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Warco WM250V Screwcutting

Problems with TDI

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Andrew Douglas 128/02/2022 21:00:39
3 forum posts

Hi all,

Im hoping someone on here can help me out. I have had a Warco 250V for about 3 months and just getting on to screwcutting.

I have set up for 1.25 pitch with a 30 tooth at the top a 75 paired with 80 tooth in the middle and a 60 tooth on the input to gearbox, gearbox set to A , all as per the table & manual. The scratch pass creates a perfect 1.25 pitch so all good.

The problem comes when i disengage the half nut and use the TDi to pick up the thread again, the label on the machine says to used TDI positions 1,6 so I am using 1 and waiting for it to come back round again. The tool does not pick up the thread and misses by a small amount eaxch time thus taking the crest off the partially ciut previous thread.

Any ideas much appreciated.

I know i can just leave the nut engaged and reverse the lathe after backing off the tool so as to get back to the statrt but would really like ton use the TDI as idid on my previous machine.

JasonB01/03/2022 06:57:45
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First question would be is it a metric or imperial leadscrew?

Secondly ensure the threading indicator is fully engaged with the lead screw and the dial on the top screwed on tightly.

DC31k01/03/2022 07:55:13
686 forum posts
2 photos

Following on as question 2, from Jason's initial one:

2) IF it is a metric leadscrew, what is its pitch?

3) How many teeth are on the gear of the thread dial indicator which meshes with the leadscrew?

Please view from 0:50 to 1:10 in this video:

https://www.youtube.com/watch?v=neMQS1TTibU

 

Edited By DC31k on 01/03/2022 08:20:51

Martin Connelly01/03/2022 08:17:11
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2137 forum posts
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To add to what DC31k has asked, how far do you have to move the saddle to rotate the thread dial one or five revolutions? Five is better for accuracy. A typical inch based one will move an exact number of inches per revolution, I would expect 2" but a metric one will move a multiple of a metric pitch so 50mm is a possibility. Doing 5 revolutions will help differentiate between the two.

Martin C

Edited By Martin Connelly on 01/03/2022 08:17:34

SillyOldDuffer01/03/2022 11:51:22
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8692 forum posts
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Assuming this is a metric WM250V, and the lead-screw is the same as my metric WM280, then one of Andrew's gears is incorrect. Where H is a spacer, he has:

H-30 (30 engages spindle gear)
75-80 (75 on outside)
60-H ( 60 drives gearbox on 'A' )

Andrew's gears aren't in my list of combinations that do 1.25 (20.32tpi) on a WM280:

85 60 50 80 5/4 1.25 20.32
30 80 50 60 5/4 1.25 20.32 (On front panel)
75 80 50 60 5/4 1.25 20.32
65 80 50 60 5/4 1.25 20.32
75 60 50 80 5/4 1.25 20.32
65 60 50 80 5/4 1.25 20.32
60 80 50 60 5/4 1.25 20.32
30 60 50 80 5/4 1.25 20.32
20 60 50 80 5/4 1.25 20.32
85 80 50 60 5/4 1.25 20.32
70 60 50 80 5/4 1.25 20.32
45 80 50 60 5/4 1.25 20.32
60 60 50 80 5/4 1.25 20.32
70 80 50 60 5/4 1.25 20.32
45 60 50 80 5/4 1.25 20.32

Is Andrew's 75 correct? Try:

H-30
50-80 (same as Andrew except 50, not 75)
60-H

I don't think it's a metric/dial counter problem because my metric dial counter works on this pitch. It allows 1-6 rather than 1,6 (i.e engage half-nuts on any mark rather than 1 or 6 only) but Andrew engaging on 1 only should be fine.

I cut this 1.0 pitch thread last week and the dial indicator didn't work at all. It's a mess:

dsc06572.jpg

Double operator error: I misread the gear combination off the front of the lathe AND the number of teeth stamped on one of the gears. (Iffy eyesight and the gears are lightly stamped) Had me going for longer than it should because I often cut 1.0 pitch threads and was convinced the banjo was correct.

Check everything.

Dave

 

Edited By SillyOldDuffer on 01/03/2022 11:53:06

Hopper01/03/2022 12:20:51
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Not familiar with your lathe but some metric lathes have different gears to put on the bottom of the TDI to work on different pitches. Worth checking on yours perhaps.

JasonB01/03/2022 13:02:18
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The metric 250s have a 3mm pitch, imperial 8tpi leadscrews

Fixed 40T on the spindle that will be driving the 30T, A on the box is straight through 1:1

As SOD says I can't make that gear train work for 1.25mm pitch

Edited By JasonB on 01/03/2022 13:03:26

Mick B101/03/2022 18:04:34
2192 forum posts
122 photos

Mine's a metric WM250V and it has a 2mm leadscrew pitch.

Most of the threads I cut are Imperial for the steam railway, and I just leave the nut engaged and reverse back to the thread start. It's simpler and safer.

In my early days with this lathe I played about with the chasing dial on both metric and imperial threads, trying to get some sense out of it, but decided life was just too short. In any case, my experience with its predecessor, a Myford Speed 10, led me to think you can waste just as much time waiting for the lines to come round the dial as you would reversing back.

I've wondered if chasing dials might be a fossil feature, from the time when reversing lathes was more time-consuming than it is now, if it was possible at all - and manufacturers just put 'em on 'cos they've always been there.

Edited By Mick B1 on 01/03/2022 18:08:00

JasonB01/03/2022 18:11:13
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Thanks Mick, looks like the latest V has a 2mm, earlier ones were certainly 3mm. Out of interest what is the gear train shown on your machine for 1.25mm?

Mick B101/03/2022 18:23:36
2192 forum posts
122 photos
Posted by JasonB on 01/03/2022 18:11:13:

Thanks Mick, looks like the latest V has a 2mm, earlier ones were certainly 3mm. Out of interest what is the gear train shown on your machine for 1.25mm?

I've got:-

H 30

......|

75 80

|

60 H

and the pitches are C=0.62, A=1.25, B=2.5 for the feed settings.

Incidentally, I notice the manual advises to leave the nut engaged, and the chasing dial doesn't appear in the photo on the front of the manual or in the parts lists!

It's just a sales gimmick IMO.

Edited By Mick B1 on 01/03/2022 18:54:29

Andrew Douglas 101/03/2022 19:46:50
3 forum posts

HI All

well firstly thank you for all your responses , i did not expect so many so quickly.

toi answer some questions

1/ the leadscrew is definately 2mm pitch (checked with thread gauge and eyepiece)

2/ The TDI gear is 48 teeth and the divisons on the dial ar 12 (odd numbered).

The changegears

H 30

......|

75 80

|

60 H

and gear box set to A (1.25)

The scratch thread is 1.25 so nothing wrong with the changegears.

The problem is picking up th thread with the TDI using the indicated positions 1, 6 , the thread is NOT picked up and i end up removing the crests of the already cut threads.

Any help appreaciated

Tony Pratt 101/03/2022 20:22:33
1963 forum posts
12 photos

Guess you will have to leave the half nuts engaged at all times, I just had a Google on cutting metric threads on a metric lathe using a TDI & apparently it may not be straightforward, others on here may be able to elaborate I only just had a quick skim read on the subject.  

Tony

Edited By Tony Pratt 1 on 01/03/2022 20:28:25

Tony Pratt 101/03/2022 20:43:50
1963 forum posts
12 photos

If anyone has a few hours to waste, try this link, well nearly a linksad

https://bbs.homeshopmachinist.net/forum/general/46983  -why-doesn-t-a-thread-dial-work-with-metric

Tony

Edited By Tony Pratt 1 on 01/03/2022 20:44:20

Edited By Tony Pratt 1 on 01/03/2022 20:45:42

Mick B101/03/2022 21:08:20
2192 forum posts
122 photos

My manual contains no reference to or picture of the chasing dial, nor is there any label indicating how it should be used. IIRC I even tried engaging at the same marking each time, and still found it didn't pitch in accurately.

I've taken mine out of leadscrew mesh, and I don't miss it.

Martin Connelly02/03/2022 00:47:45
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2137 forum posts
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You need a gear with 40 teeth on the TDI. This will allow you to go 2mm x 20 = 40mm per half revolution which is 32 pitches of your thread. This is why metric TDI systems come with multiple gears for the TDI.

You could also go 1/4 turn of the TDI for 16 pitches (20mm) with this 40 tooth gear.

To use the 48 tooth gear on the TDI you would have to move 5 of your 12 divisions or multiples of 5 such as 10 or 15 which would move you in 20mm steps (16 pitches of 1.25mm). This is likely to be error prone which is why the 40 tooth gear is the best option.

Martin C

DC31k02/03/2022 10:56:25
686 forum posts
2 photos

Martin Cleeve's book, 'Screwcutting in the lathe' covers the thread dial indicators very well (Section 5 entitled 'problems and analysis of repeat pick up'. It is concise, correct and unbiased. It is prudent to read the US forum cited above with some wariness as many of the discussions there are motivated by a desire to denigrate the metric system

Think of the thread dial indicator as a counter - the fact that it is gear-shaped is not really relevant. It is no more sophisticated that what you see if you Google 'click counter'. Every 'tick' of the counter, instead of registering someone's entrance into an event, counts one leadscrew pitch (2mm in your case).

So, if the gear has 48 teeth, it will measure 96mm per rotation. The dial that is mounted on top of that gear will not have 48 graduations on it (corresponding to one graduation per tooth) as that would be confusing. Instead, it has 12 graduations (corresponding to 4 gear teeth). Thus, each graduation on your dial will measure 96/12 = 8mm.

For the thread dial indicator to be of any use to you, the distance it measures in one revolution has to equal an integer number of pitches of the thread you are cutting. If you are cutting 1.25mm pitch, 96/1.25, or (96x4)/5 is not an integer, so the dial is no use for that thread pitch.

I am assuming the threading chart on the lathe in the Youtube video above is the same as your own. In which case, your indicator will be useful for 0.2, 0.4, 0.8, 0.3, 0.6, 1.2, 0.5, 1.0, 2.0, 0.75, 1.5, 3.0mm pitches. That is 12 out of the 18 possible ones*. It will not be any use for 0.625, 1.25, 2.5, 0.875, 1.75, 3.5mm pitches.

Martin suggests a 40t gear above, and for the 1.25mm column, that will work. But more importantly than a specific number of teeth, what it is necessary to understand is that any gear that has teeth that are a multiple of 5 will do. Similarly, for the 1.75mm column, a gear that has teeth that are a multiple of 7 will be needed. The disadvantage of the 40t gear suggested is that it will not be useful for 0.3, 0.6, 1.2, 0.75, 1.5, 3.0mm pitches as those need a gear that divides by 3, which 40t does not. The other columns need a gear with an even number of teeth, which the 40t does have so it can be also used for those ones.

If you want one gear that will do all the missing threads that the standard 48t will not do, a 35t one would do both columns, but it also has the disadvantage that it will need two different dials for the top of it - one divided into 5 and the other divided into 7.

So, the thing to take away from this is that no single gear will work with all metric threads. What you can do is assess which threads you will be cutting most often and choose a gear that maximises efficiency for those threads.

* We could perhaps discuss how many of the 'possibles' are actually useful - I do not have many screws with 0.3, 0.6 or 1.2mm pitches.

Andrew Douglas 104/03/2022 10:03:45
3 forum posts

Thanks all once again for your help.

So it looks like i need a 40 tooth and a 28 tooth as well as the 48 tooth.

The stated Module is 0.64 and i have searched high and low to find 28 & 40 tooth gears with such a Module.

I find that the nearest DP is 40 but again i cannot find any gears to suit

My next thiought was to cut a couple of gears and i can fend the neccessary involute cutters however most seem to be 14.5 degree PA as opposed to 20. I am guessing my 2mm pitch leadscrew being metric will need 20 degreePA or does it matter ?

Once again i value all your help and input

Martin Connelly04/03/2022 10:36:24
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2137 forum posts
222 photos

Since you are running against a 60° thread with a flank angle of 30° a theoretical gear to suit would have this as the pressure angle. There is no real power transmission taking place so even with a mismatched pressure angle there will be minimal wear. Just buy a plastic gear that is 40DP and make it fit and it will do the job. You may not even match the helix angle when they meshing

Martin C

Hopper04/03/2022 10:41:01
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6397 forum posts
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Posted by Andrew Douglas 1 on 04/03/2022 10:03:45:

Thanks all once again for your help.

So it looks like i need a 40 tooth and a 28 tooth as well as the 48 tooth.

The stated Module is 0.64 and i have searched high and low to find 28 & 40 tooth gears with such a Module.

I find that the nearest DP is 40 but again i cannot find any gears to suit

My next thiought was to cut a couple of gears and i can fend the neccessary involute cutters however most seem to be 14.5 degree PA as opposed to 20. I am guessing my 2mm pitch leadscrew being metric will need 20 degreePA or does it matter ?

Once again i value all your help and input

There is no need for the gears on the bottom of the TDI to be an exactly perfect involute gear tooth profile. They are a straight cut gear engaging with a thread (worm) so will be in point contact only, and loading on the gear is as close to zero as it's possible to get. And pressure angle likewise is not so important.

So probably 0.6 module or 40DP might work ok providing the gear is not jammed up hard against the screw. Or cut your own with whatever the nearest cutter you have. They can be made from Delrin or from 2mm thick brass or aluminium plate. Or you could make a disc with holes drilled around the periphery and small pins stuck in the holes for that matter. All they are is a place holder, or FPO as they say, for positioning only.

Edited By Hopper on 04/03/2022 10:41:59

DC31k04/03/2022 11:58:03
686 forum posts
2 photos
Posted by Andrew Douglas 1 on 04/03/2022 10:03:45:

So it looks like i need a 40 tooth and a 28 tooth as well as the 48 tooth.

Something else to take into consideration before selecitng a tooth count is the space available. A 28t gear will be approximately 50% of the diameter of the existing 48t one, so you might have to do some gymnastics with the indicator body in order for it to mesh with the leadscrew.

If the existing gear is 48t, a nearby one containing a factor of 5 would be 45 or 50 (45 good as it also contains 3; 50 good as it also contains 2). 49t is near 48t and contains the 7 you need, but its other factor is another 7 so it will be a one trick pony. If 42 will fit, that might do as it has 2, 3 and 7.

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