Turning a taper between offset centers

Ball bearings have been used that way for a century or more.

You probably don't have any white lead. That's the traditional lubricant for dead centers. I still have an ounce or so, and I use it.

HOWEVER, it's no damend good at high speeds. It was the standard lubricant for centers when speeds were much lower.

On 1/7/2018 12:44 PM, Ed Huntress wrote: > On Sun, 7 Jan 2018 12:25:04 -0700, Bob La Londe > wrote: > >> After much adieu over nothing I suppose (since at the moment I only have >> one to do) I decided to turn a tapered mandrel between centers. The >> first one turned out pretty good except I screwed up the finish pass(es) >> and came in under sized. No I can't just shorten it up. So I still >> have just one to do. LOL. >> >> I picked up one of those little offsettable centers and it worked well >> enough, except the tip melted. I kept it lubricated, but it might have >> just been to high of an rpm for the method. I can turn it slower, but >> if I get to do more of these I sure don't want to go slower. >> >> I have two thoughts: >> >> I can offset the tail stock and use a ball bearing center. >> >> I can machine off the melted center, center drill it, and insert a ball >> bearing between it and the center drill on that end of the stock. The >> ball bearing certainly won't melt, and it will more evenly distribute >> the load from the stock to the center. It would also have more contact >> with the drilled out center. It would also allow me to have a small >> reservoir of grease behind the bearing. >> >> I really don't have a strong feeling for which approach will work out >> better other than I dislike the time spent adjusting my tailstock back >> in. I can leave the offset center setup for this taper indefinitely. >> Ok, maybe I hope the ball bearing method will work out atleast as well. >> >> The taper is pretty shallow. .157 over 10 inches. > > Ball bearings have been used that way for a century or more. > > You probably don't have any white lead. That's the traditional > lubricant for dead centers. I still have an ounce or so, and I use it. > > HOWEVER, it's no damend good at high speeds. It was the standard > lubricant for centers when speeds were much lower. >

I don't know what white lead is. I've got 99.5% pure (used for testing molds before I ship them) and some hard alloy I mix in from time to time for special applications.

It's lead carbonate. It's white powder. Red lead is lead oxide. Both were used in paints before 1978. Both forms are highly toxic.

White lead mixed with litharge (metallic soap) or oil forms an extreme-pressure lubricant. There are higher EP lubricants available today, but the pasty version of white lead, sold for such uses as lubricating tailstocks, will resist an extraordinary amount of pressure without puncturing the lubricating film.

Not recommended today, because of the toxicity. Clowns used it for a makeup base. They lost their minds and some died from it.

BTW, is you have a really old lathe, like my 1945 South Bend, you'll have a little tapered brass plug that fits loosely into a hole in the tailstock. There is a little pocket under the plug. That's for white lead. The plug is a dauber to spread a dab of white lead on your dead center.

Bob,

I think it was me that suggested those offsettable centres so shame the centre melted. I think if it was me I would try and modify it to have a rotating centre. Maybe you can fit a housing to the movable part to support the outer races of a couple of ball races back to back and fit a centre into those so you'll have a well supported rotating centre. I've done similar before but not for a centre application but the application is still going well, it was actually the rotating mass bearings for a vibratory table, the housing was fixed to the vibrating table and the centre shaft connected to the motor via a bit of flexible hose.

So how much RPM do you think a ball bearing can take?

I thought I was turning at 300 RPM the first time, but it turns out I was actually turning at 460. Oops. The only ball bearing I have that I know is hard is one of the little bearings out of the ACBs I pulled out of the Hurco mill. Its only about .20 so I know it doesn't have enough mass to sink much heat.

~~~ time lapsed ~~~

I finished it at 115 RPM. Wow. Talk about slow. On the other hand it looks really nice after three passes with the vertical shear and then hand finishing with fine grit.

I did have the grease around the bearing start to smoke once so I let it cool for a while and then used another bearing with fresh grease. In my mind a larger bearing would have worked better, and taken longer to overheat.

If you make a set of ball races in a cup for the offsetable center, I would suggest that you also make it tiltable towards the center line too. You want the axis of rotation for the workpiece to be parallel to the axis of rotation of the center. Otherwise, you get lots of friction as the angle of the center changes in the drilled center in the workpiece. And if the workpiece is a soft alumin(i)um, you will wallow the center hole out as you work unless the axis of rotation of the tailstock center matches that of the workpiece.

I think that the amount of angle change at the headstock center would still be a problem, so inlay a ball in that center so it has maximum contact with the drilled center in the workpiece. (The same problem exists with the offset tailstock, too.) Even worse was the way the Unimat SL-1000 (and DB-200) did it, where the headstock was pivoted, so the angle of contact of the headstock center with the drilled center hole was worse, since the headstock and the workpiece axes were in opposite directions.

Good Luck, DoN.

I used plane old waterproof wheel bearing grease, and it worked fine. Not great, but fine for high pressure low RPM. I was going to try some MolyKote BG-20, but I couldn't find my tube of the stuff. Of course this morning when I walked into the shop... there it was on my desk. Right where I thought it as. LOL.

I may buy a few carbide balls to keep in my lathe tool cart for future projects, but like I said before. If find myself needing to do a bunch of these I'm going with to make a linear rod taper attachment.

I've got some other tapers I want to make for myself down the road anyway.

I'm glad you got it done Bob. I used to do lots of between centers work. Dead centers for some of it. Ball centers too. When I used a ball center I would usually first use a same size ball and a hammer to form a depression in the drilled center to fit the ball. This results in more surface area for the lube and lowers the pressure per area. So less heat is generated. When needing to turn the work at relatively high rpm with dead centers I would lube the center and leave it a little loose. Then spin up the lathe and let the part warm up. And check the tightness of the center in the part during the warm up. And once machining commences the center tightness needs to be checked on a regular basis. This all done by feel but it doesn't take too long to learn. I tried all sorts of high pressure lubes and mixed some of my own. Eric

On 1/7/2018 2:47 PM, David Billington wrote: > On 07/01/18 19:25, Bob La Londe wrote: >> After much adieu over nothing I suppose (since at the moment I only >> have one to do) I decided to turn a tapered mandrel between centers. >> The first one turned out pretty good except I screwed up the finish >> pass(es) and came in under sized. No I can't just shorten it up. So >> I still have just one to do. LOL. >> >> I picked up one of those little offsettable centers and it worked well >> enough, except the tip melted. I kept it lubricated, but it might >> have just been to high of an rpm for the method. I can turn it >> slower, but if I get to do more of these I sure don't want to go slower. >> >> I have two thoughts: >> >> I can offset the tail stock and use a ball bearing center. >> >> I can machine off the melted center, center drill it, and insert a >> ball bearing between it and the center drill on that end of the >> stock. The ball bearing certainly won't melt, and it will more evenly >> distribute the load from the stock to the center. It would also have >> more contact with the drilled out center. It would also allow me to >> have a small reservoir of grease behind the bearing. >> >> I really don't have a strong feeling for which approach will work out >> better other than I dislike the time spent adjusting my tailstock back >> in. I can leave the offset center setup for this taper indefinitely. >> Ok, maybe I hope the ball bearing method will work out atleast as well. >> >> The taper is pretty shallow. .157 over 10 inches. >> >> >> > Bob, > > I think it was me that suggested those offsettable centres so shame the > centre melted. I think if it was me I would try and modify it to have a > rotating centre. Maybe you can fit a housing to the movable part to > support the outer races of a couple of ball races back to back and fit a > centre into those so you'll have a well supported rotating centre. I've > done similar before but not for a centre application but the application > is still going well, it was actually the rotating mass bearings for a > vibratory table, the housing was fixed to the vibrating table and the > centre shaft connected to the motor via a bit of flexible hose.

Its ok. They atleast paid lip service to it needing to be hard. Based on how it machines it was case hardened. Very hard start, and then right through for a pretty face. Its a neat idea, but the boring head might have been better. The offset center tool is really wide for what it is and you have to use longer than ideal tool stick out from the tool post. I have no hard feelings about it. I learned quite a bit in a very short time by using it.

I socketed the center for a ball bearing and got the job done using some waterproof wheel bearing grease. I may never use it again, but then... I might. LOL. I've mentioned two or three times now. If I have to turn a lot of tapers I'm going to invest a day into making a proper linear rod taper attachment.

On 1/7/2018 9:09 PM, Gunner Asch wrote: > On Sun, 7 Jan 2018 12:25:04 -0700, Bob La Londe > wrote: > >> After much adieu over nothing I suppose (since at the moment I only have >> one to do) I decided to turn a tapered mandrel between centers. The >> first one turned out pretty good except I screwed up the finish pass(es) >> and came in under sized. No I can't just shorten it up. So I still >> have just one to do. LOL. >> >> I picked up one of those little offsettable centers and it worked well >> enough, except the tip melted. I kept it lubricated, but it might have >> just been to high of an rpm for the method. I can turn it slower, but >> if I get to do more of these I sure don't want to go slower. >> >> I have two thoughts: >> >> I can offset the tail stock and use a ball bearing center. >> >> I can machine off the melted center, center drill it, and insert a ball >> bearing between it and the center drill on that end of the stock. The >> ball bearing certainly won't melt, and it will more evenly distribute >> the load from the stock to the center. It would also have more contact >> with the drilled out center. It would also allow me to have a small >> reservoir of grease behind the bearing. >> >> I really don't have a strong feeling for which approach will work out >> better other than I dislike the time spent adjusting my tailstock back >> in. I can leave the offset center setup for this taper indefinitely. >> Ok, maybe I hope the ball bearing method will work out atleast as well. >> >> The taper is pretty shallow. .157 over 10 inches. >> >> > What grease are you using? White lithium isnt going to cut it on this > sort of job...just saying... > > Red Lead or a really good axle grease... > > > --- > This email has been checked for viruses by Avast antivirus software. >

At 115 RPM plane old waterproof wheel bearing grease worked ok. The job took a couple hours at that speed, but the bearing didn't start to get really hot until about 3/4 of the way through the job. I replaced the bearing and the grease and finished the job. It was pretty frustrating to go so slow. This was a 416 stainless shaft, and it will actually machine much faster.

One problem with turning between centers is that the workpiece length increases as it heats up. A setting which is right at the start of the job can be too tight part way along.

If you can make the center both a ball bearing mounted center, and spring loaded to keep about the same pressure throughout the job, you probably won't get the overheating you've observed, and maybe can run faster, too.

Such centers are made for cylindrical grinding (where the workpiece is more likely to heat up and seize if the spring loading is not provided. Of course, these centers are likely not made for offset turning -- though there may be some for grinding tapers like Morse Taper shanks.

Enjoy, DoN.