Myford drawbars
by Adrian Garner

MORSE tapers provide a secure mount for drill chucks and lathe centres as the pressures force the taper and its socket together.  Milling chucks and boring heads, however, require a draw bar to ensure their taper remains secure in the headstock.  Importantly, the drawbar only has to secure the taper – it must not be pulled up tight with a nut and spanner.  An over tightened morse taper can be very difficult to undo and excessive use of a mallet to release the taper can potentially damage the headstock bearings.

Depending on the tool, the draw bar requires either a 3/8” BSW or a 10mm metric thread.  A design for an imperial version incorporating small knurled knobs to prevent the over-tightening problem is shown.  It is easily made from a length of 3/8” free cutting mild steel rod which should be checked for straightness by rolling on the bench.  The threads are best cut in the lathe using a single point tool ground to 55 degrees.  The final rounded tooth form can be formed by a light final skim with a die.  The outer ends of the draw bar should have a 45 degree chamfer cut slightly deeper than the thread depth.  A cut about 1/16” deep with a small parting tool makes a neat job of the inner end of the thread.

The two knurled knobs and the bush can be made of mild steel but aluminium alloy is perfectly adequate provided a standard 3/8” steel washer sits between the inner knurled knob and the bush.  Alloy has the advantage that it does not have to be oiled to prevent rust.

In use the bush is inserted in to the rear of the mandrel which aligns the draw bar with the threaded hole in the tool.  This avoids much fiddling about trying to find the threaded taper at the other end of the large bore.  The outer knurled knob is used to screw the drawbar to full depth in the taper and the inner knurled knob finally tightened against the bush.

To release, the inner knurled knob is loosened and the end of the drawbar tapped with a soft (leather) mallet.  As the drawbar uses all the thread available in the taper, little harm to the thread is likely during the release.  The design allows for differing lengths of thread in the taper.

A draw bar for the Myford small bore Super 7 lathe

Prior to the big bore lathes, the Super 7 was bored to clear ½” stock.  This lathe had a shorter mandrel and, unlike the big bore lathes, the inside of the bore of the rear end of the mandrel was not ground.

The dimensions shown on the attached drawing (right) reflect these differences.  The construction notes provided for the big bore apply but with one change.  Before making the bush which is inserted into the rear of the mandrel, the dimension to clear this hole should be measured on your lathe.  Turning this bush to 0.619” provided an easy slide in fit on my lathe which had a hole through the mandrel of just under 5/8”. Check the size for the lathe you own.  Aim for 3 or 4 thou play so that it is an easy fit.

Draw bars are only intended to ensure tapers remain secure in the headstock.  As mentioned last time, the draw bar must not be over tightened.  Morse tapers are designed to be secure and over tightening can make them very difficult to undo.  Avoid excessive use of a soft mallet to release the taper which can potentially damage the headstock bearings.

Drilling the 3/8” hole in the alloy bush.  The external diameter is a loose fit in the ground bore of the rear end of the mandrel.

Drawbar for Myford Connoisseur large bore lathe.

Drawbar for Myford Super 7 small bore lathe.