Split cotter vs split casting

[whateg01]

It seems like split cotters are mostly a thing of the past. I still see them used for tail stock quills, but I rarely see them elsewhere. Nowadays, a split casting seems to be the more common method of locking something to a shaft or column. The exception being cheap machines where a grub screw is used.

Is that because it's just to costly to make them vs cutting a slit up the side of a housing? Or is a split cotter's holding power less than a split housing? Or is there another reason?

 

[F-22]

Sorry but I don't quite understand what you mean, could you add some photos?

 

[RoninB4]

I don't quite understand either. Most every tail stock "quill" I've seen uses a taper. Photos/drawing would help my limited imagination.

 

[DadsTools]

I just recently read a thread about an old Craftsman drill press having a quill with play, but there was no means to tighten it. Apparently, the way to repair this is to cut a slot into the housing and then install bolts so tightening them will serve to bring the sides of the cut slot closer together so that the quill can then be tightened to eliminate the play.

OP, is this the kind of locking means you are talking about?

 

[whateg01]

This page shows the making of an improved split cotter.

HomeShopTech

He's a picture of two of them.

 

[whateg01]

I don't quite understand either. Most every tail stock "quill" I've seen uses a taper. Photos/drawing would help my limited imagination.

The taper does not lock the quill. The taper is the interface for the arbor for a drill chuck or the matching taper off a drill.

 

[paulsomlo]

Atlas lathes use a split cotter to lock the tailstock quill.

 

[whateg01]

Atlas lathes use a split cotter to lock the tailstock quill.

I would hazard a guess that most lathes use a split cotter. It's one of the few places I still see them used. Hence my question

 

[dutchgray]

My DSG lathe uses a split cotter to lock the tailstock barrel (also the barrel in the drilling bracket.
I guess they are not commonly used elsewhere due to cost, its more parts to make over slitting a part and using a couple bolts to cinch it up.

 

[DadsTools]

This page shows the making of an improved split cotter.

HomeShopTech

He's a picture of two of them.

Thanks for the clarification. It looks like I might have understood about the "split casting" which is what I think the drill press repair description I had read was talking about making. I was clueless about the split cotter. It would seem one of your suppositions might be correct in that the split casting sure looks like a cheaper way to make it.

 

[RoninB4]

To the OP- Under the heading "A picture is worth........" I understand what your question now and agree with others that the slit sawed casting is less parts, faster, and easier to make than the fully split, threaded bushings. Haven't torn down any newer high end machinery lately so I can't comment on what's commonly used now. Thanks for photo clarification.

 

[paulsomlo]

I would hazard a guess that most lathes use a split cotter. It's one of the few places I still see them used. Hence my question

Not South Bend - at least not on my 9".

 

[whateg01]

Must have been too expensive for that machine. I believe both 10s use a split cotter. Even the little 6" Craftsman I fixed up a few years ago used a split cotter.

 

[Provincial]

The reason for not using a split casting on a tailstock is the presumption that squeezing the split to lock the quill will cause a minute deflection. Any deflection, however minute, is unacceptable to precision machinists, so the split cotter, which floats in the tailstock casting and doesn't deflect the quill, has become the preferred method of locking a tailstock quill.

 

[F-22]

I'm not from th3 US and had no idea "cotter" also means a sort of wedge (and was thinking about the split pins/cotter pins and the post made no sense to me...).

I agree it's probably a cost vs performance question. The cotter design means the tailstock is perhaps a bit more rigid but just cutting it is a lot easier and cheaper and does not really make much of a difference anyway.

 

[Firebrick43]

Not South Bend - at least not on my 9".

Don't know your lathe very well huh? Just because the casting is split doesn't mean it doesn't have split cotters(which it does).

As for the OP, I don't know that they have fell out of use per se, Its the machines that used them (except the lathe) have. There are relatively few new Bridgeport size milling machines sold these days due to CNC in idustry, but most of the ones that are still use split cotters.

Drill presses, even high quality ones such as the powermatic and deltas, never really did use split cotters.

 

[whateg01]

Drill presses, even high quality ones such as the powermatic and deltas, never really did use split cotters.

Maybe that's it. I've been messing with a couple older drill presses lately. The WT 1100 I just bought uses them in a few places - table lock and the head to column. My old Craftsman 150 does the same. So if my newer drill presses are less expensive. One is a Taiwan built 15" floor model and the other is harbor freight. Both use set screws to lock the head to the column and the clamp for the table is split. Come to think of it, the grizzly that I have worked on a few times at a friend's is the same way.

 

[2oolhound]

I really enjoyed this thread. And your link to the "Making of" split cotter was excellent.

Thanks for posting it!

 

[whateg01]

The reason for not using a split casting on a tailstock is the presumption that squeezing the split to lock the quill will cause a minute deflection. Any deflection, however minute, is unacceptable to precision machinists, so the split cotter, which floats in the tailstock casting and doesn't deflect the quill, has become the preferred method of locking a tailstock quill.

I've heard that said before, too. But I don't see how it can be true. The split cotter is literally a wedge that squeezes the quill against the far side of the bore. In a loose fitting bore, the quill could move quite a bit.

 

[Provincial]

I've heard that said before, too. But I don't see how it can be true. The split cotter is literally a wedge that squeezes the quill against the far side of the bore. In a loose fitting bore, the quill could move quite a bit.

True, if the bore was loose. The fit of tailstock bore and quill is never loose when new (even on "entry-level" machines), and rarely develops significant wear in normal use. If the tailstock housing is "split" to allow a clamping fit, it weakens the housing somewhat, and allows it to distort if forces are applied to squeeze it tighter.

Mostly, this is theory, but machinists are pretty conservative when it comes to possible deflection and the resulting lack of precision.

 

[Maui]

I had to build one of these split cotters for a Craftsman 109 lathe tailstock that I was restoring a while back. Fairly easy to fabricate, and works quite well. When I had finished restoring the lathe I ran a trial piece that I turned longitudinally to determine the amount of taper that would result from using the tailstock as a center support. I achieved 0.0001” taper on the diameter per inch of length. Pretty impressive for a lathe that is usually badly maligned by most people on these message boards.