Hand crank for threading

February 2015

This is the most satisfying project I have completed on the lathe. The individual parts are easy to make and the finished item makes threading easy and safe.

For those not familiar with the MD65/SD300, the lowest spindle speed available is 250rpm. This is way too fast for many threading operations, and makes threading under power a buttock-clenching experience as the carriage approaches the chuck at an alarming speed. A low speed attachment (extra pulley) is available from Essel Engineering however a hand crank offers more control.

I based the design of the hand crank on an article in the June 2007 (#126) issue of Model Engineers Workshop. The design is also similar to this one by data_plumber1 (Christopher) in the Prazi Machining forum.

The basic concept is that the end of the spindle insert expands and grips the inside of the spindle. Dimensioned drawings are available here.

Hand crank - SW drawing

All parts are made from EN1A, except for the drawbar which I made from silver steel, and the handle which I fashioned from a piece of leftover 15mm copper pipe. The handle is screwed in place both ends with M6 countersunk screws. The crank is made of 3mm steel flat bar – at first I thought this would be too light but it is well-proportioned when in use.

Making the parts is pretty straightforward. You will need a long series 6mm drill to get all the way through the spindle insert. You will also need a slitting saw. I used Loctite 603 to attach the spindle insert to the boss, and also to attach the drawbar to the locking knob.

There are seven main parts:

1/ spindle insert: the long part that goes inside the lathe spindle

  • made from 1/2 inch stock – although this is bigger than the 12mm spindle bore you should be able to insert it far enough in to machine both ends (thanks to the MT2 taper in the spindle)
  • machine the handle end first, then flip it over and machine the spindle end – when machining the handle end make a 12mm diameter by 3mm long step in the end to locate the crank (see drawings)
  • step up drill sizes to progressively drill out the spindle end – when you get to 6mm size use the long series drill bit to drill all the way through (this takes a little fancy footwork and some ‘safety’ cutting fluid, due to the short throw of the tailstock)
  • next, set your compound slide to approx. 20 degrees using the index markings on the side and machine out the inside taper – the angle does not need to be precise, provided you don’t move it before turning the draw plug
  • lastly, use a slitting saw to cut four slits in the spindle end – as the material is quite thin and will not expand much, there is no need to drill ‘crack-stop’ holes

Spindle insert - endLathe-end of the spindle insert, showing the slitting and internal taper.

2/ draw plug: this part is drawn into the spindle insert, expanding it and gripping the inside of the lathe spindle

  • made from the same 1/2 inch stock as the spindle insert
  • drill and tap to M6, then machine a taper to match the one on the inside of the spindle insert

Draw plug

3/ boss: butts up against the end of the lathe spindle, and provides a mounting point for the crank

  • made from 40mm stock
  • face the large end first, then turn it around in the chuck to turn down the smaller diameter, and then centre drill and drill out to 11mm
  • the boss needs to be a close sliding fit on the spindle insert: from the 11mm internal diameter bore out to fit the spindle insert
  • mark out the two M5 holes for mounting the handle, drill and tap to M5 – these need to be reasonably precise as they need to match the handle crank, and also as we are using countersunk screws (I used XY coordinate drilling on the mill table)
  • lastly use Loctite 603 (or 601) to attach the boss to the spindle insert – take care to make sure that the 3mm long step at the end is left exposed

Boss - outsideCrank end showing countersunk screws, and the step in the end of the spindle insert used to locate the crank (the dark ring in the middle – I accidentally glued it to my workbench, that Loctite 603 is strong stuff)

Boss - insideSpindle side…

4/ drawbar:

  • easy enough: cut silver steel to length and thread both ends

5/ locking knob: turn by hand to pull the draw plug into the spindle insert, locking the assembly into the lathe spindle bore

  • you could just as easily use a ready-made plastic knob, however I had a mystery piece of steel scrounged from a scrap bin and wanted to see how it would turn, so I made mine from steel
  • tap M6 to suit draw bar
  • I made a hash of knurling mine – but the marks give enough grip to turn it, and not much force is needed to lock it inside the spindle
  • check the length of the draw bar: assemble the components 1 to 5 listed above, with the draw plug resting inside the spindle insert, make sure you have 4 threads available on the draw plug end (the locking knob will need 3 to 4 turns to lock the assembly inside the lathe spindle)
  • use thread lock to attach the draw bar to the locking knob

6/ crank:

  • made from 3mm x 20mm mild steel bar
  • cut to length, drill and countersink holes for the three screws, drill a 12mm hole to suit the step at the end of the spindle insert
  • I rounded the corners on my rough grinding wheel – not terribly pretty… you can do better

7/ crank handle:

  • there are many ways to made the handle – I currently have a thing for countersinking, and love the look of copper so that’s how I made mine

I will add some video of the hand crank in action. It’s a great little item, very quick and easy to fit and remove, and it turns the lathe spindle very easily with plenty of control.

Installed Installed top view

3 thoughts on “Hand crank for threading

  1. Andy from Workshopshed

    Nice result Baz, I could do with something similar for when I get around to doing the threading for my little vice that I started when working at Dowty.
    My only concern with this design is that if you leave it in and apply power. I have heard people suggest making the handle in the form of a large wheel.

    Reply
    1. bazmonaut Post author

      Hi Andy – thanks for the comment. I guess a disc would be safer, but it seems like a hell of a lot of material. I have a wall-mounted master power switch for my lathe (out of reach of little fingers), I just switch the master off when I insert the handle.

      B

      Reply

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