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the kid
04-15-2015, 09:00 PM
So I had several buggered up centers laying around, after having a case of buttery fingers the other day I dropped and ruined my last good one, so I decided it was time to put them all back in to a usable condition. I took a page right outta the south bend book and used a bon fire to anneal them, the next day I retrieved them from the ashes and got to work, the fire scale was cleaned with a wire wheel and the shanks were cleaned and high spots removed via a file and emery cloth, the compound was set, and, making several test cuts untill the taper was dead on when checked with a fish tail gauge I was ready to go. I proceeded to recut them, things went smoothly and the hss tool I used had no trouble removing the metal. Now that the machine work is complete I've got a question, and that is how to best reharden, should I make the whole thing glass hard or just the end, and whether to strait harden it or to draw it back to a temper. I also am ignorant as to whether to use water, brine, or an oil quench in this application. What to do what to do... So I ask on here, how do I go about finishing this little project up so I can have usable centers again?

boslab
04-15-2015, 10:53 PM
There are many ways, one I like is with oil, get a bowl of oil, light oxy acetylene, hold torch in left hand, get the back of the centre in RH, immerse the centre with the tip sticking out, heat the tip to red heat and then pull it under the oil quickly, or water if you prefer, only the tip gets hardened, I think the toolmaker who showed me called it spot hardening, only the bit you want hard gets hard, I'm not sure if it's the right way but it works for me.
Mark

Edwin Dirnbeck
04-15-2015, 11:01 PM
So I had several buggered up centers laying around, after having a case of buttery fingers the other day I dropped and ruined my last good one, so I decided it was time to put them all back in to a usable condition. I took a page right outta the south bend book and used a bon fire to anneal them, the next day I retrieved them from the ashes and got to work, the fire scale was cleaned with a wire wheel and the shanks were cleaned and high spots removed via a file and emery cloth, the compound was set, and, making several test cuts untill the taper was dead on when checked with a fish tail gauge I was ready to go. I proceeded to recut them, things went smoothly and the hss tool I used had no trouble removing the metal. Now that the machine work is complete I've got a question, and that is how to best reharden, should I make the whole thing glass hard or just the end, and whether to strait harden it or to draw it back to a temper. I also am ignorant as to whether to use water, brine, or an oil quench in this application. What to do what to do... So I ask on here, how do I go about finishing this little project up so I can have usable centers again?

Why anneal them. Just speed it up and use carbide. I do this all the time. Edwin Dirnbeck

the kid
04-16-2015, 12:16 AM
Didn't mean to shock anyone lol, I just was doing what I read in a hundred year old book, no carbide or fancy steels avalible back then, and none of the centers in question are by any means modern, they all came with old machinery, and the centers them selves are equally old, 1920s tech for the most part. They've already been annealed and remachined and are ready for rehardening. Perhaps carbide would have been the way to go, but Ive very little carbide tooling. A tool post grinder would have been my first go to option but i don't have one, so annealing and remachining was the best option I had at hand. It sounds like an oil quench is going to be best, I keep an old bucket of used motor oil for the purpose, boslab's suggestion thus far seems like the best way to go, but I don't plan to get any torches out tonight as I'm rather tired so the rehardening will be waiting untill tomorrow. Untill then please keep the suggestions coming.

otherworlds
04-16-2015, 12:31 AM
kid, I believe most hardened steel centers today are made from something like 52100 steel--a chromium alloyed, bearing steel. THIS (http://www.suppliersonline.com/propertypages/52100.asp) is a quick spec' for hardening. It lists oil for quench and 1500 F for heating temperature.

I have no basis to make this judgement, but I think I would fully harden it and use it as-is. I'm not sure why I say that; it is just my illogical hunch in this instance.

macona
04-16-2015, 12:34 AM
These guys will regrind your centers for $7 each. Hard to beat that:

http://www.millermachineandfabrication.com/products.html

boslab
04-16-2015, 04:28 AM
Im thinking toolpost grinder was made for this
Mark

Forrest Addy
04-16-2015, 05:12 AM
New ones are cheap if price is the critical factor. Or buy carbide,

Lew Hartswick
04-16-2015, 08:03 AM
Or buy carbide,And then WHEN you drop it what does it cost to "repair" or "replace" it?? :-)
...lew...

philbur
04-16-2015, 08:36 AM
I've got a question, and that is how to best reharden, should I make the whole thing glass hard or just the end, and whether to strait harden it or to draw it back to a temper. I also am ignorant as to whether to use water, brine, or an oil quench in this application. What to do what to do... So I ask on here, how do I go about finishing this little project up so I can have usable centers again?

Harden and temper just the tip. Water is good. Straight hard is to brittle and you don't want/need the shank hard as there is more chance of it damaging your lathe tailstock taper.

Phil:)

dp
04-16-2015, 01:21 PM
Obviously dead centers are dirt cheap to replace but you are to be congratulated for wishing to use your damaged set as an opportunity to learn something about metalurgy and metal heat treating, and learning to bring dead tools back to serviceability. There are a lot of first-principles involved and any time you can do that in the shop it's a good day. Some before/after photos would be nice. Ignore the detractors and get'r done.

rohart
04-16-2015, 04:01 PM
This is a good application to practice using a toolpost grinder on. And a good reason to make a toolpost grinder if you haven't got one.

It's not so easy to clean up a live centre though, unless it comes apart easily.

oldtiffie
04-16-2015, 08:56 PM
So far as I can see this topic seems to only assume that the problem is only with regard to the 60 degree "point" of the "centre".

It is just as likely that the "bruise" "dent" or "ding" is on the morse taper shank.

Either way I'd just use a hand-held diamond honing "stick" on either taper of the centre - and if needs be "blue" test the morse taper to the head-stock taper.

Its all too easy.

Don Young
04-16-2015, 09:56 PM
When I got my well-used AA Products lathe, I made a clamp to fasten a Dremel tool to the toolpost and reground the centers. Crude but effective. Probably made the centers the best parts of the whole lathe!

oldtiffie
04-16-2015, 11:14 PM
All of this too assumes themorse tapers of the head-stock spindle and thetail-stock quill are in good shape and fit for use with a dead or live centre.

The head-stock MT is more likely to be OK but the tail-stock MT is a lot more likely to be "burred" or "bruised/dinged" and to have been "spun out" when drills or chucks or what-ever in the TS MT are "spun out".

If you are going to be concerned about the tapers on dead and live centres - at least check the internal morse tapers too.

And while you are at it, check the drills, chucks and what ever that were in the morse tapers and spun out as well.

A good job is all the job whereas half or part only of a job is no job at all.