Need help to figure out gearing to cut metric gears on standard lathe

dantech · Oct 3, 2019

I have been trying to figure this out for the past 2 days but getting old sucks and my brain doesn't work like it used to.

Here's what I have;
11" Harrison lathe model L5A. The gears now to cut standard threads are as follows; 25tooth driving an 80 tooth driving a 100 tooth.
I need to know which gears I need to convert this machine to be able to cut metric threads.
FYI, I do have a 127 tooth gear for this machine, but I believe I'm missing 1 or 2 more gears. I have no idea how to calculate what I need.
Any help would be greatly appreciated.
Thank-you.

dantech · Oct 3, 2019

I guess a picture is worth a thousand words.
First picture shows the spare gears I have, the second picture is how the lathe is setup for cutting standard threads. I have tried all different combinations to try and cut a metric thread but have had no luck.

Limy Sami · Oct 3, 2019

The conversion factor / ratio (whatever) on my Boxford (SB clone) is 127/100 - the 100T being pinned to the 127 to work at part of a compound gear train.

Do I would say you are at least a 100T gear, ………...the exact gear train and g'box position for a given metric thread should be in the handbook for your lathe.

Dan from Oakland · Oct 3, 2019

or a 50 tooth and make up the 2:1 difference elsewhere in the gear train.

johnoder · Oct 3, 2019

Useful to know

The 25 and the 100 make 4 to 1 (the idler has no other chore other than connecting the 25 and 100) - its tooth count is immaterial

Metric threads will require an additional ratio to combine with the 4 to 1

The 50 and 127 will make 2.54 to 1

To get that extra pair of gears in train often requires a DIFFERENT "quadrant" or "sector"

If one got all that lashed up and working the lathe will cut a few metric pitches - like in the 10 TPI position, it would cut 1 mm pitch

If you got yourself a 100 tooth, you could build a 1.27 to 1 compound - and then the lathe could cut 2 mm pitch in the 10 TPI position

A normal way to get a bunch of pitches is to have a stack of "stud" gears to replace the 25 tooth stud you have. There would also be several larger gears to replace the 100 - or maybe some more choices for the smaller gear of the compound

Clive603 · Oct 3, 2019

I have a PDF copy of the screw cutting data for the L5 and L5A lathes out of a manual publication number WH/1MR/10/65/JB. Not sure of the year but as the product list on the back includes Boxford machines its probably fairly late in the L5A production period. PM me if you'd like a copy of that page or even the whole manual.

According to that listing the standard gear train for imperial threads is 50 on the top shaft, 50 on the intermediate stud and 100 on the gearbox giving threads from 4 to 60 TPI. Which is rather different from what you give in the first post. There are alternative arrangements for coarse threads from 2 to 30 TPI and fine threads from 8 to 120 TPI. The latter needing a second 100 tooth gear pinned to a 50 tooth gear as a compound stud gear to introduce a divide by 2 into the train.

The listed metric capability is only 15 threads from 0.75 mm pitch to 7 mm. The the given top shaft driver is 50 and gearbox input is 127 for all threads. The intermediate compound gear on the stud is either 40-60 or 40-63. An appropriate TPI setting on the gearbox being used to get the various threads.

Looks like the standard set for machines covered by that manual was two 50's, one or two 100's, 40, a 60, a 63 and a 127.

Clive

HuFlungDung · Oct 3, 2019

dantech said:
I guess a picture is worth a thousand words.
First picture shows the spare gears I have, the second picture is how the lathe is setup for cutting standard threads. I have tried all different combinations to try and cut a metric thread but have had no luck.
View attachment 266456 View attachment 266457

In your photo showing the 80 tooth gear, that is where you need two gears pinned together (one being the 127t) or otherwise driving on a common hub. That is what the guys are calling a compound gear. This means you'll need to move the bottom most gear, the one into your quickchange, out by the width of one gear so it can engage the outer gear of the compound set. Probably all the extensions and bushings to make this happen were included with the lathe when it was new. Scrounge around, you might find you have the stuff without making it.

dantech · Oct 3, 2019

That was great info, and thank you for taking the time to explain it to me. Unfortunately I bought this lathe many years ago from a high school, and never got a manual or other gears. I got into this problem of having to cut a metric thread a few years ago, and made the 127 tooth gear thinking that's all I would need. It never did work and now I'm finding out why.

dantech · Oct 3, 2019

Update;

I was able to stack the gears as shown in the picture, and with the quick change gear box set for 10TPI, I can accurately cut a 1mm pitch thread !! Awesome !!
Here's my other stumbling block;
I need to cut a 1.5 mm pitch thread !! I assume that means I would have to increase the speed of the 127 tooth gear 50%.
It seems like my only option to do this is to use a 75 tooth gear instead of the 50 tooth gear currently installed in the picture. Does this make sense, or is there another way of doing it?
Thanks

Edit; I just tried to see if I could purchase an off-the-shelf gear, and was told that this is an oddball gear. I'm assuming these gears are 14 DP with a 14 1/2 pressure angle, but was told that 14 DP is not a standard. Real confused as to what my next step will be.

awake · Oct 3, 2019

dantech said:
Update;

I was able to stack the gears as shown in the picture, and with the quick change gear box set for 10TPI, I can accurately cut a 1mm pitch thread !! Awesome !!
Here's my other stumbling block;
I need to cut a 1.5 mm pitch thread !! I assume that means I would have to increase the speed of the 127 tooth gear 50%.

If I'm thinking about this correctly ... not guaranteed by any means! ... then you have it backwards. A 1.5mm pitch thread is coarser than a 1mm thread by a factor of 3:2. Unfortunately, your gear box is not set up in terms of units-per-thread (i.e., 1 mm / thread); it is the opposite, threads-per-unit (i.e., threads per inch). All of that is to say, you don't want to speed up your leadscrew by 3:2 (which is what would happen if you changed the 50 tooth gear to a 75 tooth gear in the picture above); instead you need to slow down the movement of your leadscrew by 2:3.

To do that, you either need to change one of the driver gears (the 25 or 50) to one with 2/3 of the number of teeth - problematic, that - or change one the driven gears to one with 3/2 of the number of teeth - problematic for the 127 tooth gear, but if there is room, you could change the 100 to a 150. Alternately, you could change the 100 tooth gear to a 75 tooth, and change the quick change gear setting to 5 tpi.

dantech · Oct 3, 2019

awake said:
If I'm thinking about this correctly ... not guaranteed by any means! ... then you have it backwards. A 1.5mm pitch thread is coarser than a 1mm thread by a factor of 3:2. Unfortunately, your gear box is not set up in terms of units-per-thread (i.e., 1 mm / thread); it is the opposite, threads-per-unit (i.e., threads per inch). All of that is to say, you don't want to speed up your leadscrew by 3:2 (which is what would happen if you changed the 50 tooth gear to a 75 tooth gear in the picture above); instead you need to slow down the movement of your leadscrew by 2:3.

To do that, you either need to change one of the driver gears (the 25 or 50) to one with 2/3 of the number of teeth - problematic, that - or change one the driven gears to one with 3/2 of the number of teeth - problematic for the 127 tooth gear, but if there is room, you could change the 100 to a 150. Alternately, you could change the 100 tooth gear to a 75 tooth, and change the quick change gear setting to 5 tpi.

Hmmm.. If I speed up the lead screw by 50%, wouldn't that mean that my thread pitch would increase 50% ?Therefore if I have a 1mm pitch, it would then increase to 1.5mm which is what I need.

johnoder · Oct 3, 2019

If you changed the the 50 - 100 compound to 60 - 80 you would be there. Seems like you would still have the 80 laying about

My figuring...

Current over all ratio 10.16 to 1. This comes from 5.08 times 2 OR 2.54 times 4 - depending on how you combine the gears for math operations

1.5mm divided by 1mm = 1.5

10.16 divided by 1.5 = 6.77333 = the desired over all ratio

6.77333 divided by 5.08 = 1.33333 = the ratio desired in the compound

1.33333 times 60 = 80

PDW · Oct 3, 2019

dantech said:
Edit; I just tried to see if I could purchase an off-the-shelf gear, and was told that this is an oddball gear. I'm assuming these gears are 14 DP with a 14 1/2 pressure angle, but was told that 14 DP is not a standard. Real confused as to what my next step will be.

True which is why I have a mill, dividing head and a complete set of 14DP gear cutters, having been faced with a similar problem in the distant past.

Think of this as an excuse to buy more tooling rather than a problem.....

PDW

Clive603 · Oct 4, 2019

According to the Harrison manual I have 1 mm pitch uses 50, 40-60 (pair), 127 as the gear train with the gearbox set to 30 TPI.

For 1.5 mm pitch the given set up is 50, 40-60, 127 with the gearbox set to 20 TPI.

Its instructive to compare your 1 mm set up with the book and treating the 50, 127 pair as a constant.

Your 25, 100-50, 127 train has a 4 to one reduction followed by a 10 TPI setting on the box.

The book 50, 40-60, 127 train has a 1/2 rd step up followed by 30 TPI on the box

Taking the step up out of the book train its effectively direct drive onto 20 TPI with the conversion ratio in the way. Your set up is 4 to 1 reduction onto 10 TPI also with the conversion ratio in the way. So the book setting gives twice the pitch that yours does.

Similarly your imperial threading gear train is 4 to 1 reduction, 25 drives 100 whilst the book is 2 to 1 reduction, 50 drives 100.

Its all rather odd. Are you sure the two small levers on the right of the gearbox are set correctly? These, I think, control a 2 to 1 step up and step down gear set with intermediate straight through and disengaged positions

Harrison majored in totally confusing gear setting arrangements and provided minimal (= none) data in the manuals. Its pushing 45 years since I drove an L5A and I can't find any decent pictures of the screw cutting gearbox and data plate. If I recall things correctly setting both little levers pointing left gives double the feed (halves the TPI) that you get with both pointing right and setting them pointing away from each other halves the feed. Pointing towards each other disconnects the drive.

Picture of the standard gear train for Imperial threading halfway down this page :- HARRISON LATHES L5A, (9" & 11") Later Models

Clive

macchi7 · Oct 4, 2019

Does the L5 not have a "chart" on the inside door which covers the gears. If I recall, this gives all of the set ups for cutting metric pitches. (Don't forget to keep the leadscrew engaged and reverse the machine to get back to your start point). You are right about the gears, they are non standard so you have to buy them from Harrison (no longer doing parts) or make them. As an aside, in the 1960's every secondary school metalwork shop had an L5 as well as a few Boxfords. As kids, we thought the L5 was a huge machine!

awake · Oct 4, 2019

dantech said:
Hmmm.. If I speed up the lead screw by 50%, wouldn't that mean that my thread pitch would increase 50% ?Therefore if I have a 1mm pitch, it would then increase to 1.5mm which is what I need.

Well, crud, I'm the one who is backwards. I TOLD you I might not be thinking about this correctly. You are correct, you want to speed up your lead screw, not slow it down. I always have to think about these relationships 5 times before I get them right ...

Lowe.Buuck · Oct 4, 2019

If my calcs are correct and your QCGB has the required tpi settings,

With your current 25- [100-50]-127 gear train you can also cut:
.25mm using 40 tpi
.5mm using 20 tpi
1mm using 10 tpi
1.25mm using 8 tpi
2mm using 5 tpi
2.5mm using 4 tpi

If you acquired a 60t gear and it will fit in place of the 50t => 25-[100-60]-127 you should get:
.25mm using 48 tpi
.5mm using 24 tpi
.6mm using 20 tpi
.75mm using 16 tpi
1mm using 12 tpi
1.5mm using 8 tpi
2mm using 6 tpi
3mm using 4 tpi

Alternately if the 60t won't fit, a 30t => 25-[100-30]-127 should give you
.25mm using 24 tpi
.5mm using 12 tpi
.6mm using 10 tpi
.75mm using 8 tpi
1mm using 6 tpi
1.5mm using 4 tpi

Hopefuldave · Oct 5, 2019

14DP, 14.5* is the standard on early Colchester Student / Master and Bantam / Chipmaster, which were rebadged Clausing-Colchester for the north American market - you may find what you need that way (but needing bushing/boring to fit), or HPC gears in England can supply 14DP 20* in a choice of materials FAR cheaper than ordering from Colchester through the 600 Group.

Dave H. (the other one)

dantech · Oct 6, 2019

Clive603 said:
According to the Harrison manual I have 1 mm pitch uses 50, 40-60 (pair), 127 as the gear train with the gearbox set to 30 TPI.

For 1.5 mm pitch the given set up is 50, 40-60, 127 with the gearbox set to 20 TPI.

Its instructive to compare your 1 mm set up with the book and treating the 50, 127 pair as a constant.

Your 25, 100-50, 127 train has a 4 to one reduction followed by a 10 TPI setting on the box.

The book 50, 40-60, 127 train has a 1/2 rd step up followed by 30 TPI on the box

Taking the step up out of the book train its effectively direct drive onto 20 TPI with the conversion ratio in the way. Your set up is 4 to 1 reduction onto 10 TPI also with the conversion ratio in the way. So the book setting gives twice the pitch that yours does.

Similarly your imperial threading gear train is 4 to 1 reduction, 25 drives 100 whilst the book is 2 to 1 reduction, 50 drives 100.

Its all rather odd. Are you sure the two small levers on the right of the gearbox are set correctly? These, I think, control a 2 to 1 step up and step down gear set with intermediate straight through and disengaged positions

Harrison majored in totally confusing gear setting arrangements and provided minimal (= none) data in the manuals. Its pushing 45 years since I drove an L5A and I can't find any decent pictures of the screw cutting gearbox and data plate. If I recall things correctly setting both little levers pointing left gives double the feed (halves the TPI) that you get with both pointing right and setting them pointing away from each other halves the feed. Pointing towards each other disconnects the drive.

Picture of the standard gear train for Imperial threading halfway down this page :- HARRISON LATHES L5A, (9" & 11") Later Models

Clive

My lathe looks identical to the Clearing lathe close to the bottom of the page.I'm sure I have the levers in the correct position.
I think I've narrowed down my gear choices to 40 and 60. I will machine the blanks for these in the morning and send them out to get the teeth hobbed on. I can get both gears done for $150 CND, which I feel is a very reasonable price.

Lowe.Buuck · Oct 6, 2019

dantech you may also want to consider a 42t gear in addition to the other 2 gears. If the following gear trains will physically fit in your lathe and you have the QCTP tpi settings available then:

25-[100-42]-127 should give you:
0.7mm using 12 tpi
0.6mm using 14 tpi
0.35mm using 24 tpi

25-[80-42]-127 should give you:
1.75mm using 6 tpi
1.5mm using 7 tpi
0.75mm using 14 tpi

25-[60-42]-127 should give you:
3.5mm using 4 tpi
2mm using 7 tpi
1.75mm using 8 tpi
1mm using 14 tpi
0.7mm using 20 tpi
0.5mm using 28 tpi
0.35mm using 40 tpi
0.25mm using 56 tpi

In theory you would only be missing 0.45mm pitch (M2.5x0.45), but should cover all other metric standard and fine from M1.6 to M36.

If you make a couple cardboard gear templates, it should be easy to evaluate whether the gear trains will fit.

It may not cost much more for them to cut a third gear if their machine is already set up.

Need help to figure out gearing to cut metric gears on standard lathe

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