Bodger Bill’s screwcutting issues

[Bill AndrewsParticipant @billandrews30971]

Good morning all.

Further to my original question on 10/12 Perhaps I should have added my own calculation which didn't work no matter what I did with it. leadscrew tpi 12×5=60 :-2=30

tpi required 26×5=130 :-2=60 60/30 on the gear arm…Where did I go wrong??

best rgds. Bill.

[SillyOldDufferModerator @sillyoldduffer]

Jason

The revised version (imperial) may be found at link.

Thor,

As I originally produced the table for my own metric 280 it's very easy to make it available!

I've added a copy to the same dropbox folder at link. There are three versions of the same information: txt & odf files that can edited by spreadsheet or word processor, and a pdf for viewing.

There's also a copy of the program for any Pythonistas who might find it useful.

Please let me know if you find any mistakes!

Dave

[SillyOldDufferModerator @sillyoldduffer]

Posted by Bill Andrews on 19/10/2016 09:48:54:

…Where did I go wrong??

best rgds. Bill.

Hi Bill,

I think you went "wrong" by asking a hard question!

It's stretched my poor brain considerably. Calculating backwards from a target TPI to find the combination of gears that will produce that tpi isn't straightforward. Not to me anyway.

The method involves first expressing the tpi as a continued fraction then using that to converge on a combination of gear wheels that closely approximate the target tpi. Given a set of gears it's possible that there may be many answers or none at all. Clear as mud.

I found the calculation so tedious and error prone that instead I used a computer to tabulate all the TPI that could be possibly be produced by your lathe with the gears you have available. The maths to do this is much easier, the downside is that it generates lots of answers.

It is easy to use though. To find a combination for a particular tpi, one pages down through the table until the required tpi (or one suitably close to it) appears in the rightmost column, then use the 4 gears that produced it. You may have to look for a combination of gears that physically fits into the lathe. The table is sorted to make this easy, TPI and metric pitch on the right, gears needed on the left. It looks like this:

A  B  C  D   Ratio   metric   TPI

35 33 25 66 125/3267 0.9718  26.14
50 66 25 33 125/3267 0.9718  26.14
25 50 30 52   1/26   0.9769     26
85 60 45 65   1/26   0.9769     26
55 52 45 75   1/26   0.9769     26
40 52 42 70   1/26   0.9769     26
80 60 45 65   1/26   0.9769     26

etc etc …

The gears are mounted as represented in the Manual:

H A
. |

C  B
|
D  H 

The table for your lathe and gears is HERE,

Hope it makes sense.

Dave

[Thor 🇳🇴Participant @thor]

Hi Dave,

Thank you very much for posting the link to the metric version for the 280 lathe. It will come in handy.

Thor

[JasonBModerator @jasonb]

Thanks Dave just downloaded the three files.

Jason

[HopperParticipant @hopper]

Posted by Bill Andrews on 19/10/2016 09:48:54:

Good morning all.

Further to my original question on 10/12 Perhaps I should have added my own calculation which didn't work no matter what I did with it. leadscrew tpi 12×5=60 :-2=30

tpi required 26×5=130 :-2=60 60/30 on the gear arm…Where did I go wrong??

best rgds. Bill.

YOu made a slip up when you divided 130 by 2 and came up with 60. It should be 65.

So your overall ratio between the headstock spindle and the leadscrew needs to be 30/65.

Normally, on a "normal" lathe, you would put a 30T on the head stock spindle and a 65T on the leadscrew with whatever idlers needed in between to simply fill the gaps and away you would go.

But because you have a 40T permanently attached to the spindle, you need to calculate a compound train beginning with 40T that has an overall ratio of 30/65.

It's tough one. But if you multiply 30 by a factor of 44, you get 1320. (Which is divisible by 40, our first gear)

If you multiply 65 by a factor of 44, you get 2860.

This means that 30/65 = 1320/2860

1320 and 2850 are nice round numbers that can be broken down into change gear sized factors.

1320 = 40 x33 (Your set as posted on the chart above has a 33T homemade included I believe)

2860 = 52 x 55 (Your set also has a homemade 52T ISTR)

So: 12/26 = 30/65 = 1320/2860 = 40 x 33 / 52 x 55

So you need set up your gear train with the 40 tooth headstock gear (or the 40T fixed gear it drives) driving a 52T gear that is compounded on the same stud as a 33T. This then drives, through whatever idlers of any size that fit, a 55T on the lead screw.

So the gear train would go 40 – 52/33 – 55 plus idlers as needed.

The 55 and the 52 could be swapped with each other and you still get the same correct ratio.

So the penciled in chart you posted is correct, except one of the 55s read as 52.

Edited By Hopper on 20/10/2016 03:48:57

Edited By Hopper on 20/10/2016 03:51:57

[HopperParticipant @hopper]

Then there is the easy way of doing it.

On looking back I am not sure if the chart with the extra homemade gears is yours or someone elses?

Anyway, you could greatly simplify your life by running a 40T gear in position B on your chart. This would then run at a 1:1 ratio to the headstock spindle. So any gear compounded on to this stud in position A  could be regarded as the headstock spindle gear for any conventional gear train calculation.

So to cut 26tpi, using the above caluclated 30 and 65T gears, you will run the 40T in position B on your chart with the 30 on the same stud, keyed to it. Bang the 65 on the leadscrew and fill in the gaps with whatever idlers fit. Away you go.

Using this method, you can cut any TPI you need with a conventionally calculated train with variable size first gear.  Just forget those first three 40T gears are there. And no need for the oddball 52 and 33 tooth gears.

That's the way I would do it for simplicity's sake. . Any standard gear chart for a 12tpi leadscrew will work with this set up. Must be one in Machinery's H'book or the Net someplace I'm sure. Nobody does their own calculations. The above post would be the first one I have done since leaving trade school 40-odd years ago.

 

 

 

Edited By Hopper on 20/10/2016 04:19:07

[Bill AndrewsParticipant @billandrews30971]

Hi,Hopper,

Thank you, thank you. I'm sure that I can handle that. I'm glad that you spotted my deliberate mistake. I already had 30/65 on the lathe, which is still there , with a setup that didn't work. I'm certain that this will, and I can keep what remains of my hair. I'm off out to the shed to give it a try. Thanks again.

Bill.

[HopperParticipant @hopper]

Posted by Bill Andrews on 20/10/2016 16:44:35:

Hi,Hopper,

Thank you, thank you. I'm sure that I can handle that. I'm glad that you spotted my deliberate mistake. I already had 30/65 on the lathe, which is still there , with a setup that didn't work. I'm certain that this will, and I can keep what remains of my hair. I'm off out to the shed to give it a try. Thanks again.

Bill.

You're welcome. Let us know how it works out.

What are you making with 26tpi? Old motorbike parts with Cycle thread?

Edited By Hopper on 20/10/2016 23:37:45

[Bill AndrewsParticipant @billandrews30971]

Hello Hopper,

Yes, it works perfectly,thanks again. And yes I needed to cut some studs for a neighbour who is renovating an old Vellocete. Looking back through the postings I see that others have tried to tell me the same as you have but I just couldn't see it. I'm not from an engineering background, I just like to fiddle with things mechanical. Thank you all for your help. Over and out Bodger.

[HopperParticipant @hopper]

Great stuff. Glad you got a result. Sometimes you have to come at this stuff from a couple of different directions before it "clicks". There's a reason nobody (Sparey, Tubal Cain, Bradley et al included) does their own double-compound calculations unless absolutely desperate!

Carry on bodging and make sure you get a ride on that Velo.

Edited By Hopper on 21/10/2016 11:45:01

[CirclipParticipant @circlip]

"Old Velocette?" Haven't seen a new one for about fifty years. Cycle threads in frame?

Regards Ian.

[Bill AndrewsParticipant @billandrews30971]

Hi Ian, Yes, and everywhere else seemingly. He rode it to the Isle of Man last week. He bought the studs from a Velocette club site. He didn't tell me until he got back. For me, you guys have been great, you sorted out a problem that has niggled me ever since I bought this blessed lathe. Thanks again to all. Bill. (Bodger)

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