(PLEASE NOTE: The design of the plate is only possible because I use 9mm (3/8¡± approx) risers under both ends of the lathe. I use a (not-guaranteed-flat) plywood base and this avoids the middle support grounding on the ply and potentially bending the bed. It¡¯s a simple enough mod to make on most installations, Or you could re-design the plate.)
?
I tried making the copy of the geared Unimat 3 threading option (there are plans in the Files section) but I had issues with the meshing and found that the gears slipped. Maybe they simply weren¡¯t accurate enough (I had them 3D printed by a commercial company). I thought that there must be other ways to drive the spindle and leadscrew in conjunction, and came up with this:
?
?
As you can see, it uses T5 x 10mm pulley wheels, and a double-sided belt. The three smaller pulley wheels are idlers; the slots in the plate allow adjustment to take up the slack in the belt. I¡¯ve used two 26 tooth wheels here for the ¡®drive¡¯ and ¡®driven¡¯ wheels, as I had them to hand.
Using this system it is easy to create reverse threads by simply taking the belt around the outside of the cog driving the main leadscrew.
?
?
Here¡¯s a description of the components. I don¡¯t do CAD drawings (I prefer paper and pencil!) but I¡¯ll scan a drawing of the 4mm mild steel plate layout and include it at the end of this document.
- Two 26 tooth (or whatever other combination - see below) T5 pulley wheels, tapped M14 x 1. Remember to also bore some clearance (4mm or so deep) to accommodate the non-threaded portion of the spindle end.
- Two spacers to get the plate away from the headstock. I made these from alumininium, 24mm diameter x 8.5mm or so thick. Two M6 x 20mm bolts.
- Three T5 pulley wheels as ¡®idlers¡¯. I used 12 teeth pulleys. Three M6 x 40mm bolts, six M6 nuts, and enough M6 washers to space the pulleys appropriately to align the belt.
- Brass leadscrew shaft. I made this from some 16mm round bar, and it ended up 67mm long overall. More details as this is the only slightly complex item!
The end to take the T5 pulley is threaded M14 x 1. This section is about 21mm long, and is also threaded internally M6 to take a short (9mm or so) securing bolt with washer.
The section with flats (the original diameter) is 7mm long.
The section that fits in the hole in the plate is just under 4mm long.
The M12 threaded section is 15mm long.
The slot to fit onto the flat of the leadscrew is 11mm long.
?
- I was planning to use an M12 locknut (Nyloc) to secure the leadscrew shaft but this proved to be too large. As did a normal M12 nut - I faced it down to 8mm thick clear the bed of the lathe.
?
Here¡¯s a side view of the general arrangement. As you can see there¡¯s not much clearance in places. Note the use of washers to get the idler pulleys aligned.
?
?
The next photo shows how the plate mounts onto the headstock, with the spacers.
?
?
A somewhat shaky mount of the fiber tip pen in the ¡®swiss cheese¡¯ holder, but all looks good!
?
?
The double sided belts can be a bit hard to find. I¡¯m in the UK and I got them from Totally Belts (totallybelts.com - who are Orbic Bearings Ltd in Croydon). They are described as 'Optibelt ALPHA Power High Performance Polyurethane Double-Sided Timing Belts'. I bought two sizes - a DT5 515 and a DT5 410 (their product codes TB00041364 and TB00041404 respectively).
?
As we are effectively using gears we can alter the ratios to get other thread pitches. The design allows a fairly large range of sizes (and one or two of the idler wheels could always be removed) but it is probably wise to stay fairly close to the sum of the drive wheels - 2 x 26 = 52.
Some combinations I came up with, given the U3 leadscrew pitch of 1mm:
?
18:36 = 0.5mm,?? 18:30 = 0.6mm,?? 21:30 = 0.7mm,?? 21:28 = 0.75mm,?? 24:30 = 0.8mm,?? 26:26 = 1.0mm,?? 30:24 = 1.25mm,?? 30:20 = 1.5mm
(could somebody check these are correct please? Thanks!)
?
Here is a scan of the template for the plate, and a ruler for scale. I'll add a PDF of all this into the Files section. Enjoy!
?
?
