4 Jaw Chuck

I decided it would be nice if I had an independent 4 jaw chuck.  This is probably due to Adam (Abom79) on Youtube who always uses one.  They’re useful because you can move the jaws separately from the others which allows you to perfectly center a piece or they can grip rectangular objects.  I looked around for a used one because I didn’t want a cheapie and can’t afford a fancy one.  I ended up finding an 8″ Craftsman on OWWM that was probably made decades ago for a good price.  Here’s a pic of it.


There are several different methods used to attach a chuck to a lathe.  My Hendey has a threaded spindle nose which requires the chuck to have a matching thread.  Most decent chucks have a removable piece called a back plate that goes between the spindle and chuck body.  This allows the chuck to be used on different machines by swapping or making new back plates.  This chuck came with a back plate but it doesn’t fit my lathe.  So, I’ve got to make a new back plate.


When cutting threads inside of my new back plate I cannot remove it from the chuck to test fit it on my spindle.  It’s possible that I could remove the entire chuck holding the back plate and test fit it but that would be very cumbersome.  The better option is to make a duplicate of the threads on my spindle.  To do this, I’ll cut threads that match my spindle in another piece of steel which I’ll call the plug.  Then, as I’m cutting threads in the back plate, I can check my progress by trying the screw the plug into it.

First, I need to determine the size of the threads on my spindle.  From research online, I found out that they’re 1.975″ x 8 TPI.  It turns out Hendey made the spindle a tiny bit smaller than the normal size of 2″.  They probably did this to keep people from using other manufacturer’s parts on their lathes so you’d have to buy from them.  1.975″ x 8 may have been what the spindle was when new but that was 100 years ago and there has been some wear over time.  To measure my spindle I used the three wire method because I don’t have thread mikes that are large enough.  This method uses three wires and a micrometer to take a measurement of the thread.  Two wires go on top and one on the bottom in the threads to provide a flat surface to measure.  You almost need three hands to do this.  I stuck the wires to a piece of tape to help hold it.  A number is subtracted from the measurement, as specified in the instructions, to calculate the pitch diameter.  Pitch diameter is a little complicated but it is the diameter of a point half way up the side of an ideal sharp V thread.   The difference in pitch diameters of the internal and external threads determine how well the two threads fit given that the major and minor diameters have some clearance.

I took four measurements down the length of the spindle nose and found that I had wear ranging from 5 to 26 thousandths going towards the end of the nose.   Though there is a slight taper to the nose, I’m going to cut straight threads on my plug at the size of the smallest wear.  To do this, I just need to cut threads into my plug until I get a three wire measurement that matches the largest value I measured on the spindle nose.


To start my plug, I found some cheap steel on ebay that would be large enough.  This ended up being some 2-5/8″ 1018 steel.  It’s a little larger than I really need but I can fix that.  I cut a piece down to 3″long with the power hacksaw and got to work.


I faced and center drilled the end of the plug and proceeded to remove a little bit of material to give me a clean surface to grip.  I found that despite varying the feed rate and depth 1018 doesn’t really leave a nice finish.  I was expecting this but decided to see if I could figure out how to get a good finish.  Here’s a pic of the poor finish.


I looked around online and came across a tool profile called the vertical shearing tool.  I’d forgotten about this tool and found that I had one hidden away.  It’s pretty simple to grind as it only requires a steep front edge and a little relief on the side.  I touched up mine on the grinder and gave it a try.


The tool is a finishing tool and is only capable of taking a light 1 to 3 thousandths cut.  It works well though as seen in the picture below.  Unlike other tools, this one produces long straight shavings that are twisted along the length.


With that side done, I flipped the plug over in the chuck.  Flipping it over will result in both sides not being concentric but that doesn’t matter for this part.  I proceeded to reduce the diameter down to the major diameter of the spindle nose’s thread.  As an aide:  I borrowed an indexable carbide tool from Gill and found it worked beautifully on my machine.  I think I’ll have to get one myself.


Next, I cut down to a little under the minor diameter at the shoulder to give the threading tool a place to stop.


To cut the threads I made several passes until the threads started to have thin crests.  Then I started taking three wire measurements until I got to the value I was aiming for.  To finish off the treads, I ran a file over the top and polished them with a 3M pad (another technique from Adam’s videos).


To see how I did, I removed the three jaw chuck and threaded the plug into it.  It fits!  Of course it should due to the math, but it’s always good to see it work. FJ8

I also upgraded the motor on my lathe.  Since I restored it, I’ve been running the 1/3 hp repulsion motor that came with it.  This motor has worked well, but it would noticeably slow under load and makes a ticking sound while running that grows with load.  A while back, Gill gave me a non functioning motor he had sitting around.  I fixed it after I got it by swapping the bearings and adding a start capacitor.  I intended to put it on my lathe and finally got around to it.  It’s a 3/4 hp Century motor that was made in St. Louis.  It runs much quieter than my old one.  It also doesn’t slow under load which is a big plus.


Up next will be making the back plate.

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