Lathe Collets - 4: Collet Tricks
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WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.
An alternative to precision, high quality collets, would be to make collets specific for the job at hand. This is recommended only when the correct 5C collet is not available or if the lathe does not have a collet holder.
A home-made collet can be made using a split- bushing method (shown on the right) where a cylinder of steel is bored or reamed in the 3-jaw chuck to the exact size of the work piece. The cylinder is then marked so it can be replaced back in the 3-jaw in the same position. It is then split lengthways with a slitting saw. The slitting allows the collet to contract around the piece when pressure is applied. Work gripped in this bushing in the 3-jaw runs very close to concentric and is one method of utilizing an older, less precision chuck.
Used in a pinch, a round collet can be selected or machined so that it fits the across corner dimension of square bar stock, (for 1/4", 3/8" and 1/2"), useful for quick turning of square stock. It is important that only work of the correct size be inserted into the collet. Any attempt to grip work slightly smaller than the correct diameter is going to damage the collet.
Round 5C collets come in 1/64 increments. However if you don't have the exactly correct size you can splay, squeeze, or shim to fit. To splay a collet oversize tap the stock in from the back side. This will open the collet up oversize and it can still be chucked down hard. Undersize stock can also be squeezed to a degree. Shim can be used to bring an oversized collet down to nominal. R.S.
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label: lathe collet, C5, drawbar, collet chuck, collet variations, machining tricks