New To Machining With A Question

boostin53

boostin53

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Hi y'all! This will be a sort of long first post, so please bare with me as I'm totally new to all of this. Been browsing this site for a few weeks now in preperation to begin a new hobby/addiction (machining). I decided to start small with a Jet BD-920N lathe that I got for a very good price. The lathe came with a bunch of extra stuff and including some basic tooling. It was too good to pass up considering it was basically free.

I have been practicing facing and reducing on bolts and some scrap 1/2" steel bar stock of unknown to me metal from work. Today I decided to face and center drill a 12" length. I started by only hanging about an inch and a half out of the chuck, faced and center drilled. I then put in my live center on the tailstock and extended the work piece out.

I began turning down the steel to my desired diameter of .415". While confirming measurments during the turning process, I noticed some variations on the measurments. The front half measures in at .416" while the back half measures in at .427". I then thought maybe I didnt turn in my tailstock enough to support my work, causing some slop. But it was good and tight. So to wrap this essay of a post up, what can I do to eliminate this and get the same diameter on the entire length I'm turning?
 
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What does the middle measure? Depending on your lathe it could be your tailstock is not truly centered. It could the offset is passed center cutting a taper. Also for stock that long take light cuts to keep the it from flexing on you. Good to learn you are practicing on bolts and scrap steel.
 
Most if not all tailstocks have an adjustment for centrally horizontal. Your first thing to do is to chuck a short piece of stock/scrap in the chuck and turn a point on it. The finer the better. Then, bring the tailstock with a center in it up to about 1/4" away from the point you just made. Clamp the tailstock to the lathe, put a bit of snug on the tailstock spindle and crank it up to almost touch the point. Look at the two points from above, and adjust the tailstock so that it as good as you can get it. While there, check for vertical alignment point to point. There's not a lot you can do about vertical alignment, but it should be good.
Next, cut a piece about five inches between chuck and tailstock. Take a skin cut and measture at each end. If the right end is off .004, move the tailstock (use an indicator) .002 in the correct direction. (If it's big on the right end, move the tailstock toward you, if it's small, move it away.
Your twelve inch piece is too long to not flex when you're cutting it, it needs a steady or follow rest to support it from the cutting pressure.
 
boostin53 said:
So to wrap this essay of a post up, what can I do to eliminate this and get the same diameter on the entire length I'm turning?
Click to expand...

Exactly how much deviation from the desired dimension is too much? If you require .003" error on a .415" Diameter 9 inches long with a nice finish, this is not going to happen in a conventional lathe, this would be asking a lot of a box tool as well.
 
Wreck™Wreck said:
Exactly how much deviation from the desired dimension is too much? If you require .003" error on a .415" Diameter 9 inches long with a nice finish, this is not going to happen in a conventional lathe, this would be asking a lot of a box tool as well.
Click to expand...

I can live with a fair amount of error. I'm just trying to get my machine to cut as accurate as possible. The closer to my target dimensions, the better. And I didnt think about the tailstock being off center. Im not able to get to my lathe at the moment, but I will do what you guys advise as see where Im at.

So in reality, what type of tolerance is to be expected on these lathes? Like I said, I can live with error. Just curious to know.

Edit: I would also like to see consistent dimentions across the work piece. Regardless if Im off of my target dimention.
 
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On a four inch piece, chuck on one end live center on the other, from 1/2 inch to 2 1/2 inch diameter you ought to be able to hold .0005, maybe better, any better than that, you might want an OD grinder. Yes, better is possible, when I need better I work with a block of wood behind some 400 paper and take my time.
 
Here's the links to a couple MrPete222 vYoutube videos that might help with checking your tailstock alignment.

He has a ton of videos well worth watching.
Somewher on this forum is a list of the videos and links, not sure where it moved to.
 
Firstly I agree to check the alignment of the tail stock. I would do this by placing a center in the head stock and moving the tail stock with the center in it up to it and aligning them by eye.

I quote http://www.smithy.com/machining-handbook/chapter-3/page/20

"The most accurate method of checking alignment of centers is by mounting the workpiece between centers and taking light cuts at both ends without changing the carriage adjustments. Measure each end of this cut with calipers or a micrometer. If the tailstock end is greater in diameter than the headstock end, the tailstock is moved toward the operator. If the tailstock end is smaller in diameter than the headstock end, the tailstock is moved away from the operator. Take additional cuts in the same manner after each adjustment until both cuts measure the same."

Also there might be a runout issue with the chuck. I generally like to turn down stock between centers. If the centers are aligned properly it tends to be quicker than trying to minimize runout from the chuck.
 
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So as it turns out, my tail stock was off by a few degrees. I dont know how much since the ink on the dial indicator plates are worn off. Took some trial and error to get it centered. Also, my three jaw chuck had a fair amount of run out. Removed the chuck to clean and inspect the mating surface. The previous owner managed to slightly ding the chuck where it mounts to the headstock. Took a file to it and lightly removed material until it sat flush. Im now skinning material evenly across my stock.

Now another problem. I just found out i wasnt given the keys for the 4 jaw, and its an inverted key unlike the 3 jaw i have. Using a pair of sockets and wrenches for now. Now i need to figure out how to broach without the proper tooling so i canmake myself two keys. Tried searching online with no luck. Either my google-fu sucks or im missing it.
 
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