Bodger Bill's screwcutting issues

Does anyone out there know how to calculate gearing to cut threads on this lathe, a warco wm240. The gearing chart is ok up to a point, But if you want to gear for threads not on the chart, I can't work it out . I have always used old lathes that don't have a gear chart and have always worked out my own gearing. This lathe is imperial/metric. Lead screw is 12tpi gears rising by 5's. Warco can't,or won't tell me. Help!! before the rest of my hair goes.

Bill

You work it out in the same wayas an old lathe so you should be OK. Yopu will have to allow for the fixed first driver on the spindle (usually 40T) and then just work it out from there for your 12tpi leadscrew as you have done in the past.

Hello Bill,

You say the lathe is imperial/metric----certainly a 12 tpi leadscrew is imperial so which bits are metric? Is the screwcutting chart for imperial of metric threads?

​If you will list the gears you have and which threads you want to generate I can work the gearing out for you, but no doubt someone else will give you a link to one of the calculating sites that allow you to input the information yourself and see the gearing result instantly.

Regards
Brian

You can also get advice from books such as Sparey's " The Amateurs Lathe", or Martin Cleeve's "Screwcutting in the Lathe" (Workshop Practice Series No 3). A lot of this stuff will refer to 8 tpi leadscrews (as Myford 7 Series) but it should not be too difficult to multiply or divide the No of Teeth on one of the gears by 21/8 to arrive at a suitable change wheel set up.

On my lathe, together with a Norton box, there are three different sized wheels which can be fitted to its Input shaft.

By selecting the right one, it is possible to cut pitches that the manual does not mention, such as 3.5mm.

Grab your pencil, paper and calculator, (or set up a spreadsheet on EXCEL)!

Howard

Hello Bill

just bumping this message

Neil

I wonder if Bill missed Brian Wood's offer to provide the answer?

Responses may have been subdued because readers don't know what gears a WM240 comes with. (How many each of what?).

Re Brian's comment about metric/imperial: these lathes are one or the other, but both versions come with tables of gear combinations to approximate a range of both imperial and metric threads. The imperial lathe approximates metric and vice versa,

Attn; JasonB. Many thanks to you and others for the postings. I still can't work it out. How do I allow for the for the fixed 1st driver which is indeed 40t. Just one example please. eg. 26tpi. The chart shows H 80-52 33-52 H. H is a spacer. I have a full set of gears 25 to 90 rising x5,s A full set was not provided so I cut what was missing plus spares on this lathe. I read Martin Cleeves Screw Cutting in the Lathe avidly, I can't find the answer there . Best Rgds. Bill.

My favorite online change gear calculator here **LINK**  gives the below four choices for 26TPI with a 40T headstock spindle gear and 12tpi leadscrew:

First column (A) is the headstock spindle gear. (Or the output gear on the tumbler reverse assembly that is the same size as teh headstock spindle size) The following two gears (B and C) are compounded together on one stud. The final gear (D) is the leadscrew gear. Not sure on the set up on your lathe but you may need to put an idler gear on your second stud, of any size that fills the gap, to complete the train.

There are pictures on the calculator website that make it clearer, here http://littlemachineshop.com/reference/change_gears.php

Edit: Hmm, second linkee no workee. But if you pump in your numbers and do a calculation, when it shows the answers, there is a photo there and explanation of what gears are what.

Edited By Hopper on 14/10/2016 12:54:06

Hello Bill,
​
​I tried to work out what the figures you quoted from your chart would yield as a thread pitch, the result I got was 12.291 tpi to 3 places of decimals?! Maybe I am missing something here but if that example is what your charts do for you then it is little wonder you are struggling to make any sense of it.

​I would go along with the on line gearing calculator that Hopper kindly listed; it is clearly more successful.

​However, my offer remains open for whatever pitch you want to try and achieve, you might need to make a special tooth count gear or two to get the full range of possibilities. Either that or buy them from Arc Euro, I imagine your gear set will be Module 1 and they will probably have similar bore sizes as well.

Regards
Brian

The warco site doesn't even have a non blurry image of the lathe let alone a picture under the changewheel cover. Is this one of those lathes with some hidden gears between the spindle and the leadscrew itself. There was a thread last year on one lathe like that.

How can the chart show 80-52 etc if the gears are in steps of 5? typo or wrong chart completely?

Working from first principles of reasoning.

You want 26tpi, that is finer than your 12 tpi leadscrew so you know you want ‘speed up’ gearing. The ratio is 26/12 that is > 1 so will speed up if the bigger gear is on the spindle(mandrel) and the smaller one on the leadscrew.

Lets factor that : 2x13/2x6. Remove the common ‘2’ from top and bottom to give 13/6. Oh dear we’ve got a bigish prime but no worry.

We know we have only got gears in steps of 5 so numbers ending in 0 or 5 ie 20, 25, 30, 35. Start multiplying that awkward 13 until we get a number ending in 0 or 5. First one is 5x13 or 65 and that’s a gear we’ve got. Hooray. Multiply the 6 on the bottom of the equation by 5 as well and we balance our equation:

5x13/5x6 equals 65/30. So we could use 65/30 with some idlers in between if it weren’t for the fixed 40 on the spindle, that would be on the top of the equation.

Ok we can cancel that out by putting the same gear , 40, on the leadscrew ie the bottom of the equation. 40x65/40x30.

But that means we need a 40. What if we don’t have one? Then what we do is adjust the 40 and 30 on the bottom of the equation (because we can’t change the 40 on top as it is permanent) keeping the result 40x30 the same. eg 80x15, or 20x60 or even 25x48 except we don’t have 48.

To get 40x65/20x60 onto the lathe the two on top have to be drivers, and the 40 is on the spindle which is a driver anyway. So the 40 drives the 20 which is keyed to the 65 which drives the 60. Note if the leadscrew ends up going backwards you will need an idler in there to reverse it without changing the ratio.

I think Bill is looking at the chart for a 2.0mm metric lead screw. The manual shows both, best to look at the front of the machine

From the ratios he gave

40 drives 80 = 0.5

80 drives 52 = 1.538

33 drives 52 = 0.635

0.5 x 1.538 x 0.635 = 0.488

0.488 x 2.0mm pitch = 0.976

26tpi = 0.977

SIMPLES

Edited By JasonB on 14/10/2016 15:30:25

Hi Bill,

Doh, Jason got in first with a similar suggestion, but this is what I said:

Have a look at this calculator which allows you to put in lists of gears and to specify different lead-screws.

It may be a coincidence but the calculator says that your gear combination 80 52 33 52 (plus 40 for the headstock gear) would generate 26.68tpi from a 3mm leadscrew.

Lathes with swarf guards make it difficult to do, but have you been able to confirm that your leadscrew really is 12tpi? If it's not 12tpi, then the mathematical confusion is explained.

Thanks for asking the question. Getting my head round DIY change gear ratios has been on my to-do list for yonks.

Cheers,

Dave

Edited By SillyOldDuffer on 14/10/2016 15:57:11

Having spoken to Bill, I think his problem isn't working out the normal gear ratio, it's dealing with the extra 40T gear.

Most lathes have a stud driven by a gear driven 1:1 from the spindle for the first gear in the train. This lathe has a 40T idler on a stud instead.

My suggestion is to always start with a 40T gear as the driven gear in the first gear pair, but leave it out of the calculation.

Neil

So, there is a permanent 40 tooth gear at the start of the chain as Bazyle says. How bizarre, that would seem to me to be extremely limiting. Being a Myford man that kind of limitation simply doesn't apply. Is that true for both imperial and metric versions of this lathe?

​I am beginning to regret my offer now !!

What seemed at first to be relatively easy has become a minefield of confusion leaving me out of my depth working blind with important bits of the information concealed. Jason has to be right for the wheels listed, nothing else makes sense.

I think the calculator sites can have it after all, they obviously know about these little pitfalls and have correction factors built in.

Regards
​Brian

Golly, no wonder I'm confused. I've been making assumptions again. A picture of the gear chain would help.

Dave

You and me both Dave, I am losing the will to live now!

Brian

This is the same basic set up on my WM280, the 40T on the spindle is easy to see.

Ant just to confuse you more there is a little lever or knob on the front of these machines which will give the lead screw or drive shaft a 2:1, 1:1 or 1:2 ratio so that same gear train will also cut 13tpi and 54tpi, you can just see in teh bottom left how each gear setup has three different tpi listed below

Just to potentially toss a spanner in the works. Are we totally confidant the lead screw is 12tpi? Not 2mm by any remote chance? ......

Still messing with numbers. Could Bill's 26tpi chain - given as H 80-52 33-52 H - be a misprint for H 80-52 33-55 H?

Allowing for the 40t headstock I make the latter:

I'm going to treat myself to a small sherry if that's right.

Dave