kitno455
Titanium
- Joined
- Jul 9, 2010
- Location
- Virginia, USA
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
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
Last edited: