Metric threads on South Bend

[GordonL]

After doing a lot of playing around with calculations and spread sheets I have pretty much determined that the 127/120 transition is about the only good way to go on the 13" South Bend. I certainly have not come up with a combination which works. 127/120 = 1.058 and 62/59 = 1.0508. I am not sure if that is close enough. In any case I think that I would need an idler gear in the mix to make up the span between the spur gear and the shaft gear. I have not checked that yet. Probably just purchase the 3D printed gear set from eBay for $199. My printer will not print something that big. Probably not going to happen because I do not need metric threads very often.

[Richard P Wilson]

You might not need metric threads very often, but like a lot of things, when you need it, you need it, often NOW----.

[J Tiers]

After doing a lot of playing around with calculations and spread sheets I have pretty much determined that the 127/120 transition is about the only good way to go on the 13" South Bend. I certainly have not come up with a combination which works. 127/120 = 1.058 and 62/59 = 1.0508. I am not sure if that is close enough. In any case I think that I would need an idler gear in the mix to make up the span between the spur gear and the shaft gear. I have not checked that yet. Probably just purchase the 3D printed gear set from eBay for $199. My printer will not print something that big. Probably not going to happen because I do not need metric threads very often.

Those numbers end up with an error of 0.75%. You'd be much closer with one of the other combinations such as 47/37. That would give less than 1/10 the error of the 62/59 combination.

[GordonL]

Those numbers end up with an error of 0.75%. You'd be much closer with one of the other combinations such as 47/37. That would give less than 1/10 the error of the 62/59 combination.

The problem is that it must match the 127/120 ratio not the 127/100 ratio. The 13" SB has a 6TPI leadscrew and the others have a 8TPI leadscrew so that means the that the input ratio must be different.

[J Tiers]

It seems that is true only if the gears have to do "double duty".

The ratio set up in the idler train to the gearbox should already fix the ratio. Then you should be able to use the 127/100 combination.

Only if you need one set of gears to do both the metric conversion and the 6tpi ratio conversion should you need to have the other ratio. You can split the conversions between two combinations, which just means adding the metric conversion to the gear train that already correctly creates inch threads.

[GordonL]

It seems that is true only if the gears have to do "double duty".

The ratio set up in the idler train to the gearbox should already fix the ratio. Then you should be able to use the 127/100 combination.

Only if you need one set of gears to do both the metric conversion and the 6tpi ratio conversion should you need to have the other ratio. You can split the conversions between two combinations, which just means adding the metric conversion to the gear train that already correctly creates inch threads.

I will have to play around with that. The existing gear train gives an input of 1 to .375 of spindle to screw gear so if I add a combo gear in that ratio I should be able to use the 100/127 or equivalent. There is already an 80 tooth gear in the train so adding a 30 tooth combo should accomplish that. I will have to do some playing around with CAD to see how things fit.

[mikey553]

The problem is that it must match the 127/120 ratio not the 127/100 ratio. The 13" SB has a 6TPI leadscrew and the others have a 8TPI leadscrew so that means the that the input ratio must be different.

Can you use the 91/86 gears like in my lathe? Its ratio is very close to 127/120.

[Baz]

I think the reason for not using 91/86 is that it's a lot of teeth so going to cost more than one of the other versions. Also most of the time you can make do with a 50 instead of the 100 (or 60 for the 120) and put in a 2:1 in the gearbox thus using a cheap common gear. If you can't fit a 127 on your printer at those prices it is better to buy a bigger printer; an Ender 3 will print 8 ins.

[GordonL]

I ended up 3D printing three gears (34T, 43T & 50T) and they seem to work, at least the lathe goes round and round. I have not actually cut any threads yet but I am confident that things will work good enough for my needs. Thanks all. At least it generated some interesting discussions.

[Paul Alciatore]

The thing about metric conversion is that the final result come out with a desired accuracy. In searching for all threads that my lathe could cut I found that, even with a 127::100 tooth pair there were some standard metric threads that would require additional change gears that are not in the original SB set. This applies, even more so, when using a QC gear box because the available combinations of gears are much more limited.

If you do a computer search for a combination of gears that allows the maximum number of metric threads to be cut with a standard set of change gears or with a QC gear box, it should be no surprise that there may be some combinations that are not 127::100 or any number that is even close to that ratio. Such a search is actually looking for gear ratios that "accidentally" produce a number of metric threads as well as those that contain the exact conversion factor or a number close to it. You will find the 1.27 ratio or something close to it buried in the gear choices for each of those metric threads. And it may be in different ones in different ways.

If you consider gear pairs with tooth counts between 15 and 150, there are over 18000 combinations I would not be surprised if there are some in that group that are yet undiscovered.

Mine isn't a Grizzly but it came from the same factory under a different distributor name. It has a 91/86 combo gear.

I see a lot of focus on trying to match the 2.54cm/inch or the 1.27:1 ratio idea. But that does not have to be the case.

So my 91/86 (checked and it is in fact 91/86) is a 1.058 ratio.

When using that ratio and the levers are set to C and 6 it is supposed to provide me a 1.0mm pitch. C6 in imperial with is 24 TPI. 24TPI gives us 1/24=0.0467" lead per thread. The 91/86 gear is inserted to provide an 86/91 reduction. Multiplying that by the 24TPI lead per thread of 0.0467" equals 0.0394" of lead per thread. And that works out to 1.00076mm according to my Windows conversion calculator.

But to add to this I've also got a 36T gear which is supposed to replace one of my 45T gears for some of the other lead values. So clearly the 86/91 setup by itself is not the be all and end all.

So there's more than one road to Rome on this quest for metric threading pitches from an imperial gear box. And it explains why there's a few different options.

[mikey553]

I ended up 3D printing three gears (34T, 43T & 50T) and they seem to work, at least the lathe goes round and round. I have not actually cut any threads yet but I am confident that things will work good enough for my needs. Thanks all. At least it generated some interesting discussions.

What material did you use for these gears?

Also have you designed these gears from a scratch or is there an easier way?

[GordonL]

I used ABS. There is not much load on the gears. Did the basic design with FreeCad and then added the bore and keyway with Alibre. FreeCad has an add on to do gears but Alibre does not.

[RMinMN]

PLA works pretty well for change gears on a lathe as they don't need to transmit a big torque. Most modern CAD programs have the ability to generate gears which can then be exported as an .stl file and sliced and finally 3D printed. The advantages of that are that the CAD program can generate a metric or imperial measurement gear of any number of teeth (to a practical limit). If you find that you are seeing more wear on the gear than you like, print another. As cheap and easy as this is, print 2 or 3 so you have spares.

[RMinMN]

I used ABS. There is not much load on the gears. Did the basic design with FreeCad and then added the bore and keyway with Alibre. FreeCad has an add on to do gears but Alibre does not.

Why did you not do the bore and keyway too while you had the file open in FreeCAD?

[GordonL]

I am not familiar with FreeCad and could not figure it out. I am sure that I could have figured it out but I knew how to do it in Alibre.