Download Drawing - First draft of the prototype.
A recent project - notes and drawings are still under construction.
Basic specifications are that the holder has 2 separate heads for holding dies or taps/reamers. The body is nominally 2MT - if your tailstock is different then adapt to suit. A taper shank is not a great idea unless a drawbar is used as there is a tendancy for it to slip under load. The design (adapted from a commercial holder I obtained some time ago) can make use of a bolt-on key to prevent the body rotating on the shaft. This key can be removed allowing the body to rotate, a short handle being used to prevent rotation instead of the key. The differences between my version and every other is that the tap/reamer head is 'free-floating' using a sort of radial universal joint to allow maximum accuracy (particularly when reaming). Normally, a reamer held in the tailstock chuck is held rigidly and is most probably not exactly parallel nor on a centre line with the work in the chuck. The result of this is that the reamer will cut oversize and likely produce a bellmouth hole (the extent of which is directly related to the degree of inaccuracy of alignment). The floating head allows a small degree of movement enabling the reamer to align itself with the work - the trick is to achieve this and still maintain a positive drive. The prototype is still under construction but my working diagram should be available on the drawings page real soon. BTW, a mandrel handle would be a very useful companion to this attachment, so if you haven't got one then go make one!
Taper Shank:
This is turned between centers from a 7" length of 3/4" diameter rod. Silver steel would be nice as it can be hardened later (a few knocks are likely during the tool's lifetime), case-hardened mild steel would substitute but free-turning steel won't harden well so is best avoided. It's important that concentricity is maintained on this piece. The tapped hole for the drawbar is only useful if you can actually get a drawbar through the tailstock (I can't on the S7 without removing the handle). Mill the flat for the driving key after the body has been made as it's difficult to judge the exact depth of cut required at this stage.
Die Holder Body:
Simple boring/turning job from a length of 1 3/8" dia f.c. mild steel. Undercut the corners of the die seats a little to ensure the die sits squarely in the recess. Mill the cutaway afterwards to the dimensions given, then insert the taper shank to measure the exact depth of cut for the 1/2" long flat on the end. You want a couple of thou clearance between the key and die holder body to prevent binding so don't cut the flat on the shaft full depth. You will need 6 2BA hex bolts to adjust and hold the dies, machine a 60 deg point on the end to locate in the split and dimples of split dies.
Tap/Reamer front cover:
Machine this first as it's easier to match the male 1 1/8" screwcut thread on the holder body to it later.
Tap/Reamer Holder Body:
Same basic design as the die holder, and is quickly interchangable with it. One end has a T-slot machined across the end to hold the slipper block - use a 1/2" woodruff cutter to achieve this (easily made from silver steel if you don't have a commercial HSS one). Screwcut the male 1 1/8" thread using the previously made cover as a gauge for a nice fit.
Slipper Block and Nose Piece:
The slipper block is best made from bronze but f.c. mild steel would do the job. It would be easier to mill one T-section first while still attached to the stock, then part off to length and mill the other end - easy to hold square by the first T-section held in the machine vice. For the nose piece, the threaded portion should be turned and screwcut first (the thread given fits the female threaded 1/2" chucks), parted off, then the T-slot milled (use brass shim to protect the thread whilst holding it in the machine vice).When the 3 parts of the floating head are assembled check for binding - the nose piece should slide smoothly in all radial directions without any axial wobble (or much anyway, a little is to be expected and probably necessary). A total of 1/16" radial movement is allowed for which is ample to compensate for the sort of off-centre operation designed for.
(c) Chris Heapy 1996.
|
Back to previous page |