Conversion LPI / Pitch / TPI

[mischmerz]

Howdy fellow trolls:

Lathe feed screws are measured in LPI. However - i am looking for a linear actuator and they usually advertise in TPI (threads per inch) or pitch (metric). Is there any way to convert LPI to TPI or pitch?

Thanks.

Michaela

[markrob]

Hi,

The lathe feedscrews are marked with the resulting LPI when used in the intended lathe (e.g Presto 6N). The actual TPI of the feedscrew depends on the ratios of the system. On the 6N it takes 8 turns of the platter to cause one turn of the feed screw. So to cut at 160 LPI, you use a 20 TPI screw.

Mark

[mischmerz]

Hi,

The lathe feedscrews are marked with the resulting LPI when used in the intended lathe (e.g Presto 6N). The actual TPI of the feedscrew depends on the ratios of the system. On the 6N it takes 8 turns of the platter to cause one turn of the feed screw. So to cut at 160 LPI, you use a 20 TPI screw.

Mark

Oh .. I see .. in order to calculate LPI one has to take the speed of the platter and it's relation to the rpm of the feed screw into the equation. Got it. So .. at 33 1/3 rpm the platter turns .. well .. 33 1/3 revolutions. Say my feed screw is being turned at 5 rpm and has a pitch of 1mm (0.039"). In order to move the carriage 1 inch I need about 25 revolutions of the feed screw - about 5 Minutes. In this time, the platter has turned 166 times. So I should have an LPI of 166 .. is that correct?

Michaela

[markrob]

Hi,

I think you have it.

Mark

[mischmerz]

Hi,

I think you have it.

Mark

Super. Thanks for your help.

m.

[PiercedEarsRec]

Re-igniting this thread as I am trying to also figure out the LPI of my feedscrew. Recently picked up an 8N off a friend but he was not sure of the LPI. The things I know, I can cut just over 14 minutes a side (thats probably not useful info), and the TPI of the feedscrew is 16. The above math totally confused me, but what Im gather is I need to count the rotations of the platter to the cutter head moving 1 inch of the feedscrew? and then plug that in somehow?

[displacedsnail]

Is it just over 14 minutes on a 12" disc? If so, that's probably around 136 lpi. I didn't do the math, I've just cut lots of discs with different feedscrews, LOL

[displacedsnail]

Actually, assuming the 8N has the same platter as the 6N, and using Mark's calculation and my assumption, a 136 lpi feedscrew would have a tpi of 17. And since you know yours is 16, it must be 128lpi. Right?