metric thread on my imperial lathe

[SillyOldDufferModerator @sillyoldduffer]

Posted by chris mcnicoll on 27/04/2017 18:03:24:

sorry Dave, im rubbish with computers as you can tell

Not at all Chris – it's working now.

Just in case anyone else needs them, Neil's photo posting instructions are here.

Thanks,

Dave

[chris mcnicollParticipant @chrismcnicoll83540]

right all i need now is to work out that calculation and select the gears i guess

OK maybe sit quietly and let someone help me

[BazyleParticipant @bazyle]

The top gear in photo two is probably rotating at the same rate as the spindle. Check this by rotating the spindle by hand.
Also it looks like that gear is 25 tooth which would be a common value.
Also you have a partial gearbox with settings ABC. You can check what each setting gives as a reduction ratio. Probably one is 1:1, so what are the others?

Now your leadscrew is 4 tpi so each full turn gives you about 6mm pitch. But you want 1.5mm so roughly you want a 4:1 reduction between the spindle and leadscrew. This is a rough sanity check.
The actual gears (if your little gearbox is set to 1:1) are 25:50:60:127. That is 25/50 X 60/127 which is roughly 1/4 or a four to one reduction. The 50 and 60 are on the intermediate stud.

[chris mcnicollParticipant @chrismcnicoll83540]

So I need 4 gears? Forgive my newbie questions

[Les Jones 1Participant @lesjones1]

Hi Chris,
I recon if the first gear is running at spindle speed then fitting a 30 tooth gear in that position and driving the 127 tooth gear directly from this (Or via idler gears just to make up space.) should give you 1.5mm pitch with a $TPI leadscrew and the gearbox set to 1:1 ratio.

30/127 =0.2362204724409449 x 0.25" = 0.0590551181102362"

0.0590551181102362" x 25.4 = 1.5mm

Les.

[HopperParticipant @hopper]

First thing to do is check the ratio between the headstock spindle and that first gear sticking way out there. It SHOULD be 1:1, but who knows for sure unless it is checked?

Also, check to see if that round knob sticking out the front of the headstock in that same area is a tumbler reverse mechanism, ie does it reverse the direction of rotation of that first gear when the knob is moved?

If all is good, one way, with useful application when cutting other Metric threads on your lathe, might be to use that secondary gearbox on your leadscrew with the A, C, B labels on it. If one of these settings reduces the drive to the leadscrew by 2:1, (ie two turns of the drive gear becomes one turn of the leadscrew) then you have in effect an 8tpi leadscrew instead of a 4tpi. You can then use a standard gearing chart (such as Cleeve page 45) for an 8tpi lead screw. This method would give you a gear train for 1.5mm pitch of 50 – 25/30 – 127 (plus of course the 2:1 reduction in that final ABC gearbox).

Otherwise if you just want the quick set up for one job, the 30 – Any Idlers – 127 suggested by Les Jones above is the way to go. On Cleeve's chart for 8tpi it is listed for .75mm pitch, so will give 1.5mm with 4tpi leadscrew. You can always use Cleeve's chart this way for other sizes too, if like me you are mathophobic.

Best thing to do is set up a test piece and try it out and check with a metric thread pitch gauge before tackling the job itself.

Edited By Hopper on 28/04/2017 01:58:57

Edited By Hopper on 28/04/2017 02:00:14

[chris mcnicollParticipant @chrismcnicoll83540]

The knob on the front near the gears turns the lead screw into forward or reverse, the other gear that is at the top with the lever folds up and that stops the chuck from turning

[chris mcnicollParticipant @chrismcnicoll83540]

Just had a quick look in the gears box but no 30 gear

[HopperParticipant @hopper]

Posted by chris mcnicoll on 28/04/2017 06:44:11:

… the other gear that is at the top with the lever folds up and that stops the chuck from turning

That will be your back gear. It is used to slow the headstock spindle down for screw cutting. There will be a pin or dog of some sort that disengages the belt pulley unit from the spindle so the gearing is reduced through that gear with the lever on it when engaged.

You should see if Tony at lathes.co.uk can supply you with a manual for the lathe, or if one for the later L5 would suffice.

Did you work out what that lever at the bottom, labeled ACB does? Any reduction in leadscrew revolutions there, or is it merely to engage/disengage leadscrew/feedscrew??? That could make a big difference to things.

[chris mcnicollParticipant @chrismcnicoll83540]

The a b c lever has a chart under it and on the chart it shows the changing it for different tpi etc on imperial threads, I'll get photos after work

[Michael GilliganParticipant @michaelgilligan61133]

lPosted by Hopper on 28/04/2017 07:05:51:

Posted by chris mcnicoll on 28/04/2017 06:44:11:

… the other gear that is at the top with the lever folds up and that stops the chuck from turning

That will be your back gear. It is used to slow the headstock spindle down for screw cutting. There will be a pin or dog of some sort that disengages the belt pulley unit from the spindle so the gearing is reduced through that gear with the lever on it when engaged.

.

Chris,

Further to Hopper's wise words ^^^ … you may find this page useful:

**LINK**

http://www.lathes.co.uk/latheparts/page4.html

MichaelG.

 

Edited By Michael Gilligan on 28/04/2017 07:14:51

[HopperParticipant @hopper]

Posted by chris mcnicoll on 28/04/2017 06:46:58:

Just had a quick look in the gears box but no 30 gear

That makes things a bit awkward. You could go to this online change gear calculator **LINK** and go down to the "other lathes" section at the bottom, type in what gears you do have and it will give you a variety of combinations to achieve the desired pitch filled in at the top.

Alternatively if you look at Cleeve page 55 for how to cut metric on imperial change gears with an 8tpi leadscrew and use the gearing for a .75mm pitch (which gives you 1.5mm with a 4tpi leadscrew) there is a suggestion for 40 – 38/35 – 60/25 – 65, if you have all those gears in the box. It would require two studs on the quadrant to mount the two pairs of compounded gears.

Edited By Hopper on 28/04/2017 07:18:09

Edited By Hopper on 28/04/2017 07:18:42

[HopperParticipant @hopper]

All this is assuming of course that the first small gear sticking out the side of the headstock turns at a 1:1 ratio with the headstock spindle. Have you checked this?

Also, have you joined the Yahoo Groups group for Harrison Lathes? There might be a gear chart etc in the FILES section of that group, and maybe a manual.

Edited By Hopper on 28/04/2017 07:36:06

[chris mcnicollParticipant @chrismcnicoll83540]

[HopperParticipant @hopper]

Posted by chris mcnicoll on 28/04/2017 07:53:24:

OK. that is right handy. We can deduce from this chart that the top gear's shaft rotates at 1:1 with the headstock, because the gearing for 4tpi with a 4tpi leadscrew is 60-idler-60, ie 1:1. Presuming that C is 1:1 also.

To cut an 8tpi thread you use the same gears but put lever in position A. We can deduce that position A therefore doubles the ratio to the leadscrew.

Position B appears to increase the leadscrew rotation by a factor of 1.5, as in the 6tpi setting where the gears used create a 1:1 ratio the the lever increases that to 1:1.5.

So I guess you would leave it in position C for metric screw cutting. ie 1:1 and use the gear train for ratios.

But I may be wrong so you need to check it out in acutallity by rotating the large gear at the bottom one revolution and noting how many turns the leadscrew actually turns. Ditto the headstock spindle and first gear, just to make sure they are 1:1.

[HopperParticipant @hopper]

Just another stray thought (sitting in the armchair with the flu today, so lots of them about) is an alternative to using the 30-idler-127 combination would be to substitute a 60T gear for the 30 and then use a compound gear in the idler position to get the 2:1 reduction to have the same effect as a 30.

This could be 60 – 50/25 – 127. So you would have a 60 on the top shaft meshing with a 50 on the quadrant stud. The second gear on that stud, keyed to the first gear, would be a 25 which would mesh with the 127.

All this is assuming your lathe has change gears that can be compounded two to a stud? Not sure from the pics if that is so? If not, buying a 30T gear on eBay might be your only option.

[Neil WyattModerator @neilwyatt]

If you use a 63 tooth gear as input and 50-tooth gear as output you will get a pitch of :

63/100 x 0.250" = 0.1575" = 4.0005 mm, which is as close to 4mm pitch as you can expect any workshop lathe to achieve.

A 50T output gear would give 8mm pitch.

If you can extend use a compound gear in the middle instead of an idler and settle on 63 and 100 or 50 as your input and output gears you can achieve almost any useful metric pitch easily by using the compound gear's ratio to multiply 4mm or 8mm.

Examples:

1mm 63:80 20:100 (80/20 = 4 4mm/4=1)

1.5mm 63:80 30:100 (80/30 = 2.66667 4mm/2.66667 = 1.5

5mm 63:40 50:100 (40/50 = 0.8 4mm/0.8 = 5

All to an accuracy of about 1 part in 10,000 or one wavelength of red light.

Simples!

Neil

[Martin ConnellyParticipant @martinconnelly55370]

Hopper, doesn't the selection of A halve the speed of the lead screw compared to position C to give 8tpi instead of 4tpi?

Martin

[HopperParticipant @hopper]

Posted by Martin Connelly on 28/04/2017 09:31:40:

Hopper, doesn't the selection of A halve the speed of the lead screw compared to position C to give 8tpi instead of 4tpi?

Martin

You're right, I think. Told you I had the flu! My head is mush. If C is 1:1 the leadscrew will turn one  turn for each turn of the headstock spindle in order to cut a 4tpi thread. Selecting A to cut an 8tpi thread means the leadscrew must turn half of a turn per revolution of the spindle, ergo half.

So you should be able to select A and then set your change gears as per Cleeve's charts for 8tpi leadscrews. Which still means you need either that 30T gear for the direct 30/127 ratio or the long studs and keys to run compound gearing with two gears on one stud. Or get one of Neil's 63T gears!

If you had to, you could turn up longer studs to mount the change gears on and drill them for small drive pins that engage in a hole in the other gear, the way Drummond change gears do.

Edited By Hopper on 28/04/2017 10:39:35

[chris mcnicollParticipant @chrismcnicoll83540]

Where is the best place for buying new gears if I don't have the correct ones in my collection

[Les Jones 1Participant @lesjones1]

Hi Chris,
If you list the gears that you have and the ratios available from the gearbox then we may be able to find a way to use them. For example if you had a 60 tooth gear to use in place of the 30 tooth and you set the gearbox for 2:1 ratio that would give the same result as a 30 tooth and the gearbox set to 1:1

Les.

[chris mcnicollParticipant @chrismcnicoll83540]

Ah right I will do that tonight

[BazyleParticipant @bazyle]

From the right hand end of the table (left in upside down picture) we see the first three rows have the same gear configuration but the lever is set to C,B,A. which clearly gives you those ratios.
Interesting that it is designed for 1tpi to 20 tpi. Big stuff only none of that tiddley stuff model engineers do and cleverly minimises the number of loose gears it needs.

So the lathe originally only had 20, 40, 50, 55, 60, 60 again, 70, 80. 30 wasn't needed. Also it always used the intermediate gear as an idler so no need for a compound arrangement in the middle.

To allow for the gears you (should) have the train is now 20/40, 60/127 using the 40 and 60 linked together on a new intermediate stud for a compound pair ( which you would have to make) and the gearbox in the 1:1 position. However luckily for you you can get the 1:2 ratio provided by the 20/40 from the position C of the gearbox.

So the final configuration is 60 on top shaft, anything on the intermediate stud, 127 on the input to the gearbox, gearbox position C.

[chris mcnicollParticipant @chrismcnicoll83540]

Cheers I'll be busy trying Tomorrow morning

[Ralph sheheenParticipant @ralphsheheen88046]

I have a 1940 triumph that cuts good imperial threads. I am getting a 127 for metric. Looking forward to going modern.

[Author]

Posts