Expanding mandrels

[Bob La Londe]

This might qualify more as a learning project than a working shop project.

I've got the small lathe and the mini lathe setup for using 3C collets (most of the time) with a screw on draw tube. They work fairly well, but occasionally a work stop inside the collet would be handy. The 3C collets I have do not have an internal thread. I had the idea to make an expanding mandrel or a dozen that can be tightened with a countersunk head machine screw (* also called a flat socket head screw) to tighten the mandrel in the collet. I see some issues with the idea, but I thought maybe Doozer, Jrouche, and some of the more helpful members could jump in here and tell me how to do it right.

* Yes I have factory boxes that say that.

[Toolguy]

In my experience with 5C collets that are unthreaded, the expanding type collet stops that I've used tend to slip during a part run. That is bad, because then you have to inspect every part to see if there are out of spec ones, then remake or rework those. If you have a threaded left side of the spindle, you could make a universal collet stop with some all thread and spindle adaptor.

[Bob La Londe]

In my experience with 5C collets that are unthreaded, the expanding type collet stops that I've used tend to slip during a part run. That is bad, because then you have to inspect every part to see if there are out of spec ones, then remake or rework those. If you have a threaded left side of the spindle, you could make a universal collet stop with some all thread and spindle adaptor.

Slippage from popping in the part and cutting forces was one of my concerns. The idea of a threaded outboard side of spindle doesn't really work. There is a draw tube with a hand wheel through the spindle. I guess as long as the hand wheel never slips on the tube I could thread the inside of the draw tube and have a really long stop rod. In my recent instance where a stop might have been handy it was a 1/8 pin. I'm not sure a 3/32 (to clear grip surfaces of the collet) stop rod would work very well over that distance. Or maybe I'm just not a good enough machinist to make it work. LOL.

Anyway, that's why I posted the thought. To find out all the things that won't work, and maybe learn something that will. Thanks for your feedback.

Interestingly all my 5C collets have an internal thread... and I actually do have a lathe with a native 5C spindle. It even has a lever drawbar that works. Unfortunately I have not made the time to get that lathe working again. Its still sitting on the movers over next to the bending brake where I parked it after unloading it.

[BCRider]

Have a look at what Randy did for a dead length stop for his 4C machine in the 5C Collet Stop thread.

I'm wondering about the pulling power of the collet too so I'm thinking that if Randy's straight shaft were replaced with some nice quality all thread rod and the hole with set screw for a smaller stop pin put in the other end then it could be a threaded rod adjustment for setting the length. And the centering/support bushing for up near the collet would be an easy to make item too.

[djc]

What does the back end of the 3C collet look like? Or, do all of the 3C collets look the same?

Could you loctite in an internally-threaded bush and thus make them look like a 5C?

[rickyb]

Modify your draw tube end to hold an internal adjustable rod. May need a small spider on the rod at the collet end to center it for ease of assembly through the back of the collet..

[Sparky_NY]

Modify your draw tube end to hold an internal adjustable rod. May need a small spider on the rod at the collet end to center it for ease of assembly through the back of the collet..

My cnc engine lathe (18x40) actually came with a rod that mounted in the outboard end of the spindle to serve as a depth stop. It was quite long and had some spiders to keep it centered in the spindle bore and at the collet end or even with a chuck. Doing similar in your drawbar/handwheel should work similar.

Of course, the obvious is to get that 5C equipped lathe that you have going. I just love using 5C collets and only resort to a chuck when I have no choice. I know.... never enough hours in the day to whittle away at the future project list.

[Doozer]

https://www.dunhamtool.com/miser-mandrels/

[Bob La Londe]

https://www.dunhamtool.com/miser-mandrels/

I've got a small set of commercially made expanding mandrels, and probably twice as many I've made as needed.

[Noitoen]

If the draw tube's thread is longer than the collet's thread a small threaded disc can be screwed into the draw tube until it bottomed out the tube's thread. There would still be thread left to tighten the collet. A small threaded hole in the centre of this disc would allow a threaded rod to serve as stop and will always register the correct length independent of the tightening of the colet.

[BCRider]

If the draw tube's thread is longer than the collet's thread a small threaded disc can be screwed into the draw tube until it bottomed out the tube's thread. There would still be thread left to tighten the collet. A small threaded hole in the centre of this disc would allow a threaded rod to serve as stop and will always register the correct length independent of the tightening of the colet.

I like your idea other than there's no easy way to set the pin with a part in the collet. And it would require a proper shoulder in the tube with a clearance groove for the thread ending so the disc could seat consistently against the shoulder. Randy's concept linked earlier avoids this by making the adjustment of the rod available at the outboard end.

[Noitoen]

I like your idea other than there's no easy way to set the pin with a part in the collet. And it would require a proper shoulder in the tube with a clearance groove for the thread ending so the disc could seat consistently against the shoulder. Randy's concept linked earlier avoids this by making the adjustment of the rod available at the outboard end.

No need for real precision in my opinion since the draw tube is always registered in relation to the spindle back side and any difference can be compensated by "zeroing" the tool position. Anything that is clamped will always be at the same distance from the other end of the draw tube. Later I can make a "crap o cad" drawing to explain if needed.

[Illinoyance]

The socket flat head screws have a poor geometry for expanding your collet stop. Machine the heads to a 20 to 30 degree included angle. That will give a lot of expansive force when you tighten the screw. I believe the screws used on the Dunham expanding mandrels have a very shallow angle on the head.

[JRouche]

This might qualify more as a learning project than a working shop project.

I've got the small lathe and the mini lathe setup for using 3C collets (most of the time) with a screw on draw tube. They work fairly well, but occasionally a work stop inside the collet would be handy. The 3C collets I have do not have an internal thread. I had the idea to make an expanding mandrel or a dozen that can be tightened with a countersunk head machine screw (* also called a flat socket head screw) to tighten the mandrel in the collet. I see some issues with the idea, but I thought maybe Doozer, Jrouche, and some of the more helpful members could jump in here and tell me how to do it right.

* Yes I have factory boxes that say that.

I sense some facetiousness but Ill bite I have a threaded draw tube for a lathe in 5c. I needed to make an internal stop fast and easy. I ended up using a hard wood dowel that would bottom out at the handwheel end of the draw tube (it was a closed end tube) and have the collet end where I needed the stop. The dowel was cut to length so when the work was put into the collet it would hit the wood. I used wood for ease making it because I turned the dowel to fit very close to the ID of the collet. Not supper accurate I guess, you could use metal. Aluminum would work. JR

[Bob La Londe]

Of course it was facetious, but thanks for playing.

I'm leaning towards the drawbar (tube) mounted assembly since the hand wheel always registers against the back of the spindle. I'm thinking I might make a new hand wheel anyway to make tightening the collets a little easier.