Project: Lathe Banjo
Wednesday, May 31 2017
Bigger lathes have a gearbox with levers that let the operator select the ratio of turns of the spindle to turns of the lead screw. This is used when threading to change the pitch of the threads or how many threads will fit in an inch for US threads. Mini-lathes like mine instead have a handful of gears and an adjustable bracket to hold the gears in the proper position. Gears are also used to power carriage movement.
The bracket that holds the gears is often called a lathe banjo – probably because the bracket looks a bit like a banjo. That is not the case with the mini-lathe.
That black bit of stamped metal in the picture is the change gear bracket for the mini lathe. It is a pain to adjust. You need to mount the gears to set the appropriate spacing, but with the gears mounted the adjustment screws are hard to reach.
The aluminum piece is the roughed out shape of the replacement banjo I’m making. I’ve just bored the hole that will go over the end of the bracket that holds the left side of the lead screw.
It fits. Next will be to make a slit along the end of the banjo and drill and tap a hole for the screw that will clamp the banjo to the bracket. Clamping the banjo to the bracket lets me remove the hex block and stud that the banjo is leaning against in the picture.
Thursday, Jun 1 2017
I used a slitting saw on the mill to cut a slit in the banjo. I actually made two cuts with the saw to get a wider opening. I still need to drill a clearance hole on the part of the banjo above the slit and a threaded hole on the part below the slit. That will let me use a screw clamp the banjo to the lead screw bracket in the desired position.
I want to be able to adjust the position of change gears on the banjo and lock them in place from the front. To do that I made a sleeve out of some scrap steel that will hold the change gears and their keyed bushing such that they can rotate freely. A screw will clamp the sleeve to the banjo. I’ve some slots milled on the back of the banjo to hold the screw head. A washer and nut accessed from the front will hold the gears on the sleeve without binding while locking the sleeve to the banjo.
I need to either buy or make some 6 mm hardware before I go much further. I don’t have any on hand in the needed styles and lengths.
Friday, Jun 2 2017
I wanted a screw with a square head to fit in a slot I’d milled on the back of the banjo. Instead of wasting time looking for exactly what I wanted I found some stock that was the right size and made the screw that will hold the sleeve with gears. Aluminum wasn’t my first choice in materials, but it’s what I had in the appropriate size.
After drilling and taping the hole for the clamping screw I test fit the banjo to the lathe with gears. Turns out I had a 6 mm screw in my scrap parts that was close enough to what I wanted. It was easy to fit and adjust the gears. However, the nose of the banjo is too thick. If I was using a small enough gear I’d not be able to rotate the banjo high enough to engage the stud gear.
I milled about 4 mm off the back of the banjo nose and re-cut the slot that holds the square end of the gear/sleeve clamping screw. I had to reduce the thickness of the head of the clamping screw a bit, too. Finally I spent some time with the banjo and files breaking all sharp edges, rounding the nose, and removing most of the corners.
The banjo is ready to use. I’ve removed the old bracket clamping stud and mounted the the feed gears on thwe banjo and lead screw. I’ll put the old parts in a zip-lock bag and store it with original tool holder – something else I no longer use.
I do need a larger washer. It is possible for a gear to slide off the bushing and over the washer. I’m using a second m8 washer until I find or make one of the desired size.
Friday, Jun 9 2017
I found a bit of 1/8" steel in my scrap pile. It is thicker than I wanted, but it will do. I drilled a 6 mm hole and then turned it around the hole to make a washer. I didn’t bother taking any pictures.