Thread: imperial threading

Evening All I have a Hafco AL960b lathe, has been used for turning shafts and bushes and comes with a imperial lead screw 8tpi the thread
chasing dial has32 teeth so 1 complete dial turn equals 4 inches
Cutting Metric threads I leave the split nuts engaged and reverse the motor and have no problems.
I want to cut a 1 1/2 x 8TPI t her in lies my problem every thing is set up correctly. I should be able to engage the thread chasing dial at any graduation . I start the lathe engage the wait till the thread chasing dial gauge come around to any number and engage the half nuts first cut is checked with thread gauge and is good 8 tpi repeat the procedure and the 2nd cut is off .3rd cut is off.
Note the thread chasing dial has 1-8 numbers . have tried all 1 2 3 4 as well as2 4 6 8 to no availe
could leave the half nuts engaged an do it that way but I have a thread chasing dial and I want to use it any help would be most appreciated
PS in Perth W.A Checked with HARE & FORBES Belmont suggested leaving half nuts engaged no use to me

What's your leadscrew pitch?
If it wa 8TPI, then you can only engage it on the same number, since it might turn at the same rate as your spindle. If it was 4TPI running at twice spindle speed, you could engage at numbers opposite each other and so on. Conversly. if you were turning 16TPI on a 8TPI leadscrew same thing and on a 4TPI one you could engage at any number 90 deg apart. See how it goes?
It depends on your leadscrew pitch. It gets a bit more involved if you have say a 10TPI leadscrew and your doing threads that are not divisible into 10.
There are tables to show all that. Google is your friend ...

If you are turning 8tpi you should be able to engage the half nuts anywhere as the spindle and the leadscrew are turning at the same rate. If the first thread is correct then all the following threads should be, which makes me think that it is not quite correct. I haven't got the same lathe but if you have been turning metric threads there was a 120:127 tooth gear in the train. When turning imperial threads this is unused and the 120t gear is just an idler. I would be checking that the gears on the headstock are as per the diagram on the threading chart first. Leaving it in would give you an 8% error on your pitch which may be difficult to see with just one pass(it would be around 0.25mm)

Michael

Many thanks for the reply MIchael yes the gear system has a 120/127 gear the threading chart shows a 40 tooth driving the 127 then that driving another 40 into a enclosed gear box . The gearbox levers are shown as per chart to be in position B -1 have double checked against the chart and all is ok I can not think what I am doing wrong again thanks for all your replies.

Oops, Michael is of course right. It hadn't even occurred to me that you might have a metric lead screw.... Apart from the fact that I was quit wrong with my understanding of the thread indicator positioning.... Sorry.

Cheers,
Joe

There goes my fat fingers again with that like button....

Have you actually checked the leadscrew pitch? As said since the TPI you are cutting is the same as the leadscrew it is impossible for it not to be correct, so i would say there is either something wrong with your gearing or the leadscrew is not 8tpi.

Ew

hi

3mm is pretty close to 1/8" , you might have to measure the leadscrew pitch and double check if its metric ? Mike

To all those who replied to my post thanking you very much It was me having a seniors moment ,Michael gave me the clue
had been cutting metric thread a couple of days ago ,and the bottom gear was still engaged on the 120 tooth gear not the 127 tooth gear I looked at it several times but it did not register in my ageing brain. some times you need different input to take the brain out of its mind set

Originally Posted by confused

I looked at it several times but it did not register in my ageing brain. some times you need different input to take the brain out of its mind set

nothing unusual for me to do same

glad all is ok....I didnt know so was unable to help...if it turns it works..if it doesnt turn its broken...its about as much as of a lathe I know...but I am learning