# 7x14 lathe spindle and intermediate shaft distance and 3D print the combo gears



## ARhodes01075 (Aug 1, 2020)

I recently broke the 2 speed selector combo gears in my Grizzly 7x14 lathe.  I successfully replaced them with new plastic gears.

This got me thinking.  These gears are very amenable to 3D printing.  I can easily reverse engineer the gears and my CAD program has a tool to produce gear profiles.  It needs the number of teeth, the diametrical pitch, and the pressure angle.  Obviously I know the pressure angle.  But I am not positive of the diametrical pitch because the teeth on the original gears appear to be very similar to that produced by my car program, but not identical.  The teeth on the original gear are a bit taller (or conversely cut deeper) than those produced by the CAD program.  

As long as the teeth mate properly, I believe it is not critical to have a certain specific profile.  So, I need to make the sum of the pitch diameters (calculated from the number of teeth and outer diameter or vice-versa) equal to the center-center distance of the spindle to the intermediate shaft.

Of course I didn't measure this when I had it apart, and I may have not been able to measure it accurately enough anyway.

Does anybody know this distance?  How about the diameter of the intermediate shaft.  I think it is 12mm, but I could easily be wrong.  Again, I didn't measure it when I had the thing apart.

Once I have the data, and create the 3D printable file for the gears, I will post them on Thingiverse, and here too if possible

Thanks in advance for the info!


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## ARhodes01075 (Aug 2, 2020)

By measuring the original gears and testing their meshing, it appears that the center-to-center distance of the spindle to the intermediate shaft (AKA the H/L shaft), is about 45.3mm (1.783").  that is very close to the imperial 1.75", but i can't be sure what the actual C-C distance should be!

This lathe's measurements appear to be a mixture of metric and imperial, but It is hard to reverse engineer this stuff by taking measurements of plastic parts with an unknown amount of wear and deformity.

ALl insights into the C-C distance would be apprecated!

-Tony



ARhodes01075 said:


> I recently broke the 2 speed selector combo gears in my Grizzly 7x14 lathe.  I successfully replaced them with new plastic gears.
> 
> This got me thinking.  These gears are very amenable to 3D printing.  I can easily reverse engineer the gears and my CAD program has a tool to produce gear profiles.  It needs the number of teeth, the diametrical pitch, and the pressure angle.  Obviously I know the pressure angle.  But I am not positive of the diametrical pitch because the teeth on the original gears appear to be very similar to that produced by my car program, but not identical.  The teeth on the original gear are a bit taller (or conversely cut deeper) than those produced by the CAD program.
> 
> ...


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## Lo-Fi (Aug 2, 2020)

The terminology you're looking for is "profile shifted" teeth. I suspect that's what you're seeing here. The PCD stays as it should for that tooth count (regardless of working in module or DP) , then the addendum (distance from PCD to the outside overall diameter) is increased, while the dedendum (circle touching the bottom of the spaces between the teeth) is decreased, all assuming positive profile shift. The opposite is true for negative. 

The reasons to do so are many: often in gearboxes with multiple meshes where the centre distances can't be the same for all the ratios, or to make teeth shorter, fatter and stronger (negative) or occasionally to allow a wide range of centre distances (positive) as you might find in a cheap slip roller, pasta machine or suchlike. You cannot cut profile shifted gears with a profile cutter, it will not give the correct pressure angle, which along with the involute profile are important factors in smooth, quiet operation and most importantly rolling contact for minimal wear. As for printing them, you'll need a gear profile generator that understands profile shifting or generate the tooth profiles yourself and the entire gears procedurally. However: generating involutes is not trivial in many CAD packages. 

Hopefully that points you in the right direction for further research?


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## RJSakowski (Aug 2, 2020)

Keep in mind that many gear generators for CAD programs do not create a geometrically correct gear.  They are intended to be a good simulation which is usually enough for conveying the needed information and esthetics.  

Diametral pitch is easily determined from an existing gear.  Add 2 to the number of teeth and divide by the o.d. of the gear in inches.  For metric gears, divide o.d. in mm by the number of teeth plus 2 to get the modulus.


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## ARhodes01075 (Aug 3, 2020)

I did find the formulae to determine the diametrical pitch and other constants given the OD of the gear.

I had hoped that the CAD program would give the PROPER geometry for the gear, but it does not allow variation of the adendum/dedendum, which may be a clue that this is not a "real" functional profile.

I must say that it is a bit of a challenge even defining the OD of the gears.  It vares by over 0.010" depending on where you measure it!   These lathe gears (hi-lo gearbox) do not have lots of hours on them, but they may be deformed as a result of the seizing of my parting tool and the instantaneous stoppage of the chuck.

This is why I wanted to get back to the basics and get the center-to-center distance of the spindle and the hi/lo shaft.

I did notice that the 12 tooth gear interferes against the 29 tooth gear.  Then I read that gears with fewer than 15 teeth need a modification of the "usual" involute profile.  I didn't understand why at first, but the radial divergence of the teeth for small tooth counts is greater than for the higher tooth counts.


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## addertooth (Aug 3, 2020)

I have never found 3d printed gears as strong as those which are cast out of a decent plastic material. The exceptions being those produced on machines which support carbon fiber radial filament.  A post-bake does help a lot, but still does not bring them up to the strength of the factory plastic gears.  It is certain that 3d printing has come a long ways, but it still has limitations in high-strength applications.


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## RJSakowski (Aug 3, 2020)

The o.d. of a gear can vary slightly as the tip of the tooth can be truncated.  Center to center distance will vary as well as there is alweays some clearance in the gear mesh.  Fortunately, pitch diameter and mod are always nice round numbers or low numbers fractional in the case of mod.

I find ihe calculations are accurate enought to be able to distinguish an inch gear form a metric gear and determine the diametral pitch or mod of the gear.

For SolidWorks users, the procedure on this site wil generate accurate gears.  http://thebloughs.net/involute-gear-generation-in-solidworks/

I haven't checked thoroughly, but I believe the the gears in the provided in the CAD models for McMaster Carr are accurate as well.


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## ARhodes01075 (Aug 4, 2020)

addertooth said:


> I have never found 3d printed gears as strong as those which are cast out of a decent plastic material. The exceptions being those produced on machines which support carbon fiber radial filament.  A post-bake does help a lot, but still does not bring them up to the strength of the factory plastic gears.  It is certain that 3d printing has come a long ways, but it still has limitations in high-strength applications.




i agree about the strength of printed vs cast plastic like the acetal of the lathe combo gears.  I would love to be able to COMPARE the strength of the original acetal gears vs those printed in something like polycarbonate

The measurements I am getting for the diametrial pitch are long decimals when measured in imperial units.  I will convert to metric and see if I get something close to "nice" numbers.  I do know that the overall diameter was NOT a "nice" metric number, but maybe the pitch diameter is...


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## NortonDommi (Aug 4, 2020)

Gearspec from W.M.Berg Inc. is handy.  
Kohara put out some useful information. They have some really good downloads:




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Gear Technical Reference is comprehensive gear technical documents for all machine designers. These documents are presented by KHK, a gear manufacturer.




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