Re: TTS ER-20's are killing my collets! Anyone see this before? Know what's happening
Reply with Quote
Hi....it would be interesting to see what the "slip" factor was for the amount of torque applied to tightening a collet and the amount of torque that makes a cutter slip in the collet.
This is quite easy to measure.....just put a steel rod of specific diam in the same size collet diam and then progressively torque the nut while measuring the amount of torque it takes to make the rod rotate in the collet.
The exercise would then be a measure of how much torque you need for a cutter size in whatever material you are cutting at the speed/feed rate etc.
My opinion is that with the very slow back section taper of the ER collet, the torque applied to the cutter shank would be 3 or 4 times that applied to the nut.
The nut has a fairly coarse thread, but it's the collet taper that on closing applies the frictional grip to the cutter shank.
That being so, the more surface area in contact with the cutter shank the better the holding force.........my contention is that if you have cutter holding problems then using an exact collet size for the same size cutter shank will increase the holding grip.
The moment you squeeze a collet down to the smaller size the diam of the internal bore of the collet will then only make contact with the middle section of the collet bore, possibly the large amount of toque mentioned being applied to the ER 20 nut is a prime reason why the galling is present.
I'm amazed that a 12 mm diam drill held in a keyless drill chuck can hold it's position despite the torque and back forces of the feed rate that drilling will apply......and as the drill size goes up from 1mm to the max size of the chuck, less surface area is available by the chuck jaws to the drill shank where at 12 mm diam there is only point contact where in fact you actually need it most.
Drill chucks are designed to hold drills, but the design is a flaw as you get most surface contact with the smallest drill diam and less surface contact the larger the diam of the drill when you need it most......a back to front force factor application.
With the use of ER collets for drill holding, the surface in contact with the drill shank is increased as the collet is changed up to the next drill size, even though down squeezing occurs between sizes.
The main reason drill chucks are so successful is that drill shanks are soft and drill chuck jaws are hard.......a certain amount of elastic deformation does occur, which is not the case for a completely hardened cutter shank.......hard steel on hard steel is a very slippery slope.
Ian.
Originally Posted by SCzEngrgGroup
