Lathe Change Gears Question

[garandsrus]

Hi,

I am trying to cut 7/8 x 14 threads to make the equivalent of a Lee push through sizer. This is the first time I have ever cut threads. The problem I ran into is that I don't have the correct change gears. The diagram in my lathe says that I need one spindle with both 20 and 40 tpi gears. I don't have either one!

Can I substitute another 2:1 ratio set of gears for these, such as 24x48? If I did that, would I also need to reduce the size of the next gear in half to compensate? I would think so, but it may not be possible with the gears I have.

The lathe is a Craftsman/atlas 6" lathe. The change gears I have are 24, 32 (two), 36, 44, 46, 48, 52, 54, 56, and 64.

The screw gear for the 14 tpi is 56.

Any suggestions on where to pick up a 20 and 40 tooth gear? I checked eBay but didn't find the specific gears.

Thanks,
John

[lathesmith]

Hi John, you should be able to come up with a combination that will work. 14 tpi is a fairly simple pitch. I don't quite understand your description of your spindle gear--is it 20 or 40 tpi?

I think this is a fairly common lathe, you might try doing an internet search. For lots of these older lathes and several Asian lathes guys have posted thread gear charts that list lots of combinations.

If that don't turn up anything, we'll see if we can't come up with a formula to help you compute what gears you need. If you know your spindle gear tooth count, all it will take is knowing the number of gears in your gear train and some math, a little trial and error.
lathesmith

[merrden]

hi it also may be possible to get a copy of the manual off of the internet . You will have to know the model number and possibly the year it was made. If you know who made the lathe for craftsman check their website as a start point. What sites deal with this type info? Please advise.

[725]

I just bought a few 7/8 x 14 bolts. Shop around, they are out there.

[Willbird]

You can also buy 7/8-14 allthread , it is not especially "nice" stuff to machine....you can also buy 4140 alloy steel allthread from such as mcmaster-carr and msc....I have a stick of it laying around here somewhere.

Bill

[l-weinrich]

I have the same lathe with the manual, in my manual the setup for 14 tpi is figure 2 which uses position B as an idler only. so it does not matter just choose a similar diameter for the back position (toward headstock) position A and the spindle stud gear need to be the correct gears.

[l-weinrich]

Oops, The lead screw needs to be 56 tooth change gear as well. All the back positions are driving and all the "F" or front positions are just spacers in that setup.

[merrden]

HO TAY: go to: www. quarterbore.com This guy has manual reprints on CD. That will get you the right info ASAP from the manufacturer.

[garandsrus]

Thank you for the responses!

Lathesmith - One "spindle" has two gears stacked together on it, a 40 and 20 tooth. Generally, only one is used, the other is just a spacer.

l-weinrich - I have the 56 tooth thread gear. Are you saying that I can put any combination of gears I have in position B? I really don't understand the difference between idler gears and driving gears. I do understand that one of the two gears on the spindle might not contact any other gears, so it wouldn't matter what gear was used as long as it didn't interfere with the other gears.

All - I have the gearing chart inside the gear cover, so i know what they call for, I just don't have them!

I did a search for the gears but didn't find them. I did find a complete set on eBay that closes in a few more days. The price was reasonable, but of course the auction hasn't closed yet.

Thanks for the suggestion of buying threaded rod. I might do that... If I do, how do you hold the rod in the lathe chuck so that you can turn/bore it without destroying the threads?

Thanks again,
John

[Willbird]

I just chuck the stuff up, unless you are REALLY hogging on it you will not damage the treads, you could wrap a few wraps of brass or copper sheet around it I guess, or some beer/soda can

I think the soda can would win out because I know I have some

Bill

[l-weinrich]

Yes any combination that will allow proper positioning, try to assemble the gear train and you will be able to figure it out easily. Do you know how to use the threading dial?

[waksupi]

Pennies work good as pads in jaw chucks, or those little square zinc or lead WW's you get.

[garandsrus]

Thanks guys, I will give it a try and let you know how it works out...

L-weinrich, I have read about how to use the threading dial, and have practiced engaging the half nuts when the dial reaches "0", but I don't know how much I can be off from zero without destroying the thread. I am practicing on a scrap piece before threading the real die, which is complete except for threads.

John

[l-weinrich]

sounds like you're good to go, I tried to post a picture of the page from my manual but I could'nt figure out how. The next time my daughter is home I will have her show me

[lathesmith]

garands, check out this thread I found on one of the yahoo groups:

http://groups.yahoo.com/group/atlas6...e/messages/399

Note especially post #400, the guy goes into a pretty good explanation as to how to figure gear combos other than factory. I believe this group also has a "files" section that has a thread gear chart, much more comprehensive than the original, but you will need to join the group to access the file.
BTW, accurately gripping threaded pieces without marring is easy...with a collet chuck. Jawed chucks may work OK, you'll just have to try it out and see if it works for you. The more jaws in your chuck, the less problems you will have with marring.
lathesmith

[garandsrus]

Lathesmith,

That's an excellent thread! Thank you...

John

[garandsrus]

OK, I downloaded "General Threads Calculator 1" from http://www.lathes.co.uk/screwcutting/. You will need to rename the file with a .exe extension so it can run if you download it.

I put in my gears, a 14 for the tpi and 56 for the lead screw turns per inch (which I'm sure is not correct as I just picked a number!). I guess I need to go out to the lathe and measure an inch on the lead screw and count the threads!

Putting in the numbers above results in an "answer" of:
Driven gears 24 32 Driver gears 48 64 TPI 14 error % 0
Driven gears 24 36 Driver gears 54 64 TPI 14 error % 0

However, I don't know how to read the answer

I downloaded the instructions (don't laugh). Here's the section from the manual:

THE CALCULATIONS (NthreadP)

These are the calculations on which the programme is based. They are also for assistance with some calculations required by the user.

FOR AN IMPERIAL LEADSCREW

TPI produced = DRIVEN/DRIVER X DRIVEN/DRIVER X LEADSCREW TPI

It does not matter in which order you place the driver or driven gears.

Based on this, it sounds like on one spindle (that holds two gears) I would put a 24 and 48, drive the 48 and then connect the 24 to the 64 tooth gear on other shaft, which has a 32 and 64 gear. From these two gear sets, it sounds like I could connect the 32 gear to any gear on the leadscrew and get the right pitch. Is this correct? The math would be: 14 = (24/48)(32/64)*56

Of course, l-weinrich's answer of "the middle gear doesn't matter" is probably the easiest way to implement the thread for me.

Thanks,
John

[Bret4207]

Blue Ridge Machine and several other companies offer, or offered, additional change gears for the 6" Atlas. That Zamack alloy (Zinc and aluminum IIRC) the gears are made f breaks pretty easy. Might be worth Googling "Atlas Change Gears" and see what you can find.

[lathesmith]

John, your lathe should have a lead screw thread pitch of 16 TPI--this is pretty standard, I wouldn't think it would be anything else. Even most Asian lathes have 16 TPI leadscrews these days, mainly for cutting more accurate inch thread pitches.
lathesmith

[garandsrus]

Lathesmith,

I measured the lead screw and counted the threads. I came up with 16 to the inch Thanks for the confirmation.

John