If the wanted pitch is 3.14159mm and one can achieve 3.165mm, I make that an error of 0.75%? Furthermore the pitch error is 0.0234mm, or less than a thou. I doubt that will be a problem. Either that, or I shouldn't do maths after two glasses of wine.

My lathe will cut 1 Mod threads with an error which think is 0.06%. PM me if you feel you need to use it.

Andrew

The question is, to what accuracy do you need to make an approximation to trigger the find-it fairy?

Is this likely to cause any major concerns, or is it tolerable?

Does that not depend on the intended application of your worm? You have just shy of a one thou. in pitch error. Probably OK if you are not expecting to use it in an application where the errors will accumulate (such as a rotary table or instrument rack etc).

For example, if you are planning to use the worm to drive a rotary table with 90:1 ratio, one turn of such a table with your worm would have 90.0 x 0.0009 = .084 thou in. positioning error, you can decide if that is significant or not.

If you are using your worm to drive a hacksaw or the suchlike then the error is not material.

Hello again Howard,

I can save your brain cells some, but not all, the work.

The basic gearing calculation is really not too difficult. For this example we do need to incorporate an approximation for pi/2 in the gearing, which is neatly given by 55/35 to an error of roughly 1 in 2000

The full sum therefore becomes required Pitch P = 55/35 x R x Leadscrew pitch [ R is the combined value of change wheel ratio and in your case a gearbox ratio]

Putting in the known value for P = 3.142, we can rearrange the formula to find the value of R

R = 3.142 x 35/55 x 1/ Leadscrew pitch [1/3 in this case] which gives a value for R of 0.6664

Without knowing what change wheels you have available, nor the gearbox ratios that you can call upon, it surely can't be too taxing to find values for these components in the calculation, which multiplied together amount to 2/3

Regards

Brian

Hello Howard,

How very flattering, I'll do what I can to help.

I am not familiar with your BL12-24 lathe, but I suspect it will have a 40T gear off the spindle permanently included in any change wheel set up. I don't know how comprehensive the Norton gearbox is either, so a picture or two to enlighten me on those aspects will be helpful; plus of course your spreadsheet.And a picture to show how your changewheels are arranged on the banjo would be helpful.

Do you have any feel for the ratios the gearbox provides? One way of establishing that is to take a look inside and count the teeth on the input gear, then count the teeth on the cone of gears that can be selected. I can work out what that gives as ratios if you would prefer not to.

I hope it won't be necessary for you to make other change wheels, we'll see.

My email is wood_y(at)btinternet(dot)com all in lower case, no spaces and note the underscore between d and y

Regards

Brian

Posted by Howard Lewis 04/07/2019 21:09:34

I want to cut a worm to drive a Mod 1 gear. This means that the circular pitch is pi,, unless i've miscalculated.
So effectively, I need to cut a thread with a pitch of pi mm
Have spent two days setting up, recalculating and recalculating, a spreadsheet
to cover every one of the 1278 possibles available on my lathe.
The closest that i can get is 3.165 mm. This gives an error 0.060%, which will
give a pitch error of 0.188 mm (0.0074" in old money) This seems to be a lot.
Is this likely to cause any major concerns, or is it tolerable?

Howard

Looking though Ivan Law's book gears and gear cutting on page 97 and the adjacent pages
provides some explanations on the pitch of worms.
It would not be of any use to be able to cut a worm exactly the pitch to suit the CP of the gear
(assuming that this is to mesh with a spur gear) since the worm has to be set over at an angle
in order to mesh with the gear.He does make the point that if the helix angle is low about 3 deg the
difference can be ignored.
However by adjusting the helix angle in this case to 6.9 deg the figures work out to the 3.165 mm
or .1246 inches .
As described in the book dividing the circular pitch by the cosine of the helix angle of the
worm to obtain the new pitch CP 3.14 mm or .1237 inch divided by cos 6.9 deg = .1246 in
or 3.1649 mm ,at these figures the worm would be .328 inch PCD , .407 inch OD .
It just depends if you can live with the 7 deg set over and the root diameter of about .240 inch .

I don't screw cut worms anymore , i mill them the finish is much better along with a cnc system
the pitch can be adjusted over many teeth and along with the control with rotation any size or
pitch can be achieved ,the same basic parameters need to be observed in respect of lead
helix angle and pcd.
Here is a 1 module worm and hobbed gear at 90 to 1.

John