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Adapting 820 gearbox to MW 10" lathe (Logan 200)f

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ErichKeane

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#1
In googling, I saw this thread https://madmodder.net/index.php?topic=7917.0

in how to do this, but I bought a gearbox off an 820, and have a 200. I realize I'll have to take a bit of material off the mating face and drill new holes, as well as modify the leadscrew on my machine. However, I don't actually know anything else bsides that! For example, what gear arrangement do I put in the existing manual gearbox? WHAT faces in particular do I need to remove 11/32"?

I was wondering if anyone has ever done this in a youtube series that I could watch? I'd love to prevent myself from making existing errors.
 

ErichKeane

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#3
I had seen that, though, it left me with some questions still. First, I was wondering how to adjust the gearbox left/right, though I found this: http://conradhoffman.com/metricthreading.htm
Good thing I saw those pictures, I had it set up wrong and had already center-punched the drill holes! I need to shift it over a bunch more.

Today I took the old gearset off as much as I could, then removed the leadscrew. I took the things out of the way that looked to be in the way. I put the gearbox in place with some wood proping it up, and sighted down teh leadscrew, confirming that I need to take off material, as well as WHERE I need to take it off!

Now that I came in and saw the conrad hoffman pictures, I can drill the 3 spots for 5/16-18. Once I get it mounted that way, I'll confirm the 11/32" measurement for sure and mill that off.

However, I'm left with a few more issues I'll need to solve:
1- Getting the left-gear-change cover to fit. It doesn't come close to closing around the quick change gearbox!

2- Most importantly, how do I set up the gears to thread? the diagram on the QC gearbox I bought lists lever positions for A-E and lists the stud gear, but only for a single row of 48 tooth, and a couple for 24 tooth. However, what other gears do you install? What does it mean by "stud gear"?

I guess what I REALLY want is this chart: http://www.lathe.com/990306.htm for an 820!
 

wa5cab

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#4
I spent a couple of hours last night studying the two 800 Series QCGB's without finding an answer to what the tooth count is on several gears in the QC train. There is an unidentified gear that I'm going to refer to as the "Tumbler Pivot Gear". The gear that Logan referrs to as the "Stud Gear", as far as I was able to figure out, goes on over the Pivot Gear and is locked to it by a Woodruff key. I'll stop there for now in case you're already determined that this isn't correct.
 

ErichKeane

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#5
I spent a couple of hours last night studying the two 800 Series QCGB's without finding an answer to what the tooth count is on several gears in the QC train. There is an unidentified gear that I'm going to refer to as the "Tumbler Pivot Gear". The gear that Logan referrs to as the "Stud Gear", as far as I was able to figure out, goes on over the Pivot Gear and is locked to it by a Woodruff key. I'll stop there for now in case you're already determined that this isn't correct.
I haven't really figured out anything :/ The inside of the gearbox is fine, its just how to gear it to the spindles itself. I looked at the 'metric threading' image above at least, and figured out that it uses the same bracket for the middle gears (between what the non QCGB calls a spur gear and the leadscrew gear, like here: http://www.lathe.com/990306.htm).

At one point, I suspect I'll just have to find a way to measure the actual gear ratio between the QCGB's input and output shaft at a specific setting and go from there. For example, if I put the gearbox in 8-TPI, is it truely input==1 turn, output ==8, or is it counting on some sort of gear reduction already happening before it reaches the gearbox? Since the diagram for the non-QCGB version starts at 4TPI, and the one on the gearbox starts out at 8, I presumably need to figure that out.

If you have a QCGB installed and working, a picture/gear-tooth-count of the picture here: http://conradhoffman.com/metricthreading.htm is likely what I need.
 

ErichKeane

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#6
Actually, check out this manual, page 7, figure 8: http://vintagemachinery.org/pubs/2093/6506.pdf

What I need is the tooth-count of the "Screw Gear", and the tooth count of the gear behind the "Spacer gear" (AKA, the Idler gear). The chart in Fig-7 is the gearbox I have, so hopefully one that matches that :)
 

ErichKeane

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#7
I realized last night that the one behind the 'spacer gear' doesn't matter, since it is simply transferring power, not doing any conversions.

The only ones that are important is the screw gear and the stud gear! I looked closer on teh conrad hoffman page, and his screw gear is 50 tooth (printed on the side :) ). I tried counting the teeth in the manual, but got 48. That COULD be right, but I'm going to go looking for more pictures :/

In other news, I made a TON of progress yesterday!
-I got the holes in the rail drilled and tapped for 5/16-18.
-I found that my 'gear holder' that holds the 2 gear-sets on a slider was 1/4" too wide, so I took that amount off the back of it, so now the gears mesh properly from the gearbox to those middle gears.
-I got the 11/32" milled off of the front of the gearbox. I didn't do as intense of a setup as the link in the OP, but it worked well enough.
-I cut down the leadscrew to the right length.
-I cut part of the shaft down to 5/8" and test fit, so everything looked right!

Whats left to do:
-Cut in a keyway (parts coming!)
-Cut the end of the leadscrew for 1/2-20 to hold the gear in place.
-Reinstall longitudinal feed gear
-Reassemble!

So looks like I'm getting there :)
 

ErichKeane

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#8
Mystery solved! I emailed Logan, and they responded in 10 minutes! Awesome company:
The gear ratio from the stud gear to the gearbox gear must be 1:2 for most thread cutting, and 1:1 for the coarsest threads.

The nameplate on the gearbox should show 2 different stud gears (24 & 48 or
36 & 72). Use the larger of these two for the gearbox (either 48 or 72).

The stud gear will be half the number of teeth, except for when you are cutting 4-7 TPI, when the gears will be the same.
So my answer is 48 teeth :)
 

wa5cab

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#9
Erich,

First, I don't have a Logan lathe, I have an Atlas 3996. So posting a photo of the gear setup on it wouldn't have helped you any. Second, the manual that you posted the link for is for the 900 Series, not the 800. If they both use the same QCGB and lead screw thread pitch, then it would be applicable. But I'm not familiar enough with Logan machines to be able to say that is the case.

However, you appear to have figured out how to get your machine to work for 8 TPI and above by confirming that the gear that fits in the Stud Gear position is a 24T. However, the write-up in the 900 manual that says to use the 48T gear to get 4 to 7 TPI is either wrong or leaves out some details as the 48T couldn't simply replace the 24T in the stud position as it would not mesh with the FWD or REV idler gears. So I don't know what the answer to that is. If you never need to cut 4 to 7 TPI, it will never matter.
 

ErichKeane

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#10
Erich,

First, I don't have a Logan lathe, I have an Atlas 3996. So posting a photo of the gear setup on it wouldn't have helped you any. Second, the manual that you posted the link for is for the 900 Series, not the 800. If they both use the same QCGB and lead screw thread pitch, then it would be applicable. But I'm not familiar enough with Logan machines to be able to say that is the case.

However, you appear to have figured out how to get your machine to work for 8 TPI and above by confirming that the gear that fits in the Stud Gear position is a 24T. However, the write-up in the 900 manual that says to use the 48T gear to get 4 to 7 TPI is either wrong or leaves out some details as the 48T couldn't simply replace the 24T in the stud position as it would not mesh with the FWD or REV idler gears. So I don't know what the answer to that is. If you never need to cut 4 to 7 TPI, it will never matter.
According to the research I did, the gearbox for all 10" lathes (820) and 11" lathes (920) is the same.

That image doesn't show it very clearly, but the 'stud gear' is actually at a different plane than the fwd/rev idler gears. It doesn't mesh with them on the logan, there is a fixed gear behind them.

That said, I severely doubt I'll ever cut 4-7 TPI. However, Logan's response is really helpful in that I know I can increase the size of the stud gear (or reduce the screw gear size) to do other thread pitches! Since I know the relationship for 1:1 and 1:2 ratio, I suspect I could easily calculate other thread pitches!

Each of the A-E settings seems to double the one before, so what is cool about that is if I use a 48 tooth stud gear, I believe it just offsets the chart 1x. I'd give up my ability to do 128-224 TPI though. What is interesting about that is I think with some math I could figure out how to cut the missing pitches, or even entirely new pitches! Metric become easy enough if I can figure out a 1.27:1 ratio I think, though I doubt I'll do much metric.
 

wa5cab

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#11
OK. Now knowing how the thing goes together, I can convince my eye to just make out the bottom left teeth of the Reverse Gear (so called on the parts drawings) peeking out from under the 24T Stud Gear in various photos. I went back and added a paragraph to my previous post.
 

ErichKeane

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#12
Alright, thanks for the help all, I finally succeeded! I had to order a keyseat cutter, so that took a few days to show up. I got a 5/32"x5/8" woodruff key. 5/32 because most of the other keys in the lathe are 5/32, and 5/8 instead of 7/8 because that was a touch shorter than the length of the gear. It allowed me to a use a cheaply made collar instead of using up one of the keyed ones.

Assembly was a bit frightening, lining up the keyway was very difficult since there is only about a 50-tho area where you can see the key not yet on the gear. After about 20 half-assemble/disassemble cycles getting the leadscrew into the gearbox, it 'just worked' once! I tightened down the 1/2-20 nut on the end with my shaft collar in between, and locked it all down. The rest of attaching it to the lathe went easy enough, though the auto-crossfeed gear was a total pain to get lined up right.

I put on the banjo (which I'd taken 1/4" off of the thickness, otherwise the gears didn't line up right), popped the 48 gear tooth on the gearbox and 24 on the stud gear. I used a 60 tooth in the middle (since that was what is on my adapter collar) and bolted it all together! I did a 14 tooth run and checked it with my thread gauges, and it was dead on!

I've got a new door coming soon, so I think thats my only thing left as a 'TODO' before I call this project a complete success!
 
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