New way to metric thread on 10L?

[kitno455]

New way to metric thread with double tumbler gearbox?

Metric threading on an 'Imperial' SB with QCGB usually elicits some complaints: 1. size of the compound gears 2. the large number of stud gears 3. the feeds on the regular threading chart are pretty far off with the smaller stud gears 4. the cost, particularly with the larger gear cover.

So, I've been working on some software over the past couple months to investigate alternatives.

The normal solution to #1 is to use 80/63 or 47/37 as a substitute for 127/100. These have errors of 0.0125% and 0.0213%, respectively. These are very good, but some folks balk at the error. If we also use a substitute for the 56 tooth stud gear, we get some replacements with lower error:

55,58,75 0.0010%
74,77,74 0.0043%
72,82,81 0.0028%
59,73,88 0.0047%
plus quite a few more. These are all small enough to fit inside the cover, and very, very close to perfect.

Problem #2 could be addressed if we ignore the really unusual metric threads on the south bend chart, and pick some different stud gears that each cover more metric threads. If we keep those stud gears in a range that is near the normal imperial endgear ratio, we can also address problem #3.

So, after some digging, it looks like we can get the common metric pitches with only 4 stud gears: 48, 54, 55, 56. These have the effect (in combination with the compound gear), of 'converting' the 8tpi leadscrew into 3, 3.375, 3.438, and 3.5mm pitch respectively. They also keep the gear box speed in a range from 94% to 110% of factory speed, meaning the feeds chart is still fairly accurate.

So, the cheapest I can see is to add a 63 on top of the existing 80 tooth idler, and the 4 stud gears 48, 54, 55, 56. If higher precision is required, you would need three new gears instead of the 63.

I don't see any reason this technique would not extend to other machines with different gearboxes or leadscrew pitches.

allan

 

[Finegrain]

Looks good to me! Let me know when y'all are ready to buy the gears and I'll mill up some sets . If I did these in batches, the unit cost would be much lower than singletons.

One other thing to ponder while we're outside the box is mixing up DP's to mitigate spacing problems. There's nothing wrong with having, for example, the stud gear and the part of the compound gear it meshes with be a different DP than the rest. In this configuration, the stud and the first 1/2 of the compound comprise an independent gear train, so they can have whatever DP is convenient. Same for the second 1/2 of the compound and the QC input gear (is there an idler in there as well?) -- these are their own train so they can have their own DP.

Granted, higher DP gears have smaller and hence weaker teeth, but the typical use-case for these gears, threading, implies that they don't need to have as much strength as the workaday feed gears.

Heck, you could even use 20* pressure angle gears for these special configs, which are stronger than 14.5's, and mitigate the strength concern.

Regards.

Mike

 

[hitandmiss]

allan,

Which QCGB are you working with? single tumbler, double, or wide range?

Bill

 

[kitno455]

Double tumbler, wide range, though the narrow range should be similar. The single tumbler is such a pita, you have to replace the banjo, etc.

allan

 

[beckley23]

Finegrain is correct about some manufacturers using smaller DP's for the conversion gears. Monarch uses 16 DP conversion gears on their geared machines and 32 DP on the 10EE. I just completed 2 sets of conversion gears one using an 80/63 conversion gear and the other using a 127/75 conversion set.
http://www.practicalmachinist.com/vb/monarch-lathes/metric-threads-225616/
Harry

 

[kitno455]

Sure, you can change DP, or even switch to a module gear if your lathe does not have space. I suppose you could even make a custom DP if you have a fixed center distance like the single tumbler machines do. However, keeping everything the same increases your ability to mix and match the gears to create other pitches for things like worms, etc.

allan

 

[Mcandrew1894]

Allan,

I use a 37/47 combination on my Logan with very good success. Most fastener situations have a relatively short length of engagement, and absolute pitch accuracy is not that important...leadscrews for lathes aside. .02% is plenty good enough. for a draw bar or odd bolt.

I found enough combinations by inserting my combination set in and computing the pitch from the gearbox. Even the elusive Schaublin 1.6667 mm pitch buttress threads worked just fine.

I plan on using the same on my SB 9 once it's restored.

Dave

http://www.practicalmachinist.com/v...ltham-machine-works-lathe-restoration-195807/

 

[gunsmithing1]

Try this

On a 9 or 10K lathe using only standard gears on the lathe Model A only Need a 25, 42, 44 and 46 tooth gearMetric PitchTumbler setting TPIStudScrewErrorMetric PitchTumbler setting TPIStudScrewError0.505220562.0%0.56425560.8%0.703620560.8%0.703620560.8%0.7512856404.0%0.7510446420.2%0.803220560.8%0.83220560.8%1.002620562.0%13225560.8%1.252020562.0%1.252642460.1%1.503256404.0%1.55246420.2%1.754020402.0%1.753244560.2%2.001320562.0%21625560.8%2.501020562.0%2.51342460.1%3.001220401.2%32646420.2%3.502020402.0%3.51644560.2%4.006.520562.0%4825560.8%

Metric threading on an 'Imperial' SB with QCGB usually elicits some complaints: 1. size of the compound gears 2. the large number of stud gears 3. the feeds on the regular threading chart are pretty far off with the smaller stud gears 4. the cost, particularly with the larger gear cover.

So, I've been working on some software over the past couple months to investigate alternatives.

The normal solution to #1 is to use 80/63 or 47/37 as a substitute for 127/100. These have errors of 0.0125% and 0.0213%, respectively. These are very good, but some folks balk at the error. If we also use a substitute for the 56 tooth stud gear, we get some replacements with lower error:

55,58,75 0.0010%
74,77,74 0.0043%
72,82,81 0.0028%
59,73,88 0.0047%
plus quite a few more. These are all small enough to fit inside the cover, and very, very close to perfect.

Problem #2 could be addressed if we ignore the really unusual metric threads on the south bend chart, and pick some different stud gears that each cover more metric threads. If we keep those stud gears in a range that is near the normal imperial endgear ratio, we can also address problem #3.

So, after some digging, it looks like we can get the common metric pitches with only 4 stud gears: 48, 54, 55, 56. These have the effect (in combination with the compound gear), of 'converting' the 8tpi leadscrew into 3, 3.375, 3.438, and 3.5mm pitch respectively. They also keep the gear box speed in a range from 94% to 110% of factory speed, meaning the feeds chart is still fairly accurate.

So, the cheapest I can see is to add a 63 on top of the existing 80 tooth idler, and the 4 stud gears 48, 54, 55, 56. If higher precision is required, you would need three new gears instead of the 63.

I don't see any reason this technique would not extend to other machines with different gearboxes or leadscrew pitches.

allan

 

[Mcandrew1894]

Gunsmithing1,

I think your trying to post a table of some sort....but I can't figure it out.

Is that?

Metric Pitch .75

Tumbler setting 10

TPI 44.64

StudScrew 20

Error .2%

for example?


Dave

 

[CDRIVE]

Gunsmithing1,

I think your trying to post a table of some sort....but I can't figure it out.

Is that?

Metric Pitch .75

Tumbler setting 10

TPI 44.64

StudScrew 20

Error .2%

for example?


Dave

I certainly hope that this thread doesn't end here with this question just hanging there. Actually, I just hope this thread doesn't end quite yet. What would really be beneficial (to dummies like me) would be a graphic of the back gears, indicating the gears discussed throughout this thread.

A link to the SB Metric Threading Chart would be nice too!

Pushy new guy,
Chris

BTW: Allan, thanks for the link to your thread!

 

[kitno455]

it won't end there. I'm working on an algo to generate more, simpler options. I'll try to include that really wide data too

allan

 

[coyotechet]

Metric threading

I am with Chris and I also am a dummy about how to set up and use compound gears and metric threading would like to see gear train set up.
And some kind of chart for each type of compound gear setup. Just wishing and hoping. Been trying for along time to understand what goes where using compound gearing but get lost fast.
Thanks Chet

 

[kitno455]

Well, it's a long story to make short, but I'll try.

First, understand that the gearbox and end gearing are working together to make the leadscrew turn at some fraction or multiple of the spindle speed, generally slower. This is made easier by the design of UN threads- as they are nearly all a whole number of threads per inch. Metric is not so well designed.

Second, when the leadscrew turns the same speed as the spindle, the threads produced are the same pitch as the leadscrew.

Third, the design of the SB gearbox is such that it turns the output faster than the input, even when set for 1:1 operation, like 8tpi threads with an 8tpi leadscrew. In the case of the 9"/10k, the output turns 2.8 times faster. This has to be corrected by the end gearing, which turns the gearbox input slower than the spindle, at the inverse ratio. Hence, the 20/56 stud and screw gears (simplified: 1/2.8 or ~0.357).

Fourth, the idler gear (generally 80 teeth on a small SB lathe) does not enter into the calculations at all, since it 'cancels out' mathematically. On the 9/10k: 20 drives 80, and 80 drives 56. This produces a ratio as follows:

20/80 * 80/56 = 20/56 = 0.357

Fifth, just as SB 'corrects' the gearbox speed with the step-down end gearing, we can under- or over-correct it with some other end gearing, and make threads of other measures, like metric, or DP or module worms, etc. SB does exactly this on the top row of the 9/10k, they switch the stud gear from 20 to 40 teeth, and so double the input speed. Moving the carriage faster causes coarser threads, exactly half the pitch of the row below.

SB also uses this trick on their metric sets, replacing the 80t idler with a 127/100 compound on the 8tpi machines, and moving the screw gear outward to use it:

20/127 * 100/56 = ~0.281

The important point here is that we are now turning the input of the gearbox slower than normal, so the threads will be finer than the chart suggests. In fact, they will be 1.27 times finer. So, our 8tpi setting becomes 10.16tpi. Which, just happens to be 2.5mm per turn. So, our machine now acts somewhat like it has a 2.5mm pitch leadscrew, instead of 8tpi. And all the multiples of 8tpi which the gearbox makes, are now applied to 2.5mm instead. Hence, the 'twice as fine' setting (16tpi), gives us 1.25mm threads.

Now, the problem. Metric threads are not even fractions of a common distance like UN threads. So it turns out we can only get a few threads from our current endgear setup. So the SB way is to swap the 20t stud gear for a number of other gears, each of which allows us to cut a few metric pitches.

The SB metric sets are still available used, NOS, or reproduction for the smaller machines. So if your time is short, just buy one and move on. If you are an incurable tinkerer like me, you can consider using an alternative set, which is smaller and has fewer stud gear changes. That's where my current investigation lies.

Clear as mud?

allan

 

[CDRIVE]

Allan, thanks for the very detailed information but that data was not what I was referring too. What is needed in a discussion like this is graphics! It does little good to go through all this detail without the reader knowing and understanding what gears you're talking about.

This link has the kind of graphics that I'm talking about but these graphics are not applicable to a double tumbler Heavy 10. You will note that you can mouse over all the gears and a tool tip message will appear naming the gear.

Metric Threading

Do this and our eyes won't glaze over.

 

[kitno455]

Sorry, I assumed you had looked under the cover- there are only three gears, stud (40t), idler (80t), and screw (56). A compound gear would replace the idler, with the stud gear in contact with the inner gear of the compound, and the screw gear shifted left, so that it contacts the outer gear of the compound.

pictures of end gearing should be pretty common here, lets see what google finds:

http://www.practicalmachinist.com/vb/south-bend-lathes/heavy-10-gear-whine-191067/#post1220970
http://www.practicalmachinist.com/vb/south-bend-lathes/cutting-metric-threads-156039/#post872728

allan

 

[CDRIVE]

Sorry, I assumed you had looked under the cover-
allan

Well, duh! Of course I looked under the cover.

Allen, before I make anymore comments or ask anymore questions I'm going to study those two photos and compare them with my gears. Actually, there is one thing I'd like to say...

Ten years ago, when I bought her, I was much more familiar with her than I am now. I explained how that happened on the SouthBendManual group. In 2001 I inspected those gears and replaced one with a broken tooth. This week is the first time I've seen the old girl since the tarp covered her 10 years ago. Right now I'm going though a complete re-familiarization process. I should have made some notes back then!

 

[CDRIVE]

Sorry, I assumed you had looked under the cover- there are only three gears, stud (60t), idler (80t), and screw (56). A compound gear would replace the idler, with the stud gear in contact with the inner gear of the compound, and the screw gear shifted left, so that it contacts the outer gear of the compound.

allan

Allan, is that a typo in bold? I count 40 teeth on my outer stud gear. Also, I assume you mean invert the screw gear, which puts the the long shoulder against the gear box, which raises the gear to mesh with the idler?

Chris

 

[kitno455]

The first point is an error, I corrected the original post. The second point is correct, the shoulder is actually a loose spacer. you swap it with the screw gear to shift the gear to the end of the shaft (left when operating the lathe).

allan

 

[CDRIVE]

OK Allan, I got ya. I didn't realize there was a spacer on there. By the way, remember when I posted this?

What would really be beneficial (to dummies like me) would be a graphic of the back gears, indicating the gears discussed throughout this thread.

A link to the SB Metric Threading Chart would be nice too!

Well, Jeff doesn't have this SB chart on his website but he has it in his ebay listing here. Scroll down the page.
METRIC TRANSPOSING GEARS FOR SOUTH BEND HEAVY 10" LATHE | eBay

This is really all that needed.
Thanks,
Chris

 

[kitno455]

This is really all that needed.

Yes- if you want to use the stock SB setup, that's all you need. Oh, and 7 stud gears, and the 127/100 compound, and a new gear door if you want it to close

allan