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tkollen
01-14-2013, 10:13 PM
When I decided to buy a milling attachment for my 7x16 Microlux Mini Lathe I came to realize how short the cross travel was and also how limited the use of a milling attachment was. I decided to extend the cross travel with 1" before the centerline and 1" beyond the centerline, which meant that the cross feed screw had to be extended by 2". Since I am still a novice machinist I decided to fabricate 2 extended cross feed screws, one by threading with a die and one by threading with the lathe. In the end I was going to choose the screw that fitted the nut the best.

Threading a A2 tool steel rod on the lathe was quite a challenge but also a great learning experience especially since the thread was left hand. Due to the length and the springiness of the 3/8" drill rod I was threading I had to use both tailstock and the follow rest in order to get a decent thread and consistent major diameter of the thread. Without the follow rest the diameter would be smaller in the middle of the screw due to the rod's springiness. In the end, and after several attempts, I ended up with a new cross feed screw that did fit the nut smoothly and with no noticeable backlash.

For my second attempt I did eventually find a 3/8" - 20 TPI LH Die overseas. I could of course not use my tailstock mounted die holder since it limited the thread length to a few inches only. Nor did I want to thread by hand for such a length using a die stock. Eventually I came up with the idea to make a sliding die holder that would guarantee that the die was perpendicular to the rod, both horizontally and vertically at all times. I used the steady rest, stripped from the finger assemblies and the bottom locking nut, into which I inserted a home made die holder that screws on to the steady rest. With a hand crank in the spindle I can now thread the rod and the freely sliding steady rest keeps the die absolutely perpendicular to the rod at all times.

I made the die holder out of 2 3/4 round aluminum that could fit 1 1/2", 1" and 13/16" dies. The holder was fitted to the steady rest from the back by 3 M6 screws. The mounting surface on the steady rest was slightly ground in order to make it perpendicular to the center line. Then 3 M6 holes were drilled and tapped in the steady rest.
The threading operation with the die was a breeze compared to machine threading. After a few passes and adjusting the die I finally achieved what I consider to be class 3 fit with the cross slide nut, much better than the original screw and nut. Now as an extra benefit to having an extended range for the cross slide any trace of backlash has disappeared.


Tom


http://s1287.beta.photobucket.com/user/tkollen/media/Dieholder-2_zpsd0286842.jpg.html?sort=3&o=1
http://s1287.beta.photobucket.com/user/tkollen/media/Dieholder-3_zps1e00ca5d.jpg.html?sort=3&o=0

http://i1287.photobucket.com/albums/a627/tkollen/Dieholder-2_zpsd0286842.jpg
http://i1287.photobucket.com/albums/a627/tkollen/Dieholder-3_zps1e00ca5d.jpg

vpt
01-15-2013, 08:26 AM
Very nice!

I extended the cross feed screw in my atlas as well. Comes in handy!

jdunmyer
01-15-2013, 07:08 PM
I'm glad it worked for you, but I wouldn't have bet on it.

Some years ago, I made a die holder that clamps into a toolholder on my big lathe. Tried threading a fairly long piece of rod, and the thread ended up "drunken". I still don't know why.

tkollen
01-15-2013, 09:25 PM
Actually to begin with I tried the tool holder approach too. It didn't work either. I assume the reason is that since you have to move the carriage continuously by hand it will produce varying lateral forces on the thread. With the bed well oiled the friction from the steady rest is very low and also constant.

tylernt
01-16-2013, 11:25 AM
Actually to begin with I tried the tool holder approach too. It didn't work either. I assume the reason is that since you have to move the carriage continuously by hand Why not set your gear train to the same pitch as the die, and use the leadscrew to power feed the carriage?

tkollen
01-16-2013, 11:53 AM
I tried that too but the thread quality came out uneven at various parts of the thread. I suspect backlash in the gear train is the culprit since I had to backtrack every second turn for chip breaking.