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CVA Toolroom Lathe - Metric Threading

CarbideTip

Aluminum
Joined
Sep 21, 2016
Location
Woking, Surrey, UK.
I was fortunate that when I acquired my CVA it came with some of the change gears needed for metric threading. Unfortunately my set is incomplete, but I'm aware of other owners that do not have any of the extra gears for metric threads. Like the 10EE the CVA has to use much finer teeth to fit a 127T into the space available on the change gear quadrant. The standard (inch threading) gears are 16DP. The gears for metric threading are 32DP. Both are 14.5 degree pressure angle.

MyMetricSet.jpg

At this point I should point out that another forum member does produce the set of gears needed for metric threading on a round dial 10EE, and I am told that these will also fit the CVA.

Monarch 10EE Round Dial Metric Change Gear Kit | American Machine & Gear Works

The main purpose of this post is to provide some fairly easy options for cutting close approximations to metric pitches using a few extra 16DP gears. My curiosity was roused by messages from other CVA owners who don't have any of the required gears. I'm also aware of this post here on PM.
https://www.practicalmachinist.com/vb/monarch-lathes/10ee-metric-thread-chart-364611/?highlight=metric+thread

There may be other relevant posts too.

There are at least two variants of the feed gearbox fitted to CVAs. These can be identified from the feed dial.
UK Version
UKFeedDial.jpg

Export Version
ExportFeedDial.jpg

The "Export" dial has the same pitches as the round dial 10EE.

The reason for the difference is probably due to the systems of pipe threads used in different parts of the world. Here in the UK we use British Standard Pipe threads. The pitches for BSP are: 28, 19, 14, 11, 10 and 8 TPI. In the USA NPT is the norm with pitches: 27, 18, 14, 11.5, and 8. So CVAs made for export may have had the alternative gearbox to provide the NPT pitches.

The calculations are actually fairly simple, but for convenience and speed I have written a small programme to do them. This makes it easy to try other gears. The tables that follow give the gearbox settings and STUD / BOX gears to obtain the metric pitch listed. For the dial position I refer to the 3TPI position as 1, 3.25TPI as position 2 etc etc. Pitch errors more than 0.25% are rejected. The actual pitch error is also shown. These are all for a simple gear train (not a compound train as would be required with the metric translation gears). There are two versions of the table which take into account the different ratios in the two gearbox variants.

The metric approximations listed in the tables below use only two additional 16DP gears. A 33T and a 34T. I believe that this suggested combination came from the late John Stevenson.


Table 1:
UK Gearbox - Available Gears 24 33 34 48
Pitch(mm)ABCDialStudBoxApproxError(mm)Error%
0.449792C934480.45mm-0.000208-0.05
0.499769C734480.50mm-0.000231-0.05
0.599722C534480.60mm-0.000278-0.05
0.999537B734481.00mm-0.000463-0.05
0.999537C734241.00mm-0.000463-0.05
1.247321B433481.25mm-0.002679-0.21
1.247321C433241.25mm-0.002679-0.21
1.499306B134481.50mm-0.000694-0.05
1.499306C134241.50mm-0.000694-0.05
1.746250A933481.75mm-0.003750-0.21
1.746250B933241.75mm-0.003750-0.21
1.999074A734482.00mm-0.000926-0.05
1.999074B734242.00mm-0.000926-0.05
2.494643A433482.50mm-0.005357-0.21
2.494643B433242.50mm-0.005357-0.21
2.998611A134483.00mm-0.001389-0.05
2.998611B134243.00mm-0.001389-0.05
3.492500A933243.50mm-0.007500-0.21
3.998148A734244.00mm-0.001852-0.05
4.497917A634244.50mm-0.002083-0.05
4.989286A433245.00mm-0.010714-0.21


Table 2:
Export Gearbox - Available Gears 24 33 34 48
Pitch(mm)ABCDialStudBoxApproxError(mm)Error%
0.449792C834480.45mm-0.000208-0.05
0.499769C734480.50mm-0.000231-0.05
0.599722C534480.60mm-0.000278-0.05
0.999537B734481.00mm-0.000463-0.05
0.999537C734241.00mm-0.000463-0.05
1.247321B433481.25mm-0.002679-0.21
1.247321C433241.25mm-0.002679-0.21
1.499306B134481.50mm-0.000694-0.05
1.499306C134241.50mm-0.000694-0.05
1.746250A833481.75mm-0.003750-0.21
1.746250B833241.75mm-0.003750-0.21
1.999074A734482.00mm-0.000926-0.05
1.999074B734242.00mm-0.000926-0.05
2.494643A433482.50mm-0.005357-0.21
2.494643B433242.50mm-0.005357-0.21
2.998611A134483.00mm-0.001389-0.05
2.998611B134243.00mm-0.001389-0.05
3.492500A833243.50mm-0.007500-0.21
3.998148A734244.00mm-0.001852-0.05
4.497917A634244.50mm-0.002083-0.05
4.989286A433245.00mm-0.010714-0.21

Notes & Caveats:
I should add that this is still a work in progress. I may add additional tables if other gear combinations show promise.
I have not ACTUALLY tried any of these!
33T 16DP 14.5 PA is a standard gear in Colchester Bantam/Chipmaster lathes, but will need boring and bushing to fit the CVA.
If you need really precise metric pitches (eg for a machine feed screw) you'll need the proper 127T translation gear set.
Comments suggestions appreciated.

Martin
 
Thanks - Really useful post.
I remember discussing this with John years ago and it's great to have the information collated in a simple post.
I'd love a set of metric change gears but it's far too much money for something rarely used.


Charles
 
Thanks - Really useful post.
I remember discussing this with John years ago and it's great to have the information collated in a simple post.
I'd love a set of metric change gears but it's far too much money for something rarely used.


Charles

It is also worth noting that even though the metric change gears are an exact solution since they have a 127 tooth transposition gear, you can get results from other gear combinations that are more accurate than the 10EE's inherent accuracy. The Perl program in the referenced link will calculate the accuracy for any combination of gears you want to try.
 








 
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