Reduce your lathe spindle speed

Make a lathe speed reduction gearbox 

record tachometer

Lathe too fast?

Drop the spindle speed by 50%

Budget Chinese lathes can supposedly cut threads, and you get gears to do this, but in reality the lowest speed is too high for this to be done safely.

You might want the lathe to go slower for big diameter and off centre jobs.

So how to get the speed down, and importantly not lose torque?

Here's how I did it - and it cost next to nothing.

What you need

One worn out or blown up four stroke stationary or lawn mower engine.

Dismantle it.  Keep the crankshaft, timing gears/camshaft, and ball races.

You will also need two more ball races.

What to do with the bits

Use the timing gears to make a two to one reduction gearbox and lower the final drive ratio of the lathe spindle.

The process

Cut the crankshaft near the timing side web - keep the half with the small gear (see below).

Machine the web away to accept one of the ball races, and then reduce the main shaft to a suitable diameter to fit a "V" belt drive pulley.  

The completed shaft should look similar to above.  Don't forget to machine stand offs for the bearing inners and outers.   Any existing unwanted threads as above/below will be cut off later.

With the main and a smaller bearing fitted it looks like below.  This is the input shaft.

The camshaft is generally a cast iron unit running on a steel centre shaft - as on the right.

This is the weakest part of the timing gear setup.

It needs serious modification to be useful.

Discard the hardened steel centre shaft.

Cut off the cam lobe section so that only the timing gear is left.

Face up both sides of the gear.  

It should then look similar to  below.


Bore out the centre and turn up a shaft to mate with one side of the gear, with provision for two supporting ball races.

I drilled and tapped the gear, then attached it with four countersunk bolts - Loctited in.

See below - this is the output shaft.    


You now have two shafts with strong gears supported in ball races, capable of operating on the same plane, and able to be fitted with pulleys. 

Find a place to locate the gearbox and build it up to suit. 

You can see the simple gearbox taking shape below.

Don't mesh the gears too tightly or they will increase drag and noise.

I made press fit bearing retainers rather than machine into the gearbox side plates.  This allows easy adjustment/alignment of clearances and avoids binding issues.

The bearing retainer can be a section of suitable sized heavy tubing with a couple of locating tabs bronzed on (as above), or it can be machined out of a piece of thick plate.

Either way you must allow clearance for the attaching bolt heads.  If space is not an issue the retainers are best mounted outside of the gearbox (in the usual fashion).

If the bearings are the same size it also makes life easier, but I just went with what I had. 

Some form of cross bracing is required to keep the side plates and shafts correctly aligned.

Fit it all together, trim the drive shafts to length and fit the pulleys.

Below is the gearbox greased and ready to be installed (the image is close to actual size).   

The drive shafts have been staggered to fit the available space and allow for belt clearance.  Input is on the left, output is on the right.  The pulleys were machined up as well.

The gearbox can be seen mounted below the AC motor - bolted in place.

Space is very tight in this position, but the gearbox could be made bigger and positioned lower if longer drive belts were used - it depends on space and the style and type of bench.

As speeds are relatively low, the gearbox and bearings can be left open.  Shield the box from cuttings, and lubricate it in the same manner as the change gears. 

The setup is operational and nearing completion below - testing with a temporary spring tensioner.

Instead of driving the spindle from the idler (intermediate) pulley as per the original setup (see yellow arrows), the belt now goes to the reduction gearbox and then by an additional belt to the spindle - using the two outboard pulleys.  

Finished - the gearbox to spindle belt roller tensioner now has a screw adjuster, and the bottom of the drive enclosure door has been cut away to clear the belts when closed. 

The lowest spindle speed is now 73 RPM.  Two other speeds of 116 and 171 RPM are also available with the gear box in use.

In theory the reduction with equal sized pulleys should be 50% - down to 62 RPM.

However, differences in drive belt ride height will vary the speed either way.   In my case it went down by 42% - due to the adverse effect of a deep section belt I am currently using to drive the spindle.

Seen cutting a short 40 TPI thread below - no problem.

cutting threads on a lathe

The only variation from standard use and functionality is that the motor direction switch must be reversed to allow for the effect of the gear box.

It's a simple process to change belts and drop the speed.

If another (intermediate) gear from a similar engine was put between the two gears/drive shafts, the increased clearance would allow the input pulley size to be increased and drop the speed even further.  It would also counter the directional change by the gearbox.

This is a good project to test out your engineering, fabrication and turning skills.

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