Crazy Threads...

[RJSakowski]

I did try to measure metric threads on the less screw, nothing really lined up. hoe should I be measuring it tho?
When I measured the 8tpi on the lead screw, it was almost dead nuts 8 teeth I'm the inch on the ruler

With a lead screw pitch of 8 tpi, you should have a 16 tooth gear on the thread dial. This will give you 2" of travel of the carriage for one revolution of the thread dial. Everything should be OK then. Apparently either the PO replaced the OEM gear with one from a metric set or the wrong gear was installed during manufacturing.

An 8 tpi lead screw would be very close to a 3mm metric pitch. You can tell the difference by measuring the distance traveled by the carriage through 10 revolutions of the thread dial. The distance for your setup with an 8 tpi lead screw would be exactly 17.5". For a 3mm pitch metric lead screw, the distance would exactly 420mm or 16.54". If you do have an 8 tpi lead screw, you just need to get or make the 16 tooth gear.

 

[pdentrem]

My lathe is not the same but all lathes are the same. I hope these can help. Check the gear train to be sure all is correct in the settings and the number of teeth per gear.
Here are a few photos showing what I mean. I counted the threading dial gear and it is 4X! Mine has 32 teeth and 4" of travel for one revolution. I included the threading chart so that it shows the 120/127 gear to switch between Imperial and Metric.
Pierre

 

[Hukshawn]

If the threading dial gear is really 14 tooth it has to be intended for a metric lathe with a metric leadscrew. If you have an 8 TPI leadscrew you need a 16 tooth gear. Those gears are easy to make though as they carry no load. I made the one on my threading dial out of nylon on the lathe using a dremel strapped to the toolpost and an indexer cobbled up with one of the change gears.

The 15 TPI is a puzzlement, though. Again I suggest that you set up for 8 TPI. Then everything is one-to-one and it should be easier to see what is going on.

[Edit] Could the previous owner have installed metric interpolating gears? I can imagine someone installing them so he can cut metric threads and then thinking that he should also install a metric threading dial. 15 TPI is real close to 1.75mm pitch.

if you look above, i did the 8 tpi setup and supplied a picture. it did the same thing as everything else. engaging on any position resulted in a thread that was close, but still each pass 2-3 thou off from the previous.

 

[Hukshawn]

If the threading dial gear is really 14 tooth it has to be intended for a metric lathe with a metric leadscrew. If you have an 8 TPI leadscrew you need a 16 tooth gear. Those gears are easy to make though as they carry no load. I made the one on my threading dial out of nylon on the lathe using a dremel strapped to the toolpost and an indexer cobbled up with one of the change gears.

The 15 TPI is a puzzlement, though. Again I suggest that you set up for 8 TPI. Then everything is one-to-one and it should be easier to see what is going on.

[Edit] Could the previous owner have installed metric interpolating gears? I can imagine someone installing them so he can cut metric threads and then thinking that he should also install a metric threading dial. 15 TPI is real close to 1.75mm pitch.

why would it cut a thread the machine wasnt set to cut? i was set to 16 tpi and it cut 15 tpi... regardless of the thread dial. also note, this thread was cut by not disengaging the half nuts. pulling the tool back with the cross slide and reversing the lathe back to the start. so the thread dial was not used at all.

 

[Hukshawn]

Place a metric ruler along side the leadscrew and count 10 turns. How many mm did that measure out to? It should come out to a number like 20, 30, 40 etc. It will not come out to a partial like 33 mm. Supply a photo of this showing the ruler along side the leadscrew.

On the side of the headstock, you have the gear train, if one of the gears is off by 1 tooth ALL the threads and feeds will be off. Most lathes use a 127/120 gear set to change from Imperial to Metric. Can you supply a photo of these gears? Also a better picture of the plate showing the gears to be used.
Pierre

here are those pictures. the lead screw does not line up at all with metric.
according to the label plate, i have the gears set to imperial. and i did count the teeth, they all seem right.

 

[Hukshawn]

so, here are my two main issues that i just cant wrap my head around...

first - with the half nut engaged the whole time and reversing the lathe back to the start position. i had the machine set to 16 tpi but it cut a 15... how can that be?

second - when i use the thread dial and disengage the half nut each pass, i have to use the same number each pass, without changing any depths or positions, each cut is 1-2 thou in front of the previous cut. if i carried on long enough, i would eventually over lap my first cut. and if i try to use a different number or line as you can with an even number tpi, the cuts are in a totally different place. ill end up cutting between threads.

 

[rick9345]

There is no magic bullets
RTM
reread all threading info,verify the setup,recheck the work/clean up performed in getting your lathe operating.
Most any lathe handbook on threading is quite informative.
Search for the problem , not opinions. If previous owner available,humbly ask if he used threading and any quirks with machine encountered.
Stick with it you can solve this.

 

[higgite]

so, here are my two main issues that i just cant wrap my head around...

first - with the half nut engaged the whole time and reversing the lathe back to the start position. i had the machine set to 16 tpi but it cut a 15... how can that be?

Some food for thought. If your lathe originally had a metric leadscrew with 3mm pitch and it was replaced with an Imperial leadscrew having 8 TPI (like it now has) and if it is still geared to cut 16 TPI threads with a 3mm leadscrew, it will cut a 15.1 TPI thread .

second - when i use the thread dial and disengage the half nut each pass, i have to use the same number each pass, without changing any depths or positions, each cut is 1-2 thou in front of the previous cut. if i carried on long enough, i would eventually over lap my first cut. and if i try to use a different number or line as you can with an even number tpi, the cuts are in a totally different place. ill end up cutting between threads.

Like others have already said, the thread dial doesn't match the lead screw.

Tom

 

[JimDawson]

This must be a problem in the change gears. Something is not setup correctly. With an 8 tpi lead screw, the quick change gearbox must be some integer ratio in to out when set for 8 tpi. If it's not then the problem is there. The spindle to the quick change gearbox is 1.5:1, so 12 turns of the input gear, should turn the lead screw 8 turns.

 

[Hukshawn]

One last question, I'm working two jobs now starting this week for a few weeks, so it'll be some time before I can devote any time again to solving this again..

even if it has an imperial lead screw, it appears the lathe is designed to cut both imperial and metric threads, based on the label plate... so why would it matter? that's the hard time I'm having with this. according to the label plate, I have the gears set to cut imperial, with an imperial lead screw, disregard the thread dial, using positive drive the whole way, it SHOULD work.... no?

Everyone's saying recheck the setup, check the gears, I did that. I even supplies pictures, they look right... I don't know where to look for problem.

Could the gear box be metric? should I look for a metric lead screw and change this back? I know nothing about metric threads. I'm at a total loss at how to fix this and am fearing this lathe is useless for threading.