[R.DesJardin]

I know that finding a way to lock the R8 collet seems to be a hurdle for the tool changer guys.
While running a Hardinge Tool Room lathe at work the other day I thought why not use a mechanism similiar to what is used for 5C collets in lathes, this would eliminate the load on spindle bearings worries.
The clamping force would then be adjustable.
What do you guys think?

[BlueFin]

I have no idea how a 5C thing works, but I was thinking about a hydraulic setup that would operate as a floating unit that would not place any stress on the spindle bearings at all. Please excuse my kindergarden drawing with electronic crayons, but I just did this and only spent 5 minutes on it. There would be a flange on the spindle and this unit would ride on top of the drawbar that was machined to fit inside of a socket ( I drew a single large ball bearing in there with the draw bar dished to match, but maybe there is a better way). When activated the 3 evenly spaced hydraulic cylinders would pull a plate against the flange on the spindle and cause the drawbar to be pushed down against a spring thus releasing tension on the collet. Since you can use something like 5000 PSI hydraulic pressure instead of only 150 PSI air, the system componants could be very small and there really is not much travel needed. When in-active the hydraulic cylinders would extend and allow an air gap in-between the plate and the flange. This would all mount above the drive pulley (I forgot to lable it).

[BlueFin]

Ok, scrap that last idea. I did some more thinking and came up with a much more simple device that would accomplish the same thing. But what I don't know is how much force does everyone want the drawbar to pull up on the collet? Reading the tool changer thread it was mentioned that 4,000 pounds was an estimate. I was looking for a way to convert rotational force into linear force for a given thread pitch (minus friction loss) and didn't find much.

[justgary]

Bluefin -

Play with this Bolt Torque Calculator. I quickly put in 1/2-20 and a few other parameters and got a value of 30 Ft-Lbs (350 In-Lbs) for 4000 pounds of draw load. I know that it's not actually the drawbar size, but that number seems reasonable enough.

The K Factor of 0.2 at the bottom of the page assumes that you have dry threads. It's probably best to assume that unless you plan to specify to oil them. Platings, waxes, and oil can make a huge difference in the tensile load for the same torque value.

Google "bolt thread torque" or "lubricated thread torque" or similar terms and you'll find tons of information on dry vs. lubricated threads.

Regards,

- Just Gary