My old Drummond lathe uses change wheels to cut threads but this arrangement cannot give enough reduction in feed to be used as an auto feed.
There are solutions to this problem - trying to make the reduction gear train longer, making very large compound gears, making a worm reduction gear.
All these solutions involve some rather time consuming gear cutting. To avoid this I constructed a feed based on a ratchet and pawl drive.
The advantages of a ratchet are
1) The ratchet only needs teeth about 1mm deep.
2) Each rotation of the pawl cam moves the ratchet one tooth - it is like having a driving gear with only one tooth.
3) The ratchet tooth shape is not critical.
The disadvantage of course is that the feed is not at a constant rate, but is in pulses. To minimise the pulsing I arranged two pawls, one to push the ratchet and one to pull.
The feed attachment that I built is via a double eccentric with the cams offset 90 degs. Each cam operates one pawl to push and another to pull a ratchet wheel giving a total of 4 actions per rev.
The ratchet wheel has 40 teeth giving a 10:1 reduction.
By over-lapping the action of the pawls the feed is still lumpy, but smoother than the 180 deg. single acting design.
The pic below shows all the parts. The rule in the pics gives an indication of size.
This pic shows a single set of pawls in greater detail.
This is the cam and ratchet. The ratchet was made by setting a tool sideways in the lathe and using the saddle as a shaping machine.
Here is the unit in place. For clarity the collar that is fitted to hold the eccentric to the mandrel or the change wheel train that would normally pick up the drive from the ratchet have been left off.
When in use, the pawls sometimes jumped off the side of the ratchet wheel. To prevent this I made two side plates.
Which are screwed to the ratchet
Ian.
There are solutions to this problem - trying to make the reduction gear train longer, making very large compound gears, making a worm reduction gear.
All these solutions involve some rather time consuming gear cutting. To avoid this I constructed a feed based on a ratchet and pawl drive.
The advantages of a ratchet are
1) The ratchet only needs teeth about 1mm deep.
2) Each rotation of the pawl cam moves the ratchet one tooth - it is like having a driving gear with only one tooth.
3) The ratchet tooth shape is not critical.
The disadvantage of course is that the feed is not at a constant rate, but is in pulses. To minimise the pulsing I arranged two pawls, one to push the ratchet and one to pull.
The feed attachment that I built is via a double eccentric with the cams offset 90 degs. Each cam operates one pawl to push and another to pull a ratchet wheel giving a total of 4 actions per rev.
The ratchet wheel has 40 teeth giving a 10:1 reduction.
By over-lapping the action of the pawls the feed is still lumpy, but smoother than the 180 deg. single acting design.
The pic below shows all the parts. The rule in the pics gives an indication of size.
This pic shows a single set of pawls in greater detail.
This is the cam and ratchet. The ratchet was made by setting a tool sideways in the lathe and using the saddle as a shaping machine.
Here is the unit in place. For clarity the collar that is fitted to hold the eccentric to the mandrel or the change wheel train that would normally pick up the drive from the ratchet have been left off.
When in use, the pawls sometimes jumped off the side of the ratchet wheel. To prevent this I made two side plates.
Which are screwed to the ratchet
Ian.
