Poor quality gears

I'm having trouble thinking of any machining method that would leave such marks. There don't seem to be traces of marks from a regular hob or a cnc endmill process going awry. It looks more like they were cleaned up by shot blasting with small bearings.

I have to await moderator approval before I can access that.

Not sure I would want to punish my leadscrew like that but I probably have the gears to push the pitch up a lot. I suspect I wonder about doing nasty things to a cheap compound slide and use the same gear method milling machines use. The training exercise was a perfectly formed 3/4 in dia twist drill apart from grinding the end - sadly in mild steel but i also machines some replacement vice screws the same way which could have been done on a lathe.

From time to time I regret loosing my training notes and handouts but I do have some info on milling gears produced by the machine manufacturers. I'll see what I can dig out. Your correct though, Info on many things often isn't explained very clearly.

John

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Not really any particular strain on the leadscrew, in this configuration the leadscrew is driven from the handwheel - the only strain is on my wrist

Rod

Hi Maurice, just get ur money back.....they are junk,......won't belong before they will be unusable....

.I was in a similar position as u.....found several English firms making gears around ur size ...I wanted case hardened, a matched pair 4 my job ranged from £45 - £340.....

I settled for just under £100 they were perfect......cant find the receipt....when I do I'll post it....

clogs

I suspect that they have been made on a gear shaper and maybe the gears that make up the linkage for rotating the blank etc are a bit worn. If a shaper is used the same rack form can cut any gear of it's dp / mod. If the usual disk type form cutters are used there are added complications for precise work.

Personally I would just clean them up the marks are just cosmetic. They could be checked out by running them together with a thin smear of blue. I wonder if Reeves of old in Marston Green sourced better gears. They ran into grief due to certain people producing investment castings etc that just needed cleaning up with a file. On the other hand perhaps interest in the hobby was dropping off as well.

If lapped thinking a bit more about the problem it would be better to advance the tooth mesh by a prime number not associated with the gear tooth counts and not 1. 5 might be a suitable number but I suspect you will run into grief lapping the small one against the larger one.

John

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Edited By John W1 on 13/07/2015 11:33:57

Edited By John W1 on 13/07/2015 11:34:21

As they are only timing gears on a slow running engine I'm sure they will be fine as is like Rod says.

Not sure what your supplier charges for the set but for just over £20 you could get 13/26 32DP gears from HPC which come close to teh original 7/8" ctrs at 0.862" and for a small extra charge they will make the wider width if you prefer to stick with te drawn width rather than add a spacer.

As Jason says the HPC gears will do the job, I have them on my Centaur and they don't alter the valve gear settings.

Mike.

Haven't checked but there might also be something here. HPC seems to be the obvious choice.

**LINK**

There are a number of USA on line retailers even offering stainless. They seem to refer to them as screw gears when crossed. Helical otherwise.

The book I mentioned is by Brown & Sharp. A Practical Treatise on Milling dated 1930. They introduce lead by wrapping a right angle triangle around a cylinder and from that introduce the angle. Some parts are confusing such as the lead changing as the diameter of the gear changes suggesting that any ratio other than 1:1 will have different leads - then follow with a statement that suggests they need to be the same. It does discuss a number of complications about the use of the usual form cutters on helical gears caused by the fact that they are effectively a thread. The cutter choice and cut departs from the correct one for the DP due to that.

There used to be a lot of books like this in older junk shop type used book shops around in B'ham. I'd guess most have been burnt now.

Whoops - on Milling should read on Gearing.

John

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Edited By John W1 on 13/07/2015 14:16:22

To my eye, that gear appears cast rather than machined. If so it should be rather cheaper than a machined gear.

If it's cast and it meshes and it is cheaper than a machined gear, I would be inclined to accept it - I'm sure it will survive in fitted to a light-duty 19cc engine.

Neil

56cc Neil 1.5" bore x 2" stroke

Going back to Rod's post on designing screw gears this may be of interest AND FREE till some one notices it, removes it and sells it as an e-book.

**LINK**

The problem with the site is how easy it is to waste an hour or so seeing what's there on a particular subject.

John

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The OP is talking about Centaur (1" x 1 3/4" not Wyvern.

Neil

Sorry, I confused the issue by introducing Wyvern into the conversation but Jason's correct - Centaur, the bigger engine, is 1.5" x 2".

Rod

That's very useful - downloaded and saved. Interesting to see the derivation of the calculation for selecting the cutter. My copy of Machnery's Handbook (25) gives an additional calculation for choosing the correct cutter which takes the diameter of the gear cutter into account. The problem with this is that one quickly runs out of cutters. Standard sets stop at 135 teeth and above but the selector calculation can call for cutters for e.g.250 teeth. My solution is to use the simple calculation if you're making a matching set of gears but it is probably best to use the more exact method if your making a gear to fit with an existing hobbed gear.

I've put the calculations in a spreadsheet here **LINK**

Rod

PS.  I should point out that I've used the odd size of 31.75 DP in the spreadsheet because it's 0.8 MOD and HSS MOD cutters are readily (and cheaply) available from the far east.

Edited By Roderick Jenkins on 13/07/2015 18:29:38

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John,

Thanks for the link

I don't think you need to worry too much about the Internet Archive:

although Google [for some, presumably commercial, reason] does seem to have adopted the nasty habit of claiming that there is "no eBook available" if you follow the links to their site.

The answer is use the "show all files" link on the Internet Archive page. < example >

MichaelG.

Edited By Michael Gilligan on 13/07/2015 19:41:22

I'm confused even more now. Reeves' website gives Centaur as 1 x 1 1/2"

Neil

You don't want to believe everything you read on the net

I knew about the show all files but haven't had to use it on the new layout they use but should do really to get the djvu files as they are much smaller and just as good as pdf's if scanned correctly. Searches also work on these. They don't for me on scanned pdf's on Linux. Not sure about Windows.

What has a happened in a few instances is that archive downloads have disappeared and are only available as e-books or prints of them that have to be paid for. As the copyrights have often lapsed by rather a long time I wonder how this happens. I suspect it's just the threat of legal action when there isn't really any basis.

Noticing the comments about cheaper gear cutters for ordinary spur gears there are some cheaper hobs about now. I recently noticed that a 2mod hob has a cp of 0.2474 so close to 0.250 that a bit of lapping would soon make a worm mesh correctly with a wheel. i feel gashed wheels are fine for home use for dividing etc so bought a Russian one very cheaply. Just a little more than one from China. My dividing head is the one designed for the Dore Westbury and I sometime get tempted to try something a bit larger. On the other hand the worms and wheels for dividing head kits aren't too badly priced really.

John

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I have just obtained the full set of castings from Reeves for the Centaur gas engine.I had not unpacked all of them so after reading this post I checked the supplied gears and found them to be excellent,nothing like those shown.I think that they had a bad batch.

Frank

Right, second attempt after I hit the 'wrong' key sequence and lost everything I had typed.

Out of curiousity I had a quick run through the design steps for helical gears as shown in Machinery's Handbook last night. It all seems pretty straightforward.

The key is appreciating that there are two diametric pitches (DP), one in the plane of rotation, as per spur gears, and a normal DP perpendicular to the helix angle. The normal DP is generally used for design purposes, and if it is made an integer then standard involute cutters can be used.

There are, of course, some complications. First, even for a fixed normal DP and number of teeth, the pitch circle diameter, and outside diameter (OD), vary with the helix angle. Hence the centre to centre distance of a given pair of gears will vary with helix angle. That is why there are different design flows in Machinery's Handbook depending on whether the centre to centre distance is fixed or flexible.

The choice of helix angle is also flexible. For parallel shafts, as for spur gears, the angle is largely arbitrary. It seems to be chosen just large enough to ensure that a second pair of teeth engage before the previous pair disengage. This should promote smoother high speed running, which is one reason for using helical gears, as opposed to spur gears, in the first place. For non-parallel shafts there is a constraint on the helix angles. The sum of the helix angles of the two gears must add up to the angle between the shafts. So for the engine mentioned by the OP, where the shafts are at 90º then the helix angles should add up to 90º. Presumably 45º each would be logical?

There are probably two approaches to machining helical gears in the normal home workshop. Either a universal mill (see note 1) and universal dividing head, or a 4-axis CNC mill in conjunction with software such as Gearotic. Using a universal mill has some limitations. Most tables only swivel ±45º so for cutting steeper helix angles a special swivelling vertical head is required. I don't have one of these; I'm trying to visualise if it would be possible to machine steeper helix angles by setting the dividing head at an angle and using a one axis swivelling head, which I do have. Another potential issue is the range of leads available with the gears supplied with the dividing head. The lead is the distance travelled by the helix over one rotation. If the lead required does not exactly match those provided then an error will result. Whether the error is significant will depend on the application and the width of the gear. If the error is unacceptable it is possible to calculate a gear train for greater accuracy, but this would normally require custom gears for the drive train from table to dividing head.

Back to work now!

Andrew

Note 1: It is one of many model engineering myths that a universal mill is one that has horizontal and vertical capability. Not so, it is a mill that has a swivelling table so that in conjunction with a universal dividing head, that can be driven from the table, it can be used to machine helical paths for things like helical gears or flutes on special tooling.

Edited By Andrew Johnston on 16/07/2015 12:12:26