Don't know how to thread on this lathe

[HDB]

Dear Forum members,

I recently bought a Mc. Dougall Model DT-50 lathe. Since I don't have a manual and that I can't find one on the internet, I started googling for information. That's how I found out about this forum.
I've use a Celtic 14 in the past to turn lot's of different tools and such, but I'm having difficulties with this particular lathe when it comes to threading.

The table on the lathe show the settings for both Widworth and metric threads. Nothing difficult: for 1.75mm (metric): sliding gear out - 9 - C. The question is what gears I have to put on the lathe. I've got two big ones and two smaller ones. I have to combine a big with a small gear. The question is which combination.
Since I didn't know which gear to combine with which gear, I tried every combination.
Let's say that G(gear)1 and G2 are the big ones, and G3 and G4 the smaller ones:
G1 - G3
G1 - G4

G2 - G3
G2 - G4

One of these four combinations should result in a 1.75 metric thread. But, and this is my real question, none of them give a thread that comes even close to it. In most cases it's as if I'm just turning instead of threading the piece. While the chuck is turning (lathe on...), I can use two feed speeds (handle 1 (goes up or down) and handle 2 (turns clock/counterclockwise)). I do enable the wheel on the right (the small wheel that should lock the feed, and assure a proper thread (I think)).
If I do get a threading-like result, it's not the correct one.

So my question is: Is there anyone who has an idea about what I might be doing wrong? Would there be some other thing I'm overlooking?
I really hope that the gears are the correct ones, and I guess they are, and that it's just something I'm overlooking.

Thanks in advance.
Pictures: https://www.dropbox.com/sh/j1f4oih9aqlm ... -O-Ya?dl=0

[Harold_V]

So my question is: Is there anyone who has an idea about what I might be doing wrong? Would there be some other thing I'm overlooking?
I really hope that the gears are the correct ones, and I guess they are, and that it's just something I'm overlooking.

Thanks in advance.

Welcome to the board.
I am not even remotely familiar with your machine, but one thing you said troubles me:

I do enable the wheel on the right (the small wheel that should lock the feed, and assure a proper thread (I think)).
If I do get a threading-like result, it's not the correct one.

When threading, one does not lock down a wheel. The action of threading demands synchronization of the cutting tool with the spindle, so the threading dial is generally employed to engage the half nuts at the proper interval. I am making an assumption that the lever you'd use for that purpose is the one on the right side of the apron. It's red in color, and is vertical.

With the machine not running, try engaging that handle. If it is the proper one to engage, the carriage will be somewhat limited to movement once it is engaged. You can verify that it's working properly by attempting to move the carriage by rotating the carriage hand wheel. If you find it moves, but only a miniscule amount, that's a positive sign, as it indicates that the half nuts have engaged the lead screw. Don't force the carriage, just lift the handle to the vertical position, which should disengage the half nuts.

If you have discovered that the lever is, indeed, for threading, I'd then recommend you run the machine at a slow speed, and take a light pass on a piece of material, using a threading tool, to see what thread pitch is generated. The feed rate when using the screw and half nuts will be much faster, which is why I recommend you make this attempt at slow speed, as the carriage has the potential to move rapidly.

Once you've ascertained that you now have the proper sequence for threading, you may be able to determine what system each gear combination creates.

The threading dial is critical for some engagements. Typically, if a machine is set up with an Imperial lead screw, the threading dial won't function properly when you are chasing a metric thread. In such a case, you'll have to leave the half nuts engaged and return the tool by reversing the machine. You'll have to experiment to see how yours behaves.

For Imperial threads, assuming the screw is Imperial, you may wish to experiment with the positions on the threading dial, so you'll understand when you can engage the half nuts so the thread tracks properly. If in doubt, always use the same line.

Sorry I'm not more help.

Harold

[stevec]

HDB, first off I'd like to suggest you put some indication of your worldly whereabouts in your profile, responders can gather insight from this knowledge.
I'd hazard you might be in Great Britain from your reference to "Widworth" threads, although I believe it's "Whitworth".
Once you have ascertained, following Harold's instructions, that the red lever with the black knob is in fact the one that engages the "half-nuts (you may have to "wiggle" the handwheel on the left while engaging the half nuts. If the machine is running this won't be necessary) .
The 3 position lever between the half-nut lever and the handwheel engages the cross feed when in the top position and the longitudinal feed in the bottom position and in the middle when neither feed is chosen, that is where it should be when the half nuts are used for threading.

The bottommost lever is the "clutch" to engage either the crossfeed or longitudinal feed and is also not used when threading.
I'm pretty sure your feed screw is imperial (inch) but you can verify by counting threads over an inch.

I suspect that one of the gears you will need to select will have 127 teeth and probably be the largest.

Let us know how you make out.

[HDB]

HDB, first off I'd like to suggest you put some indication of your worldly whereabouts in your profile, responders can gather insight from this knowledge.
I'd hazard you might be in Great Britain from your reference to "Widworth" threads, although I believe it's "Whitworth".
Once you have ascertained, following Harold's instructions, that the red lever with the black knob is in fact the one that engages the "half-nuts (you may have to "wiggle" the handwheel on the left while engaging the half nuts. If the machine is running this won't be necessary) .
The 3 position lever between the half-nut lever and the handwheel engages the cross feed when in the top position and the longitudinal feed in the bottom position and in the middle when neither feed is chosen, that is where it should be when the half nuts are used for threading.

The bottommost lever is the "clutch" to engage either the crossfeed or longitudinal feed and is also not used when threading.
I'm pretty sure your feed screw is imperial (inch) but you can verify by counting threads over an inch.

I suspect that one of the gears you will need to select will have 127 teeth and probably be the largest.

Let us know how you make out.

I've updated my profile.

I'm living in Belgium and would like to configure the lathe for Metric Threading. In this case I need to thread a 12mm (diagonal) bolt with 1.75mm threads. How many teeth should be on the gear for metric threads? And what about the second (smaller) wheel that I have to (re)mount also?

"the red lever with the black knob" indeed activates the "threading mechanism", but the results don't correspond to what I expect to get (it differs from the table with all the threading combinations).

Since you say that for Whitworth threads I should probably use the bigger gear, would it be just a matter of mounting the smaller gear for metric threading?

BTW: I have used a Celtic 14 lathe in the past (spent hundreds of hours on it), so I do know (at least the basics) of using a lathe/threading.

Thanks for the reply!

[HDB]

With the machine not running, try engaging that handle. If it is the proper one to engage, the carriage will be somewhat limited to movement once it is engaged. You can verify that it's working properly by attempting to move the carriage by rotating the carriage hand wheel. If you find it moves, but only a miniscule amount, that's a positive sign, as it indicates that the half nuts have engaged the lead screw. Don't force the carriage, just lift the handle to the vertical position, which should disengage the half nuts.

If you have discovered that the lever is, indeed, for threading, I'd then recommend you run the machine at a slow speed, and take a light pass on a piece of material, using a threading tool, to see what thread pitch is generated. The feed rate when using the screw and half nuts will be much faster, which is why I recommend you make this attempt at slow speed, as the carriage has the potential to move rapidly.

Once you've ascertained that you now have the proper sequence for threading, you may be able to determine what system each gear combination creates.

The threading dial is critical for some engagements. Typically, if a machine is set up with an Imperial lead screw, the threading dial won't function properly when you are chasing a metric thread. In such a case, you'll have to leave the half nuts engaged and return the tool by reversing the machine. You'll have to experiment to see how yours behaves.

For Imperial threads, assuming the screw is Imperial, you may wish to experiment with the positions on the threading dial, so you'll understand when you can engage the half nuts so the thread tracks properly. If in doubt, always use the same line.


Harold

I have no idea whether it's an imperial lead screw or e metric one. How do I check? The vertical lever is indeed for threading. I'm (was) unfamiliar with the term 'Half nut', but I guess that "Engaging the half-nut" means "Engage the lathe for threading".

Thanks for your reply

[stevec]

Since you say that for Whitworth threads I should probably use the bigger gear, would it be just a matter of mounting the smaller gear for metric threading?

BTW: I have used a Celtic 14 lathe in the past (spent hundreds of hours on it), so I do know (at least the basics) of using a lathe/threading.

Thanks for the reply!

HDB, I did not say "that for Whitworth threads you should probably use the bigger gear" I said "I suspect that one of the gears you will need to select will have 127 teeth and probably be the largest"

I would venture to say that being made in Canada years before " going metric" your lathe has an inch leadscrew, the pitch can be determined by counting the number of threads in an inch of it's length.

Once you determine an arrangement of settings for threading and are using the 127 tooth gear you should be able to do the math required to find which combination of gears you require.

Note also that in order to thread metric on your lathe you should NOT disengage the half nuts in order to return the carriage for repeated passes. You will have to stop the lathe, back the tool away from the material using the crossfeed and then reverse the lathe to return the carriage beyond the start position of your thread. then return the tool via the crossfeed plus the increase in depth of cut you desire via the compound feed. This procedure will need to be repeated until the number of passes required to reach thread depth is achieved.

Good luck and let us know how you make out.

[HDB]

Ok, I have counted the teeth on the four gears have:

127
120
45
60

I count exactly 6 threads per inch on the leadscrew. So it's an inch leadscrew, right?

So now I should be able to determine which ones are the ones are for imperial threading and which ones are for metric... Can someone explain me how, my gut says that I have to find a 2.54-factor, but I can't find a combination that gives this.

HDB, I did not say "that for Whitworth threads you should probably use the bigger gear" I said "I suspect that one of the gears you will need to select will have 127 teeth and probably be the largest"

Why would you expect me to need the 127teeth one, and for which type of threading?

Thanks in advance

Thanks

Update: I can confirm that the right red black knobbed handle engages the half nut.

[stevec]

Ok, I have counted the teeth on the four gears have:

127
120
45
60

I count exactly 6 threads per inch on the leadscrew. So it's an inch leadscrew, right?


Why would you expect me to need the 127teeth one, and for which type of threading?

Thanks in advance

Thanks

Right, it's an inch leadscrew, 6 TPI

Try dividing 127 by 2.54 I think you'll find the answer is 50, an even number and the right divisor/multiplier for conversion from inch to metric and vice versa.
keep working at it you'll get there.

You know the lathe.

[dly31]

A quick look at your threading charts shows the following: For Imperial (TPI) threads moving from 1-10 on one selector gives you 4-8 TPI (finer threads) and moving ABC on the other lever gives you 4, 8, and 16 TPI (finer threads). For metric threads moving from 1-10 gives you 8-16 MM (coarser threads) and moving ABC gives you 8, 16, and 32 MM (coarser threads).

This means the power flow through the gearbox is reversed for metric threads. A change that gave more step-down now gives less. This is done for many of the higher end lathes where both Imperial and metric threading is done as it gives useful metric threads for all positions of the gearbox. This eliminates the need for using 5 or 6 different gears to get all of the metric threads.

Therefore there needs to be another 1 or 2 controls that are shifted to get the reversal of the gearbox set up correctly. Once this is done, the 127 tooth gear with one of the other gears should get you the correct pitch if your leadscrew is actually 6 TPI. If it is actually 4 MM, the 120 tooth gear would be needed.

[HDB]

Therefore there needs to be another 1 or 2 controls that are shifted to get the reversal of the gearbox set up correctly. Once this is done, the 127 tooth gear with one of the other gears should get you the correct pitch if your leadscrew is actually 6 TPI. If it is actually 4 MM, the 120 tooth gear would be needed.

Right, it's an inch leadscrew, 6 TPI

Try dividing 127 by 2.54 I think you'll find the answer is 50, an even number and the right divisor/multiplier for conversion from inch to metric and vice versa.
keep working at it you'll get there.

So let's say that it indeed is an inch leadscrew (I'll post a picture of it tomorrow), I should use the 127 tooth gear. The the question is which smaller gear I have to chose to let correspond with the 127 one:
The 45tooth one or the 60 tooth one?

dividing 127 by 2.54 I think you'll find the answer is 50

Would this imply that I have to use a 50 tooth gear in combination with the 127tooth one? Because I don't have such a gear, which would mean that I have at least one worthless gear

Thanks for the replies.

[stevec]

The number 50 is the mathematical ratio (50-1) NOT a suggestion of gear size to use.

If you can't determine whether to use the 45 tooth gear or the 60 tooth gear mathematically, you could try a trace cut, on your lathe, using one gear, on a piece of scrap, any diameter, one pass and measure the pitch that results. If it gives the pitch you require, you're good to go, if not try the other gear.
I trust you have a metric screw pitch guage?
Good luck.

[HDB]

The number 50 is the mathematical ratio (50-1) NOT a suggestion of gear size to use.

If you can't determine whether to use the 45 tooth gear or the 60 tooth gear mathematically, you could try a trace cut, on your lathe, using one gear, on a piece of scrap, any diameter, one pass and measure the pitch that results. If it gives the pitch you require, you're good to go, if not try the other gear.
I trust you have a metric screw pitch guage?
Good luck.

I do have both an imperial as a metric pitch gauge. How would I mathematically determine the number of teeth required on the smaller gear? Some calculation should give 45 or 60 as an answer, but I don't see what calculation that would be.