Transmitting rotational motion along shaft

Hi all. I have a mechanical design puzzle which I hope the collective wisdom here can help me with. In an apparatus I'm building, I have a block on a leadscrew driven by a motor. On the block is mounted a mechanism with another leadscrew which needs to be driven at a rate directly proportional to the first leadscrew. The obvious way of doing this is to transmit rotational motion from the first lead screw to the second, but the question is how? How, as the block moves along the first leadscrew, can I get the rotational motion to the mechanism mounted on the block?
I would consider using something like a telescoping universal joint driven by gears fixed to the first leadscrew, but space retrictions prevent this.
Since the block itself will be larger than its linear movement (about 6" versus 5"), I think I can have a shaft, one part (6") of which is threaded as a leadscrew, and the other part (6") of which is splined. A splined bushing on the spline shaft, mounted to the block with an appropriate bearing (a thrust bearing?) and fastened to a gear should work, I think. I do want to minimize backlash, and I wonder how much the bushing would introduce.
Another option would be to use spur gear stock instead of a spline, and have a spur gear mounted to the block and meshing with and sliding along the length of spur gear stock. I don't think, however, that spur gears are designed to to this, and I do think this will introduce even more backlash than the spline/splined bushing method.
Comments and especially suggestions are welcome!
THeo

In article <ad6737ea.0406040838.2a0e63a4
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Inexpensive lathes often use a single shaft for both screw cutting and normal cutting feeds. Where higher class machines use two separate shafts - one threaded for screw cutting, and another keyed or splined for normal feeding - on cheaper machines the single shaft is both threaded and keyed its entire length. Seems like something like this might work for you.
Ned Simmons

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What he said
Brian W

snipped-for-privacy@uoguelph.ca (Theo Hopman) wrote in
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You may wish to look into a Saginaw shaft, or half-saginaw, half ballscrew. The saginaw shaft is similar to a splined shaft, but uses a roller nut instead of sliding for much higher accuracies and reduced friction.

Hi Theo:
If I understand your situation correctly, it appears that you might need a third shaft: a spline shaft. This shaft can be driven by the first leadscrew via spur gears or (even better) by a timing belt or chain. On the carriage, the spline bushing can drive the second screw with another gear or belt. Note that the spline bushing will need to be mounted between bearings that handle thrust (like angular-contact bearings).
Hope this helps.
Don
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