Cutting a worm wheel

[Ramon WilsonParticipant @ramonwilson3]

HI guys, in the coming days I intend to have a go at something I’ve not attempted before and that is to cut a worm wheel using a tap as a hob.

Just wondered if anyone has carried this out using this method and with what  level of success as a result. I can see that the equal (in form) number of teeth wil define a diameter but can’t quite see how one arrives at that save from a cut and see basis.

Any tips or pointers both to do, or avoid, would be much appreciated

 

Best – Tug

[Andy StopfordParticipant @andystopford50521]

It’s a bit random, the diameter makes a difference, but not predictably, +-10% maybe.

I’ve tried gashing the blank at the appropriate angle, but it didn’t seem to make any difference.

I did see someone doing it on quite a large scale in one of those Pakistani safety last videos on YouTube a while back. He turned a half round groove, about equal to the radius of the tap first, then fed it in very gently. It seemed to work well, and I got the impression it was for something which would require a predictable result (though this wasn’t specified). Unfortunately I can’t remember which YT channel it was on, but it was this year, in the summer maybe, so might be reasonably easy to track down.

[JasonBModerator @jasonb]

If you work out the circumference it will give you an idea of how many teeth you will get on the wheel for a given TPI but could go a tooth either way.

There was a long thread a while back which I will look for but this was the video I posted at the time.

Tell a lie it was gear cutting where I moved the tap along rather than just feeding in the gear blank.

[bernard towersParticipant @bernardtowers37738]

Yes Tug I did it for the steam launch feed pump and its still working. Did a few others at the time to work out tpi to circumference and it does work out. Plunged in not like the prev vid.

[FulmenParticipant @fulmen]

I’ve only done it to test the concept, but it seemed to work OK. There are a couple of challenges to watch out for though. One is the need for constant engagement on the tap, which can be problematic on certain hand taps. Spiral fluted machine taps might work better. I also think you should feed to full depth in one go to get the pitch right.

[Pete RimmerParticipant @peterimmer30576]

Remember that the cutter which cuts the teeth on the wormwheel should be the same pitch diameter as the worm you are using. If not, the engagement will be either only in the middle or on the edges of the teeth.

[Les RileyParticipant @lesriley75593]

A little bit bigger scale but similar method :

I wanted to make the steering worm and wheel for my 4″ traction engine.

I used the info in Ivan Law’s book in the workshop series.

Cut a blank and gash it at the pitch angle, Turn a worm that has enough spare to make a “tap” as well.

Then cut off the length for the worm and put 4 flutes in the remainder and case harden.

Set the wheel up on the lathe toolpost to be free spinning and put the newly made cutting tap in the chuck.

Bring the two together and gradually mesh them. As the wheel is already gashed there isn’t much to come off and you can’t get wrong on tooth numbers.

[Nigel Graham 2Participant @nigelgraham2]

It obviously worked but would it help if the home-made tap has, if not helical, at least slanted flutes for more nearly constant contact, as Fulmen indicates?

 

(My interest in this is two-fold: my steam-wagon’s steering unless I use a screw-&-nut – it is enclosed – and replacing the table drive missing from a small horizontal mill. For the latter I may be able to cut a suitable worm anyway.)

[Ramon WilsonParticipant @ramonwilson3]

Thanks for all the input guys – all duly noted and appreciated.

My requirement is quite small a wheel of approximately 16mmOD and a worm turned to suit. It is for the reversing gear on the marine engine, the tooth count is not important and once the wheel is made the worm will be cut on the relevant diameter to suit the already machined pivot points in the support bracket.

I have purchased a spiral flute tap – 12mm x 1.5 pitch and intend to use this – my visulisation is that there will be a point where the teeth form perfectly at a certain diameter – I just wondered if anyone had any advice on reaching that point by some reasonably accurate means. The worm will be a smaller diameter than the tap but I’m sure the contact area will be sufficient for the intended use.

I should be getting onto this in the next few days so will report back – good or bad result

My thanks again – Tug

[Ramon WilsonParticipant @ramonwilson3]

Just a quick note to say that two PB wheels were produced this morning without issues – I was quite surprised how easy it was. I have taken some pics so will put them up on the engine thread after the worm is cut and assembly to see if it all works
Tug

[PeteParticipant @pete41194]

A bit late since it seems Ramon already has his worm gear done. But was this the video you were thinking of Andy? https://www.youtube.com/watch?v=nilZnYHnba0 If so, maybe the link will help someone else in the future.

[Andy StopfordParticipant @andystopford50521]

Yes, that’s the one. I think the advantage of grooving the blank first may be that it ensures the teeth on the tap stay engaged all the time, even using a straight-fluted one, so it doesn’t skip and generate a random number of teeth.

[Andy Stopford]

On Andy Stopford Said:

Yes, that’s the one. I think the advantage of grooving the blank first may be that it ensures the teeth on the tap stay engaged all the time, even using a straight-fluted one, so it doesn’t skip and generate a random number of teeth.

Absolutely.  Though the method often works well, it’s not precision engineering.

The problem is slip.  Slip is always a risk because the work is turned into position by the cutter as it removes metal.   On a good day, an unsynchronised cutter will start well and then be steered “near enough” thereafter.  On a bad day, the work slips and the cutter mangles the job.  Mangling can be tooth skipping, or face erosion, or both.

Although Les Riley’s pre-gashing reduces the risk of slip considerably, a precision method turns the work and the cutter together so cuts are always spaced accurately.

Advantage of the method is it’s a simple way of making non-precision parts, and for hobby and repair work these are often all we need.

Disadvantage is that no two parts will be the same and poor shape due to slip will cause relatively rapid wear. The reject rate is likely to be uneconomically high too.  So the method isn’t suitable for manufacturing interchangeable parts made within tolerances.   When that’s required, cheaper to flash the cash on a proper hobbing machine.

Dave

[bernard towersParticipant @bernardtowers37738]

SOrry Dave  never had a problem and Ive done a few

[PeteParticipant @pete41194]

I’d read your post a few hrs before that popped up in my YT recommended videos Andy and remembered your post here. Just luck that happened.

I’ve got a few articles from my American machinist magazines showing this same process. In those, they do mention pre-gashing the worm gear blank for the reasons Dave mentioned. And over the years there has been a few posts on other forums mentioning incorrect tooth counts or a failure to maintain the same tooth spacing during each revolution using un-gashed gear blanks. For whatever reason, not using a pre-gashed blank may or may not always work. And I’d highly agree, this isn’t a precision method at all. To transmit motion or as a fairly low speed gear reduction, it should be good enough in most cases. Certainly good enough for a model as Ramon’s use for his shows. But for anyone not understanding the difference, and for something as critical as a precision worm gear used for dividing, it isn’t. The method obviously works, but there are real limitations to the level of precision it’s capable of.

[Ramon WilsonParticipant @ramonwilson3]

Some interesting views there but I can say I found this to be a much easier operation than I envisaged.

I agree – it’s not a ‘precision’ method as such but it was precision enough for the job in hand and has produced a smooth running worm wheel that works perfectly with the worm made to suit – and by that I mean the ‘diameter’ of the worm to suit the pre made bracket for good engagement.

As said I made two – I doubt very much they are identical in diameter – I didn’t check as there was no need, I only needed the one but the one matter I could not visualise beforehand was the point at which the pitch of the tap would cut a true number of teeth for a given diameter – exact number of teeth were unknown so I don’t see pregashing would make any difference – beside surely it would need pregashing at the pitch angle too.

Slip was not an issue either – the blank began to turn immediately the tap came into contact, I just fed the blank into the tap by the given depth of thread keeping a very close eye on the teeth to see if any were deformed. I think slip would only occur if the re was a distinct drag on the blank

So no, not precision, but precise enough for a small one off gear required for the engine – requirement for two would have possibly been a different matter.

This image shows it backed off after the initial ‘cut’ after touch on to see if it would work. It was then cut by constantly feeding in to depth

 

Thanks for your interest and input – always appreciated

Tug

[bernard towers]

On bernard towers Said:

SOrry Dave  never had a problem and Ive done a few

Sorry Bernard, but surely you’ve missed the point?  I said “Though the method often works well, it’s not precision engineering” and this statement is true no matter how many reasonable parts have been made by gashing.

I’m sure Bernard’s gash-made parts were “good enough” for what he needed.  But they were all approximations,  not precision made.  I suggest Bernard’s inspector had low standards, probably only checking for a reasonable fit, and then allowing the pairs to grind themselves in.

This article on the “American System of Manufactures”  describes the principles behind how almost everything is made today.  In it, interchangeability and reliability are achieved by working accurately to tolerances, requiring accurate measurement, tight controls and a different approach to man-in-shed methods.

Bodging is great, but I hope no-one on the forum believes that it represents industry best practice.  Or that an opinion based on “Ive done a few”  somehow overrides a century plus of manufacturing progress.

Boils down to a simple choice.

• If a DIY job can be satisfied without special equipment you don’t have, go for it, even if the results are somewhat inferior.  I’m a big fan of “good enough”, “value for money” and “fit for purpose”.
• But, when fit for purpose is defined by a specification, the production method has to meet the specification, and the resulting parts will be checked to ensure they are to spec.  In this context,  “Ive done a few” doesn’t cut the mustard when an independent Inspector arrives armed with GO/NO-GO gauges and a shadowgraph.

The problem with simply driving work with the tap, is slip. The method cannot produce parts to specification, unless of course the spec is wide open. Slip is prevented by synchronising the cutter and work as they turn;  requires extra gearing. For precision work there’s a lot to said for owning a gear hobber…

I thought I’d explained slip clearly so people could decide for themselves if it mattered; obviously not clearly enough, because Bernard didn’t get it!

🙁

Dave

[Phil PParticipant @philp]

Tug

I am just curious as to wether the spiral tap actually maintained contact with the worm wheel blank at all times ?
It might be just the angle of the photo, but it sort of looks like it might lose engagement still.
Phil

[EmgeeParticipant @emgee]

Dave

I think you are over complicating what Ramon wanted to achieve, he used tools and machines to hand and produced a satifactory item and then with skill produced a second part to mesh with the 1st to achieve his goal. It’s called Model Engineering, not NASA parts.

Emgee

[SillyOldDuffer]

On SillyOldDuffer Said:

it’s not precision engineering

While undeniably true it’s also a bit like beating a non-existent horse. This is a hobby forum, not practical machinist. Anybody here doing actual precision engineering should be able to figure that bit out on their own. Or am I giving people too much credit here?

The first issue is the intermittent feed. Gashing will help, so will cutting a round groove that matches the OD of the cutter. Making a custom cutter with smaller gaps should also solve this.

The second issue is the fact that only the cutting tooth will have full engagement. As SOD points out this will inevitably cause some slip. Increasing the diameter of the blank will help, but I don’t see any simple solutions that will ensure perfect alignment.

[Ramon WilsonParticipant @ramonwilson3]

Oh Dear!

No it’s not precision engineering, I don’t think anyone is implying it is but it IS precise enough for some jobs, and certainly ‘this’ job, in hand.

I did not want to go to the expense of purchasing precision gears and I certainly did not want to go to the effort of machining a hob unless it proved neccesary either. I had heard of the method so decided to give it a try and yes, for what I required of it, it actually worked well enough to be of use. I’d like to think that’s ‘model engineering’.  I spent long enough manufacturing precise components to very tight limits at work to know this was not in similar vein but the result has proved just as good as was hoped for – you’ll have to take my word that the gear works very smoothly and operates the linkage as it should without binding or resistance.  – Yes I did have to ‘run it in’ but not to wear down tight spots just to ease the fit.

Phil P – I can see what you mean but I didn’t experience any indication of it when cutting – if it hadn’t been in constant engagement I’m sure that that would have been felt as the cut was advanced. The tap was M10 not M12 as previously stated BTW – the wheel is quite small the diameter in the groove just 13 mm

Best – Tug

 

PS by the way Dave – I don’t see this as bodging – I really don’t bodge anything intentionally and definitely not on my models

[Roderick JenkinsParticipant @roderickjenkins93242]

Tug,

Nice to see that you are still cutting metal to your very high standards.

All the best,

Rod

[Ramon Wilson]

On Ramon Wilson Said:

…

PS by the way Dave – I don’t see this as bodging – I really don’t bodge anything intentionally and definitely not on my models

The one in the photo certainly looks anything but bodged ,Tug.

One thing I’d add – if possible, make the blank much wider than required to increase the chances of continuous tooth engagement by the tap.

Due meanness with materials, I didn’t do that with the one shown below and ended up with several gears, none of which had the exact number of teeth. I picked the best, but wasted far more bronze than if I’d made the blank wider in the first place. Still, it was pretty easy to do (unlike setting the lathe up to cut 7 TPI for the two-start worm. Hopefully such shenanigans will soon be a thing of the past when the parts for an electronic leadscrew arrive).

[Ramon WilsonParticipant @ramonwilson3]

Andy, I made my blank wider, not for the same reason but I take your point. The extra thickness I applied was to make up the difference of the lower support platform from centre height of the tap/centre line of the gear – all surplus being turrned off afterward.

Best – Tug

[Author]

Posts