NO 26TPI THREAD CUT SETTING

I,m a lathe newbie and am presently trying to pick my first machine, had set my mind on a Warco Wm250 V as it seemed to tick all my boxes.

Went to the warco open day to see it in the flesh ,whilst there I found it does not have a 26 tpi thread cutting setting,as this is a common pitch for motorbikes,and that is my main interest , is this as deal breaker for this lathe for me ,or could I use a metric setting that is close ,or change a gear in the box ,but maths is not my strong point so please no point in reams of over complicated formulae as my brain will melt.

I could of course choose up to the wm 280 ,but is an extra £400 .

One thing I did want to check out today was motor noise as I had viewed an identical machine badged by another company with standard motor ,the inverter model from warco is substantially quiter.

Many thanks

George

For a short thread you could probably get away with 1.0mm pitch on the metric range which would give you 25.4tpi.

It would also be possible to come up with a gear train that would give you 26tpi but may mean having to make or buy in a commercial gear to modify as the required gear sizes may not be in the set that comes with the machine.. The 280 is nice but still does not have the 26tpi, at least mine does not but thats 5yrs old and spec may have changed.

Whatever you get post a photo of the gear chart here and we can work it out for you.

J

If your main interest is restoring old motorbikes, you may do better to look for an old imperial lathe - ebay or one of the model-engineering magazines should have plenty of those for sale - cheaper too.

Hi George,
If you can't multiply and divide then there is a program called "nthreads.exe" That can be downloaded from here that will do the sums for you. All you have to do is give the program the pitch of you leadscrew and the numbers of teeth on all of the change gears that you have. One thing I have found at the exhibitions is that "Chester UK" normally have a large box of assorted gears for sale. I have found quite a number of gears that I could use on my lathe to extend the range of threads I can cut. If you use this approach you will need to find out the DP or modulus of your gears and make sure you can check that the gears will match yours. Note you will probably have to modify these gears by boring them out or bushing them.

Les.

Do you see yourself truing up flywheels? If so this will dictate the size of the lathe. (Something I cannot do on my Myford).

JA

Thankyou very much Gentlemen for your concise and rapid responses .

The 280 showed a 26 setting on its front panel ,it does look a considerably sturdier machine than the 250 with all components ,crosslides ,beds and tail stocks ect, but i did originally start looking at the 180 model and have talked myself up to the 250 as most feedback i have heard from people is to get as big as u can afford.

My interest in motorcycles is mostly vintage Japanese but when i do get to retire one day i would like the option to do British just in case. I have being watching out for old lathes on ebay ,but as an newbie i m not savy enough to know whats a good or a bad one ,worn out or not ,if i buy new i know what im getting and can put it on a business lease too.

Thankyou for the heads up on NthreadsP ,have had a look at that and looks simple enough for my brain to possibly master , So it looks like it would be possible to make a workaround for this 26tpi prob , but ooohhhh that 280 did look nice though !!

Can i see myself truing up flywheels ,not at the present , but when i have finished fixing other peoples cars everyday and retiring ,who knows,hopefully.

Thanks to all ,for your help

George

I have had a look at the VM250V on the Warco site and I cannot see anything that looks like a screw cutting gear box. In this case you will have to set up the change wheels for each thread you are attempting to cut. That, unless I have missed something.

On the basis that I am correct you may find the changewheel combinations listed on my website of help.

The lists assumes a lead screw having a 2mm pitch and changewheels in increments of 5 teeth

26 TPI equates to a metric pitch of 0.9769mm

My list shows 7 combinations at 0.9768mm but 6 do use a 21 tooth gear.

However, if you accept a very slightly greater error at 0.9778mm there are 21 combinations, many do not use the 21 tooth gear, 3 of which only use two drivers and two driven. These are obviously the ones to use if you have the required gears.

Do you realise George that when lathes are quoted as being able to cut an imperial thread on a metric lathe (or visa versa) that there will be errors in the result, that unless a 127 tooth gear is provided and few do to my knowledge

If the Lathe has other that a 2mm leadscrew, or gears in increments of other than 5, the web page explains how with a simple calculation the required result can be obtained.

Some may wonder why I list so many combinations. This is to cover the situation where a lathe owner does not have a set of changewheels

The web page can be found here-- http://www.homews.co.uk/page31.html

Incidentally, the pages also cover for metric threads on an imperial lathe.

Harold

Sure yo can:

The naysayers will tell you it can't be done, but I have a Harley that had its crank done on this rig in 1972 and has been in constant use since with no problems. Many, many more done on this rig in the intervening years.

Harold, the 250 & 280 do have a very basic gearbox which will give three different feed rates for each combination of gears on the banjo.

One is straight through 1:1, another 1:2 and the last 2:1 so for example you could cut 8, 16 & 32tpi all with one gear train ( this is for teh 280 but I assume the 250 has the same ratios)

Also both are available with metric or imperial leadscrews. Also the driver on the spindle cannot be changed.

Thanks Jason, you have resolved a feature mentioned on the Warco site that was puzzling me. This was that the control you are referring to is labelled on the photograph as Feed Rate, which I took to be the fine feeds for finishing. I could not see how they could easily bypass the changwheel output to provide a fine feed control.

It would appear however that it becomes more useful when the changewheels are set to provide a fine feed as it will allow for the feed rate to be changed depending on the machining taking place. So perhaps Warco's definition of its use is not that far off course.

The combinations listed on my website could still be used as published if the number of teeth on the fixed gear is a multiple of five. If not, then it can be compensated for by a simple calculation. Perhaps I should have foreseen this and detailed it on the website. It would be interesting to learn how many teeth are actually on the gear.

Harold

Thats it Harold using the lever you get also get three different fine feed rates for one gear train.

The 280 lathes have 40T integral gear on the spindle, 250 should be the same but I suspect the 290 has more teeth as the bore is larger.

8tpi leadscrew on the imperial ones and 3.00mm pitch on the metric ones. Gears are in 5T steps and the imperial comes with a 63T to do metric (as near as possible). Again this is for teh 280 but 250 should be the same

J

More useful and interesting stuff for me to try and get my head round, but just to clarify for me please if i go imperial , can do metric but just with less accuracy and vice versa for the metric, can do imperial to a point. As i will probably be working on more contemporary machines projects ,my guess would be to go for the metric version .

The 280 has a separate saddle lead screw i noticed ,are there any real advantages for this in day to day machining

Please forgive my ignorance on this subject but its been over 30 years since i used one at college ,and i want to choose the right one as i hope its going to be my companion for the next 25 years,if i,m lucky .

Cheers . George

Hi Jason,

for your information, my 290 lathe have 40T integral gear on the spindle.

Thor

Yes a metric lathe will get you very close on the imperial pitches.

The separate rod saves wear on the leadscrew, thus keeping it more accurate for when you want to screw cut.

Thanks Thor, I just thought they may have had to use a larger one due to the bigger bore, maybe they just bored out the same spindle

.

George,

[for the sake of clarity] Yes, that's generally true ...

But: There are many very close approximations available [and the good folks on this forum will readily assist you in finding the optimum for a particular thread].

For a theoretically accurate solution you would need a factor of 127 somewhere in the gear train; and this is only practical with some lathes.

MichaelG.

Hi George,
Just as an example (without a 127 tooth gear.) on my Chester DB10G lathe which has a 2mm pitch leadscrew I could cut 26 TPI with an 0.053% error (25.98 TPI) This is about 0.77 thou over one inch. This is using some of the extra gears I have bought.

Les.

Bear in mind that conversions based on the use of a 21 or 63-tooth wheel (mm-inch or vice-versa) have a basic error of 0.125 parts per thousand (0.0125%).

Over a 10cm length of M1 screw thread, that's an error of 1.25 micrometers or about 2 wavelengths of red light, so might struggle to measure it...

It's just over half Schlesinger's permissible error for lead screw accuracy for toolroom lathes, and a third of that for finish turning lathes...

Any questions?

Neil

.

Please Sir, Yes Sir ...

Can you explain the underlying math ?

Rounded to 3 decimal places:

126/127 = 0.992

127/126 = 1.008

... both of which look like eight parts per thousand, to me.

MichaelG.

Whatever, the error actually is, this only occurs if you are using the 63 (or 21) tooth gear as an approximation for a 127 tooth gear used as a follower. The Myford system uses 21 teeth gears as drivers and gets approximations which are to all intents and purposes exact. e.g. 1mm pitch is 1/25.4 = 0.039370". The Myford set up for this is Drivers 45 and 21, Driven 40 and 75. So the pitch from this set up for the 8tpi leadscrew is (45 x 21) / (40 x 75 x 8) which gives 0.039375". Somebody very clever worked this out in the days before computers. Nowadays we can use the computer gear calculators: they will search for the best combination in any order for any set of gears. Making sure you have a 21 in your set is very helpful in getting close.

Cheers,

Rod

Hi Michael,

Another victim of the 63 is nearly half 127 fallacy!

You use the 63 (or 21) tooth wheel to multiply a metric dimension:

25.4 * 63 = 1600.2

As the pitch cut will always be a simple ratio of two integers, then the basic accuracy achievable will be no worse than 1600.2/1600

Any ratio of leadscrew pitch to spindle rotation will have to be a simple fraction related to that basic number, e.g. using my lathe as an example. 1mm thread with a 16 tpi leadscrew:

1/16 * 63/50 * 30/60 = 0.039375 inch pitch = 1.000125mm

Close enough for RABA-WIWIT engineering!

Simples!

Neil