help with gear calculations

[Pete RimmerParticipant @peterimmer30576]

Philip can you measure the distance across the bottom of the teeth i.e the root diameter? I realise you can't get the caliper jaws directly opposite with the odd tooth number but get it as close as you can.

[Philip SewellParticipant @philipsewell33761]

Peter I am now in Norfolk working for much of this month (I am based in Leicestershire) so haven't got the part with me. I did use a gauge block fitting in the root of a tooth to get a measurement across the gear (ie caliper measurement minus gauge block width) and off the top of my head I think it came to 3.7mm, so the root diameter is (I think) OD minus 7.4mm

Hope that makes some sense to you,

Philip.

[Pete RimmerParticipant @peterimmer30576]

Posted by Philip Sewell on 01/06/2019 16:44:44:

Peter I am now in Norfolk working for much of this month (I am based in Leicestershire) so haven't got the part with me. I did use a gauge block fitting in the root of a tooth to get a measurement across the gear (ie caliper measurement minus gauge block width) and off the top of my head I think it came to 3.7mm, so the root diameter is (I think) OD minus 7.4mm

Hope that makes some sense to you,

Philip.

It makes perfect sense because I fired up Gearotic and generated a 14DP gear, added in some profile shift to bring the OD up to your 32.64mm which gives us a root diameter of 25.04. Your measurements of 32.64-7.4 = 25.24 is only 8 thou different from that so I guess it's fair to say that your gear is in fact a standard 15 tooth 14DP pinion, generated according to the long-addendum info in Machinery's Handbook.

Edited By Pete Rimmer on 01/06/2019 18:00:37

Edited By Pete Rimmer on 01/06/2019 18:02:21

[Philip SewellParticipant @philipsewell33761]

Thanks for taking the time to research this for me Peter, much appreciated.

So just to clarify, I will be able to cut a replacement gear with a 14 DP number 7 involute gear cutter.

Depth of cut =3.8.

Philip.

[Pete RimmerParticipant @peterimmer30576]

Sadly not. The profile-shifted gear must be hobbed, or else you must make a cutter to suit.

[Philip SewellParticipant @philipsewell33761]

So it's fly cutting with a cutter ground to match the unworn profile of the gear.

Could you explain the term hobbing please, how is it different from cutting with an involute cutter.

[Pete RimmerParticipant @peterimmer30576]

Hobbing is the process of generating a tooth form by rolling a blank in time with a cutter having (normally) straight-sided teeth. The rolling action generates the involute profile, rather than the profile being ground into the cutter. The advantage is that only one cutter is required for a given tooth form. Disadvantage is the machine and hobs are expensive.

Here's a slow-motion video of a small gear hobber working:

[Nigel McBurney 1Participant @nigelmcburney1]

After all the theory,machine up a tryout gear and shaft in aluminium the od of the gear the same size as the original,cut say 6 six teeth on the ali gear with the standard dp cutter to standard depth and try it out,if its a tight fit cut the teeth a few thou deeper until it fits,this gear is ONLY used to rotate at very low speeds along a rack, it only needs to be precise if a graduated dial is fitted to the saddle hand wheel.very often odd gears can be made to work by trial and error with some adjustment to depth of cut or by shaving a very small amount from one side of the tooth.I once owned a Blackstone engine built in 1899 which had the spiral 2 to 1 ratio side shaft gears cast in iron ,the teeth were just cast and then just hand fettled to fit,and they ran smoothly and quietlly for over a hundred years, I once had to make for a customer a pair of Blackstone gears,they calculated out at 9 1/2 dp ,they probably cut the pattern at a whole number and the 1/2 deviation was due to casting shrinkage ,I used a 9 dp cutter and fiddled the cutter depth until I got a good fit ,there was very little adjustment on the shaft centres. what does not work so well is trying to mesh a as cast gear to a machined fiddle gear.

[Philip SewellParticipant @philipsewell33761]

Good suggestion Nigel, I'm not really kitted out to accurately grind a fly cutter to match the gear profile. I think I'll do as you suggest with a suitable involute cutter as plan A, if that's not successful go on to trying to grind a cutter.

[Pete RimmerParticipant @peterimmer30576]

Posted by Nigel McBurney 1 on 02/06/2019 09:36:25:

After all the theory…

Comes the practice

Send me your address Philip and I'll post it on. You'll have to trim the length to suit.

[Brian WoodParticipant @brianwood45127]

That looks like a very professional job Pete, it should fit nicely

Brian

[Philip SewellParticipant @philipsewell33761]

This is a lovely surprise Peter, really kind thing to do.

I have pm'd you re address details.

Many thanks for everyone's help.

Philip.

[Philip SewellParticipant @philipsewell33761]

Just to give some context to the gear thread I've posted some images of the lathe the gear is fitted to, now I've finally finished doing it up. The gear works very well by the way so thanks again Pete.

I'm not an engineer so I've learn't a lot about how a lathe is put together. As the gears in the gearbox were pretty mashed up I decided to lock the box into one speed (I put different pulleys on as well) and fit a VFD. I mounted the potentiometer where the change wheel for the gearbox would normally be. Nearly all the original switches etc have been removed although I've kept the one for the suds pump.

I plan on adding a DRO when finances allow.

Thanks again for all the advice about the gear, it's something I want to have a go at when I get the right kit for my mill.

Philip.

[Neil WyattModerator @neilwyatt]

A halfway house to hobbing is to make a cutter ('straight hob&#39 that has the cross-sectional form of a rack of roughly five teeth.

This is used like a single-point cutter, so cutting one tooth, then indexing around. It works well and gives usable results with one set of passes but you can get even better results by indexing around half a tooth and moving the cutter across by the same amount. The end result is gears where the working faces are a series of facets.

If making just one pass per tooth, position the cutter so it makes the first cut centred over a tooth-space and cuts it to full depth. Such single passes will do a pretty good job on all but the smallest gears.

Neil

[Howard LewisParticipant @howardlewis46836]

Putting the Headstock speed gearbox into one speed, and using a VFD will provide variable spindle speeds.

BUT, for various feed rates or thread pitches, the Norton gearbox will have to be used. So lets hope that this is in reasonable condition.

Howard

[Ian PParticipant @ianp]

Posted by Andrew Johnston on 30/05/2019 17:06:02:

The results for the rack definitely seem to indicate 14DP. <snip>

The pinion doesn't seem to be that worn, not sure I'd bother replacing it at this stage, if it works.

Andrew

Being a bodger (me not Andrew) I would agree that the pinion is quite usable, it may be that it could be shimmed so that it engages more closely with the rack but I think the biggest improvement you could do is to adjust the pinion position axially so that the unworn part engages the rack.

Without knowing the bearing or handwheel arrangement its hard to know whether its feasible but its worth investigating.

Ian P

[Philip SewellParticipant @philipsewell33761]

Sorry for the confusion, the pinion gear was made very kindly for me by Pete.

I have added the images of the CVA lathe as this was the project that I needed the gear for and I all but finished it at the weekend (in hindsight perhaps I should have started a new thread).

Howard, thanks for your comments. I must confess I didn't appreciate locking the gearbox into one speed and using a VFD to control headstock speed would mean I couldn't get certain thread pitches. I assumed the speed from the gearbox controls the rate at which the thread is cut but the gearbox on the headstock controls the ratio between leadscrew and saddle speed.

Could you give me an example where you would have to set the gearbox driven from the motor to a specific speed to cut a certain thread so I can better understand you.

Many thanks

Philip.

[Pete RimmerParticipant @peterimmer30576]

I don't think Howard is suggesting that, he's saying that given the obvious wear in the handwheel pinion and the reported noisy headstock gears he's hoping for your sake that the screwcutting gearbox at least is in good condition so that you may have the full range of feeds and cutting pitches.

[Philip SewellParticipant @philipsewell33761]

I obviously misinterpreted Howard's post. I have had the lathe for 15 odd years so it's not new to me. It has always been my ambition to strip it right down and replace worn/broken parts and get the lubrication system working properly and get it looking top notch. There are no issues with the feeds/threads gearbox, the problems were in the gearbox driven from the motor.

Philip.

[Howard LewisParticipant @howardlewis46836]

Thanks Peter!

Being extremely skilled at teaching granny to suck eggs,I wanted to make it clear that the Norton gearbox would provide a number of settings, possibly in combination with a few change gears, to enable a range of screwthreads to be cut.

The VFD will obviously allow the rapidity with which the thread is cut, or the rate at which the saddle moves along the bed when cutting. But the Norton box will determine the ratio between the speed of the chuck and that of the Leadscrew, or Feed shaft into the saddle.

Or am I digging my hole even deeper?

Howard

Edited By Howard Lewis on 03/09/2019 18:16:08

[Philip SewellParticipant @philipsewell33761]

I understand what you mean now Howard, I didn't appreciate that the gearbox bolted to the headstock was called a Norton gearbox. I subsequently googled it and all became clear.

Philip.

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