Captive drawbar
The standard X1 mill is supplied with an M10 drawbar that is used to tighten the MT2 tooling into the spindle taper. To remove the tooling it is necessary to loosen the drawbar and then tap the end of it in order to break the wedging action of the taper. Once the tooling is loose the drawbar can be unscrewed from the tooling.
This system works but it does have some downsides. Firstly, although only a light tap is necessary to dislodge the tooling, after several years use the end of the drawbar starts to belly out from the repeated hammer blows. Secondly, and perhaps worse, is the shock loading of the spindle bearings from the hammer blows which probably does them no good at all.
Various methods for making self releasing drawbars have been proposed but the simplest way is to use the screw action of the drawbar itself to free the tooling.
This system works but it does have some downsides. Firstly, although only a light tap is necessary to dislodge the tooling, after several years use the end of the drawbar starts to belly out from the repeated hammer blows. Secondly, and perhaps worse, is the shock loading of the spindle bearings from the hammer blows which probably does them no good at all.
Various methods for making self releasing drawbars have been proposed but the simplest way is to use the screw action of the drawbar itself to free the tooling.
The drawbar supplied has an M10 threaded portion at one end and a square end, for a spanner, at the other. Just below the squared end is a raised collar.As the screw is tightened into the MT2 tooling the collar is drawn against the end of the spindle and this locks the tooling taper to the spindle taper. To make the system self releasing then it is necessary to provide another surface above the collar so that when the drawbar is unscrewed the tooling is pressed out.
The header photo shows a steel cap that fits over the top of the spindle trapping the collar of the drawbar.
The cap is made from a 23 mm length of 32mm diameter steel. This is drilled out 10 mm all the way through.
The steel is then bored out to a diameter of 20 mm for a depth of 18 mm to be an easy fit over the end of the spindle.
It is then cross drilled 4mm diameter 6 mm from the open end of the cap.
The draw bar is then put into the spindle and a washer with a 10 mm hole and 1mm thickness placed over the squared end so that it sits on top of the collar. The cap is placed on this and seated down on top of the washer. Using the 4mm drill the position of one of the holes is the marked onto the spindle. A 4 mm rod was pushed into this hole and the second holes was spotted through onto the spindle. The cap,drawbar and washer was removed and the spindle was drilled through at the marked points. All drilling of the spindle was done using a hand held portable electric drill.
It is then cross drilled 4mm diameter 6 mm from the open end of the cap.
The draw bar is then put into the spindle and a washer with a 10 mm hole and 1mm thickness placed over the squared end so that it sits on top of the collar. The cap is placed on this and seated down on top of the washer. Using the 4mm drill the position of one of the holes is the marked onto the spindle. A 4 mm rod was pushed into this hole and the second holes was spotted through onto the spindle. The cap,drawbar and washer was removed and the spindle was drilled through at the marked points. All drilling of the spindle was done using a hand held portable electric drill.
This photo shows the spindle after drilling the 4 mm holes.
The two holes in the cap were drilled out to 5 mm and threaded M6.
Two M6 socket head cap screws were turned as shown. The threaded portion is 10 mm long and the tip is turned down to 4 mm diameter for a length of 4 mm. The tip was well chamfered.
Turning of the screws was accomplished using the loose chuck jaws .
Turning of the screws was accomplished using the loose chuck jaws .
Final assembly was to insert the drawbar (without the washer) into the spindle, place the cap over the spindle and then insert the two screws and tighten them up. The draw bar should rotate freely.
Further development.
The mill spindle is held stationary by using a small tommy bar, supplied with the mill, in a hole at the bottom of the spindle whilst tightening the drawbar with a spanner at the top of the spindle. This procedure works but it is fiddly because it is often difficult to locate the tommy bar hole as it is not easily visible beneath the headstock.
Since fitting the captive drawbar I found that I was using the protruding ends of the M6 screws securing the cap to the spindle to hold the spindle stationary whilst tightening the draw bar. This was much easier than using the tommy bar at the bottom of the spindle.
Since fitting the captive drawbar I found that I was using the protruding ends of the M6 screws securing the cap to the spindle to hold the spindle stationary whilst tightening the draw bar. This was much easier than using the tommy bar at the bottom of the spindle.
The two capscrews have been replaced by two screw in handles, as shown in the photo. Each handle is made from 6 mm steel round bar 36 mm long. The tip is turned down to 4 mm for a length of 4 mm. The threaded portion is 7 mm long and the end of the bar is chamfered and rounded.
The handles are completely contained inside the Perspex cover when the mill is in use.
The new handles make the changing of tools easier.