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Helical gear problems - Help

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gbritnell

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Hi Andrew,
By going with the 2 different angles you can get the gears almost the same outside diameter but you will still need twice as many teeth on one as the other.
Attached is my drawing for the gears I made for a tiny side shaft hit and miss engine. The pitch is tiny but if you look at the gears and information that will give you a better idea of what you are looking. at. Using the spreadsheet that Don1966 published all you have to do is plug in you diametral pitch, tooth count and helical angles and it will give you all the information you need.
 

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teeleevs

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Oh Wow, for our side shafter we used a 9 tooth Suzuki crankshaft gear which was more like a 9 start thread than a gear, it is much larger than the 18 tooth dizzy gear. (Now you are more confused than ever ) we cut the crankshaft gear in half so now we have 2 narrow ones and put one away for future job. We had measurements and angles of the dizzy gear and cut it in brass without trying to helix it, filed it a little and it runs ok. Don't know if I have photos.
Ted from down under
 

gbritnell

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The reason the 9 tooth gear is larger than the 18 tooth gear is because as you increase the helix angle the gear gets larger. If you would take a cut through the 9 tooth gear at right angles to one tooth it would have the same profile as a spur gear of the same diametral pitch. So to evenly space this tooth you are calculating the hypotenuse of the triangle, therefore the steeper the helix angle the larger the gear gets to evenly space the teeth and spaces.
 

SmithDoor

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You may want a app for Helical gears

Dave


Good afternoon guys
I’m based in the UK and have built the Chuck Fellows helical gear jig to try to machine some replacement gears for a smiths Chronometric Speedo gearbox using my Bridgeport without tilting its head.
im trying to cut a 12 tooth 24DP 35degree helical gear with 0.625 blank diameter.
So reading the cosine rule I’m using a no5 cutter 24DP ( theoretical 21.8 teeth)
I’ve used the GTBritnell centre setting method also.
but the first cuts I take seem very wide. I’ve angled the fellows spindle 35 degrees up from horizontal.
did I do something obviously wrong here With the angles maybe ??
 

gbritnell

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An app is not necessary. Download the spreadsheet from Don1966 and it will do everything you need to calculate the gear information.
gbritnell
 

coulsea

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I have been making the chuck fellows jig and am at the stage of cutting the main shaft to length and thought that i would run a few numbers through the spread sheet to check the shaft length and am not sure if i am doing it right.
I want two gears (about 20mm dia)to run at right angle with a 1:2 ratio 1 being the crank and 2 the cam and i am using a 0.5 mod metric cutter.
so in the spread sheet i have Gear 40 teeth 30 helix angle dp 50.4 helix lead = 4.9 inches Pinion 20 teeth 60 helix angle dp 50.4 helix lead = 1.4 inches
My question is how does a template with a 4.9 inch helix lead fit on the attachment or did i make a stupid mistake in the spreadsheet.
 

teeleevs

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We began with a factory made 9 tooth gear from a Suzuki jeep engine, photo 1 , we only had to copy the 18 tooth distributor gear, zoom up photo 2. No calculations needed.
Ted from down under
 

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Misterg

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My question is how does a template with a 4.9 inch helix lead fit on the attachment or did i make a stupid mistake in the spreadsheet.
I came up with the same numbers for lead as you (4.95 & 2.47").

I presume you're looking at CF's plans where the shaft is shown as 4.375" between shoulders and the support block is 2.5" long (giving a max of ~1.8" movement). If you look at any pictures of the jig, the shaft is longer than that. I was going to do the same as you and make it to suit the longest template that I thought I might need.
 

TSutrina

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The KHK reference document "calculation of gear dimension". There are a few pages on helix gears. many articles show cutting jigs for helix gears. HMEM group web site has an article "helical gear cutting lathe attachment" It is the original document I believe. The basic setup will work for a mill also. Jul 10, 2010 by Cfellow. Mike shows one approach to making a gear cutting tool also the rack style will work.
 

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Mechanicboy

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In 2009 I created the helical gear with own made milling machine, here is the movies.. :)


 

coulsea

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I came up with the same numbers for lead as you (4.95 & 2.47").

I presume you're looking at CF's plans where the shaft is shown as 4.375" between shoulders and the support block is 2.5" long (giving a max of ~1.8" movement). If you look at any pictures of the jig, the shaft is longer than that. I was going to do the same as you and make it to suit the longest template that I thought I might need.
one solution may be to make the template and then remove half of the height. my gears will be fairly narrow and the travel across will be quite short.
 

Misterg

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one solution may be to make the template and then remove half of the height. my gears will be fairly narrow and the travel across will be quite short.
Yes - it should be possible to truncate the template - I'm trying to get my head around just how much of the template would be needed. Some travel is lost at the start because of interference between the rolling follower and the edge of the template, and some over-travel would be needed for the cutter to clear the work at either end. I think I need to sketch out a few examples, but it's making my head hurt at the moment!
 

gbritnell

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I don't remember the dimensions of the shaft from the original drawings but I made mine .50 diameter. Helical gears don't need to be made too long because they're not like spur gears in the sense that the whole width of gear has tooth contact. Helicals only have contact for a short distance because it's like laying to pieces for round stock on top of each other. I have made high helix templates and then just cut them off to give ample travel for cutting the gear. I don't use commercial involute cutters for cutting (generally). I make my own from .50-.63 diameter drill rod so I don't need as much travel of the fixture to clear the gear blank at both ends.
gbritnell
 

TSutrina

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I don't remember the dimensions of the shaft from the original drawings but I made mine .50 diameter. Helical gears don't need to be made too long because they're not like spur gears in the sense that the whole width of gear has tooth contact. Helicals only have contact for a short distance because it's like laying to pieces for round stock on top of each other. I have made high helix templates and then just cut them off to give ample travel for cutting the gear. I don't use commercial involute cutters for cutting (generally). I make my own from .50-.63 diameter drill rod so I don't need as much travel of the fixture to clear the gear blank at both ends.
gbritnell
Actually the contact of a helix gear is the same as a spur gear. The difference is that a set of spur gears contact on a line at a particular radius for each gear. contact at best moves from the tip to the bottom near the root and then back out. Helix gears due to the helix could have a line of contact that covers the full radius form the tip to the the bottom near the root and for another pair from the bottom near the root to the tip. Helix gears run quieter and with less vibration, but create a thrust on both gears. Spur gears push, do not slide. Helix gears also just push even if the axis are not parallel. Worm gear slide as they push thus are with a shallow lead angle can not be back driven due to sliding friction. Helix gears can be back driven. If you move away from involute gears you can create gears sets that can not be back driven. and generally they are helix gears that have sliding occurring.
 
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coulsea

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making progress. I was worried about how well i would be able to bend the template and what i had to make it out of, thought it would be a good project for the plastic printer. this is a 4.9 inch template printed as 2.45 for half a turn, I use Fusion 360 to draw a cylinder and then cut out a square section coil the same dimension as the wall thickness then split the body vertically to remove some of the unwanted areas.
IMG_2309.JPG
 

Misterg

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thought it would be a good project for the plastic printer.
What an excellent idea.

I've been working on a parametric template model in F360 using exactly the same method to generate it. I reckon your 40T gear only needs ~35°of useful rotation to cut an 8mm thick blank with 6mm over-travel, so some of the templates can be truncated a lot.
 
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