Myford change-gear pressure angle

Hello All

Can anyone provide a definitive answer to a long-running question? What is the correct pressure angle for Myford 20 DP change gears?

!4.1/2 degrees is often quoted, often without provenance but by reputable sources such as Hemingway, also by RDG for their gear cutters, and one would suppose that they are (now) in a position to know.

But then, equally reputable sources quote 20 degrees, including Graham Meek in his Screwcutting Simplified article in E********** in M********, who quotes the man at Myfords who used to cut the gears, and you would suppose that he also was in a position to know.

For the record, the rack teeth on my Myford are, without doubt, 20 degrees, easy to measure by digital photograph, blown up on Photoshop and laser printed.

I suppose the truth might be that, within limits, and at the speeds and loads encountered in small lathes, any old pressure angle will mesh with any other old pressure angle.

Jim

Not so, gears of different pressure angles will not mesh together. It's all down to the geometry of the tooth form, it has nothing to do with load or speed.

Regards,

Andrew

Jim,

I can't comment on the rack or it's mating pinion as I don't have any details on these but could get them if I wanted.

However as regards the screw cuting gears they are all 14 1/2 degrees PA with the exception of the first 4 on the Super 7 and like family.

The first 4 being spindle gear, the two tumblers and the gear the tumbler drives. These are 20 degrees PA.

Outboard of this they are all 14.5 degrees.

The ML7 are all 14.5 including back gear but the backgear on the S7's is special and requires a special hob as the tooth form is modified.

John S.

Edited By John Stevenson on 24/02/2012 22:41:10

Hi John S

So on my ML7, the spindle output gear is 14.5 degrees?

regards

Jim

Oh oh. This presumably means that the change gears I've converted for Graham's screw cutting gizmo are no good because they're the wrong pressure angle for the spindle gear (because the Delrin gears which link the two can't suit both)?

Chris

That's precisely why I asked the question. I need to know the PA in order to make the gears for the screwcutting clutch.

Pace the reply from Andrew Johnston, on my ML7, which I retrospectively gearbox-equipped, I had a foul-up years ago which stripped the teeth on the change gear mounted on the output from the tumbler. This was originally the input into the change-wheel train (before the gearbox was fitted). John S suggests that this stripped gear was 14.5 degrees.

I bought a cheap pattern changwheel to get me going, this was returned to the supplier as being laughably rubbish. I then made a changewheel in cast iron, using the only 20 DP gear cutter of the right tooth count that I had available. This cutter was 20 degrees pressure angle.

So apparently I was replacing a 14.5 degree gear with a 20 degree gear, and then meshing it with 14.5 degree gears. This gear has been in use when screwcutting, and quite often when surfacing (too idle to change back) for over 20 years. I have just looked at the lathe and none of the gears show any signs of distress. The meshing is not tight as it depends on the setting of the quadrant. The arrangement runs sweetly with low noise.

So I stick with my observation in the first post. Within limits of load and speed, gears of different PA but same DP will mesh without too much trouble. It may not be pretty but it works in this context.

In my days in a railway testing lab, when we often had to cut corners in order to get a testing rig built quickly, I used to say "this is fine in practice, but will it work in theory?"

Having said all this, I am still not absolutely clear what PA I have, so I will determine the PA on the gears that will be used with the screwcutting clutch modification. I will do this using the method shown in http://mikesworkshop.weebly.com/change-gear-pressure-angle.html which is accurate enough to distinguish between different tooth profiles.

Incidently, I intend to generate the gears for the clutch unit with my shaper suitably modified, using a simple rack-tooth form tool as described years ago in ME by "Base Circle", the original article can still be found on the internet.

Jim

I think the key words there are how much clearance you can give the teeth so in the context of a movable quadrant, it works. However, if you have fixed shafts at a fixed distance from each other, you may have a problem with tightness if the gears are formed accurately. Mine certainly engage fine off the shafts with no suggestion of binding plus the intermediate gears I've cut in Delrin. It will bug me enough to remake them though. Bummer.

I think it's the case, IIRC, that since there is only one involute that can be drawn from any point on a cylinder of a given diameter, involute gears of a given pressure angle and centre spacing can run at a different centre spacing and PA as long as they are the same DP. As the centres move apart the PA increases, but if running at the "wrong" spacing for a PA there will be a lot of backlash. But I may not recall correctly!

Yes, but I was assuming that both gears were designed for the same PA, not different, but run at a larger spacing. Definitely not recommended engineering practice I agree, but I think I read that one of the benefits of the involute for clocks is that pitching isn't so critical, and clocks famously only go one way!

And let's not get into involute vs cycloidal....!

Hi All

Well. I asked a question and now apparently I might not be fit to be call myself an engineer!

My bodged gear fix was (a) all I could do at the time (b) worked well enough to keep the job going (c) continues to work well many years later (d) cost next to nothing.

Sounds like engineering to me.

The whole world works on compromise, approximation and fitness for purpose. This is not to say that the best should not be strived for, but in the meantime the world needs to keep turning ( and screwcutting).

Thankyou for the answers to my question.

Jim

Jim, quite right!

As well as using gear trains for screwcutting similar gear trains are used in gear hobbing but the problem here is to cut a prime, you need the same prime, chicken and egg situation.

I have seen where people have made a 'gear' by flycutting with the tip of a triangular insert ot 60 degrees and they have been good enough for the machine to then make a correctly hobbed master..

We have proved it works in practice, we just have to see if the theory holds up

John S.