How do I machine a rod to a precise length?

[Arquibaldo]

Dear Machinists:

This is probably a stupid question, but I don't know the answer:

I own a lathe and think I know the basic operations, but I really
can't figure a simple way to finish a piece of rod (say 400mm long)
to its precise length: I can feed it through the spindle hole and
face the first end, then reverse and face the 2nd end, but I cannot
measure the precise length (I own a 600mm caliper) while it is chucked...
If I remove it to measure, then I'll have to locate the end precisely
to cut just a little more... Nothing like when turning to a precise
diameter where, after a cut, I can stop the lathe and measure with a
micrometer without disturbing the position of the work and the bit,
so my next cut can be very accurate...

If I mount the rod "between centers", then I can measure without removing,
but it's difficult to face, and I would need a special dead center on
the tailstock...

I could mount it on the mill, using 2 V-blocks, and mill the ends using
the sides of an end-mill, but I never heard of doing it this way, and I
fear the finish will be much worse than what facing on the lathe will
achieve...

Am I missing something?

Nelson

[Geof]

Face one end.

Grip this end in the chuck holding only a very short bit and support the far end in a steady rest.

Face other end and measure with the calipers; you should be able to get the caliper between the chucks jaws for the measuring.

[Arquibaldo]

Thanks a lot, Geof.

Very simple, as I suspected, but I couldn't figure it out by myself...

Nelson

[PixMan]

Alternately, you could make some kind of a plug for the back ond of your spindle with a centerspot facing the chuck, to act as a dead stop. Clean one end, flip it around and push it up against the hard stop you just made. Now you can clean off the opposite end, take it out for a measurement, and put it back in for the final cut without worry.

[AMCjeepCJ]

If you take the time to make a plug, then take a few more minutes to drill and tap it clear through. Then get a piece of threaded rod and make a second plug a few thousandths smaller than the bore of the through hole. On this second 'plug' you'll need a blind hole in the back of it for the threaded rod to bottom out in and then you have an adjustable solid stop that you can run all the way out to your jaws if needed. (Word of caution, careful about this hanging out the back of your lathe, it's a death trap if you accidentally start it up in high RPM's with the threaded rod hanging out unsupported!)

Oh yeah, double nut the rod hanging out the back or else it will loosen up and you're screwed accuracy wise~ We run ours like this for production parts ALL THE TIME...

[AMCjeepCJ]

PS

I always get metric screwed up, so I'm not sure if 400mm us 3" or 3'! JK but if your part is really long and you use a stop like we mentioned, take a few minutes to whip up a collar to slip over your stock so it doesn't droop in the spindle while you're trying to load the part against your solid stop. We had to use collars in our CNC Mori all the time on really long bars and occasionally on the manual stuff too~

[Arquibaldo]

Dear PixMan & AMCjeepCJ:

Thanks a lot for your suggestions - I like them a lot, specially because, more than a
single rod with a precise length, I'm interested in machining a few rods, all exactly the
same length.

Let me start working on the plug for the back of my spindle...

Nelson