Change gears for a 27 TPI thread

[sb327]

If my thinking is correct, a 1mm pitch with 3mm leadscrew in a,b,c,d configuration would be something like 60,90,60,120

Dave

[sb327]

In a,b,c

60,x,180

The middle gear matters not, as Bob mentioned.

Dave

[jackssmirkingrevenge]

This is the chart on the lathe:



The gears available are 30,35,40,45,45,50,55,60,60,65,70,75,80,110,120 and 127

I agree that 60, [whatever], 180 should theoretically work for a 1mm pitch from a 3mm lead screw.

B/C acts as an idler and D has 3 times as many teeth as A, so for example 40, [whatever], 120 should work too, if B/C is a single gear then as long as A:D is 1:3 there are several combinations that should work.

As you can see from the chart however, the recommended gear combination for a 1mm pitch is 60, 110, 45, so A:D is 4:3.  I've cut 1mm pitch threads with this combination so I know it works, but it doesn't seem to follow.

[sb327]

It sounds as though the lead screw is a 1.33mm pitch, if this is the case.

Are you certain on the leadscrew pitch?

Dave

Edit: it appears the leadscrew pitch is  .75mm from back figuring a 3mm pitch (which isn’t a 1:1 final according to the chart, it’s a 4:1)

[jackssmirkingrevenge]

Are you certain on the leadscrew pitch?

It's listed as 3mm in the manual and while I don't have the gauge it measure it accurately it looks to be the case:

it appears the leadscrew pitch is  .75mm from back figuring a 3mm pitch (which isn’t a 1:1 final according to the chart, it’s a 4:1)

Clucking bell, you're right!  For some reason I multiplied instead of dividing

So if to cut a 3mm pitch it is 4:1 then for a 1mm pitch it should be one third of that so 4/3:1 which is 1.333:1!

Thanks Dave 

[sb327]

It looks like a 3mm according to your rule. I was thinking last night that a 3mm makes sense because of the thread type. An acme (style) in .75 or 1.333 doesn’t.

Sooo….is there another gear somewhere in the drivetrain between the spindle and leadscrew?

Dave

[jackssmirkingrevenge]

is there another gear somewhere in the drivetrain between the spindle and leadscrew?

I suspect it is between the spindle and shaft A, I'll check it out when I'm in my workshop.  If the online calculators assume 1:1 between these two shafts then that would explain the discrepancies I was seeing.

[sb327]

That’s what I’m thinking, most calculators assume ‘a’ to be 1:1 with spindle. Since yours doesn’t directly drive from spindle, it may have one more step to account for.

Dave

[jackssmirkingrevenge]

As suspected, one turn of the spindle results in a quarter turn of gear A:

[jackssmirkingrevenge]

Going back to this calculator, I entered 0.75mm as the value for pitch (3mm divided by the 4 times reduction) and the results are now congruent with the listed combinations.  This confirms that it was failing to take the reduction into account that was giving the wrong results.

[sb327]

So the mystery has been solved. Good deal.

Dave

[rsterne]

I was going to suggest that, but didn't as you stated it was 1:1.... 

Bob