Myford super 7 CNC conversion

[hardwic]

I recently bought a Myford super 7 which I would like to have CNC functionality on.
I do a lot of one off manual stuff so I don't want to have the machine CNC only.

I was thinking of stepping out the x&y crank handles and having a timing belt connected to steppers. The idea being the steppers can run open circuit when using manual mode.
Also I am not sure to CNC the RH lead screw or the apron crank. I know the crank is course but am not sure what resolution you would get.

I also have an Emco compact 5 which would be easier to CNC but then I would be tied to both machines and also only be able to CNC small parts.

I have respect for the both lathe and don't want to bastardies them so a subtle conversion is required.


Any ideas or similar conversions?

[hardwic]

OK so I answer my own post, how sad

I have CNC'd the Myford and turned a few test parts.
I made a mounting plat to sit above the slide screw. I inserted a timing belt pully between the slide crank. A timing belt connect to a stepper mounted on the plate.
I used some old timing pulleys and belts recovered from scrap.
I think the pitch of the slide screw is 2mm. This is my X axis

For the lead screw, I made a mounting bracket to fit on the drive chain arm in the gear housing. I coupled the motor with the lead screw using standard Myford gears 20:60.
I think the pitch of the lead screw is 3.14 (8tpi). This is my Z axis.

I am quiet happy with Z axis but the X axis is not calibrated correctly.

I seem to have a 0.3mm error in about a 10mm movement on the X axis.

I think my problem lies with the timing pulleys I used. I don't know their correct ratio so I measured them with a digital vernier and used that as the ratio (11.4:21.4)
I know the ratio sounds wacky.
Is is ok to measure ratio on time gears like this or can you simply count the teeth as you would on normal gears?
Maybe I should just dump the "scrapped" pulleys and buy known items.


Will post some pic's some time.

[Bubba]

You can use the tooth count just like you would on normal gears or chain and sprocket.
The pitch diameter of a toothed belt drive is actually slightly larger than the OD of the pulley. This is where the error comes in. If your getting the necessary torque to move the axis and speeds are acceptable, the pulleys should be fine.

[hardwic]

Bubba, thanks for the reply.

Turns out I had 2 problems. I stripped down the cross screw drive to get to the pulleys to count the teeth and found something I had not notice. I thought what I had was a perfect set of pulleys but the large one was not 100% matched to the belt. If I wrapped the belt around the pull, the sync was out at the ends .
So the pulley is actually fractionally different spec resulting in tooth skip every few turns.
I replaced the pulleys with more closely inspected ones and this time counted the teeth to get the correct ratio. Now it's running spot on.

Think I will replace these once I find a good source of small timing belts in S.A. (not easy to find)

[ScottC]

Hi, any chance you could post some pics of your myford conversion. I have a super 7 ive been contemplating converting too...

Scott