Changewheel conundrum

Either I'm missing something blindingly obvious (quite possible!) or there's summat wrong with my lathe.

Here is how the wheels are set up - they've been this way since I bought the machine, and I've had no need to change anything (it's a gear head, I've never needed to exceed the range of feeds/threads offered by the gearbox with this setting before now):

Well, that's come out sideways for some reason (can anyone explain why this happens?) but what you're looking at is a 24T driving a 120T idler on the banjo driving a 48T on the input to the gearbox.

I want to cut a 2mm pitch thread, so:

just swap round the 24 and 48T and use the 127T as an idler. The only reason for using the 127T is (as far as I can see) that a straight swap of the 24/48 would lead to the outboard spacer on the 24T fouling the 127. So are you meant to turn the idler so the 127 is nearest the headstock? But then the spacing is wrong - the 120/127 wheel is made in one piece and runs on bearings which seem to be press fitted to the shaft which bears on the banjo, so I don't think I'm meant to shift it along the shaft.

I'm really hoping that I'm being stupid here and someone with the same or a similar machine will point out a simple solution so I don't have to machine parts unnecessarily.

Robin.

127/120 is a imperial / metric gear conversion set. Check the pitch of your lead screw and it will be imperial. The chart I think is showing you where the 127 wheel should go, the 120 will mesh with the other gear.

Thanks Zan. I understand the function of the 120/127 wheel for threading - the lathe actually has a metric leadscrew, and I have no problems using it to make imperial threads. My difficulty is with arranging the 127 as an idler rather than a a pitch converter - it just doesn't seem to mesh any which way!

Robin

Edited By Robin Graham on 13/11/2018 23:42:47

It makes no difference to the overall gear ratio what size idler you use. Irrelevant to the ratio whether you use the 127 or 120 as an idler. All that matters is the ratio of the first and last gears in a simple train like this.

There should be some provision to move the stud the idler gear mounts on to accommodate different sized idler gears. Have a close look at it. There should be some kind of slot, or eccentric that allows the stud to move, or the stud should be mounted on a quadrant bracket that can be pivoted to change the stud position relevant to the other two gears.

In your photo it looks like there is a quadrant, the grey piece about 20mm thick in behind the bottom gear. Pull the large idler gear off and have a look at what is going on there. If in doubt post us a photo of what is there.

Details of the make and model of your lathe would help others with the same machine offer their experience.

Edited By Hopper on 13/11/2018 23:40:18

Hi Robin,

Regarding the photos, I post mine up from an iPhone, if I take the photo portrait, it always ends up on its side in here. If I take the photo landscape, it orientates correctly. Don’t know if android is the same?

Cheers

Mark

I assume thet that the 127T gear is the one closest to the headstock and the bottom of the picture is to the right.
Remove the outer gear (The one with more than 127 teeth.) and replace it with any small gear or spacer (If supplied with your lathe. Now re fit the gears and sleave so that the small gear (Or spacer) is closest to the headstock. I recently had to make a longer stud to go on the banjo and an extra pivot point for an idler to enable me to cut a very coarse thread (About 40 mm pitch) Regarding the picture. Many cameras put information in the meta data part of the picture file which indicates which way the camera was orientated. I think some image processing software does not deal with this information correctly when rotating a picture.
You may have to rotate the spindle or the leascrew a small amount to get the gears to mesh. You will have to move the position of the pivot stud on the banjo slightly. You may also have to move the position of the banjo slightly.

Les.

Edited By Les Jones 1 on 14/11/2018 08:48:23

Edited By Les Jones 1 on 14/11/2018 08:54:20

If the Leadscrew is Metric, and you are cutting Metric threads, as long as the meshes are correct for backlash, the choice of Idler is immaterial. The important ratio is between the Driver and the Driven. The Idler fills the space between the Driver and Driven and ensures that both rotate in the same direction, unless you want a Left hand Thread.. If so you need to insert another Idler!

If you want to cut an Imperial thread, then you do need to compound the 120 and 127.

Howard

.

I beg to differ, Les

The opening post includes a clear description of 'what you're looking at'

... 24T driving a 120T idler on the banjo driving a 48T on the input to the gearbox.

MichaelG.

Edited By Michael Gilligan on 14/11/2018 23:01:25

Thanks for replies.

I should have said that the lathe is (was- now obsolete) an Axminster version of the generic '12x36' machine marketed with minor differences by Chester as the Crusader and in the US by Grizzly as a gunsmith's lathe. The late lamented John Stevenson who helped me install it said it was a Harrison M300 clone - but the arrangement of the wheels between the motor and the gearbox is quite different from the Harrison.

I understand that the size of the idler doesn't matter, the only reason for using the 127T is to give physical clearance for the other wheels.

I've had another look and I'm now pretty sure that it just can't work without without making some new bits. Not a massive problem, but I'm surprised as the machine has been faultless in other respects.

Robin.

I like that... a ,125" lead screw times 120 divide by 127 equals exactly 3mm

Last 2mm pitch I cut I used 25:40...

.125" x 25 / 40 = 1.984mm that's 16 um short.

.

... and my avatar represents a 40 um square

... sometimes the microns matter, sometimes they don't.

MichaelG.

This thread here **LINK**

includes pics posted by Mr Stevenson himself of a remarkably similar looking lathe, showing the moveable banjo plate that pivots around the leads screw boss via pinch bolt visible in his pic. There also appear to be slots in the banjo to move the gear stud to accommodate the larger gear.

Does yours not have this piece in place?

There are also other pics in later posts in the same thread showing another type of banjo mounted more centrally between leadscrew and headstock spindle.

If you don't have something like this already, you will have to make something up to approximate it. Seems odd that they would supply the 127/120 gear but no facility to move the stud to fit it?

I just looked at your original picture again, turning it right way up for clarity, and it sure looks like there is something there where the quadrant should be. That grey unpainted piece of cast metal behind the leadscrew gear? The one the idler gear stud appears to mount to? What is that?

Edited By Hopper on 15/11/2018 01:12:55

Hi Michael,
I misread the first post. I thought he was using the 127 tooth as an idler in both setups.

Les.

Sorry its nothing like a Harrison M300 take no notice. It only uses a 40, 44, 88 and a 95 tooth for any thread whether imperial or metric. The only changes needed are to swap the 44 to the 88 and vice versa thats it, any pitch up to a certain range around 4mm pitch upwards or 3 TPI is the change over point.
https://www.model-engineer.co.uk/sites/7/images/member_albums/75533/816826.jpg

https://www.model-engineer.co.uk/sites/7/images/member_albums/75533/816827.jpg

Thanks for finding that thread Hopper - JohnS did say he had a very similar lathe. I now see what the problem is, though I'm not sure how to solve it yet!

This is John's pic of the gear train on his machine taken from his thread:

There is a spacer behind the wheel driving the 127/120 compound which allows it to reach over the 120 and engage with the 127.

My machine:

The drive shaft doesn't extend far enough to allow the drive wheel to engage with with the 127T in the way shown in John's pic.

I now understand how it's meant to work, I just have to figure out how to fix it - any suggestions welcome!

Robin.

Is it not possible to swing the banjo further away from the line joining the 48 and 24 tooth gears From your picture on 15/11/18 at 22:18 I can see that swinging the banjo anti clockwise may make the 127 tooth gear touch the belts but is you swing it clockwise so that the 127 tooth gear is to the right of the line jining the 24 and 48 tooth gears it looks like there is nothing to get in the way. I have rotated your original picture to make it easier to view.

Les.

Thanks for putting the photo in the right orientation Les, it does make it clearer. The fundamental problem is that if I just swap the 24T driver and the 48T driven using the 120 idler, the outside spacer on the now driven 24 fouls the 127 wheel. No reorientation of the banjo will change that. The 127/120 compound is made in one piece so I can't just swap the wheels around.

Looking at the photo of JohnS' machine I'm pretty sure that there is a problem with the factory assembly of the gearbox. More anon I expect!

Robin

Edited By Robin Graham on 16/11/2018 00:03:28

Can you not just pull the 120/127 gear off and flip it over so so the larger 127T gear is inboard and engaging with the driver gear and the driven gear? Of course, the position of the stud and banjo would need to be adjusted to suit.

Fit the 48T on the drive with the spacer facing outwards or omit the spacer all together

Fit the 120/127 on the stud with the 127 nearest the machine

Fit the 28T on the input shaft

All three will be lined up.

You will have to swing the banjo to a new position and also move the stud along the banjo so all three mesh correctly

Edited By JasonB on 16/11/2018 08:01:08

Amazing what you find when you look in the manual!