Help Needed on a Non-concentric Shaft Setup

I'm new to machining and quickly learning work piece setup can take longer than the actual machining operation. My terminology may sound novice but it's all I know so far. I've made a couple small things but I've hit a wall on this...

I'm making a mandrel which will only be used for a babbitt pour on an old jointer. I picked up some 2 1/2" cold rolled steel at a scrap yard really cheap. Later realized it's not the greatest thing to machine - but I already have it and the price was right. I'm having trouble getting it lined up perfectly on the tail stock mainly because it's not perfectly round and smooth. I chucked it in the 3 jaw and set the opposite end in the steady rest while adjusting it by indicating the length of the shaft at the 12 and 3 o'clock positions until it appeared to be spot on. When I went to drill in the end to setup for the live center I noticed a slight wobble in the bit. I did proceed to drill and setup the live center. With an indicator on the live center I'm getting .003 wobble.

The shaft is 34" long and I expect this to create a taper, right? What's a better method to use to find the center for setting up the tailstock and is any amount of wobble acceptable?

Thanks.
Brian in NC

'Splain yourself. What does "non concentric shaft " mean?

Are you still holding it with a steadyrest? If so, take the steadyrest off. Does the center still run out .003?
If yes, you have a bad center. If no, the run out will be transferred to the OD of the bar. Turning the OD will true it up.
You will get taper if the tailstock is not perfectly aligned with the head stock.

If I understand your situation correctly, you can still turn down the shaft without a taper. You must check the tailstock's axis-axis distance to the spindle. Two ways to do this. One is to get a test bar with a well-known accurate diameter, roundness, and straightness, and set it up between centers. Then you can indicate it along its length to see if one end or the other is sticking out. The alternative way is to turn down some length of shaft, and measure the diameter at each end to see if it has a taper. Adjust the tailstock half the distance to the goal, and repeat until you are satisfied with the result.

The .003 wobble you measure right now only means one thing: you need to take a cut deeper than .006 (diametrically) to make the whole thing shiny. Once the whole thing is shiny, further work will be concentric to each other.

Edit: Booze Daily is a faster typer

tdmidget said:

'Splain yourself. What does "non concentric shaft " mean?

Click to expand...

Wobbly....downright "BUMpy"....

I am an amateur also but I noticed something others did not mention. You said that the drill wobbled when drilling for the live center. This is not a result of the bar being off center. It believe this is a result of the drill being off center due to tailstock alignment or chuck run out.

Did you use a center drill for this operation? They are stout and typically will drill with one edge if misaligned and thus make a true to center hole.

You say the live center is wobbling .003. This is unacceptable. It is ok for the shaft prior to cutting to wobble but not the live center. Check the center for slop, proper seating in the tail stock, etc.

At this point I would loosen the chuck to remove stress on the center and then tighten the chuck while the center is engaged and see if it then runs without wobble. You may have had a lot of force constrained by the steady rest that the center can not constrain.

Bill

What size do you need to finish at??? If you got plenty of stock to come off just turn it and adjust the tail stock accordingly to get the taper out. If not,chuck on a short length of stock larger than the diam of your tail stock. Turn it down to the same diam of your tail stock.Now you can run an indicator along the stock and tail stock and adjust to center it. You can also check the top of the tail stock to see if your low, in a perfect world the tail stock will be a bit high. Now that your tail stock is on center, Turn a band on the bar for your steady rest at the chuck end using your existing center to support the shaft. Next take it out of the lathe and flip it end for end. Now use your steady rest to support it using the band you just turned. Drill it and then single point your center. Next, remove steady rest, and install center. Now turn another band at the chuck end and repeat the process. Now you should be able to use a drive dog to turn part between centers. Probably an easier way, but that way works.Of course a lot of this relies on your lathe being somewhat level etc.

Put something pointy in the chuck. Turn a point on it if you do not have something. Then run the center up until the points almost touch. They should met exactly. If not adjust the tailstock as needed. Or use another thing for the center for alignmnet purposes.
Sandwich a piece of flat metal between the two points to magnify any alignment issues.

The end of the shaft should be faced square to the spindle axis before drilling. That slight wedge effect of a typical saw cut surface is enough to deflect the center drill 'down slope'. Once the dimple is begun, off center, it is a bitch to bring it back, so do it right to begin with.

A typical drill chuck does not hold a center drill rigidly. It can flex the jaws quite readily (or the chuck jaws are already bellmouthed from use), so it cannot be relied on to keep the drill in rigid alignment on an out of kilter surface. Holding the center drill in a collet chuck would be much stiffer, but, who bothers?

Drilling via steady rest should be done bringing the drill up close while the part is turning, and see if the point scratches any kind of circle on the end of the shaft. If it does, then readjust the steady to put the point in the center of that circle. I would typically then drill a dimple only as deep as the point angle of the drill (maybe .010"). Then, leaving the drill point against the part while it is turning, I go round and feel the pressure on each steady adjusting knob. Actually back each off slightly and advance to feel slight pressure, equally on all. Then actually drill the center hole.

One trick I've used to correct some center holes that are slightly off, is to take an old center drill, grind the pilot point off. Then correct the hole as follows: rig up some sort of pusher bar in your toolpost with a blade that can reach in and steady the center drill (from wobbling). I often just use a part-off tool set temporarily high on the toolpost, so the front of the blade can push lightly against the shank of the center drill. This pressure will arrest the wobbling of the center drill as it wants to find the old countersink, and will improve the centeredness of the center hole. Another thing you can do is grind away one flute of the center drill so it is a single lip cutter. Mount it so the remaining flute is horizontal, drill lightly with it. Still steady the drill with the blade already described.

These shortcuts may not give an absolutely true running part, but might be well within your requirements (like, better than your chuck can do holding on the other end of the bar). Badly worn lathe chuck jaws can also cause the part to precess around lathe centerline, and the tailstock may not be stiff enough to completely eliminate that effect. For this reason, on roughing work, I chuck on just a short length of the stock so it can flex around in the chuck a little, and not torque the tailstock quill.

If the center isn't true, while the part is still in the steady rest set your compound for 30* and bore the center true.

Thanks for all the advice. I think the first thing I need to do is check my tailstock and I hope this is where my problem is. I also like the idea of turning between centers - I had not thought about this.

The finished diameter of the mandrel should be 1 7/16" so I have a lot of play.

Its a mandrel for a babbit pour.......where does accuracy come into it?......how much margin for machining the bearing......surely ,you're not going to try to get the babbit to finished diameter from the mandrel.........generally babbit needs to be peened to cure shrinkage from the housing.

Wow, 2-1/2 stock down to 1-7/16. 34" long?
Good luck.

I think I'd just buy a bar of 1-7/16 0-1 from McMaster and start there. $75 plus shipping.