Cutting BA and imperial threads on a Cowells

[wlb]

Checked out the topic on cutting imperial threads on a Cowells and members saying they are nearly there. We are talk the opposite of cutting metric with an imperial lead screw were you need a 127 tooth gear. The say you can use a 63 tooth one but that is not a true thread and only just near. When I had to cut imperial threads on a metric lathe some years ago when I was working in heavy industry we just fitted the 127 gear and it all fell in place. Unfortunately I can not remember the formula, I will have a look for it, it is somewhere on a piece of paper put in a book. If I find it will post it for the group, I remember we had to cut our 127 gear as at that time you could not buy them which might be the same now for the Cowells.         

[Dan Rowe]

Yes it is true if you want to cut exact imperial threads with zero error you will need a 127T gear.

There is not enough room on the standard quadrant for a 32DP 127T gear and idle gears or compound studs. You could use a smaller pitch for the 127T as the Sherline gear set does but you will also need at least one other gear with the smaller pitch to transfer the motion.

Jo and I went with a compromise solution using a 37T and 47T gear. If the 37T is on the spindle and the 47T is on the lead screw then a 20TPI thread is cut with 0.021% error which is a very small error

Dan

[mklotz]

The say you can use a 63 tooth one but that is not a true thread and only just near.

Actually 80/63 is a slightly better approximation than 47/37.  The former has an error of -0.012% while the latter, as noted, has an error of 0.021%.  However, an 80 tooth gear is often too large for many lathes.

[steamer]

Marv is correct clearly

However
The error is smaller than the precision of all but perhaps the very best lathes regardless

Dave

[steamer]

....in this case the error for a 37/47 is .021%....or .00021"/ inch of thread.

That's much better than the error in the typical lathe leadscrew....which you may find to be close to .001"/inch

Dave

[Dan Rowe]

Dave,
I did say it was a very small error....I will have to send out my thread gage out for calibration now

Dan

[steamer]

Dave,
I did say it was a very small error....I will have to send out my thread gage out for calibration now

Dan

YUP...that was kinda my point...especially for the typical threads we cut which are rarely over 1 inch in length....you'll never see it.

Dave

[Raggle]

Earlier in this thread I mentioned the usefulness of a 38t gear in converting imperial lathes to metric screwcutting use. Since then we have seen a superb treatise on cutting BA threads on the small Cowells lathe. And a herculean task it has been.

I didn't make my case very clear (as is often the case with me) so with your forbearance I will try to clean it up.

                              _____________________________________________________________________________


My Exe lathe has a permanently fitted 18t gear on the spindle and drives an 12tpi leadscrew through user selected 18 DP gears. Obviously an 18t gear on the leadscrew will give a copy of the leadscrew pitch at 12 tpi.

The formula for any pitch is simple  -  chosen pitch x 3, divide by 2 gives the teeth on the driven gear (for simple trains of 2 idlers)

e.g.  12 x 3/2 = 18t
        26 x 3/2 = 39t

        40 x 3/2 = 60t    -  or 30t on leadscrew with 36/18 double and one idler

To cut 25.4 tpi (1mm pitch) needs a leadscrew gear with 38.1 teeth. Search as ye may, nobody makes one. So try 38 and it comes out as 0.9973753280839895013123  -  ok, that's the last big number but just to demonstrate, if my lathe was capable of cutting a thread length of 1 metre it would have 997.37 threads on it. More likely would be 99.7 over a 100mm (4") length. Like Dave said, you'll never see it.

Other metric pitches are handled by compounding the gear train and most standard M series are possible.

                                        _____________________________________________________________________

Now my imaginary metric lathe needing to cut imperial threads. It has a 1mm leadscrew and I can choose which gears I use on the spindle. So I put a 38t on there. I chose 40 tpi as my target thread and found a 60t gear on the leadscrew would do just fine giving 40.105 tpi  -  the error is the same as the previous example but in the other direction.

Error = either 381/380 or 380/381 depending on application

So I'm rather tickled to find that if I had such a metric lathe in its 38t driver form the calculation for driven gear is the same as for my Exe lathe: desired pitch x 3 / 2 and a good range of regular imperial pitches can be obtained.

No spreadsheets were harmed in the making of this post, nor need be in applying it.


Ray

[wlb]

Said I would get back if I found the bit of paper with cutting imperial threads om a metric lathe
With a 127 gear on the head stock
A 50 gear on the lead screw if 1 mm pitch
Then you need a 20/80 on the middle stub to give 40 TPI
                            25/80      ?         ?              ?          32TPI
                            25/65      ?         ?              ?          26 TPI
You would need an intermediate gear before the lead screw to get a right hand thread.
Have not got the BA gear train one.