Metric Threads on Generic 12x 36 lathe

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oldengineguy

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Recently bought a King 1236 lathe , probably the same as Grizzly or Jet or Precision Machine. Probably sold under many different names in Europe and Australia too. In setting up and trying out my new toy I was going through the different Metric and Imperial threads to check for accuracy. All seemed well until I got to
Metric pitch 1.5. With gears and change levers set as shown on chart , cut a test thread, and found it was not correct ! Tried .75 and 3 (change lever in 6 position as specified,the same position as 1.5) and these were wrong too. Tried a few other threads with different chage gears and lever positions and found them to be O.K. Checked and double checked my settings and found them to be as called out. I know it's not the right way to describe the problem but it cut less threads per length than it should. I have tried Imperial threads with lever in 6 position 6 12 24 and they are correct. Sorry for the rambling description of my problem , has anyone else found they can't cut 1.5 pitch on this generic Chinese lathe ? Thanks Colin
 

Scott_M

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Hi Colin
It sounds like you did not change the idler gear from 40 to a 32 tooth.
On my 12x36 "Import" lathe to cut that range of threads requires a change of one of the 40 tooth gears to 36 tooth. See pic of my thread/gear chart.

Here you can see that the lower 40 tooth gets changed to a 32 this rides on the 120. You can see the imperial chart on the right and all thread sizes use two 40 tooth gears. And to cut the full range of metric threads you need to swap some gears ( 4 different ways )
This is what happens when the lead screw is imperial.
Also, and very important, you must leave the half nut lever engaged from start to finish. back out , stop motor , reverse motor , back up and take another pass. If you disengage the half nut it will loose position.

Scott
 

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oldengineguy

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Thanks Scott. On this version to cut Metric threads the lower gear (input to quick change box ) is replaced with a 60 tooth from the 40 used for Imperial and the upper (drive) gear is replaced with gears 26,27,35,45 or50 tooth from the40 tooth used for Imperial, depending on pitch used in conjunction with quick change levers to get desired pitch. For 1.5 the combination is 45 driver 60 driven and box levers set at B 6. This has been done and re checked but will not give desired pitch, I wish it were as simple as the wrong gear. I have contacted the importer ,King Canada they promised to get me an answer , but have not yet. Only 1 business day so far but they told me "a couple of hours" As stated in my first post ,the other combinations give the pitch called for - just not 1.5 Sorry, can't post a picture as you have, of the gear change chart. Thanks Colin
 

oldengineguy

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Meant to say in last post that I am not disengaging the half nuts but stopping, backing out and reversing as you state Thanks again Colin
 

Scott_M

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Hi Colin
Sorry ,you do seem to understand :) I hope I did not come off as condescending :(
Do you know the pitch of the leadscrew ? and the tooth count of all the gears ? We can figure this out, it is just a matter of ratios.
My leadscrew is 8tpi. And the you can see my placards showing the gear changes.
It could be as simple as your placard is wrong and calling out the wrong gear combinations. You need the carriage to move faster per rev , correct ?

Scott
 

Jasonb

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You also need to make sure the middle gears are not just being used as idlers, upper gear needs to drive the 91T and lower gear needs to be driven by the 86T. Look at the image on the chart rather than read "ANY"

Page 27 of a similar Grizzly https://cdn0.grizzly.com/manuals/g4003_m.pdf

Without a 127T gear in the train all the metric pitches will have to be an approximation so you will get 1.503mm pitch rather than an exact 1.5mm but unless it's a long thread engagement unlikely to cause problems.

For a sanity check that gear train gives a ratio of approx 0.71. so taking the 12TPI that is in position B6 when setup for imperial you will get 0.71 x 12/1 x 25.4 = 1.503mm pitch
 
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oldengineguy

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Scott: I do not think you are codecending at all; I thank you for your help and concern. A post by Jasonb, post #6 may explain what I am doing wrong. The manual that comes with this machine is very poor in comparison with the Grizzly that Jasonb has posted a link to. I read my book to indicate using "any" gear between driver and driven instead of driver to 86 then 91 to driven, Can,t wait for sun-up so I can get into the shop and try this! Thankyou ,will report, Colin
 

Scott_M

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Great !!
I think that will put you back on the path :)

Good luck with your threading

Scott
 

Bamban

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I have a 1236 Taiwanese lathe, to thread 1.5, just flip the lower 40 T. Loosen the 120/127 combo to engage the 120T, adjust lash.

Levers in C-1 positions.

We just got through threading a barrel tenon for a Howa 1500 with this set up.
 

Jasonb

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Colin's lathe does not have a 120/127 pair like yours and Scotts
 

Guzphile

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Hi Colin, I found on my Chinese lathe that some of the change gear setups were simply wrong in the supplied manual. Now I just calculate the ratios on a spreadsheet to check. You can also mess around with combinations of the change gears you have on hand, that give the closest ratio and still fits on the change gear banjo. My lathe uses a 63 tooth gear as an approximation of the 127 tooth gear normally needed for an exact translation from an imperial leadscrew.
 

oldengineguy

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Went and tried Jasonb suggestion but quickly looked at the chart on my lathe and set up with top gear driving 86 tooth then 91 driving lower gear opposite to what Jasonb had posted. Wrong! I will ignore set up as shown in my booklet and set up exactly as Jasonb describes and try again, Thank you all for your patience with my seeming inabilty to follow even the simplest of instruction, will report when this test complete as the previous set up was a failure. Thanks Colin
 

oldengineguy

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Just finished another test piece, YAHOO! amazing what happens when you finally get the set up right. Time to stop beating myself up re.my innabilty to figure this out and get on with the job. Thanks,Jasonb and all other folks for your help. What a great forum. Colin
 

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