Imperial thread on a metric lathe

[Dave Harding 1Participant @daveharding1]

I'm sure this question will of been asked before but bare with me I am brand new at this. I want to cut a imperial thread in 7/8"x14. My lathe is metric.

On the front of the Lathe is a chart showing metric and imperial gear wheel set ups. Do I use the metric T numbers and ignore the imperial ones and if so how do I set it up to cut 7/8"x14 TPI.

[Dave Harding 1Participant @daveharding1]

[JasonBModerator @jasonb]

Use the imperial chart and place the gears as shown to get 14tpi (assuming 14 is on the chart) You will most likely have to keep the half nuts engaged and run the spindle in reverse to back out teh carrage.

It would help if you said what the lathe is and also a photo of the chart would help us guide you

 

Edit looks like you were reading my mind, from your photos you need to use the set of table on the left and the lower one n/1" and arrange the gears shown under the 14 (tpi) If you are not sure how to arrange the gears ask again and I'll sketch it out and show how the various ratios give 14tpi off your metric 2mm pitch lead screw

Edited By JasonB on 30/09/2014 18:53:30

Edited By JasonB on 30/09/2014 19:05:42

[Dave Harding 1Participant @daveharding1]

Hi Jason I have replaced the chart in my album the original one was from my lathe manual it was not the same as the one on the lathe.

The pictures in the album are of my lathe and the chart on the front of the lathe.

How do I set the lathe up to cut 7/8"x14 TPI. I'm totally confused as to how to go about the whole thing.

[JasonBModerator @jasonb]

OK you need to use the lower of the two charts headed TPI.

If you remove the cover from the end of the lathe you and look at it from the front you will see the edge of the various gears that run between the spindle and the end of the lead screw, these will be arranged to give a fine feed rate so you need to change them to give 14tpi.

Looking at the chart under 14tpi there are three rows of gear teeth numbers , the top row is 66, H this means you need a 66T gear and a spacer on the banjo side, the second row shows you need a 33T and 60T on that peg with the 60T towards the banjo and the last line shows you need a spacer and 80T gear next to the banjo.

So looking at the side of the new gear train it should look like this

As you change the gears you will have to loosen the various pegs and slide them along so the different gears mesh, put a strip of copy paper between the teeth and this will help set the right gap. You may well also need to pivot teh banjo forwards to do this and onve all the gears are set on the banjo pivot it back and mesh th etop 66T gear with the 40T of the spindle.

Hope that helps ask if not but it will have to wait until morning. I'll explain the maths then.

J

[“Bill Hancox”Participant @billhancox]

Posted by Dave Harding 1 on 30/09/2014 18:44:25:

I'm sure this question will of been asked before but bare with me I am brand new at this. I want to cut a imperial thread in 7/8"x14. My lathe is metric.

Dave

Appears you are fixing to machine something relative to the common reloading thread pitch. A few years ago I picked up a USA 7/8X14 tap that provides a nice snug fit for most reloading tools such as powder measure stands, although a 7/8X14 nut welded to a bracket works every bit as good (although not pretty). I turn all 7/8X14 external threads on my Imperial Myford Speed 10 (very light cuts) using a Pratt-Whitney WWII vintage thread chaser. You no doubt already know that there is quite a variation in the tolerances of reloading tools amongst manufacturers so some items will fit well while others will be quite sloppy. Good luck with your project.

Bill

[Clive HartlandParticipant @clivehartland94829]

You can buy 7/8 x 14 studding which I have used quite happily to make reloading dies and swaging tools. As Bill says there is quiet a variation in swaging and reloading dies from different manufactures some tight in the thread and some really sloppy but matter snot if the lock nut is tightened.

Clive

[JasonBModerator @jasonb]

Right, just to explain the maths down the right hand side

The 40T gear on the spindle drives the 66T gear, this 66T gear then drives the 33T which is keyed to the 60T which is driving the 80T on the leadscrew. If you divide the driver by the driven you get three ratios

40 divided by 66 = 0.606

66 divided by 33 = 2.000

60 divided by 80 = 0.750

Next multiply these three ratios

0.606 x 2.000 x 0.750 = 0.909turns

The 0.909 is how much the lead screw will revolve for one turn of the spindle, as the spindle has a pitch of 2mm you get

0.909 x 2 = 1.818mm per rev which is the closest you can get to 14tpi with the gears available, to check divide 1" in metric by 14

25.4 / 14 = 1.814mm so you can see that you are quite close and unless it was a very long thread the erorr is hardly worth bothering about.

J

[Johnboy25Participant @johnboy25]

Hi.. I suffer from a type of dyslexia which makes hard work of reading and understanding things not to mention writing emails and the like! Recently I've been struggling with understanding the gear ratios to get the right combination of stud gears on my metric gearbox and lead screw. I have researched various books and googled the subject. But by reading the example that JasonB has rendered up I'm please to say that the pennies dropped! Just by seeing this example has helped tremendously. It just goes to show how useful these forum are – thanks Jason for this contribution. It's help me greatly.

John

[Dave Harding 1Participant @daveharding1]

Thanks for that Jason I set the gears using your sketch. I managed to cut a very reasonable 7/8"x14 Tpi thread thanks very much. The maths example helped as well. That's the first time I have ever cut a thread I am well chuffed.

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