Re: Problem making metric threads on a Jet GHB1340A lathe


<dougcasey@xxxxxxxxxxxxx> wrote in message
news:1128442432.088687.191700@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> This lathe is advertised as being able to cut english and metric
> threads. I have had no problems making english threads.
>
> According to the manual, in order to do metric threads you must do the
> following:
>
> Change gears - i.e. for a 1.5mm thread, select the two 48 teeth gears
> and run one on the 120 main gear and run the lower on the 127 gear
> (machine has 120/127 double main gear for converting to metric).
>
> Select the proper combination of levers - i.e L,T,A,C according to the
> chart printed on the front of the lathe.
>
> Disengage feed selector to the "O" posotion.
>
> Now, when cutting english you use the dial indicator and
> engage/disengage the half-nut lever. On metric, the manual says to
> engage the half-nut lever and leave it engaged during the whole process
> of cutting the thread.
>
> However, the manual fails to tell you how to stop the machine at the
> end of the cut so that you can reset the depth on the cross-feed. You
> cannot reverse the direction of lead screw travel as suggested by an
> authorized jet technician for three reasons. One, there's a large
> warning on the front of the machine telling you not to do this. Two,
> it doesn't work since you are temporarily changing gear teeth when
> moving the selector know, therefore you are off when reversing and
> coming back into the cut. Third, changing the lead screw direction
> goes from cutting right-handed to left-handed threads when changing
> directions.
>
> It seems that the procedure involves leaving all gears continually
> engaged. Then you cut the forward/reverse lever on the apron section
> to the neutral position to cut power to the travel. The head continues
> to rotate due to momentum until it comes to a rest and the magnetic
> latches release on the motor. Then you back out the cross feed, jog
> the machine backwards using the jog button, reset your depth and do
> another cut. This appears to work, but what do you do to stop the
> drift after stopping? If you need to thread a half inch piece that has
> a 1/4" relief and then projects into a 60 degree bevel towards the
> chuck, you risk crashing the tool into the part when trying to guess
> where the momentum will stop the carriage. This machine does not come
> with a brake and jogging the machine backwards immediately after
> disengaging the feed start lever does not work since the motor does not
> disengage the latches until it comes to a complete stop.
>
> I have read other scenarios about reversing with other machines, but
> this is designed to run 220V single phase. Any help would be
> appreciated.
>
> DC
>

Simply make a dry run with the threading tool clear of the workpiece. Cut
power when you think it is far enough along and see how far it coasts.
Adjust the switch point accordingly. If it coasts more than 1/4", reduce
the spindle rpm. You will always be safe because it will not coast as far
with the tool engaged in a cut.

Randy


.

Loading