Imperial thread cutting on a metric lead screw

[hamotech]

Hi everyone

I am having trouble cutting a 14 threads per inch on a metric lathe , so far after reading about this I have found out that you can’t use the thread dial and I have read that you have to leave half nut engaged and run the lead screw in the opposite direction on each cut is this correct as I’m not getting it to cut along the same thread path , any help would be very appreciated

 

[EPAIII]

Pictures of your set-up and results may help.

If you keep the half nut engaged and reverse the motor so the HS spindle and lead screw both back up together, you almost have to maintain synchronization.

Are you using the compound for the in-feed between passes? If so, what angle is it set to? It should be 30 degrees (OK, 29.5 degrees) from parallel to the cross feed's direction of feed. Many beginners rely on the angle scales on their lathes but those scales can be numbered two ways and the numbers can be 60/60.5 instead of 29.5/30. So align the compound with the cross feed and then change it by 29.5 degrees.

 

[technocrat]

You can use the thread dial if you put a carriage stop on the ways. So long as the carriage is in the same place every time and the thread dial is on the exact same indication you are ok. On an 0-4 thread dial, if you engage on 0, you must do that every time, you can't use 2 or 4. Set the carriage in any position with the thread dial on 0 and the flaf nuts engaged (motor stopped) then fit the carriage stop. Reversing is a pain.

 

[modelmakerblue]

Hi everyone

I am having trouble cutting a 14 threads per inch on a metric lathe , so far after reading about this I have found out that you can’t use the thread dial and I have read that you have to leave half nut engaged and run the lead screw in the opposite direction on each cut is this correct as I’m not getting it to cut along the same thread path , any help would be very appreciated

If you don't disengage the half nut the tool must follow the same path, as has been said are you moving the compound slide and also start the tool some way to the right of your job to allow for backlash.

 

[jariou]

Hamotech,

Yes, threading inch on a metric lathe is like threading metric on an inch lathe, the thread dial is not as helpful as you might think.

Conceptually, the easiest way is to keep the half nut closed and reverse the spindle. Make sure you go back far enough past the beginning of the thread to eliminate backlash when reversing again.

However, if you are threading close to the chuck or to a shoulder, you may not feel comfortable leaving the half nut closed and you want to be able to open it quickly so you don't crash into the chuck or a shoulder. So, here is a way to do that.

1) Engage the half nut when the dial is say at 0.

2) Make a pass.

3) At the end of the thread, release the half nut as you always would, retract the tool, and then stop the motor. Try to do three things fairly quickly, one after the other, but make sure you first release the half nut. The chuck will coast for a few seconds and come to a stop. The carriage is not moving anymore, but the thread dial is. Hopefully, it does not go past one revolution.

4) Now, start the spindle in the reverse direction and watch the thread dial.

5) Close the half nut when the dial comes back to 0 and leave it engaged you are at step 3 of the next pass.

6) Let the tool travel right until it has well cleared the thread.

7) Stop the spindle, feed the tool in a bit.

Now repeat steps 2 to 7 until you are happy with the thread depth.

Practice on a bit of soft material, or even just in the air, above the material, just to get the movements down.

Good luck,

Jacques

 

[Conrad Hoffman]

I use the above method for most metric threading and it works great, but remember that the "retract the tool" part is really important. I've never investigated how the gearing needs to be set up to get from metric to imperial, but I assume the same conversion gears can do the job, but probably reversed?

 

[hamotech]

thank you everyone for your help , I was not using the compound slide for feeding in I was using it straight and was revering the lead screw feed , after reading you guys post where you said to reverse the spindle i realised that reversing the lead screw is still going to have problems with play in the half nut , is this correct , sorry for asking dumb questions but this is the first time I have tried to cut a thread on a lathe , I did do some research cutting threads but it was metric threads and when I found out it was a imperial thread that through a spanner in the works .

 

[jariou]

Hamotech,

You can use the compound slide or the cross slide it's of no special relevance here. However, if you really mean that you are flipping the lead screw feed direction rather than just spinning the spindle in reverse while keeping the half nut closed, I could easily see that you might not get back into the same groove.

There might be some kind of leadscrew reversing mechanisms such that you don't lose synchronism when reversing, but I doubt you have that on your lathe. My expectation is that the feed screw reverse lever just moves a sector holding two gears of same size. In one position, the gear train goes through only one of those gears while in the other position, the gear train goes through both gears.

When you flip from one position to the other, there will be a moment where neither of the two gears are connected to the spindle gear and you may (or should) lose synchronism right there.

So, moral of the story, the spindle has to reverse, that will automatically reverse the direction of the leadscrew. But, the gearing of the leadscrew to the spindle should stay connected at all times. If you do that and follow the instructions above, you should have success.

If I have misinterpreted what you are telling us, my apologies. If so, please try to explain better what you are doing or not doing. We'll try to help out.

Good luck,

Jacques

 

[EPAIII]

You were reversing the lead screw's direction of rotation? But with the spindle (and object being threaded) either still (motor stopped) or still rotating?

Here is what you need to understand about threading. There is a rotational relationship between the object being threaded and the threads on the lead screw and the linear motion of the carriage, which holds the cutting tool. If you do not strictly maintain this relationship, then the tool can and probably will take a different path on a successive pass. The object being threaded is held by the chuck or other means to the head stock's spindle so that spindle must be held in that relationship to the lead screw and then to the carriage and cutting tool.

That strict relationship WILL be lost if the gear train from the rear end of the spindle to the lead screw or the connection by the half nuts is disconnected AT ANY POINT. So, opening the half nuts loses that relationship. But interrupting things at any other point will do the same thing. Removing the object being threaded from the chuck and then remounting it will lose that relationship. Taking a change gear out of the train and not replacing it in exactly the same relationship will lose it. Shifting the gearing in a quick change gear box and then putting it back will change it. AND using the tumbler reverse to change the direction of rotation of the lead screw to move the carriage back to the starting point WILL LOSE THAT RELATIONSHIP.

A threading dial can allow you to re-establish that strict relationship after it is lost if certain conditions are met. However, every threading dial that I have ever seen will not work if the lead screw has a thread pitch in one measurement system and the thread being cut is in another (English vs. metric).

So, threading dials are only used in a metric - metric situation or in an English - English situation.

Here is what you need to do with your metric lead screw, step by step:

1. Set your gearing to cut the English thread pitch needed.

2. Make the first pass.

3. When the tool reaches the end of the thread on the first pass, STOP THE MOTOR or disengage a clutch that STOPS THE SPINDLE. That should also stop the lead screw and the motion of the carriage. Do not interrupt the mechanical connection at any point between the object being threaded and the cutting tool.

4. Using the cross feed on the carriage (or the compound) back the tool out of the cut to allow it to clear the work as it moves back to the right in step #5. Remember it's position on the dial first.

5. Now, with the tool backed away from the work, either by reversing the motor's direction or by just turning the spindle by hand, back up BOTH the spindle and the lead screw with that mechanical connection still connected. Both of them should turn together and the carriage and tool should move to the right. Stop when the tool is at the initial left-right starting position or even a bit further to the right. The exact left-right position is not important, just the relative positions of the work and the tool. This maintains the mechanical relationship between the work and the tool.

6. Advance the carriage back in to the position you noted in step # 4.

7. Using either the carriage or the compound, advance the tool in for the amount you wish to remove in the second pass.

8. Perform the second pass as you did the first by either turning the motor on or engaging the clutch from step #3. Return to step #3 above and continue to loop through steps #3 through #8 until your thread is finished.

In step #5 you need to back the tool up enough for any backlash in the gearing or half nuts to be taken up before the tool reaches the work in step #8 for the second pass.

thank you everyone for your help , I was not using the compound slide for feeding in I was using it straight and was revering the lead screw feed , after reading you guys post where you said to reverse the spindle i realised that reversing the lead screw is still going to have problems with play in the half nut , is this correct , sorry for asking dumb questions but this is the first time I have tried to cut a thread on a lathe , I did do some research cutting threads but it was metric threads and when I found out it was a imperial thread that through a spanner in the works .

 

[john.k]

You must reverse the entire mechanism....AND eliminate all backlash as well,before the thread will correspond..........in simple terms .you must return the tool well beyond the start point before going forward again.

 

[bob]

dont you need a 127 tooth gear somewhere

 

[EPAIII]

In theory, for a perfect conversion, yes. But for many threads where absolute precision is not required, there are other gear pairs that will provide a usable thread. One common one is 49 and 39 teeth.

Many lathes are actually equipped with such, approximate ratio gears.

dont you need a 127 tooth gear somewhere

 

[EPAIII]

I agree that the backlash or lost motion in the gear train must be taken up before cutting begins. But, if the carriage and tool are simply brought to a point to the right of the part being threaded, then that backlash will automatically be taken up before the carriage/tool begins to move.

So you really don't need to worry too much about the exact distance as long as the tool is to the right of the part. A short distance is wise just so that things in the gear train can jiggle together, but more than that is just a waste of time.

You must reverse the entire mechanism....AND eliminate all backlash as well,before the thread will correspond..........in simple terms .you must return the tool well beyond the start point before going forward again.

 

[rogertoolmaker]

Hi everyone

I am having trouble cutting a 14 threads per inch on a metric lathe , so far after reading about this I have found out that you can’t use the thread dial and I have read that you have to leave half nut engaged and run the lead screw in the opposite direction on each cut is this correct as I’m not getting it to cut along the same thread path , any help would be very appreciated

 

[rogertoolmaker]

About two months ago I cut a half dozen metric studs on an old America lathe by leaving the gear engaged, stopping the spindle, reversing the spindle by hand and making the next pass. This will work on a metric lathe to cut your threads.

 

[Conrad Hoffman]

The method of opening the half nuts and stopping the motor does work; I do it all the time. BUT (big but) you need to have some experience cutting threads in the lathes native system so you understand how everything normally works. Then do non-native threads without disconnecting the mechanical connection, the way everybody normally does it. Only then learn the half nut trick so you can thread to a shoulder without risk.