron blanks are not expensive). After you cut the threads, the rest is easy.

Paul Alciatore
05-20-2009, 02:53 AM
Making a new backplate is not that hard, it was one of the first things I did with my lathe. As others have said, maintaining concentricity with a plug may be difficult. It is far better to cut the thread mount first and then finish the front while mounted on the spindle. That way you are almost guaranteed concentricity.

By the way, with a four jaw, you mainly want the mounting surface for the chuck to be perpendicular to the axis to keep the axis of the chuck parallel to the axis of the lathe. If it's a small amount off center it will not matter very much as work is centered in a four jaw each time it is mounted. So if you do go with a plug, just take a light cut on the face of the backplate to ensure that it is perpendicular to the lathe axis.