How tight is too tight to lock down a tailstock?

[graham-xrf]

I think the cause of the break, while discovered by having a longer lever, was actually because the stress up the centre line of the casting was being taken to the point where none of the cross section was in compression anymore. As you tighten up, the casting is mostly in compression, except for a line along the topside, which goes into tension. Tighten some more, and the point where compression changes to tension gets deeper. Go at it enough, and you end up trying to bend it all the way through.

Of course, it doesn't actually bend much, but it does not have to. Cast semi-steel does have some strength in tension, but only a bit more than plain cast iron, which will break with hardly much of a tug. Your steel through-bolts repair will have the bolts taking bending stress while you tug upwards, their cross section area replacing the area you super-glued. The tailstock would only feel big pressure to ride backwards if you were taking a cut left to right, which is, I guess what you were doing.

I am with @Aukai here.
A simple cut plate of steel with a hole in it, thick enough to stay unharmed while you strip the big nut, or break something else.
If it needs a guided shape, add another thinner plate, narrower, to fashion a composite section like the original casting had.
Have the screws that hold them together be placed only on the corners. Avoid the line up the middle.

The difference here is that your steel replacement would be twice as thick, or more, and made of stuff that has strength in tension!

 

[MyLilMule]

Well, you could have saved yourself some trouble by just using a drive dog. Easy enough to make. I have often discovered that breaking something is due to impatience which leads to negligence.

But, based on the pictures, I guess it's no surprise. Clean your chip tray every now and again. Take care of the machine. Take some kind of pride in it. Good gravy.

 

[Ken226]

If your using that much force in lieu of a driving dog, are you not worried about damaging or causing excessive wear on the spindle bearings?

With the mechanical advantage of the tailstock handwheel, and all the force being in the axial direction, it's a lot of force on the tapered roller elements.

 

[jwmelvin]

Axial force will tend to bow (buckle) the work piece too.

 

[Aaron_W]

Logan 9", not sure of vintage, I believe it's from the early '60s.

I just torqued head bolts last week with a calibrated torque wrench so I have valid de-facto calibration cert on my right arm, and according to its muscular strain gauge the thing broke at around 30ft-lbs. I was surprised too, still am.

I think something had to have been jammed in there to upset the balance of forces and make it break. I'm sure I've put more torque than that on it before and it didn't break.

It is also easy to forget these are precision machines and despite weighing a lot more than us, they were not made for King Kong to crank on them. I've sheared the split pin on my mill drawbar by cranking it down to hard. Might be hundreds of pounds of steel and iron but they only need a gentle touch to operate.

 

[benmychree]

Is this any way to run a railroad?

 

[RJSakowski]

I broke the bottom plate on my Atlas/Craftsman 6x18 many years ago. I made a replacement from hot rolled steel. It hasn't broken again. Asd to how tight, it should be as tight as necessary to accomplish the task at hand. Aside from the bottom plate, everything is essentially in compression.

 

[strantor]

Well, you could have saved yourself some trouble by just using a drive dog. Easy enough to make. I have often discovered that breaking something is due to impatience which leads to negligence.

But, based on the pictures, I guess it's no surprise. Clean your chip tray every now and again. Take care of the machine. Take some kind of pride in it. Good gravy.

Thank you for the constructive criticism. I didn't take it well at first. My first inclination was to tell you where you could cram your opinion. I didn't bother typing that one. My second inclination was to justify my neglect of the lathe by telling you all about how it's a clapped out hoop-dee of a machine that cuts 10 different tapers depending on where the saddle is, can't part off plastic without rattling all it's bolts loose, requires 3 gib adjustments to make it from one end of the cross slide to the other, and how I'm probably going to turn it into a cutter grinder once I get my other lathe working. I did type that one up but I didn't post it because when I read back to myself it sounded like a load of b.s., and that's because it is b.s.. The truth is I've neglected it for too long, and while it was a freshly painted turd when I bought it, I've done nothing but make it worse.

So for the past 2 days I've been cleaning it. I took it completely apart, everything short of exposing the spindle bearings and taking the headstock off. I scrubbed all the moving parts in the parts washer and stoned all the machined surfaces.



In doing so I found the reasons for its binding in the cross slide and a big part of its lack of rigidity. Nothing was flat. There were multiple dings on the side of the saddle that extruded the cast iron upward and impeded the cross slide. Ex:


There wasn't a single flat spot under the compound:


The bottom of my toolpost wasn't even close to flat:


The yeti chomps on my ways that I thought were just superficial, weren't:


Yeah, I used a Harbor Freight knife sharpening whetstone on it. And sandpaper, where the high spots were too much work for the stone. I don't have precision ground flat stones or a precision straight edge or anything to do this job properly. The ways need to be put on a surface grinder and then scraped, but that's not going to happen. I think what I did is not typically advisable, but I don't see how I could possibly have made anything worse. I can get from one end of the cross slide to the other without any gib adjustments now, and the carriage moves freely from one end to the other, no loose spots or tight spots. Best condition it's been in since long before it was mine.

And all it took was a little kick in the ass from you. So again, than you. I needed to hear that.

 

[MyLilMule]

I could have phrased everything I wrote better. Thanks for reminding me of that. Maybe it wouldn't have had the same effect? Who knows. But I am glad it helped in any case, but apologize for coming across as a self righteous ass.

I think you also discovered another reason to keep the lathe clean. All lathes will wear out at some point, but no sense in helping it along by letting chips get underneath the sliding surfaces. My Bridgeport has a huge score under the table way. But I was able to stone the edges of it flat again. Its not perfect but it's better than what it was. I don't have the budget to have it ground and scraped. I suspect the previous owner neglected it - there was other evidence of that.

My South Bend 13" is nowhere near perfect, but I put in hundreds of hours cleaning it and painting it, I keep the way surfaces and the saddle as clean as I can. And I can't stand to let it sit more than a couple weeks before I clean the chips out of the chip tray.

 

[strantor]

I could have phrased everything I wrote better. Thanks for reminding me of that. Maybe it wouldn't have had the same effect? Who knows. But I am glad it helped in any case, but apologize for coming across as a self righteous ass.

Nah I think you hit the nail right on the head. No worries.