Seadog
Save the planet. It's the only one with rum!
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That looks like the makings of a good Christmas pudding
- Thread starter Pete.
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Agroshield
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Looking for something else, I found these:
They are an old series of booklets on gearing by the Association of Engineering and Shipbuilding Draftsmen.
It is good to see them preserved in electronic form.
They are an old series of booklets on gearing by the Association of Engineering and Shipbuilding Draftsmen.
It is good to see them preserved in electronic form.
Martin Hocking
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Can someone tell me whether gear helix angles are that unique to their application?
Sorry I can’t think of a better word to explain. The gear in Pete’s example using the calculator he has found has a helix angle of 15.15 degrees. Would this be something you’d encounter commonly? Why would this not just be 15 degrees? Pressure angles seem to be set to standard angles what does the extra .15 gain?
My naive first impression was that there would be the fixed gear sets for whole degrees (which I think is what it says in manual) and bobs your uncle helix away my friend.
(Apologies in advance for expressing an angular measurement in decimal hail Mary’s have been said)
Sorry I can’t think of a better word to explain. The gear in Pete’s example using the calculator he has found has a helix angle of 15.15 degrees. Would this be something you’d encounter commonly? Why would this not just be 15 degrees? Pressure angles seem to be set to standard angles what does the extra .15 gain?
My naive first impression was that there would be the fixed gear sets for whole degrees (which I think is what it says in manual) and bobs your uncle helix away my friend.
(Apologies in advance for expressing an angular measurement in decimal hail Mary’s have been said)
Pete.
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The pitch diameter of a helical gear increases slightly with the helix angle. So if you have two gears at fixed centres (perhaps on a common shaft as in a gearbox) you can fine-tune the mesh/backlash of the gears by increasing or decreasing the helix angle which will bring the gears closer or further into mesh.Can someone tell me whether gear helix angles are that unique to their application?
Sorry I can’t think of a better word to explain. The gear in Pete’s example using the calculator he has found has a helix angle of 15.15 degrees. Would this be something you’d encounter commonly? Why would this not just be 15 degrees? Pressure angles seem to be set to standard angles what does the extra .15 gain?
My naive first impression was that there would be the fixed gear sets for whole degrees (which I think is what it says in manual) and bobs your uncle helix away my friend.
(Apologies in advance for expressing an angular measurement in decimal hail Mary’s have been said)
Also you can cut gears deeper or less so to ensure pairs of gears on a common shaft mesh, whilst using the tooth number needed for a particular ratio.
But the answer generally is that you can have any helix angle - for example original mini gearboxes were helical (no idea what angle, even if they were all the same), then came straight cut, which in theory are weaker, but lose less energy, and then someone came up with semi-helical, essentially a low number helix.
But the answer generally is that you can have any helix angle - for example original mini gearboxes were helical (no idea what angle, even if they were all the same), then came straight cut, which in theory are weaker, but lose less energy, and then someone came up with semi-helical, essentially a low number helix.
Martin Hocking
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Thanks chaps,
Plenty of food for thought there. I’ve spent most of today staring at tractor tyre tread trying to get my head around how the helix angle changes where the gears meet.
This thread has been a very helpful incentive to try and relearn some of the maths I used to know, and the Greek alphabet for that matter!
Plenty of food for thought there. I’ve spent most of today staring at tractor tyre tread trying to get my head around how the helix angle changes where the gears meet.
This thread has been a very helpful incentive to try and relearn some of the maths I used to know, and the Greek alphabet for that matter!
julianthegypsy
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Off topic slightly, but I used to work on industrial centrifugal separators. They were driven by a worm gear running backwards. Typically driven off a 10kw or more motor driving a big bronze gear through a centrifugal clutch to the shaft that held the bowl with the worm gear on it. The bowl would weigh about 250kgs and spin at about 8000rpm. There was a lot of force on that worm gear! It took 3 minutes to get up to speed and eight to stop using the brake, or about half an hour if the operator forgot the brake...
westfalia or Alfa-laval?
julianthegypsy
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Westfalia. I spent four years as service manager for their local subsidiary in North Sumatra.
julianthegypsy
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Yep. You can dial in how much fat you want left in the milk, but usually they take all the cream out then add whatever amount back to the skimmed milk to make semi skimmed. We had a heap of machines doing raw palm oil, latex, some doing marine diesel plus assorted oddballs doing wine, recycling lube oil in a steel rolling mill etc etc. Oh and a couple cleaning viscose fibre out of caustic solution. They are a rabbit hole when you get into all the different types and uses.
Martin Hocking
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It looks great pete well done. But are you absolutely sure you don’t want it to be just a little more kork ballington?
Martin Hocking
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Sorry my bad a very self indulgent reference to Kawasaki green, which is doing the business on petes tos mill. Kork ballington or mick grant on kr gp race bikes is the first thing that I think of when I see it. I’ll get back in my box…
Pete.
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So, the time has finally come to take the plunge and cut my first helical gear on this machine. When I think back to 7 years ago when I got my Mikron hobber and how it took me a while to figure out the workings to cut straight gears I realise that this is simplicity itself compared to cutting helicals. I can't imagine how complicated it must be to cut bevels or spiral bevels/hypoinds. Not that I'm ever going to find out because this has taxed me to be honest.
So I'm going to try to run through the theory, basic maths and setup to show what's involved. I chose a gear with simple numbers to start with:
40 tooth gear
1 Module pitch (20 degrees pressure angle not that it matters)
10 degrees helix, right hand
To tell the hand of a gear you lay it on it's side. go up from the bottom and if it angles upwards to the right, it's a right hand gear, to the left it's a left hand gear. This can be confusing when setting up because if it's angled to the right when you look at it then the back side is angled to the left so you have to swing the lead to the left. Easy to mess up.
First I had to cut the blank. For my first gear I chose brass because I'd like to keep it as a momento and it's easy and forgiving to cut in case I messed up. For a spur gear the OD is easy, it's diameter is tooth count + 2 x module, so 40+2 x 1 = 42mm. Helical gears are slightly bigger though depending on the helix so my 10 degrees helix needs an OD of 42.617mm. I bored the blank, mounted it on the hobbing arbor then turned the OD.
Now I have the blank mounted I set the angle on the hob. When cutting a spur gear with a right hand hob you tilt the head to the right the amount of the helix angle. For my 1 mod cutter this is 1 degree 58 minutes (2 degrees). But for a 10 degree RH helical gear you tilt the head 10 degrees to the left AFTER applying the 2 degrees tilt for the hob helix. So that means I tilt the head 8 degrees and 2 minutes to the left for my gear.
There's a handy page in the manual that helps you visualise the head tilt, and the rotation of hob and blank for the hand of helix being cut.
I set the head angle according to the drawing for RH hob and RH helix:
Anyway, now I have the blank and hob mounted it's time to do the gears. There are three sets of gears to figure out:
Index gears - sets the tooth count for the gear
Feed gears - sets the feed rate/cutting speed
Differential gears - sets the amount the index is increased by according to the gear tooth pitch and helix angle being cut
Each one has it's own calculations and I'll run through them one by one
Index is easy, you look it up in the manual. If your tooth count isn't listed in the manual there's a formula you can use which involves the machine index constant (which is always a whole number, for this model of machine the index constant is 12). Anyway the manual says for 40 teeth you use 24 - idler - 80 so that's what I fitted. I have to fit an idler in between them and the manual says for a RH helix using a RH hob I also need an idler in the coarse gears. Also, note the top coarse gears has the dogs visible. For spur gears you'd have the gear flipped with the dogs engaged to lock the differential but becuse we're using the differential they have to be disengaged or it'll smash up the gear train.
So I set the gears up 24 - idler - 80. The extra gear in front is just a spacer.
Next is feed gears. The manual suggests .040" as a good feed rate so I went for that except the 90T gear in the set is the one gear that's damaged to I used a 80T instead for .045" feed. The feed is "feed per revolution" and it's important to remember that because it means a 20T gear will have double the cutting depth of a 40T gear. For low tooth count gears you use a finer feed especially if it's a tough material or a coarser tooth pitch.
I set the feed gears onto the quadrant 30 - 40 - 36 - 80.
So I'm going to try to run through the theory, basic maths and setup to show what's involved. I chose a gear with simple numbers to start with:
40 tooth gear
1 Module pitch (20 degrees pressure angle not that it matters)
10 degrees helix, right hand
To tell the hand of a gear you lay it on it's side. go up from the bottom and if it angles upwards to the right, it's a right hand gear, to the left it's a left hand gear. This can be confusing when setting up because if it's angled to the right when you look at it then the back side is angled to the left so you have to swing the lead to the left. Easy to mess up.
First I had to cut the blank. For my first gear I chose brass because I'd like to keep it as a momento and it's easy and forgiving to cut in case I messed up. For a spur gear the OD is easy, it's diameter is tooth count + 2 x module, so 40+2 x 1 = 42mm. Helical gears are slightly bigger though depending on the helix so my 10 degrees helix needs an OD of 42.617mm. I bored the blank, mounted it on the hobbing arbor then turned the OD.
Now I have the blank mounted I set the angle on the hob. When cutting a spur gear with a right hand hob you tilt the head to the right the amount of the helix angle. For my 1 mod cutter this is 1 degree 58 minutes (2 degrees). But for a 10 degree RH helical gear you tilt the head 10 degrees to the left AFTER applying the 2 degrees tilt for the hob helix. So that means I tilt the head 8 degrees and 2 minutes to the left for my gear.
There's a handy page in the manual that helps you visualise the head tilt, and the rotation of hob and blank for the hand of helix being cut.
I set the head angle according to the drawing for RH hob and RH helix:
Anyway, now I have the blank and hob mounted it's time to do the gears. There are three sets of gears to figure out:
Index gears - sets the tooth count for the gear
Feed gears - sets the feed rate/cutting speed
Differential gears - sets the amount the index is increased by according to the gear tooth pitch and helix angle being cut
Each one has it's own calculations and I'll run through them one by one
Index is easy, you look it up in the manual. If your tooth count isn't listed in the manual there's a formula you can use which involves the machine index constant (which is always a whole number, for this model of machine the index constant is 12). Anyway the manual says for 40 teeth you use 24 - idler - 80 so that's what I fitted. I have to fit an idler in between them and the manual says for a RH helix using a RH hob I also need an idler in the coarse gears. Also, note the top coarse gears has the dogs visible. For spur gears you'd have the gear flipped with the dogs engaged to lock the differential but becuse we're using the differential they have to be disengaged or it'll smash up the gear train.
So I set the gears up 24 - idler - 80. The extra gear in front is just a spacer.
Next is feed gears. The manual suggests .040" as a good feed rate so I went for that except the 90T gear in the set is the one gear that's damaged to I used a 80T instead for .045" feed. The feed is "feed per revolution" and it's important to remember that because it means a 20T gear will have double the cutting depth of a 40T gear. For low tooth count gears you use a finer feed especially if it's a tough material or a coarser tooth pitch.
I set the feed gears onto the quadrant 30 - 40 - 36 - 80.
Pete.
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Now that we have the index and feed gears set up, if we were cutting a spur gear we would be done. For a helical gear though we have to use the differential. This is exactly what it says, a differential set indide the machine that can add a percentage of turn to the index wheel to increase or decrease it according to the tooth pitch and helix angle being cut. This uses a 4-gear train and has to be calculated to a minimum of 5 decimal places for accurate gear cutting. To work out the gears you need the following:
Gear tooth pitch (also known as normal pitch) - I chose 1 module as it is a smallish tooth so puts a low stress on the machine and also makes the calculations a little bit simpler.
Helix angle - I chose 10 degrees just because
Machine constant - this is a number given by the manufacturer for the purpose of these caculations. It's specific to the model of machine and for my hobber the machine constant is 0.6366. Not having this number makes life VERY difficult so any machine which it is not known for it all but useless unless you put a lot of work into discovering it.
The manual gives a table of change gears for DP pitch gears and common helix angles but I am cutting a module gear so I have to use the formula given:
So the formula is 1.616964 x the sine of my helix (10deg) dividied by the module (1). Luckily you can get a full set of natural sine tables by downloading old gear cutting books. I got mine from Brown & Sharpe's book on milling operations published about 100years ago. The table gives the natural sine for 10 degrees as 0.17365.
So my decimal ratio for figuring out the 4 gear train is 1.61694 x 0.17365 (Divided by 1 which does nothing here) = 0.2807857986
Now there is a way of figuring out 4 gear trains from a decimal, and there are books with hundreds of pages of tables to look them up, but I don't need to use these as there is a clever guy who has written a program to do the job and shared it for free. It's called decimal to change gear. You put in the decimal, tick the boxes for all of the gears you have available and it will give you as many combinations of change gears as it can generate. I have all gears from 22T to 80T so I tick all those and put in my number and get the following result:
So now I have my 4 gear train: 29 driving 76 coupled to 39 driving 53. I try to set this up on my machine and find that the 39 won't reach the 53. No problem because on any gear train, any driver can be swapped with any other driver and any driven can be swapped with any other drive without changing the overall ratio. So I swap the 76 with the 53 and it all fits.
So, now I have spent hours reading and understanding, watching youtube videos, scouring old forum posts, downloading books and doing research. I spent an hour getting the 3 gear trains set and the blank mounted, the hob fitted and head angle set just right. It was time to hit the power lever.
It starts cutting and the tooth spacing looks good:
I watch it carefully and it does look like it's cutting the gear properly. If it was set wrong in any way it would be very apparent by now.
And then in just about 4 minutes it's done and the gear is cut. A bit of an anti-climax in the end which is how it should be. Gear hobbing is all stup then a few minutes cutting.
Anyway here it is, my first gear cut n this machine ad my first properly-cut helical gear. I shall keep it as a trophy
Gear tooth pitch (also known as normal pitch) - I chose 1 module as it is a smallish tooth so puts a low stress on the machine and also makes the calculations a little bit simpler.
Helix angle - I chose 10 degrees just because
Machine constant - this is a number given by the manufacturer for the purpose of these caculations. It's specific to the model of machine and for my hobber the machine constant is 0.6366. Not having this number makes life VERY difficult so any machine which it is not known for it all but useless unless you put a lot of work into discovering it.
The manual gives a table of change gears for DP pitch gears and common helix angles but I am cutting a module gear so I have to use the formula given:
So the formula is 1.616964 x the sine of my helix (10deg) dividied by the module (1). Luckily you can get a full set of natural sine tables by downloading old gear cutting books. I got mine from Brown & Sharpe's book on milling operations published about 100years ago. The table gives the natural sine for 10 degrees as 0.17365.
So my decimal ratio for figuring out the 4 gear train is 1.61694 x 0.17365 (Divided by 1 which does nothing here) = 0.2807857986
Now there is a way of figuring out 4 gear trains from a decimal, and there are books with hundreds of pages of tables to look them up, but I don't need to use these as there is a clever guy who has written a program to do the job and shared it for free. It's called decimal to change gear. You put in the decimal, tick the boxes for all of the gears you have available and it will give you as many combinations of change gears as it can generate. I have all gears from 22T to 80T so I tick all those and put in my number and get the following result:
So now I have my 4 gear train: 29 driving 76 coupled to 39 driving 53. I try to set this up on my machine and find that the 39 won't reach the 53. No problem because on any gear train, any driver can be swapped with any other driver and any driven can be swapped with any other drive without changing the overall ratio. So I swap the 76 with the 53 and it all fits.
So, now I have spent hours reading and understanding, watching youtube videos, scouring old forum posts, downloading books and doing research. I spent an hour getting the 3 gear trains set and the blank mounted, the hob fitted and head angle set just right. It was time to hit the power lever.
It starts cutting and the tooth spacing looks good:
I watch it carefully and it does look like it's cutting the gear properly. If it was set wrong in any way it would be very apparent by now.
And then in just about 4 minutes it's done and the gear is cut. A bit of an anti-climax in the end which is how it should be. Gear hobbing is all stup then a few minutes cutting.
Anyway here it is, my first gear cut n this machine ad my first properly-cut helical gear. I shall keep it as a trophy
Last edited:
Windy Miller
Semi-Professional Potterer!
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