WM250 thread cutting

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WM250 thread cutting

This topic has 38 replies, 7 voices, and was last updated 23 September 2017 at 17:57 by Mark P..

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21 September 2017 at 13:49 #317978
Mark P.
Participant
@markp
Hi all, I have been trying to set my WM250 lathe up to cut a 20TPI thread. I fitted the change wheels as per the chart supplied ie starting from the top spacer,30T driving a 50T with a 45T on the outside driving an 85T on the leadscrew with a spacer on the inside. The problem is that the 30T gear is no where near the headstock gear. Am I missing something fundamental? The lathe is the metric one.
Regards Mark P.
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21 September 2017 at 13:49 #13052
Mark P.
Participant
@markp
21 September 2017 at 14:06 #317982
Les Jones 1
Participant
@lesjones1
If the 30 tooth is only a spacer then it has no effect on the ratio so just use a larger gear to make up the space between the headstock gear and the 50 tooth gear.

Les.

Edited By Les Jones 1 on 21/09/2017 14:07:04
21 September 2017 at 14:30 #317991
Mark P.
Participant
@markp
No Les the spacer is for locking the 30T gear to the ‘banjo’ spindle.The 30T gear drives the 50T wheel from the headstock.
Mark P
21 September 2017 at 15:46 #318003
SillyOldDuffer
Moderator
@sillyoldduffer
Hi Mark,

A picture would help. I think Les means that the 30 tooth gear is used only to transfer rotation from the headstock to the other gears. As a gear used that way doesn't alter the ratio, you can use any gear. Just pick one big enough to physically mate with the headstock without worrying about how many teeth it has.

My metric WM280, which I believe to be similarly geared to the WM250, suggests 60t where you have the 30t.

Dave
21 September 2017 at 16:08 #318009
Mick B1
Participant
@mickb1
Looking at the screwcutting chart for my WM250V, the 20 TPI config is:-

H 30

…..| (had to put dots in to get the connector in the right place )

75 80

|

60 H

…with feed setting A.

I've found the charts to be correct for screwcutting on those that I've done.

Is the WM250 without the 'V' a different animal?

Edited By Mick B1 on 21/09/2017 16:10:15
21 September 2017 at 16:15 #318011
JasonB
Moderator
@jasonb
Have you swung the banjo on its pivot so that the 30T engages with the gear on the spindle?

Edited By JasonB on 21/09/2017 16:21:40
21 September 2017 at 16:37 #318015
Mark P.
Participant
@markp
Jason,yes I have but the 30T gear doesn’t contact the gear on the spindle. It’s not on an eccentric pivot at the bottom is it?
Mark P.
21 September 2017 at 16:45 #318019
Mark P.
Participant
@markp
Mick B1, just had a look at the chart and it goes from the top-
H 30
45-50
85-H (leadscrew).
H is a spacer. Think the WM250 is different to the 250V.
Mark P.
21 September 2017 at 17:04 #318023
JasonB
Moderator
@jasonb
What is the leadscrew pitch on yours?

That gear train with a 40T on the spindle would give 1.275mm pitch which is about right for 20tpi at 1.27mm if it were 3mm pitch.

You could put a 60T in place of the 30T and use lever position C which would give same ratio but the 60T may reach the spindle
21 September 2017 at 17:36 #318026
Journeyman
Participant
@journeyman
Mark, I have the older (2007) wm 250 with 3mm pitch leadscrew. I have cut threads with the 30 tooth gear at the top and it definitely meshes with the spindle 40 tooth gear. I know it works with

H 30
60 80
85 H

You have pivoted the banjo over to the left? As Jason says in this case the top gear is just an idler and you couls fit a larger gear without problem.

John
21 September 2017 at 17:52 #318031
JasonB
Moderator
@jasonb
It's not an idler as changing it to some other tooth count will alter the ratio. I suggested making it a 60 and also selecting position C which will counter the effect of using a 60 instead of a 30, one doubles the ratio and the other halves it so cancelling each other out.
21 September 2017 at 18:22 #318042
Mark P.
Participant
@markp
Jason, yes it is a 3mm pitch leadscrew. I’ve pushed the banjo as far to the left as it will go but the 30T gear will not contact the spindle gear, will try the 60T gear tomorrow. John mine is 2008. Also I could do with cutting 26TPI threads as well it will do threads either side but not 26!!
Thanks Mark P.
21 September 2017 at 18:24 #318043
JasonB
Moderator
@jasonb
Mark, Can you post a photo of the gear chart off the front of the machine
21 September 2017 at 18:33 #318046
Journeyman
Participant
@journeyman
Posted by JasonB on 21/09/2017 17:52:33:

It's not an idler as changing it to some other tooth count will alter the ratio. I suggested making it a 60 and also selecting position C which will counter the effect of using a 60 instead of a 30, one doubles the ratio and the other halves it so cancelling each other out.

Beg to differ my maths is pretty shoddy but I came up with this:-

It took me ages to get to grips with this, doesn't work if you have a five gear setup so safer to use all the ratios!

John
21 September 2017 at 18:38 #318047
Journeyman
Participant
@journeyman
Not the plate but the manual page:-

Note the entry for 1.75 pitch should read:

H 30
70 80
60 H

or similar the plate on my lathe annd the manual is wrong for the M12 thread. Will have to think about the 26TPI one.

John

Edited By Journeyman on 21/09/2017 18:42:13
21 September 2017 at 18:55 #318052
Mark P.
Participant
@markp
http://www.model-engineer.co.uk/albums/member_photo.asp?a=43809&p=766112
Jason.
21 September 2017 at 19:00 #318053
JasonB
Moderator
@jasonb
yes sorry, 3 in a vertical line makes the middle an idler
22 September 2017 at 09:58 #318097
Journeyman
Participant
@journeyman
Mark, I notice that your lathe plate has the same error as mine for M12 thread. I had a go at working out the 26TPI gear train. My maths is always suspect so check carefully.

26TPI = 1.023622mm pitch
this requires an overall gear ratio of 0.341207 (spindle to leadscrew) if you use the following:-

H 30
45 80
65 H

you get a ratio of 0.34615 giving a pitch of 1.03846mm which is only about 1% too big. As threads go this is pretty close and unless the thread is particularly long should be OK. (I haven't actually checked to see if this gear combo fits)

John
22 September 2017 at 10:49 #318102
Journeyman
Participant
@journeyman
I told you my maths was ropey I got the first line wrong I think 26TPI should in fact be 0.97692mm pitch. The gear set up should be:-

H 30
45 80
70 H

this gives an overall ratio of 0.32143 as opposed to the required ratio of 0.32564 and gives a pitch of 0.96429 which again is about 1% too small.

Sorry for the confusion perhaps someone with better maths skills than me can double check….

John

Edited By Journeyman on 22/09/2017 10:51:33
22 September 2017 at 11:54 #318109
Mark P.
Participant
@markp
Thank you John, couldn’t understand why 26TPI was not listed on the plate or in the manual. Maths have never been my strong point.

Mark P.
22 September 2017 at 12:38 #318111
Journeyman
Participant
@journeyman
Posted by Mark P. on 22/09/2017 11:54:

Maths have never been my strong point.

Nor mine! You could use this program from a recent thread to find something nearer perhaps.

John
22 September 2017 at 13:36 #318114
SillyOldDuffer
Moderator
@sillyoldduffer
Posted by Mark P. on 22/09/2017 11:54:32:
Thank you John, couldn't understand why 26TPI was not listed on the plate or in the manual.

…

Mark P.

I've always thought it's because 26tpi is 0.9769mm which is pretty close to metric 1.0

Once you know that all you have to do is set the gears up for 1.0 on the mm table, ie:

H 30
50 80
75 H

Dave
22 September 2017 at 14:01 #318116
Mark P.
Participant
@markp
Thank you chaps, job done. This is what I get when I set up as per the charts http://www.model-engineer.co.uk/albums/member_photo.asp?a=43809&p=766181 the banjo contacts the head stock spindle.

Mark P.
22 September 2017 at 15:06 #318123
Cabinet Enforcer
Participant
@cabinetenforcer
Colour me confused Mark, but that photo is not of what you described in your first post, the 30t is clearly driving the smaller of the two gears?

I only read this post since my 280 chart is full of lies, I think I got a better one off a yahoo group.
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