Making a live center

[marfaguy]

This weekends project

It's going to be about a month before the shop remodel
http://madmodder.net/index.php?topic=4496.0 gets going
so I've still got time to do some stuff in the shop before I've got to
shut it down and move it all out. Knowing I will eventually need a
live center, this seems like an excellent time to make one.
I'm looking at this build/mod;
http://mikesworkshop.weebly.com/revolving-centres.html
So here's the plan;

1. Get 3/4" stock. Which leads me to first question.
 a) There's a close by supplier. The web site listing for materials for what they have
http://www.metals4u.com/info.html
I would think the A36 Mild steel would be fine for this but please tell me if I'm
about to drive off a cliff. Would any of the Aluminum stocks work just as well?

2. 3/4" Stock in hand, true it up to in the lathe to whatever it turns out to be. I would suspect
about .7 or .65x something would work.

3. Cut it off, clean up the ends and remount it in the lathe. 

4. Turn down one end to 3/8" (or so) for about 1".

5. Also central drill this end just a bit for the ball bearing (size and depth depending on available ball bearings).

6. Flip the piece around and chuck it up on the just turned  3/8" (or so) shaft.

7. Carefully cut the MT2 taper.

So that's the basic plan. Any and all comments/suggestions/refinements greatly appreciated.

[BillTodd]

Any piece, like shafts arbours etc. that needs to be concentric is  best turned between centres.

Rough turn to size in the chuck first and drill the centre holes. Place a piece of scrap bar in the chuck and turn a 60' taper to make a live centre (btw the thing you are making is a rotating or low friction centre) leave it in the chuck 'till the job's done. Put a dead centre in the tail stock and check the alignment with the live centre.

mount the work piece between the  centres (lubricate the dead end). Use a dog against the chuck jaws to drive the work piece .

Turn the bearing end (you can demount the work piece to check the fit.)

Turn the work piece around, driving from the  bearing end (protect the surface) and cut the taper.

Bill Todd

[Lew_Merrick_PE]

This weekends project
<Snip>
1. Get 3/4" stock. Which leads me to first question.
 a) There's a close by supplier. The web site listing for materials for what they have
http://www.metals4u.com/info.html
I would think the A36 Mild steel would be fine for this but please tell me if I'm about to drive off a cliff. Would any of the Aluminum stocks work just as well?

A36 is low carbon steel meeting a minimum set of strength characteristics.  It is generally available, but not really any better for this application than any low carbon steel (usually sold as Merchant Stock or Merchant Steel).  It will be slightly harder (as is) than aluminum.  It will be 3X as stiff (for any given section) than aluminum.  It will also have about 3X the mass of aluminum.  They (untreated low carbon steel and most common aluminums) will have about the same yield and ultimate strengths.  Aluminum will lose most of its strength when it gets above 450°F (230°C) and start behaving very erratically.  You have to get most steels into the 2000°F (1100°C) range before this happens to them.

Untreated low carbon steel will have to be handled and stored carefully as (also true with aluminums) they will ding really easily.  You can get a leg up on that to a certain degree with aluminums by having them anodized -- which creates a thin layer of hard aluminum oxide on the surface that ends up being close to Rc-60.  Case hardening can be done to the steel to make it harder to ding.  The "trick" to case hardening steel is a very even heat and a very even quench.  Done right, you get virtually zero distortion.  Done wrong, you are in a world of hurt.  This is probably more of a "learning experience" than you want to deal with at this point in time.

My recommendations to you are: (1) Make several of each part as you go as this is set-up intensive work such that making 2 or 3 spares as you go won't cost you that much in either $$$ or time.  (2) Finish your parts with some type of "tool black" finish (a dissertation in its own right, but several techniques are posted in the discussions here) such that "dings" stand up and shout at you.  (3) Make a nice wooden box to store your completed parts in so that you minimize the chances of "dinging" them.  (4) Carefully examine your parts each time you take them out or put them back into the box.

I have tools made from low carbon steel that have been in service since the late-1960's.  I have had other low carbon steel tools that rolled off a bench and were "dinged" into non-usefulness within hours of being made.  The difference has more to do with "luck" than with "care."  Taking the "care" increases your "luck" value, but sometimes all the "care" in the world can be overruled by "luck" in a small fraction of a second.  You can often restore "ding damage" so long as you know it is there.