Thread Dial

[Kpar]

I finally decided to have another crack at single point threading following my past failures.
I have an AL-320G lathe with a metric leadscrew. I changed the gears for a 1.5 pitch on a 16mm rod.
I ran into problems engaging the half nut, then decided to leave it engaged and reverse the lathe.
Finally got the thread I required.
My questions are.-my thread dial has 6 numbers (not 4) and seems to have 2 engaging spots between numbers easy to select the wrong one ?
Also how much end play should be in the leadscrew ?. With the machine turned off & half nut engaged if I move the carriage back & forth
the lead screw moves 1/8" is this normal ?.

Kpar

 

[SmithDoor]

I finally decided to have another crack at single point threading following my past failures.
I have an AL-320G lathe with a metric leadscrew. I changed the gears for a 1.5 pitch on a 16mm rod.
I ran into problems engaging the half nut, then decided to leave it engaged and reverse the lathe.
Finally got the thread I required.
My questions are.-my thread dial has 6 numbers (not 4) and seems to have 2 engaging spots between numbers easy to select the wrong one ?
Also how much end play should be in the leadscrew ?. With the machine turned off & half nut engaged if I move the carriage back & forth
the lead screw moves 1/8" is this normal ?.

Kpar

There are a lot threading you need to use the forward and reverse.

Dave

 

[petertha]

This is just a guess & I am unfamiliar with that lathe, but if you have metric leadscrew & metric transposing gears, that pretty much makes it a metric lathe from threading standpoint. I think you should be able to engage/reengage the clamshells on a specific threading dial number. Some pitches may you require you to engage on specific number(s) others may be any number. There should be a chart. Sometimes things can drift or vary where they like to engage just before or just after the line, but ideally it should be close & consistent.

The usual difficulties that generate questions are threading metric on an imperial leadscrew, for example 8 TPI which is common. Those have no dial number indication or if they do its coincidental. So often people leave it engaged, which isn't a requirement technically, but that's a different subject.

Now the question is, was your lathe converted from IMP to MET? Do you have nominal metric dial graduations? Reason I ask is if someone swapped in a metric leadscrew, hopefully the changed out the engagement clamshells, the indicator assembly & adjusted them correctly

 

[djc]

My questions are.-my thread dial has 6 numbers (not 4) and seems to have 2 engaging spots between numbers easy to select the wrong one ?
Also how much end play should be in the leadscrew ?. With the machine turned off & half nut engaged if I move the carriage back & forth
the lead screw moves 1/8" is this normal ?

You need to give us more information in order to receive a sensible reply.

First, what pitch is the leadscrew?

Second, how many teeth on the thread dial gear that engages with the leadscrew? I am not asking of the dial at the top - it is the gear underneath that rotates when driven by the leadscrew.

A photo of the dial itself would be useful.

It would be good to read Martin Cleeve's 'Screwcutting in the lathe'. You only need to read Section 5, page 71 onwards. Try very hard to gain a good understanding of what he calls 'minimum synchronisation distance'.

The play in the leadscrew is not great, but so long as you start your cutting pass sufficiently far away from the end of the work for all the play to be taken up, it is useable in that state. Clearly, you have to withdraw the tool fully before you reverse.

 

[Kpar]

This is just a guess & I am unfamiliar with that lathe, but if you have metric leadscrew & metric transposing gears, that pretty much makes it a metric lathe from threading standpoint. I think you should be able to engage/reengage the clamshells on a specific threading dial number. Some pitches may you require you to engage on specific number(s) others may be any number. There should be a chart. Sometimes things can drift or vary where they like to engage just before or just after the line, but ideally it should be close & consistent.

The usual difficulties that generate questions are threading metric on an imperial leadscrew, for example 8 TPI which is common. Those have no dial number indication or if they do its coincidental. So often people leave it engaged, which isn't a requirement technically, but that's a different subject.

Now the question is, was your lathe converted from IMP to MET? Do you have nominal metric dial graduations? Reason I ask is if someone swapped in a metric leadscrew, hopefully the changed out the engagement clamshells, the indicator assembly & adjusted them correctly

I purchased the lathe new and nothing has been changed. It seems for metric threads the engage points are very close
on the dial's 6 point markings.

 

[petertha]

With the machine turned off & half nut engaged if I move the carriage back & forth
the lead screw moves 1/8" is this normal ?.

Do you mean it appears the leadscrew can free rotate 1/8" like in a 12:00 to 1:00 manner? If you are saying the carriage can float axially along the bed 1/8" with half nut engaged, that sounds both suspicious & problematic. I can see a little bit of backlash but nowhere to that extent. Basically impossible to cut a thread which you've indicated you have? The only thing driving your carriage in threading mode relative to spindle rotation is the engagement of leadscrew & half nut, basically a bolt & nut principle.

I don't want to second guess your abilities or knowledge but you are sure you are engaged in threading mode vs power feed mode? Has the half nut subassembly somehow loosened? The half nut inserts themselves are moving maybe? Usually they retained by set screws but this varies. The gears are fully engaged & orientated as they should be? Often times you have to flip positions or readjust the banjo for proper gear engagement if its that style. Usually leadscrews are engaged to headstock output shafts with a shear pin through a coupler whereas separate power feed bars may have a clutch coupler assembly. If you have a clutch coupler, is engaged properly? Some pictures or excerpts from the parts diagram would be helpful. I'm just laying out general lathe information principle, not specific to your lathe model.

 

[Kpar]

You need to give us more information in order to receive a sensible reply.

First, what pitch is the leadscrew?

Second, how many teeth on the thread dial gear that engages with the leadscrew? I am not asking of the dial at the top - it is the gear underneath that rotates when driven by the leadscrew.

A photo of the dial itself would be useful.

It would be good to read Martin Cleeve's 'Screwcutting in the lathe'. You only need to read Section 5, page 71 onwards. Try very hard to gain a good understanding of what he calls 'minimum synchronisation distance'.

The play in the leadscrew is not great, but so long as you start your cutting pass sufficiently far away from the end of the work for all the play to be taken up, it is useable in that state. Clearly, you have to withdraw the tool fully before you reverse.

Thanks djc, Leadscrew is 24mm Dia, Pitch 3.0mm Dial Indicator gear 24 teeth.
I guess this is how you measure pitch "center to center of thread"

 

[Kpar]

Do you mean it appears the leadscrew can free rotate 1/8" like in a 12:00 to 1:00 manner? If you are saying the carriage can float axially along the bed 1/8" with half nut engaged, that sounds both suspicious & problematic. I can see a little bit of backlash but nowhere to that extent. Basically impossible to cut a thread which you've indicated you have? The only thing driving your carriage in threading mode relative to spindle rotation is the engagement of leadscrew & half nut, basically a bolt & nut principle.

I don't want to second guess your abilities or knowledge but you are sure you are engaged in threading mode vs power feed mode? Has the half nut subassembly somehow loosened? The half nut inserts themselves are moving maybe? Usually they retained by set screws but this varies. The gears are fully engaged & orientated as they should be? Often times you have to flip positions or readjust the banjo for proper gear engagement if its that style. Usually leadscrews are engaged to headstock output shafts with a shear pin through a coupler whereas separate power feed bars may have a clutch coupler assembly. If you have a clutch coupler, is engaged properly? Some pictures or excerpts from the parts diagram would be helpful. I'm just laying out general lathe information principle, not specific to your lathe model.

Peter what it is if I engage the halfnut (lathe off) and attempt to move the carriage back & forth with the handwheel
The halfnut clamps to the leadscrew and holds firmly but the leadscrew has about 1/8" end float ?

 

[kf2qd]

The backlash in the lead screw is not a problem. Alllathes have some backlash there. As you are only cutting in one direction the backlash will be taken up before the carriage moves.

To check out the threading dial, take a piece of material and paint it with some cheap paint, and just lightly touch the surface to leave a mark, and then back outm and release the half nuts and bring it back to the start and then put the tool close to the surface and as the spindle turns engage the halfnuts and see how it matches up. Different threads will have different combinations of lines where it will chase.

 

[petertha]

The plot thickens. I would call backlash the the amount of free rotation between the threaded surfaces before one acts on the other 7 starts to displace movement. Specifically the leadscrew & half nut threads. That should be minimal & entirely related to wear beyond how well the threads are cut. But sounds like what you are describing is when they are engaged, you can move the entire leadscrew 1/8" relative to the bed. That I think is a problem because translates into the thread starting in potentially different position each threading pass. The pitch may be correct once engaged, but the point may occur anywhere within that 1/8" zone. That might also explain why the threading dial is indicating slightly different positions and/or not lining u to a number.

This might be related to what I was calling a coupler or collar. Something must join the leadscrew to its mating output shaft inside the headstock. I am guessing something is slipping there to allow the float. It might be as simple as a set screw acting on the shaft. Can you find a schematic in the parts manual? It could also be float at the tailstock end that is intended to make up free play, so have a look there too. Typically the leadscrew is turned down on the TS end & runs in a bushing within a bolt on block, something along those lines.

 

[petertha]

Found what I think is manual?
https://images.machineryhouse.com.au/products/L141/PDF/L141 - Instructions Manual.pdf
Left = headstock side. I'm assuming 53 tapered pin is the shear pin that goes through coupling sleeve into leadscrew. It doesn't show what the vertical retention component is, presumably a roll pin into mating gear shaft? One would think this arrangement to be no movement, but I guess that's what needs to be checked - where is the free play originating?

Right = tailstock side. Hard to tell from sketch but can any of the parts take up slack movement by adjusting?

R

 

[petertha]

Now I'm assuming the leadscrew is a solid because that's how it appears in parts view. In the picture it looks like slightly different color than segment going into HS. Its not an external sleeve coupler is it?

 

[Kpar]

Found what I think is manual?
https://images.machineryhouse.com.au/products/L141/PDF/L141 - Instructions Manual.pdf
Left = headstock side. I'm assuming 53 tapered pin is the shear pin that goes through coupling sleeve into leadscrew. It doesn't show what the vertical retention component is, presumably a roll pin into mating gear shaft? One would think this arrangement to be no movement, but I guess thats what needs to be checked. Where is the free play originating?

Right = tailstock side. Hard to tell from sketch but can any of the parts take up slack movvment by adjusting?

View attachment 145324View attachment 145325
View attachment 145326
R

Thanks Peter, Yes the bearing block at the TS end has elongated holes so adjustment is available.
Will do that tomorrow. If i don't reply, All is Good.

 

[djc]

That I think is a problem because translates into the thread starting in potentially different position each threading pass.

I am not sure what you are saying is correct.

Start with the carriage a sufficient distance from the end of the workpiece. Engage the halfnuts. The drag of the carriage on the ways will always take up the slop in the same direction.

You will only ever have a problem if you are not consistent in your technicque on any given thread and try to cut it both towards and away from the headstock.

The problem is no different to backlash on a mill. You take up the backlash before the cutter enters the work and cut in a way (i.e. do not climb cut) so that the resultant force from the cutting action does not change anything.

 

[djc]

Leadscrew is... ...Pitch 3.0mm Dial Indicator gear 24 teeth.

OK. Thanks.

For the 1.5mm pitch you describe in your initial post, you do not need the thread dial indicator at all. You could remove it from the machine altogether and still cut that thread properly.

If the leadscrew pitch is 3mm, every leadscrew pitch contains exactly two 1.5mm pitches. That means wherever you engage the half nuts, the threads will line up.

You can do the same exercise with other metric pitches. For instance, the 3mm leadscrew pitch contains exactly three 1mm pitches. So for 1mm pitch threads, the indicator is not needed.

If you go through every reasonable metric pitch, you will find roughly 50% of them do not need the indicator at all.

The following pitches do not need the indicator:

0.2, 0.25, 0.3, 0.5, 0.6, 0.75, 1, 1.5, 3

For other pitches you do need an indicator.

Let us pick 1.25mm pitch (M8). How many 1.25mm pitches fit into a multiple of 3mm pitches? The lowest integer that will divide 3 and 1.25 is 15. Thus five 3mm pitches contain twelve 1.25mm pitches.

Hence, for this pitch, you would need a thread dial indicator that has marks every 15mm of travel. Since each tooth on the gear attached to the dial counts/measures 3mm, you need marks that occur once every five teeth of the gear. You also need a gear that has a tooth count divisible by five (so the marks do not change every revolution). Your gear has 24 teeth, so it is not suitable for 1.25mm pitch. If you want to cut that pitch, you have to use the full reverse method.

So what pitches can you use your current TDI for? 0.4mm, 0.8mm, 2mm, 4mm and 6mm. The synchronisation distance (q.v. Cleeve) for these threads is 6mm, 12mm, 6mm, 12mm and 6mm respectively, so your dial will work if its marks occur once every four teeth of the gear.

Finally, let us look at the gear requirements for pitches not yet mentioned. 0.35mm needs a gear with teeth that are a multiple of 7; 0.45mm multiple of 9; 0.7mm multiple of 7; 0.9mm multiple of 9; 1.25mm multiple of 5; 1.75mm multiple of 7; 2.5mm multiple of 5; 3.5mm multiple of 7; 4.5mm multiple of 9; 5mm multiple of 5; 5.5mm multiple of 11.

 

[roncohudd]

There are a lot threading you need to use the forward and reverse.

Dave

Once I get the threads started, I never take it out of gear. I let it run back a way to check the threads. If I'm using the tail stock, just slide it back. Saves me a lot of rework.

 

[petertha]

I am not sure what you are saying is correct.

Start with the carriage a sufficient distance from the end of the workpiece. Engage the halfnuts. The drag of the carriage on the ways will always take up the slop in the same direction.

You will only ever have a problem if you are not consistent in your technicque on any given thread and try to cut it both towards and away from the headstock.

The problem is no different to backlash on a mill. You take up the backlash before the cutter enters the work and cut in a way (i.e. do not climb cut) so that the resultant force from the cutting action does not change anything.

Everything you are talking about assumes that this is a THREAD backlash problem between the lead screw & half nut threads. Yes, if for example 0.050" thread backlash exists, then leadscrew is free to rotate until such point that backlash is taken up, meaning at some point thread to thread contact occurs, then away goes the carriage according to pitch. What the OP appears to be describing is something different. His entire leadscrew is able to displace 0.125" independently. It is floating axially & nothing to do with thread engagement. I think this is a secondary problem & a bigger problem because the reference between spindle & leadscrew is lost or at least highly variable relative to thread backlash. 1/8" (as per post#8) is 3.175mm of float, more than the pitch of his leadscrew.

Without further complicating matters using you mill analogy, lets assume zero THREAD backlash between leadscrew & nut. Its perfect. Now if you pulled on the table you would expect it to stay put. But if it slides back & forth 1/8" hopefully you would agree this is a different problem because by definition we said zero thread play. So maybe the leadscrew nut is no longer tied down properly to the casting or whatever. In any event we cannot trust the DRO in any position within the error band, in this case 1/8".

 

[djc]

But if it slides back & forth 1/8" hopefully you would agree this is a different problem

No. Because during thread cutting it cannot slide both back _and_ forth. After you engage the half nuts, it will slide forth and will stay slid forth as much as it wants to slide forth until the direction of motion is changed.

If the amount it slides forth is inconsistent and varies each time, sure, it's a problem but the magnitude of the forth does not matter in and of itself.

It can never slide back from its forth until the thing (gearbox) that is pulling it or the saddle that is resisting it changes direction.

On my lathe, to use the taper turning attachment, I have to release the cross slide screw from the handwheel. I can still use it for normal turning with no axial restraint (i.e. limitless opportunity for back and forth) between screw and handle so long as I am only putting on a cut (moving the cross slide towards the lathe centreline). The lack of connection only becomes a problem when I want to withdraw the cross slide (i.e. change direction). If I pull the cross slide back by hand, I can send it forth to exactly the same place every time.

Glue two blocks of wood to your table top. Put a piece of threaded rod that is shorter than the distance between the blocks in between the two blocks. Put a nut on the rod and apply some resisting force to the nut to stop it rotating. Turn the rod. It will move until one end or the other hits a wood block. Then it will stop moving itself and start dragging the nut. No matter how much slack is in the system and no matter if the slack is between rod and blocks or between rod and nut or a combination of the two, it will always become tight at exactly the same point. The point at which it becomes tight will differ depending on direction, but in the same direction, it will always be the same.

By no means am I saying that it is a good situation for him to be in but the leadscrew play will only cause a synchronisation issue if it is not the same each time.

 

[petertha]

No. Because during thread cutting it cannot slide both back _and_ forth. After you engage the half nuts, it will slide forth and will stay slid forth as much as it wants to slide forth until the direction of motion is changed.
If the amount it slides forth is inconsistent and varies each time, sure, it's a problem but the magnitude of the forth does not matter in and of itself.
It can never slide back from its forth until the thing (gearbox) that is pulling it or the saddle that is resisting it changes direction.

I think I understand what you are saying but maybe where we differ is the implications & numerical extent. Lets say we know for a fact he has 0.025" of thread backlash between half nuts & leadscrew. And he has 0.125" of leadscrew axial float, say the leadscrew is sliding within the collar connecting HS gear box. I think what you are saying is: doesn't really matter because when he engages half nuts outside the threading zone on each pass, the rotating lead screw will eventually close up both gaps. So max 0.025" due to thread fit + max 0.125" due to axial play = max 0.150" (3.81mm). And at this point the carriage will commence travelling along dictated by the pitch.

But what is also happening during this dead band period? The leadscrew continues to rotate, which spins the thread indicator dial, which now conveys increasingly erroneous information of phasing. Which is its purpose. It might be OK if it happens to be a thread that can engage on any numerical position which is another way of saying disconnect the thread dial altogether & just engage the half nuts. But if its a pitch that dictates a specific number before each threading pass, I cant see how this degree of variability could be good. 0.150" = 3.81mm. That exceeds the pitch of his leadscrew. Am I missing something?

 

[petertha]

The point at which it becomes tight will differ depending on direction, but in the same direction, it will always be the same.

How can you be so sure? What if the collar is slowly slipping over time? Maybe the new machine was something acceptable like 0.050" float, today its 0.125", tomorrow its 0.150"...