Silly 4th Axis Question

[IHateMayonnaise]

Howdy Folks,

I've got an RF-25 clone working as a funcitonal CNC, and at some point I have dreams of having a 4th axis. Now.... I have a number of 5c spin index fixtures, including the Master-Grind from Suburban Tool as well as the HV-4 from Hardinge. I use neither of these, and I recently restored them both and they are in great shape. What are the chances that I can turn one of these into a 4th axis? It's easy enough to get a 5c 3/4-jaw chuck of decent quality, and adding a direct-drive stepper on the back end doesn't seem all that unreasonable. Direct driving would mean that I would lose linearity in torque, but WHO CARES with a 4th axis? And with microstepping, I find it hard to believe that I would not have the resolution that would satisfy my (hobby) requirements. I could always add a harmonic-drive gearbox etc, but for my applications I think this would be overkill, as direct driving a stepper would minimize this anyway (with the appropriate coupler etc).

I don't usually ask these types of questions because usually I've found that someone has already asked them, but I can't find much on the subject, which leads me to believe this isn't such a great idea in and of itself. Maybe this will help someone in the future.

Thoughts?

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[cncuser1]

If you do direct drive then the holding torque on the stepper would limit the type of work you can do. The gearboxes multiply the holding torque so you can do machining away from the axis of rotation without fear of loosing steps. Most of the time the slow RPMs due to the gear reduction aren't a deterrent to this implementation. Backlash is always an issue with gear reduction on 4th axis.

doing a word search in these forums can yield a wealth of information quickly.

[IHateMayonnaise]

If you do direct drive then the holding torque on the stepper would limit the type of work you can do. The gearboxes multiply the holding torque so you can do machining away from the axis of rotation without fear of loosing steps. Most of the time the slow RPMs due to the gear reduction aren't a deterrent to this implementation. Backlash is always an issue with gear reduction on 4th axis.

doing a word search in these forums can yield a wealth of information quickly.

Thank you for your response. I was intending my question to be more focused towards using these index fixtures that I have laying around. It's a lot easier for me to get an oversized motor (1600+ oz in) with a holding power that is, and you'll have to take my word on this, more than sufficient for my purposes, than it is to do gear/belt.

[CL_MotoTech]

Used harmonic drives aren't that expensive, but if you are just doing engraving or something low force you probably don't need it.

[cncuser1]

Do these units have bearings or bushings in the spindle? I'd prefer bearings if I knew my work had a lot of small back and forth movements.

The size of stock or workpiece would make me prefer a face type of work holding compared to a collet style work holding. How are these unit configured?