Feeling a little restricted.

[Richard.]

Ok. So confusion has now set in.
My lathe behind the cover has all the combinations detailed for putting the gears in place for both metric and imperial threads. I also have all the gears available for all the combinations. Am I right in thinking it will only cut one or the other or can I cut both by simply setting the change wheels right. If I wanted to swap from metric to imperial or viva verca do I need to make drastic changes like lead screw. I thought it was only done on feed speed but it sounds like there may be more to it. I haven't yet checked whether my screw is metric or imperial. The dials are all metric so I assume it's a metric lathe. Sorry for all the questions guys but I'm trying to get my head around it all and I guess I thought the screw cutting was a simple case of if you had the gears it would cut it.

 

[Brad93]

Exactly what I've been doing. Cheers fellas. Need some tooling now mind and a DTI. Any links to either would be good as I don't want to be buying twice. Is it worth me buying the hss tools and leaning how to grind them up? Should learn about it really.

DTI wise you've got Baty, Mercer (not around now I think but can pick up used ones), Mitutoyo, Kennedy, Starett. Most of my measuring equipment is Mitutoyo and it's always been good quality so I would go for that.
Cromwell's usually have everything in stock by them. You need to choose and applicable resolution to what you are doing though and the relevant accuracy. I.e. some have 25mm of travel some only 1/4" but may be accurate to 0.0001"

 

[MattH]

Ok. So confusion has now set in.
My lathe behind the cover has all the combinations detailed for putting the gears in place for both metric and imperial threads. I also have all the gears available for all the combinations. Am I right in thinking it will only cut one or the other or can I cut both by simply setting the change wheels right. If I wanted to swap from metric to imperial or viva verca do I need to make drastic changes like lead screw. I thought it was only done on feed speed but it sounds like there may be more to it. I haven't yet checked whether my screw is metric or imperial. The dials are all metric so I assume it's a metric lathe. Sorry for all the questions guys but I'm trying to get my head around it all and I guess I thought the screw cutting was a simple case of if you had the gears it would cut it.

A lathe with either Imperial or Metric leadscrew can cut both thread types with the appropriate change gears or gearbox settings. If you have a Metric leadscrew and Threading dial then you must leave the halfnuts engaged throughout when cutting Imperial and vise versa.

 

[Richard.]

So what your saying Matt if I leave the half nuts ingaged throughout the cut and fit the right gears in accordance with the diagram and make sure the tdi is always set off at the same place it will cut the right thread dispite it been metric or imperial.
So all I need to do for any thread in its range is change the gear ratio no matter what lead screw type I have and tdi pitch?

 

[MattH]

Yes essentially. FYI if the halfnuts are left engaged the TDI is unimportant, the TDI is just for making sure you re-engage the halfnuts in the same position on the leadscrew on each threading pass.

Have a read or this:

http://www.lathes.co.uk/myfordml10/

 

[Wallace]

Richard, if it helps, try thinking of the DTI number as a timing mark for TDC on the start of the thread like cam timing.

 

[Richard.]

Richard, if it helps, try thinking of the DTI number as a timing mark for TDC on the start of the thread like cam timing.

Yes that makes sense mate. So the lead screw pitch is also irrelevant just so long as the timing marks are all alined before you start the engine you shouldn't bend any valves.

 

[Pete.]

That's an interesting read Pete.
I'm unsure as to whether I can change the number of teeth on my thread dial gear as it has 2 gears both sides of the lead screw and they appear to be fixed not adjustable so changing the size of them in my mind would mean they wouldn't physically fit in. Could be wrong mind.

If your threading dial has more than one gear on it there will be a nut on the bottom. You remove the nut and slide off the stack of gears and swap them around to use the one you need. The dial should be on a swing-bolt so loosen the bolt and you can adjust the engagement. It doesn't have to be a close fit at all since backlash etc doesn't affect how a threading dial works so don't be too fussy about setting the right running clearance.

the fact that it has more than one gear makes it a fully metric machine (dials and leadscrew are all metric). It WILL cut imperial gears no problem, but not using the dial unless you want to go through a bit of a rigmarole.

 

[Richard.]

Yes essentially. FYI if the halfnuts are left engaged the TDI is unimportant, the TDI is just for making sure you re-engage the halfnuts in the same position on the leadscrew on each threading pass.

Have a read or this:

http://www.lathes.co.uk/myfordml10/

If the half nuts are always engaged I guess the tdi will never move or need to be worried about.
To take the slide back for the next cut do I just reverse the lathe or disengage, take it back and reengage on the mark while its spinning. The dog drive was handy cos I could use that to engage while the half nuts stayed engaged but I've been told not to do that.

 

[Richard.]

If your threading dial has more than one gear on it there will be a nut on the bottom. You remove the nut and slide off the stack of gears and swap them around to use the one you need. The dial should be on a swing-bolt so loosen the bolt and you can adjust the engagement. It doesn't have to be a close fit at all since backlash etc doesn't affect how a threading dial works so don't be too fussy about setting the right running clearance.

the fact that it has more than one gear makes it a fully metric machine (dials and leadscrew are all metric). It WILL cut imperial gears no problem, but not using the dial unless you want to go through a bit of a rigmarole.

So do I need to change the tdi gear Pete for various threads. Thanks for the info on the lathe so it's defo a metric. This is a good thing so it should cut all metric threads with just wheel changes.

 

[Richard.]

When I say the tdi has more than one gear I mean there is another gear below the lead screw. The tdi gear is a direct drive on the screw.
Like the sketch below. Sorry I should of been clearer.

 

[MattH]

If the half nuts are always engaged I guess the tdi will never move or need to be worried about.
To take the slide back for the next cut do I just reverse the lathe or disengage, take it back and reengage on the mark while its spinning. The dog drive was handy cos I could use that to engage while the half nuts stayed engaged but I've been told not to do that.

Yes, so if cutting imperial you would leave the halfnuts engaged (ignore the tdi), at the end of the cut you would stop the lathe (so the relief cut and a slow speed are important), backoff the cross slide a set amount, reverse the lathe until the tool is back at the start of the cut, stop the lathe and reset to forward, advance the cross slide back the same amount as it was backed off, put your cut on with the compound and then restart the lathe.

 

[Richard.]

Yes, so if cutting imperial you would leave the halfnuts engaged (ignore the tdi), at the end of the cut you would stop the lathe (so the relief cut and a slow speed are important), backoff the cross slide a set amount, reverse the lathe until the tool is back at the start of the cut, stop the lathe and reset to forward, advance the cross slide back the same amount as it was backed off, put your cut on with the compound and then restart the lathe.

You make it sound easy mate.

 

[Newlands]

Ok. So confusion has now set in.
My lathe behind the cover has all the combinations detailed for putting the gears in place for both metric and imperial threads. I also have all the gears available for all the combinations. Am I right in thinking it will only cut one or the other or can I cut both by simply setting the change wheels right. If I wanted to swap from metric to imperial or viva verca do I need to make drastic changes like lead screw. I thought it was only done on feed speed but it sounds like there may be more to it. I haven't yet checked whether my screw is metric or imperial. The dials are all metric so I assume it's a metric lathe. Sorry for all the questions guys but I'm trying to get my head around it all and I guess I thought the screw cutting was a simple case of if you had the gears it would cut it.

I'm assuming you have a metric Myford. As far as I know even the metric Myfords are fitted with an imperial leadscrew, but if I'm wrong it doesn't matter too much, provided you know exactly what you have. So first check the leadscrew pitch as accurately as possible as it is probably 8 tpi but it could be 3 mm pitch which is very near but not the same. Then make a list of all the gears you have so you know what numbers you have to play with.

Now for the fun bit......
Imperial, TPI = driven/driver x driven/driver x leadscrew TPI
Or:
Metric, Pitch = driven/driver x driven/driver x leadscrew pitch in mm.

Using the imperial as an example, and just guessing at what gears you have, if you fitted an 80t at the top, followed by a 25, then a 63 on the same shaft as the 25, driving a 80t gear on the leadscrew shaft, you would cut a thread tpi as follows:
Tpi = 80/25 x 63/80 x 8
Tpi = 20.16, ie 20.16 teeth per inch of thread, which converts to 0.794mm pitch
If you were shooting for a 0.8 mm pitch then you'd only be 6 thou out or 0.75% which would probably be fine for anything but the longest thread (certainly ok for a normal nut and bolt, or a plumbing fitting).
If your first guess at the right gears to use is wrong, have another go by altering one or more of the gears to increase or decreas the tpi accordingly.

On a practical note, when you fit the gears, mesh them together with a strip of newspaper between the teeth so you get the right clearance between the teeth - too tight and they will bind which is very bad, too loose and they'll clatter like hell which is not quite so bad.

If you post what gears you have, plus the lead screw tpi or pitch details and the thread you want to cut, we can all have a play with the numbers. But bear in mind that the gear combination that looks good on paper has to actually fit and mesh under the headstock cover!

 

[MattH]

Or, don't worry about screwcutting until you have your Bantum with a nice gearbox! Sooooooo much easier.

 

[Richard.]

Or, don't worry about screwcutting until you have your Bantum with a nice gearbox! Sooooooo much easier.

That's just throwing in the towel.
Something ive never done throughout my life. This little Myford will cut threads when I know what I'm doing. I've ordered myself a lump of 40mm Alu solid round as Alu is better to learn on than steel. Copper and brass are too expensive to turn into swarf with no purpose or product at the end of it. Now I'll have to wait for tonight before I measure the lead screw but my gears are as follows.
1x20t
2x21t
2x25t
1x30t
1x35t
1x40t
1x45t
1x50t
1x55t
1x60t
1x65t
1x70t
1x75t

 

[Newlands]

Surprised you haven't got a 63t among that lot - that's the gear needed to cut metric threads with an imperial leadscrew (actually a 127t is better, but might be physically too big to fit). Anyway, you've got a good selection.

There are a couple of thread cutting programs here http://www.lathes.co.uk/latheparts/page14.html which might be useful, along with a load of good info on the subject.

 

[Richard.]

I don't think I've got a 63 but will confirm for sure later on this eve.
Got no chance with a 127.
Surprising though as none of the threads suggest using those sized gears on the chart.

 

[Richard.]

This is what I have sitting behind the gear cover on my lathe. There is no mention of those gears on it. Are these combinations any good.

 

[Pete.]

Can we have a better pic please?