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Metric threads on a Jet BD-920N issue

4thgen

Plastic
Joined
Dec 28, 2022
I have a jet BD-920N lathe that has change gears to cut metric threads. This is my first time trying to single point cut metric threads. I changed the gears according to the chart provided so I could cut a 1mm pitch. I am getting some heavy copper plating done and need to make a plug with metric threads to keep the internal thread free of plating.
The issue I'm having is that it seems to be cutting between pitches. When I test the threads I cut it starts to bind, even if I make the threads more loose than I want.
I just wanted to know if anybody also had this issue and if there is a fix for it or if it just has to do with being a cheaper lathe.
-Thanks in advance!
 
Sorry if these sound like basic questions, but you never know what someone knows. What specific pitch are you trying to cut? What tool/insert are you using to cut them? Are you using the half-nut lever and not the power feed? Lastly, you're not disengaging the half-nuts until you're done and not in between passes right?
 
Confirming that there is actually a pitch error should be your first step.

Pitch error can be easily measured with a vernier - set your vernier to 10mm and it should line up nicely over 10 threads. 20mm over 20 threads etc.

If you have a progressive pitch error it's very easy to see it with this method.

If there is actually a pitch error present, use this very useful website to double check your geartrain: https://bilar.co.uk/cgi-bin/change-gear-calculator.pl
 
Even a lower budget lathe should not have an issue like that due to any sort of inherent accuracy issues. Something else is probably going on. I would follow Gregor's advice and double check your gearing. An easy way to check pitch is to set a dial indicator registering against the tool post and rotate the chuck one complete revolution by hand with the half nut engaged, then checking how far the carriage has moved. Be sure to get the slack out of the train before counting that revolution. Use a pointer of some sort that is set to a feature on the chuck like the edge of a jaw to make your one revolution accurately.
 
Thanks for all the quick replies! I'm trying to cut a 1mm pitch and it seem to be between 1mm and .9mm pitch. Taking another look at the gear diagrams I think I just put the gear on the wrong side of the 127/120 combo gear. I will adjust and post my result.
 
Been there done that. Many years ago I was attempting th turn a 1 7/16-12 internal thread. I had the combo gears backward and was actually turning an 11 1/2 tpi thread

I had no idea what a thread like that was for until I needed to make some fittings for a garden hose. It turns out 11 1/2 tpi is standard for garden hoses
 
Thanks again for the replies, I just had the change gear on the wrong side of the combo gear. I changed that and ran into a fitment issue that the 127 combo gear would bottom out on the spacer below it. I had room so I just took the spacer OD down enough to make the clearance. Got the plugs I needed made now. Its nice to finally be able to cut metric now.
 
127 is the metric/imperial conversion gear. 2 x 127 = 254. 1"=25.4mm

Instead of multiplying by 2 and dividing by 10, you can reach your final answer a little quicker by noting that 127/5 = 25.4

As change gears usually have an integer number of teeth, thinking in fractions (an integer divided by another integer) rather than decimals makes the maths closer to the mechanics.
 
Instead of multiplying by 2 and dividing by 10, you can reach your final answer a little quicker by noting that 127/5 = 25.4

As change gears usually have an integer number of teeth, thinking in fractions (an integer divided by another integer) rather than decimals makes the maths closer to the mechanics.
true, but I'm attempting to point out as simply as I can how the gears work as mechanical calculator.
 
Did you turn the raised diameter and then mill the straight knurls or are those cut with a straight knurl cutter?
Those were cut with a straight knurl. I had to make a special holder which put the knurling roll on center with the toolpost to prevent rotation of the toolpost. It required way too much pressure. I will NEVER do that again.
 








 
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