Assumptions behind thread plate calculations

[steamingbill]

See attached jpeg showing the thread cut for various configurations on my Hercus 9"

I see on the web that various other lathes have a similar plate.

What I dont understand is the assumptions behind the calculated values.

From what I have read, I currently think that the critical factors are
1. TPI on leadscrew
2. Number of teeth on stud gear
3. Number of teeth on final gear
4. Whether or not a compound gear is involved
5. Gearbox ratio (if you have a gearbox)

The way I see it, its reasonable to assume items 1, 2 and 5 were known by the people who calculated the values on the plate, (different values for item 2 are listed on the table)

They must have assumed something for items 3 and 4 ?

I've set up a spreadsheet for my lathe change gears and gearbox ratios and can calculate the TPI for various configurations and these calcs agree with VernonV's Latheworks program, so I think I understand the basic maths but I don't understand how anybody could print that plate without saying what they had assumed for items 3 and 4.

Any recommended web sites I should visit to help me understand better ?

Havent cut any threads yet - still reading - does the answer become blindingly obvious after I have had a go ?

Bill

[Stustoys]

Hi Bill,
Not that I have a Hercus lathe but I'll take a shot.

Isnt the stud get the one on the input of the QCGB?

The "other" gear(driver) cant be changed on a Hercus if I recall correctly. :)

They would "assume" no compound gears when cutting thread the same as the leadscrew.

Stuart

[Michael G]

To cut a thread (or change your feed), all you are doing is changing the relative speed of the leadscrew with respect to the spindle - at one setting the leadscrew may revolve 2 twice for every rotation of the spindle; at another setting it may revolve 3 times and so on.
When a lathe manufacturer makes up a chart like that they have calculated the ratios of the gearing between spindle and leadscrew. They have assumed the number of teeth on the final gear and any compounding that may take place - typically the items you have called 3 & 4 are the standard gearing that the lathe ships with. A compound gear may be standard, especially if you cut metric threads with an imperial leadscrew lathe. This is normally explained in more detail in the manual for the lathe (if one exists). The plate is a shorter and more convenient way of representing that in that will not get torn, lost, grubbed up or used for writing down lunch orders.

On my lathe to cut metric threads I must include compound gears into the train so I have two plates. One tells me how to cut imperial threads and the other shows me the settings and necessary gearing to cut metric. There are other combinations that can be used to cut BA threads and worm gears. These are not plated. I have to go to the manual to find these out (or sometimes calculate them). When I cut Simon's worm I even had to make up the gears because they were not part of the standard set.

So yes, there is an assumption made but it is not a guess.

Michael

[steamingbill]

Havent cut any threads yet - still reading - does the answer become blindingly obvious after I have had a go ?

Bill

Ok,

Went and played with lathe today. Managed to answer my own question.

Used a pencil and a piece of wooden dowel for starters to see what was going on - seemed to be easier and quicker than cutting and counting threads - see jpeg

Looks like the 80 tooth gear will never fit on the end of the gearbox because it hasnt got a keyway - so its only the 40 or the 56 and of those two the 56 gives me the threadcount on the plate and the 40 gives me something much coarser.

So it looks like every now and then I have to actually go and do something with the lathe instead of just reading about it in books.:U

Bill

[Stustoys]

Ok, I'm confused. Do you have a picture of the change gear set up?

Stuart

[morrisman]

Bill You might like to read

"Screwcutting in the lathe " by Martin Cleeve ..its No. 3 in the Workshop practice series . Mike

[steamingbill]

Ok, I'm confused. Do you have a picture of the change gear set up?

Stuart

Stu,

Attached is my current setup - this is also what I believe to be the "standard" setup that will produce the threads shown on the thread plate.

My confusion was caused by buying the lathe second hand and not knowing if I had all the relevant gears and what setup the thread plate was written for. Being a total newb I didnt know what setup the lathe was shipped with.

I found this post by Pipeclay here http://www.woodworkforums.com/showthread.php?t=129670

"If you have an Imperial 9" lathe you will have a 20 tooth gear or 40 tooth gear attached to the shaft below your reverse tumblers,this is known as the Stud gear.

The Stud gear will in turn run on your 80 tooth idler which inturn will run on your 56 tooth gear,this is known as the Screw gear.

In standard configuration as the Lathe left Hercus you would only need to interchange the 20 and 40 tooth gears to be able to cut all TPI listed on the Quick Change Box.

If you are running the 20 tooth gear the 40 tooth gear would be located on the Screw Gear shaft next to the 56 tooth gear,the 56 tooth gear sits on the Screw shaft closest to the Gearbox and if you were running the 40 tooth gear on the Stud shaft the 20 tooth gear would sit in the position where the 40 tooth gear was."

I reckon if I look hard enough any questions I think of will already have been asked and answered.

Bill

[nadroj]

Nice picture, Bill.
I think you and Pipeclay got it right there.
Hercus should have done that, but their documentation on this point is lacking. Otherwise, it's excellent!

Jordan