BSW

Hi all i am trying to cut a 5/8 BSW thread on lathe, it is a chinese cd 210v lathe i have changed gear wheel to suit 11 tpi but when i cut thread it comes out at 2.5 on the thread gauge any idea what i am doing wrong please

2.5 what ? Are you using a metric thread gauge?

Roy

I have similar lathe . i have never tried to cut imperial threads on it but i did work out the pitch the lathe actually creates for the settings in the thread cutting table. ( using maths rather than practical engineering).

A 11TPi should have a pitch of 2.309mm . In fact the lathe, with the gears suggested by the table( A= 72, B=H, C=50, D= 60, E= H, F=52), creates a pitch of 2.585 mm and that equates to 9.8 TPI.

I have never tried to work out the gears needed , nor am i sure they are supplied, but , if your lathe is the same as mine, then the thread cutting table is a little inaccurate for imperial size threads..

I guess you have a metric lathe, so it will cut metric threads.

My guess is, you are confusing metric and imperial threads. they are different thing in how they are defined.

5/8 BSW has a thread pitch of 11 TPI (threads per inch), which is approx. a pitch of 2.309 mm.

Maybe a setting of 2.3 mm pitch may be close enough for what you want, depends how long the thread is and how accurate you need the thread to be.

If you wish to cut imperrial threads on a metric lathe you will need a conversion gear set in the change wheel set, and you will have to keep the saddle engage to the lead srew and reverse the drive after each pass.

Do not know if you can get more change wheels for your lathe.

Conversion gears aren't usual on these lathes, which instead approximate Imperial from a Metric leadscrew, or Metric from an Imperial leadscrew. The approximations are generally good. I suspect Julian's problem is the gear combination printed on his lathe for 11 tpi is a mistake, and he needs to calculate what it should be.

I think the lathe has 84, 80, 72, 66, 60, 52, 40 and 30 toothed gears. I don't know what the leadscrew pitch or spindle are.

Dave

Hello Julian,

If I am correct in that your lathe has a 1.5 mm pitch leadscrew, the following gear train will cut an 11 tpi thread for you

80 teeth on the spindle----driving a 40T wheel coupled to a 50T wheel as a pair----which in turn drives a 65T wheel on the leadscrew

I hope that helps you

Regards Brian

Julian,

You have a PM waiting for you

Brian

Gerard O'Toole,

There is a personal message waiting for you, it might be of interest.

Regards

Brian

Measuring an Imperial thread in Metric units is liable to lead to confusion. You will spend your time punching the keys on a calculator

Your thread gauges probably don't include a 2.3 mm pitch, the closest is likely to be 2.25 mm, which is inaccurate for what you want, and 2.5 mm is even less accurate..

If the work is is Imperial, Try to work in Imperial units.

Your 5/8 BSW thread is 11 tpi ( 0.0909 inch pitch which calculates as 2.30886 mm)

Thread depth is 0.0582 inch. (1.47828 mm ) 1.5 mm will be too deep. You will be able to get close if you have a DRO reading to 3 places of decimals in Millimetres.

You won't get a perfect 11 tpi on a lathe with a Metric Leadscrew, unless you use a 127T gear in the train.

Brian's recommendation will give an error of 11.007 tpi, or 0.007 inch over a one inch length. which equates to a pitch error of 0.000636 inch, or 16 microns. Roughly one tenth of the thickness of a human hair.

You are unlikely to cut a thread so that the internal and external threads are so close that that error will cause binding.

Unless you work to single figure microns, your conversions from Metric to Imperial will not be exact

Howard.

Could you kindly point me to any standard metric thread anywhere in the world that has a 2.25mm pitch.

If you are unable to fulfil the above, perhaps you could ponder on why any set of thread pitch gauges might include that pitch.

Howard,

I was making a lot of assumptions with my calculation. I now have Julian's photo of the headstock [with a fixed 56 T gear on the lathe spindle] and showing the threading tables; he tells me his leadscrew pitch is 2mm

I have had to recalculate accordingly, the value one gets from the machine plate is 9.84 TPI

I am hoping Gerard O'Toole will get in touch, he has the same lathe and my new tables might be of interest to him as well

Regards

Brian

There’s a Devil in that approximation

... try working to a few more decimal places

.

I believe the 09 sequence repeats to infinity [but I can’t be sure ... yet]

You are, of course correct about the 127 conversion

MichaelG.

Edited By Michael Gilligan on 10/04/2021 20:01:22

Why don't you use a BSW die to cut the thread?

MC

DC31k

Thank You for adding to our collective store of knowledge.

Since many Metric thread pitches increment by 0.25, I assumed that there would be a 2.25.

Checking my Starrett thread gauges, I find that there is no 2.25 mm gauge present.

Not that it materially affects the matter under discussion, since the object is an 11 tpi thread.

Howard.

Edited By Howard Lewis on 11/04/2021 07:58:55

Hello again Howard,

I now have every reason to suspect this new crop of lathes from China are fitted with 12 tpi leadscrews and not a metric thread of 2mm pitch as Julian said he had measured.. The very similar lathe on the Grizzly website is equipped that way but the threading tables are very different of course with a 40 T spindle gear. Using a 12 tpi leadscrew in the calculations does give realistic results for that machine.

However, trying this new approach on the CD 210V gearings as listed gives a completely incorrect set of results for the whole range of pitches, both metric and imperial so I am now withdrawing from this unrewarding pursuit to leave it to others with perhaps more stamina to figure out the wily Chinese mind.

I think MC Black is right to suggest cutting the thread with a die instead!!

Regards

Brian

I took this screengrab of the change gear table on a CD 210V lathe currently on eBay. I do very little screwcutting, so hopefully I am wrong, but the table looks spot on for 0.5, 1.0 and 2.0mm threads with a 2mm leadscrew (which leads me to fear I may be right).

It doesn't do as well for the others. The worst metric thread is 2.5mm which I think comes out at 2.121mm (56/80 x 50/33 x2 mm). It would be closer (2.545mm) if B & D were swapped, so perhaps this is a simple typo?

Imperial screwcutting appears totally chaotic, with only 20tpi coming close. Like Brian Wood, I can't get the numbers to work with a 12tpi leadscrew either, so I don't think they took this part of the table from an imperial lathe.

PLEASE will somebody prove me wrong, I really don't want to believe I am right.

Brian G

Thanks for all your working out for my lathe , the conversations got to complicated for me so I bought a BSW die problem solved. But thanks for all your replies it has helped me immensely to understand the Chinese lathe. Cheers Julian

There's a handy little program on the Lathes.co.uk website called NthreadsP which will solve all your screwcutting problems.

Russell

Hello Russell and Brian G,

I don't think that will solve the dilemma for Brian G or my understanding of this lathe for that matter until we know what the correct leadscrew pitch is.

I did ask Julian to do some tests for me to establish it properly so that I could send revised tables to him but having bought his BSW die that now sounds like a dead end..

If Brian G can establish the truth, one way or another, I have a complete and extended set for both languages ready to go based on a 2mm pitch leadscrew. It is not a lot of added effort to rework those based on one of 12 TPI. Send me a PM if that would help you.

Regards Brian

Hi Brian

I am fairly confident that whatever pitch leadscrew the lathe has in practice, the metric table at least is based on a 2mm leadscrew, which agrees with Julian's measurement of what was meant to be an 11 tpi thread.

I had already tried 12tpi in my spreadsheet and according to the plate in the photo the metric threads work (at least approximately, apart from 2.5mm, which looks like a typo) if the leadscrew is 2mm pitch, but none of them come close at 12 tpi:

Pitch, 2mm L/S,12tpi L/S

0.5 mm, 0.500 mm, 0.529 mm

0.75 mm, 0.741 mm, 0.784 mm

1 mm, 1.000 mm, 1.058 mm

1.25 mm, 1.244, mm 1.317 mm

1.5 mm, 1.493 mm, 1.580 mm

1.75 mm, 1.750 mm, 1.852 mm

2 mm, 2.000 mm, 2.117 mm (56/84 x 60/40 = 1:1, suggesting 2mm leadscrew)

2.5 mm, 2.121 mm, 2.245mm (2.545 mm with 2mm L/S if 50t & 60t gears swapped)

The imperial threads are a mess with either pitch leadscrew, it is as if only the gears for metric threads are supplied.

tpi, 2mm L/S, 12 tpi L/S

10 tpi, 9.4 tpi, 8.9 tpi

11 tpi, 9.8 tpi, 9.3 tpi (2.585mm with 2mm leadscrew)

14 tp,i 15.1 tpi, 14.3 tpi (Pretty close if using a 12 tpi leadscrew)

19 tpi, 13.6 tpi, 12.9 tpi

20 tpi, 19.8 tpi, 18.7 tpi (Close again but with a 2mm leadscrew)

22 tpi, 18.9 tpi, 17.8 tpi

28 tpi, 30.2 tpi, 28.6 tpi

38 tpi, 34.3 tpi, 32.4 tpi

40 tpi, 41.6 tpi, 39.3 tpi (Close with a 12 tpi leadscrew)

44 tpi, 38.1 tpi, 36.0 tpi

Either there is something very wrong with my calculations (I rather hope for the sake of anybody with one of these machines that is the case), or the designers copied parts of the table from another lathe, perhaps unwittingly or else confident that only a small percentage of customers will try screwcutting before it is too late to return the lathe.

For comparison, the least accurate imperial pitches claimed by my son's metric Chester lathe are 8, 16 and 32 tpi, which all have the same percentage error, coming out at 8.03, 16.05 and 32.1 tpi respectively, which I think is reasonable from a metric lathe, whilst all the metric pitches are correct.

Brian G

Edited when I discovered that my attempts at tabulation were a dismal failure 

Edited By Brian G on 11/04/2021 17:43:43