Where to start?

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I have a slug of 2" diameter aluminium 4" long that I bought just to learn on and play with. Ends are not cut exactly square and when I chuck it in my 3 jaw, about 1/32" from being all the way into the chuck and bottoming out, it has 0.037 runout. I tried repositioning in the chuck but couldn't do much better on runout. I've checked other round stock and get between 0.011 and 0.015 runout, which sounds high. One piece is something I turned and the other is a piece from a tool i just bought. How do you start trueing up the slug? Could the slug be that out of wack or is it just how it is chucked?


That would explain things. Going back to my question, how would you start trueing it? Should I find the center as best as I can if it isn't perfectly round and then use a center in the tailstock? Or try to face one end and then seat it all the way in the chuck and start turning it true? Or something else?
Sorry read that kinda wrong. I would square the one end up best I could flip it around and square up the other end center drill it. And center it in my tailstock. Idk if that’s right or wrong but that’s what I would do.
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Hi Guys,

I would start by putting it in an independent four jaw. Using a dial gauge to adjust the piece until it was as true as I could get it. Check it close to the chuck and also at the free end. Once you are happy with it, because of its length, put a center in it and support it with a live center ideally, but a hard center will work fine. Just use a little lubrication on it.

If you now take a fine cut along its length you will see just how out of round extruded bar can be.

If you just want to test your chuck for run out get a short piece of ground bar, say 1/2" or 3/4" and use a dial gauge to measure it. A good 4" chuck may have two thou or more from new, getting worse as the chuck wears and also slightly different run out values at different diameters. If you really want precision use a collet chuck or soft jaws.
0.037 is pretty high for a 3 jaw, even 11-15 thou is high. Is there reason to believe that the chuck is thrashed? Sure extruded bar is not perfectly round, but it is not out 0.020"+.

Have you been through the exercise of tuning the mounting of you chucks? Most any 3 jaw, even a poor one will do better than 0.005" run out. 4 jaw chucks also benefit from tuning the mounting - in a 4J the RO is not an issue, but ensuring the axis of the mounted chuck is parallel to the axis of the bed is important.

Your numbers are so bad I suggest the issue is something more than just a problem with the chuck proper. There is something else going on.
I think I would do similar to killswitch above, but after I faced the first end I would put a center in that and then turn the outside diameter to clean it up to round. Then flip end for end and chuck on my newly turned cylindrical section with the square end. You'll end up sacrificing some dimension on the diameter, but if this is just a playing excercise that won't matter too much.

At that point you have a choice to either turn the rest of it to round or just go about playing with it as you wish. Sooner or later it will get round, that's what the lathe does, it just depends on how round you want all of it to be and at what point in the process. Just remember for first facing cuts take light passes to clean up the end enough to get a clean centre drill in. Once it's supported, you can be a bit more aggressive with the cuts. Three inches of stick out on a two inch bar is not that extreme.

You might want to check out that chuck though. Make sure all your jaws are in the right order and no bit of chip got in behind the mounting. It's amazing how much one little chip can throw something out.

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Sounds like the stock is out of round, which as stated above is common with extruded aluminum round bar. And you may have a chuck problem also. But grab onto it as best you can and face a small area in the center about 1/2 inch dia. Then center drill and support with the tailstock center. Then finish facing. Now you have a square face. Take a cut on the OD and get as much as you can round(ish). Now you can flip it over and face and center drill the other end. You now have something to work with. :)
Could the slug be that out of wack or is it just how it is chucked?
I say both.

How do you start trueing up the slug?
Depends on what you want to true up. A common 3 jaw scroll chuck should not be expected to repeat very accurately so when the slug is flipped and a second centre is drilled as Jim wrote, then you have to turn the slug between centres to get the OD cylindrical and both ends parallel and perpendicular to the OD. This will likely require a half centre in the tailstock. It might also require a square centre in the headstock to drive the work. When working between centres the work can be flipped without loosing register.
A worthwhile exercise in my books.
Hi Guys,

Going back to the original question, "How do you start truing up the slug?". Here in the UK we would call that a "Billet".

If you simply place the slug in a three jaw and face the free end you will not end up with a face that is truly square to the length of the slug !
Even starting off using a four jaw, it isn't going to be perfect, but at least you will get a center that is nearer to the real center of the material.

Flip it and do the same for the other end, using a dial gauge to get it a true as possible. As said above, put it between centers and take a skim off. You will soon see how out of round it really is, and when you face the ends they will be square.
With 3 jaw material goes in and stays until finished.

To make true insert to almost seated and tighten 3 places gradual tighten.

Face of end then flip and seat against face of Chuck and repeat.

Toss in bin for use later...

If making something between centers then after facing center drill.

Does not matter if Chuck running exact...place material in Chuck and perform all cutting then part off.

Facing parted face last.

Sent from my SAMSUNG-SM-G930A using Tapatalk
tq60, that is exactly what I did last night except my chuck only has one pinion to tighten. I have a second chuck that I posted about last night that has 3 pinions.

Maybe a dumb question but can you use a center in the tailstock and drive with the chuck or does it have to be between centers? Reason is that I don't have the setup to do it between centers but I do have a dead center for the tailstock. If you can do it, I assume I would set the tailstock end up on the center first while the other end is being loosely held in the chuck and then tighten the chuck. Yes...No???
Hi DiscoDan,

Yes that will work to drive the workpiece.

Usually if you want to work between centers you would put a center into the lathe mandrel and one in the tailstock, then use a lathe dog on the work driven by one of the chuck jaws to turn it.
Dan, here's some really basic stuff in no particular order that will hopefully clarify:
  • A 3 jaw chuck will turn a work piece concentric with your spindle centerline as long as you don't remove it from the chuck as the other guys have told you. The work piece does not have to have a finished surface for you to turn an accurate part; an extruded surface will be fine as long as you don't take the work piece out of the chuck.
  • A work piece should not extend from the chuck more than 1-1/2 to 2 times its diameter without tailstock support, that being a live or dead center. So, if its short then just face and turn it. If its long, face it, center drill it and bring up your tailstock center for support. This is so the force of the cutter doesn't rip the work piece out of the chuck or cause inaccuracies as you turn it.
  • You do not have to face one end to turn a piece. The end face inside the chuck doesn't even have to bottom out in the chuck, although it is best if it does. Just close the jaws until they touch the work, then rotate the work as you tighten the jaws. This will get it close. You can then snug it down. If you want it as accurate as possible then use an alignment tool with a bearing on the end. More on that some other time. In any case, once the work is clamped in the chuck, face the end that is sticking out and have at it.
  • When clamping a work piece in a 3 jaw chuck, high quality chucks are always marked on one of the three pinions. These chucks have a rated or specified accuracy level and that accuracy level is achieved only when the work is tightened with that marked pinion. The mark will be a hash mark, zero, straight line or dot. Use that pinion. It is not a good idea to tighten a 3 jaw chuck by cranking down on all three pinions; this throws the accuracy of the chuck off. If your chuck is not marked then mount a short piece of ground rod and lock it down with one pinion and check with an indicator to see what the run out is. Repeat this with each of the other pinions and mark the most accurate one. Use that one every time you use the chuck.
  • Turning between centers is the most accurate way to work but it is rarely used by most of us. It requires careful work, with carefully drilled centering holes. You must use a drive dog to drive it, a dead center in the spindle to locate that end and a dead or live center on the other end. I would guess only 1% of my work is done this way. Most of the time I use a 3 jaw, sometimes a 4 jaw and a collet chuck only when appropriate.
  • Most work will be done in a 3 jaw and you can use a tailstock center, a dead or live center, to support a long work piece. The chuck drives the work, the center supports the other end. So yes, you can use your dead center. You need to face the work, center drill it with a 60 degree center drill, then lube the dead center before engaging it in that hole. You apply enough pressure with the tailstock to just enable the chuck to turn freely by hand, then lock everything down. As you work, the work piece will heat up and enlarge so watch the pressure exerted by the dead center. You need to adjust it if things get too tight. Also, periodically re-apply lube to the dead center. When you tire of this, buy a live center.
Hope that helps.
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