cutting spur gears on a mill

Its the unified thread angle

Pressure angle and thread angle are not the same thing

… But I’m sure you must know that.

MichaelG.

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Useful animation towards to bottom of this page:

https://www.chiefdelphi.com/t/turning-down-the-od-of-a-gear/137417/20

Edited By Michael Gilligan on 17/09/2021 10:27:08

Yes, of course I know that thread angle and pressure angle are not the same. I misunderstood Brian, sorry.

Jan

Pressure angle and flank angle of an involute rack are the same. the thread is (to all intents) a rack wrapped around a cylinder. (squint a bit and ignore the helix for that approximation).

Thread angle is the included angle, but pressure angle is half that - so a 60 degree thread form should make a 30 degree pressure angle involute.

Dave

Why do you say "A rack and pinon uses a 20geg pa, "a" rack & pinion could just as well use 14.5pa and is what is likely on your DW and certainly on the Myford

Andrew did point out very early on ( first page) in the thread that you would get something like 30pa using a tap.

Edited By JasonB on 17/09/2021 12:57:28

I just read somewhere that a 60 thread angle - Unified produced a 20deg involute pressure angle and a straight line. the hobbing cutter has straight sides but its curved action produces the required 20 deg slope, I wish Id kept the reference, so the rack has straight sides

I had a look under Maureens apron (she a bit coy about that sort of thing - girls born in the 50's were like that)

As you can see the rack, straight sided teeth 60deg

That animation of meshed gears in the link MG has done my head in

Another avenue of pointless pursuit ahead of me - "life, life, dont talk to me about life" said Marvin the depressed robot

Really, how are you measuring that?

Copy your picture into a CAD program and measure the angle. Not 60°. I've done it but will let you find out what it is for yourself.

Martin C

Jason posted just after I started doing it.

Edited By Martin Connelly on 17/09/2021 16:00:39

Hmm well I was going on what I read, maybe Maureen got a faulty rack?

How do you justify the angle of 67deg?

Now if you postulate Maureen would have had a PA of 14.5deg then the combined angle should be twice that ie 30deg

so if PA was 20deg then this should be 40deg

so this is a long way off?

but note that the rack form has straight sides no involution here. A rack is treated as a gear with infinite PCD

Now PA=14.5 was the historical value from victorian times as giving quiet operation and long wear (but only for medium loads and speeds), The AGMA had better ideas and chose 20deg since 1980 for greater power, reduced pinion teeth before undercutting, wider base (greater no of cycles before fatigue failure), but greater wear.

Whats not to like

I was thinking more that the DW gears were 14.5pa rather than any rack.

Quite possible they beefed up the rack and it's pinion to 20pa which is not far off the 22deg measured allowing for the angle of the photo possibly being a bit off

Yes a 20pa cutter would have an included angle of 40deg and a 14.5 pa cutter one of 29deg hence why it was pointed out that your bolt/tap would produce about 30pa due to the 60deg included angle.

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Could you try nudging that a little, Jason … and maybe get 20° instead of 22.44°

MichaelG.

[ requested before I saw your last post ]

Edited By Michael Gilligan on 17/09/2021 19:03:43

nudging numbers, I thought that was more Brian's sort of thing

There is some distortion, if you look at the notches at each side of Brian's original photo they are leaning so can only take a guess. However we don't really need to do that as if you look under "the Apron" in this old catalogue you can see that they did indeed use 20pa for the rack and it's pinion, must have been thinking of all those model engineers who wanted to use the carriage to plane keyways.

EDIT, here you go with the others measured and a token 20deg line for good measure.

Also note that the teeth seem to be slightly concave not "straight" as proclaimed by Brian, maybe after 70yrs of use she is showing some wear and is a bit bow legged under the apron. Click for larger image

Edited By JasonB on 17/09/2021 19:21:00

she is showing some wear and is a bit bow legged under the apron.

kerriste dont say that in front of her, she'll have a cow

So there's another mystery solved, rack and pinion 20deg PA 40deg apex angle

now to de-bumf the base angle and unclutter the pitch circle from your diametral pitch and pitch circle. OMG do they make hard work of it all, mixing nomenclature with the cousins

Are thread forms independent of pcd, so a small pinion has the same tooth form as a large spur for the same circular pitch.(teeth per inch) I dont think it does

Of course the tooth (not thread) form is different as the PCD changes. That's why we have numbered gear cutters, they are different shapes.

I can see this thread being sent up to the beeb as a classic comedy series The MEWD Modellers - like the Detectorists with Gareth keenan, imagine himself with a huge horizontal ex WD mill called the Honey Monster which dims the street lights every time he switches it on

Ruston Horniman made a 1/4 scale Fowler steam traction engine in his front room. Erindoors wont let him get up a head of steam cos the soot will foul the Dralon drapes

and so it goes

Edited By brian jones 11 on 17/09/2021 20:12:27

For at least one participant in this thread it's more likely to a a reality TV show than comedy

I suspect that the fact that:

sin(14.5° ) = 0.250

is no coincidence regarding the use of 14.5° as a pressure angle.

A hob does not generate an exact involute, it produces an approximation consisting of a number of facets. Of course in many cases the tooth form created is perfectly satisfactory. But it interesting to note that in the professional world hobbing is often regarded as a roughing operation. Look at the literature for a geared head lathe and great play is made of the fact that the gears are shaved and/or ground as finishing operations.

Andrew

Edited By Andrew Johnston on 17/09/2021 20:21:54

this thread it's more likely to a a reality TV show than comedy

never watched that mucky stuff

is that where young persons with nothing on, go to an island and play lewdo. We machinists never take our boiler suits off for that. I prefer Radio 4 and Mrs Dales Diary (I think Jim is having an affair?)

but for the benefit of the noobs (and those who never knew how little they knew) we are unearthing the mysteries of gears and exposing things hobbers dont want you to know

must press on

PS

sin(14.5° ) = 0.250

is no coincidence regarding the use of 14.5° as a pressure angle.

now thats a revolution

Edited By brian jones 11 on 17/09/2021 20:39:03

.

Thanks, Jason

Seems entirely reasonable, given the duty that the rack serves …

with a crisper photo, we could probably confirm the size of the pinion.

MichaelG.

rack thread pitch is 0.1566" so assume PA 20deg what is the pcd