Improving a lathe spindle head.

[metric_taper]

I am not a fan of screw on chucks, I much prefer a taper fitting like my old Colchester Student had.

My other lathe has a D1-8 spindle nose. I like that I can power turn in both directions.

I had to build a bridge crane as the 12" chucks are now getting beyond my strength limit of back damage.

I also like keyless chucks, but they have this same issue.

I hope you document your CNC lathe conversion. I've thought of the same, I'm curious about methods of attaching a shaft encoder to the spindle. Keeping it out of the way for long stock though the spindle hole appears to need some sort of cog belt and timing gears. Then a quality encoder with ball bearing and such is needed, and there must be wear to worry about in all this.

[tonyfoale]

The spindle thread was a real bastard at 50mm diameter, 8TPI. I recently found 2 backplates on eBay that fit this thread. They were slightly too tight of a thread fit, so I got them centered in a 4 jaw, and retimed the thread so I could use the lathes power feed with an internal threading bar. I manually rotated this assembly to scrape the interference fit. So much for mass manufacturing with interchangeable parts.

I just checked. My fitted imperial spindle is 2.25" with 0.125" pitch thread, the metric one has a 62 mm mounting diam and a thread diameter of 60 mm with 3 mm pitch.

[Tonyg]

You will not have a problem with the steel and concrete on the lathe, the thermal coefficients are so close and in the order of 10 to the power of -6/degree C. This combination is used on a lot of the very large CNC machines that I install, build and maintain.
It is the closeness of these Thermal Coefficients that makes it such a good match for reinforced concrete.

[tonyfoale]

I hope you document your CNC lathe conversion. I've thought of the same, I'm curious about methods of attaching a shaft encoder to the spindle. Keeping it out of the way for long stock though the spindle hole appears to need some sort of cog belt and timing gears. Then a quality encoder with ball bearing and such is needed, and there must be wear to worry about in all this.

I sometimes need to fit an encoder for a cam measuring setup but that method does not allow long work pieces.

Click image for full size

I am not convinced that you need a small resolution encoder for most lathe operations. Screw cutting is probably the most severe test. The Mach3 CNC control software only accepts one pulse per spindle rev. That sounds too coarse to me, although when screw cutting the tool load over a revolution should be close to constant and so there should not be much speed variation over a revolution. It would be very easy to mount a pickup to get pulses from the spindle gear.



If Mach3 gets away with a single pulse per rev then the number of pulses from the gear should be pretty good. Then the question is finding compatible software.
My mill has servos with linear encoders but if I do the lathe then for simplicity it will use open loop steppers which limits software choices. There is Mach3 and 4 (I do not know whether M4 accepts an arbitrary number of pulses), then LinuxCNC but I have no idea what that accepts. The Centronics Acorn looks like a good controller and software option but once Centronics have you hooked they seem to pile on the charges for the extras needed to make it useful. It is hard to know what the final cost will be.

[tonyfoale]

You will not have a problem with the steel and concrete on the lathe, the thermal coefficients are so close and in the order of 10 to the power of -6/degree C. This combination is used on a lot of the very large CNC machines that I install, build and maintain.
It is the closeness of these Thermal Coefficients that makes it such a good match for reinforced concrete.

Thanks for that confirmation. I had no real worries about it, I have used the technique before to stabilise various measuring devices but never before in a machine tool. I did not know that it was common practice.

[metric_taper]

I am not convinced that you need a small resolution encoder for most lathe operations. Screw cutting is probably the most severe test.

Tony, what I would like to do, is put an encoder on the spindle, and one on the main feed screw, and fix the timing as the thread dial has shown to be wrong either for metric or Imperial threads. What I do now, is keep the half nut engaged, and reverse the lathe. This gets tiresome. So I'm thinking I can set up an Arduino with the two encoder inputs, and have it give me an engage light when they are timed for the start of a thread. I have not thought this through with all the gearing and such, but this seems like it would enable reliable half nut engagement, so I don't start new threads in the part.

[tonyfoale]

Tony, what I would like to do, is put an encoder on the spindle, and one on the main feed screw, and fix the timing as the thread dial has shown to be wrong either for metric or Imperial threads. What I do now, is keep the half nut engaged, and reverse the lathe. This gets tiresome. So I'm thinking I can set up an Arduino with the two encoder inputs, and have it give me an engage light when they are timed for the start of a thread. I have not thought this through with all the gearing and such, but this seems like it would enable reliable half nut engagement, so I don't start new threads in the part.

The problem only arises when you want to cut metric threads with an imperial lead screw or vice versa. Remember that you cannot engage at will, the half nut will only engage when the screw is lined up. It should be feasible but depending on the thread pitch and the lead screw pitch you may have to wait some time before everything is back in sync. It will also depend on just where the saddle is when you want to engage. You would have to withdraw the saddle to a stop. If you are serious about making mixed system threading easier I think that you should look into motorising the lead screw (changing to a ball screw is preferable but not essential).
There are a ton of ELS (Electronic Lead Screw) projects on the net. Some are excessively complex but there are some quite practical examples.
If and when I do mine, I'll share it here with Arduino code if that is how I do it.