Re: [CAD_CAM_EDM_DRO] dovetails for cnc

I used to have an old collet lathe like that, I think it was a Rivett if I remember correctly. The dovetail is usually perfect on these as they were never used much. The tapered gib and one side of the female dovetail machined to match is probably the best route to go similar to dovetails on a B/port. I would use the same length to width ratio for the female dovetail as B/port designing the cariage. Normally a lathe carriage has a very wide mating surface with the bed for stability. Using a longer carriage on a narrower male dovetail should give the same effect. Straight dovetail ways and gib rely on a row of adjusting screws for tolerance. Adjusting these and getting them right is about impossible without one portion of the gib being tighter than the rest. Resulting in uneven wear. I would just machine the dovetail tapered on one side and tapered gib then grind the tapered gib to mate. With a good CNC mill and 60 degree cutter it should be fairly easy. Then lap the straight side of
the female dovetail. Something else I would consider is mounting the ballscrew possibly on backside of the machine as close to the dovetail as possible to prevent a binding effect when driven. The center would be ideal but a bear to keep clean. If you notice on a conventional engine lathe the carriage rides on a "V" and the other side just glides along on a flat, the leadscrew is directly under that V to prevent the carriage from "walking" on the ways. One design element I would incorporate in the carriage is machine the top flat with T-slots. Which would recieve the cross slide assembly and servo. That way you can remove the crosslide assembly and mount weldments or blocks to bore with a boring bar between centers. On a 20" lathebed you can bore very accurately about 10" deep. Excellent for deepholes.

Ron

skykotech <rick@...> wrote:
I am toying with cnc'ing a small hardinge dv-59 lathe. This lathe
does not have a longitudinal feedscrew so I will have to add a
leadscrew or ballscrew. The bed is a ~3.5" wide dovetail (60
degree) way. I want to machine a mating carriage to ride on this
dovetail. I have the correct carbide insert dovetail cutter on
order, but I am wondering what is the correct way to make the
dovetails and gib. Does it matter for cnc if I make the gib
straight instead of tapered? There is not a lot of information on
why gibs are tapered or how they compare to non-tapered gibs. Also,
I have been told that you need to taper one of the four dovetails if
you use a tapered gib. I took off the cross-slide on my 7x12 lathe
and it just uses a straight dovetail and straight gib. Of course
this may not be the highest quality thing I could have examined.

Opinions? I will keep all my cad files in case others want to do
something similar to a dv59. They can be found dirt cheap because
they are not really usefull to the home shop user as-is.

Thanks,

Rick



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