Material(s): Aluminum and Steel
This is Part 1 of 2. Part 2 can be found here.
The first step in starting a job on the lathe is figuring out how to hold the work. The G0602 (and probably most hobby lathes) comes equipped with two chucks, a six inch independent chuck and a five inch scroll chuck. Not a bad setup for most jobs. I started my hobby using a Harbor Freight 7×12 lathe which still sees occasional use. One of the chucks that came with the HF lathe was a 4″ inch four jaw chuck. Being a much lighter and smaller chuck I thought it might be useful for much of the work I do on the G0602 and started looking about for a way to adapt it to the larger lathe. At the same time I was envious of a friend who had adapted a set of ER32 collets to his lathe. I like the ER type collets because of their continuous range. At the confluence of these projects was the need to be able to create adapters that would mate with the 1.750 x 8 TPI spindle thread.
Long story short, I made an adapter for the HF chuck and another for the collets and find them both to be essential. The lighter four jaw is a breeze to mount and true up. The ER collets (I used ER40) give me a continuous range from one-eighth inch to one inch with little to no measurable runout. I thought others might have an interest in this type of modification; probably most of this audience has 7×12 of one flavor or another with a four jaw chuck. Since the G0602 has a uncommon spindle thread (1.750 x 8 TPI) there is a good chance at some point in the future you may need to make an adaptor that will mate with the spindle to mount other devices. So the first step is to make a copy of the spindle shaft. The photo below shows the one I made. I plan to post these projects in two segments. Part one will cover the copy of the spindle and the four jaw adapter while part two will cover the specifics of making an ER40 adapter.
To get started you will need a 4-5” piece of 2” diameter aluminum or other suitable material to be used to make the spindle copy. You could use steel or brass ($$), but I had some aluminum left over from another project and used that. Start by measuring the diameter of the spindle sleeve on your lathe. Mine was 1.810”; we want a close fit between this sleeve and the adapter so the spindle copy should be as close to identical in dimension as you can make it, this will be our plug gauge. Start by turning one end of your spindle blank to this diameter for a distance of about 0.500”, we will use this end to test the opening in the chuck adapter. When this is complete, reverse the blank end-for-end and complete the threaded end including a close fitting sleeve section.
This spindle copy will no doubt be used in the future for other projects so tuck it away in a secure place when done. The spindle mating shoulder and the sleeve are what register the chuck to the spindle; the threads only apply the force to keep them together.
The drawing will give you an idea of what we will end up with (click to enlarge). The length between the mating shoulder of the spindle and the threaded end should be the same on the spindle and the copy. You can use the chuck that is not in use to verify the fit as you go along. This is threaded at 8 TPI and since there has already been considerable discussion on threading I will take it that this needs no further input from me. At the base of the sleeve place a shallow groove, perhaps 0.020-0.040” deep, this will remove any radius that your turning tool might have left. You can easily do this with your cutoff tool; rotate the tool horizontally ever so slightly so that the edge of the tool does not scrape the shoulder of the spindle copy. Once this is complete we can move on to the adapter.
For the adapter I used a short piece of 3” diameter 1018 CRS. The adapter is about 2.500” in length, if you have a longer piece of stock you can cut it off later and then face that end. If the piece you have is more or less the needed length then true it up in the four-jaw chuck. Once we finish the inside we will mount it on the spindle and have the spindle hold the work while we finish the outside. The drawing shows the chuck adapter plate that goes on my 4 inch four-jaw chuck and its position relative to the spindle adapter and the key dimensions.
Step drill the material to open it up to 1.000”, when this is done take a light cut with a boring bar to clean up the inside. Next open up the inside bore to 1.605”; this should correspond to your calculation of the minor diameter for a thread pitch of 8. The depth of this larger bore is 1.407”; this will leave about 0.300 of material at the end of the bore. The last step prior to threading is to open up the forward portion of the bore to 1.810”; the depth of this bore is 0.566”, this will leave a section about 0.841” long to thread at 8 TPI. The diameter of this sleeve portion is very important, use your spindle copy to test the bore diameter as you get close to the final dimension.
You are threading up to a shoulder so slow and easy cuts are best. If you have a DRO that will help keep you out of harms way, if not then use a dial indicator to keep track of your depth. I ground my threading cutter from a 5/16” tool blank; all of my other internal threading tools had too much overhang. Keep in mind this is an internal thread and you are cutting right to left so the cutter edge closest to the spindle must have the rake, seven degrees or so should do the trick. I rotate my compound around so that it addresses the spindle axis at 29.5 from perpendicular and feeding toward the operator. I feed with the compound and when I reach the end I use the cross slide to retract the cutter and then reverse the motor to back out. Use plenty of lubricant; MolyDee is my choice and my benchmark for all cutting fluids. If this is your first internal thread this is a good project to start with since the threads and the bore are large giving good visibility.
Use your spindle copy to test the fit as you go along. When it is complete you should be able to thread the copy completely to the end with very little side-to-side play. When the end is reached it should seat squarely against the spindle copy shoulder.
Once you have threaded the adapter and you are happy with the fit then mount it on the spindle and we can finish the outside. Take a clean-up cut off the outer diameter and face the end. At this point you will have to remove the backing plate from your 4” chuck and determine the measurements that are applicable to your adapter. The above drawing shows how mine was configured. No dimensions are shown since yours will probably be somewhat different.
The idea is to design an arrangement that will allow you to fasten the chuck backing plate to the adapter and hold it in alignment. There was a recess machined into the back of my chuck mounting plate which registers the chuck to the spindle of my mini-lathe, I made a corresponding mating piece on my spindle adapter. A close fit between these two surfaces will keep the chuck in alignment while under load. To fasten the chuck mounting plate to the spindle adapter I drilled and tapped four 10 x 24 socket head screws into the spindle adapter.
The mounting screws that hold the chuck mounting plate to the chuck interfered with my spindle adapter so I scalloped out the side of the spindle adapter to provide clearance. This allows me to mount the chuck mounting plate on the spindle adapter and tighten everything down before mounting the chuck plate to the chuck.
One safety note. The outside diameter of the spindle adapter where it mates to the spindle shoulder was not chosen at random. It should be the same diameter as the spindle shoulder and thus you can drill and tap the adapter for the chuck restraining clamps. I do not use these since I have replaced the original motor with a variable speed and the controller for this uses a “soft start” and “soft stop” thus I do not have worry about a chuck spinning at 500 RPM coming loose. Assess your situation and takes the steps that are appropriate. The stored energy contained in ten pounds of mass rotating at 500 RPM is considerable.
Next week I’ll post part two of the project, making the ER40 collet adapter.
Jim About the Author