Modify ML7 gearbox to do longer lead lengths

[Richard Howells 2Participant @richardhowells2]

[Richard Howells 2Participant @richardhowells2]

Hello all, first time posting here! I am lucky enough to have an ML7, capable of 4mm leads when screw cutting with the gearbox's various gear configurations. I thought that would do me for the rest of my days, until I needed to do a 7mm lead (yes 7 full millimetres) of trapezoidal m40 on an internal thread. This is almost double the lead lengths of the ML7, but could it be easily modified to do this? If it would be more work and risk of damage to the lathe then sending off the part to a professional with bigger toys might be the best thing to do…

Thank you all in advance of any help, wisdom or just whitty jokes you leave here!

[Howard LewisParticipant @howardlewis46836]

7 mm pitch will impose quite a load on the gearbox and gears, you are asking to drive the Leadscrew a lot faster than the chuck.

The usual advice given by others for situations such as this is to attach a handle to the Leadscrew, and to provide the drive manually.

HTH

Howard

[JasonBModerator @jasonb]

Posted by Richard Howells 2 on 15/11/2020 20:52:21:

………., but could it be easily modified to do this?

I would have though selecting 3.5mm pitch on the gear box and then playing about with the external gears to double up the rate that the gearbox input shaft turns at would give the pitch but as said manually turning the lead screw would be safer though it's still a heavy cut at whatever pitch.

[egaParticipant @ega]

You would clearly be looking to reduce the weight of cut in any way possible: free-cutting material, a sharp tool and light cuts. I don't remember ever having used the angled-in feed method for internal threads but it might be helpful here.

As has been indicated, you don't need to modify your lovely gearbox, only the input gearing!

7mm pitch is about twice the normal maximum – something to think carefully about.

[Brian WoodParticipant @brianwood45127]

Hello Richard,

If you have the metric banjo you can set up a gear train, including the gearbox, to give close to your desired pitch

With the gearbox set for 8 tpi the external gearing will be 55 [ mandrel] —-25 [gearbox input] with idlers between the active gears. Alternatively, with the gearbox set to 10 tpi, those gears become 55—-20, again with suitable idlers to link them

In both cases the pitch achieved will be 6.985 mm. But, as Howard has pointed out and no doubt others will say the same, using the lathe to cut a screw pitch of less than half that of the the leadscrew puts a very heavy load on the gearing and the drive should be made using the leadscrew as the power input to have any chance of a successful outcome.

If you can get a quotation you are happy with, I would farm the job out to someone with a big lathe which could handle the job with greater ease.

Regards Brian

[Nigel McBurney 1Participant @nigelmcburney1]

Putting the job out must be better than than risking the cost of a wrecked gear train or gear/box,There was a similar problem with spiral milling where very short leads cause great strain of the gearing ,Browne and Sharpe sold a short lead attachment for their universal milling machines,similar to driving the lathe leadscrew by hand,or to get a steadier drive,make a motorised drive .

[Dave HalfordParticipant @davehalford22513]

Is the back gear slow enough to you to control the cut? My lathe requires pitches of 4 to 7 tpi to be cut at 50rpm. These are the fastest available on the leadscrew gearbox.

[JasonBModerator @jasonb]

You don't need to worry about the spindle speed if driving the leadscrew backgear or not.

[Dave HalfordParticipant @davehalford22513]

Posted by JasonB on 16/11/2020 12:09:29:

You don't need to worry about the spindle speed if driving the leadscrew backgear or not.

It doesn't affect the helix obviously, but the apron speed might be interesting at a 1000 rpm if you need to stop on a mark.

[Brian WoodParticipant @brianwood45127]

Posted by Dave Halford on 16/11/2020 15:40:37:

Posted by JasonB on 16/11/2020 12:09:29:

You don't need to worry about the spindle speed if driving the leadscrew backgear or not.

It doesn't affect the helix obviously, but the apron speed might be interesting at a 1000 rpm if you need to stop on a mark.

Hello David,

I think you are forgetting that the driving power for coarse thread cutting is coming from the leadscrew so stopping on a mark just means letting go of the winding handle if you are doing this purely manually.

Professor Chaddock, the man who devised the Quorn tool and cutter grinder went even coarser at 3/4 inch pitch for the height adjusting thread on the column and for a Myford lathe, or any other with 8 tpi leadscrew, that requires gearing back to the headstock in a ratio of 1:6

Regards Brian

Edited By Brian Wood on 16/11/2020 16:40:40

[JasonBModerator @jasonb]

As Brian says by hand driving the leadscrew it just everything stops dead, two hands and a better handle would help but this is 32mm pitch. Infact you would probably be best disengaging any drive between motor and spindle as there is no point rotating the motor as well.

You could probably hand crank the leadscrew at about 60rpm which would mean the spindle is turning at around 27rpm if leadscrew is 8tpi.

Edited By JasonB on 16/11/2020 16:59:55

[egaParticipant @ega]

Jason B:

I see I am not the only one who drops Allen keys!

That said, I think there is a kind called "Magic Ring" that stay in the socket.

[Dave HalfordParticipant @davehalford22513]

Posted by Brian Wood on 16/11/2020 16:39:38:

Posted by Dave Halford on 16/11/2020 15:40:37:

Posted by JasonB on 16/11/2020 12:09:29:

You don't need to worry about the spindle speed if driving the leadscrew backgear or not.

It doesn't affect the helix obviously, but the apron speed might be interesting at a 1000 rpm if you need to stop on a mark.

Hello David,

I think you are forgetting that the driving power for coarse thread cutting is coming from the leadscrew so stopping on a mark just means letting go of the winding handle if you are doing this purely manually.

Professor Chaddock, the man who devised the Quorn tool and cutter grinder went even coarser at 3/4 inch pitch for the height adjusting thread on the column and for a Myford lathe, or any other with 8 tpi leadscrew, that requires gearing back to the headstock in a ratio of 1:6

Regards Brian

Edited By Brian Wood on 16/11/2020 16:40:40

Can't hand drive the lead screw on mine.

[old martParticipant @oldmart]

I have done similar things on the Smart & Brown model A at the museum. The Norton QCGB only goes to 8tpi as standard, so to do 4 and 5 tpi, I had to double the gear ratios and select 8 and 10tpi. There is a safety link in the gearing, and as I was concerned about that breaking part way through, I disabled it. Great concentration was needed to avoid a disaster. I used the back gear, but the option of hand winding is safer, as you have time to think. You can easily make a handle, or there are many ready made ones for the Myford lathes.

Edited By old mart on 16/11/2020 19:14:47

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