cutting spur gears on a mill

A rack has no PCD but the gear or pinion that runs against it does. There is a clue in the abreviation Prescribed Circle Diameter

A rack also has no thread pitch, it will have a DP size or MOD if metric.

It has a Pitch which is equal to the circular pitch of the mating gear/pinion. At 0.156 that would suggest 20DP which is 0.1571"

Edited By JasonB on 18/09/2021 07:19:18

Um, pitch circle diameter?

How does the fact that sin45 = 0.2503800041 help Brian?

It doesn't, it's the sine of 14.5, not 45. I suspect that 14.5 was chosen as a PA not as a result of experiments that showed it had the characteristics posited by Brian but because the sine was a round value, at a time when calculators were unheard of and many people wouldn't have known how to use trigonometric tables.

Mathematically a rack does have a PCD, it just happens to be infinite.

Andrew

Oops! Meant 14.5...

I guess my implied question is, what does someone do with the sin of the pressure angle, whether it comes from tables or wherever? 

Edited By John Haine on 18/09/2021 10:38:57

Maybe because it’s easy to set the angle using a sin bar?

Dave

Somehow doubt if BJ 11 would be interested in cutting a rack.
This one here took over 5 hours to cut the 159 teeth.

Was definitely a "OMG what a painfully slow process as you mill each one"
John

For a Quick and Dirty job, a bike chain can make a rack . I always remember how inefficient RnP can be from misspent youth traversing canals and their locks. These had sluice gates to be operated with a hefty handle hauling on a victorian RnP

I like the cloth cap answer for 14,5deg, on a 5" sine bar its 1-1/4" height. That would suit a draftsman

much more plausible than my postulate of trial and error comfort (given by some rubbish from google)

more work needed here but Maureen is beckoning

BTW JB whats this

There is a clue in the abreviation Prescribed Circle Diameter

I am appalled at the way the industry presents its essential features and I am trying to de-clutter the nomenclature into categories of "need to know" and "nice to know"

I blame AGMA for this prolixity after all why use 5 words when 100 words looks good and sounds important and you can charge more money for it - geddit

For me this is the most useful post in this thread. I can feel another little wizard coming along here - having a method to cut gears on a straight CNC machine with A axis would be very useful. Yes, one can use a straight end mill but very small and fragile cutters are needed for small moduli. But if one could make a single cutter rather like a standard gear cutter but with straight sides rather than curved, that could do a wide range of tooth counts it would be very useful. Martin, it would be very interesting to know what the cutter dimensions are in terms of the modulus for this.

I'm not sure why we have to restrict ourselves to non-CNC solutions - if so, then why not restrict ourselves to a file and cold-chisel?

Would also be interesting to know the increments that the A axis was moved in. The minimum would be one position per tooth which would only give a couple of facets to thegear's teeth as the teeth of the cutter above and below ctr height take a little off. More increments with a small Z height movement would give smaller facets but you would probably not want to go so small that you end up rubbing more than cutting.

John, if you really have an idle moment you could possibly do something for bevel gears too that reduced the depth as the cutter got closer to the ctr of the gear by moving in Y.

Well the OP set out to find a simpler way of cutting gears for the humble hobbyist

I'm not sure why we have to restrict ourselves to non-CNC solutions - if so, then why not restrict ourselves to a file and cold-chisel?

CNC guys should really stick to the senior common room with much more room for comfortable armchairs (I see are necessary to mind a CNC m/c) because you are operating in a far different orbit to mere hobbyists who will probably never own a such a beast - its way out of our league

I just marvel at the way you guys can cut a hole with a set of xy instructions - no more large drill stock etc. We cant do this on small manual m/c not without considerable effort with a face plate etc

Like Morris Minor with a Lamborgini Clarkson

Pass me the 4lb mallet, glove puppet, the EMS is playing up

Posted my Me 18/09/2021 11:40:12

Somehow doubt if BJ 11 would be interested in cutting a rack.
This one here took over 5 hours to cut the 159 teeth.

Was definately a "OMG what a painfully slow process as you mill each one"
John

Should have pointed out the machine is part Cnc but he rack is about
25 inches long, table traverse about 21 inch so done in 3 stages ,
reset to do each 8 inch or so length ,plenty of time for a snooze
in an armchair, machine beeps at end of sequence so no need for
alarm clock , pumped oil system so no laborious squirting
of oilcans to disturb slumbers.

John

Piccie of the armchair, big trough in the squab is there?

Cant really see what happening at the business end, the spindle is clamped and you motor the y slide in and out to cut the tooth, then index along x wise for the next tooth is it?

5 hours phew, were you compensated by the hour?

Could this duty not have been done with a motorbike chain?

Now for those who believe a lot of productive work is done sitting in a comfortable chair.

Here is a conundrum for the SCR that is bothering me

Why was the concept of DP Diametral Pitch introduced when the CP Circular pitch addresses the same size issue and is physically realisable ie you can measure it with calipers

CP is also analagous to screw thread pitch and is intuitive

Even in the Gear Hobbers vade mecum by Dr. Rainer Hessmer only the CP , PA and No of teeth are required, DP does not figure at all

I suspect foul play by AGMA

Edited By brian jones 11 on 19/09/2021 23:06:54

Well Brian, I want to find a simple way to cut a range of gears as a humble hobbyist who happens to have a CNC mill, which didn't cost me a lot of money since I got the mechanics cheap on eBay and built the electronics myself. Since they are for clocks I'd like them to have a well defined ratio, and the predetermined diameters. Your method seems to fail on both counts.

Indeed you need precision gears for clocks with minimum friction - what PA would be used?

My method not suitable

No one has yet spotted the fundamental limitation with my method

I admit the PA will be 30deg. given a unified thread on the cutter but thats not it (noisy and inefficient).

.

DP is much more convenient than CP when you are laying-out gear-trains.

CP is convenient for worm-gearing an for rack & pinion drives

… Each can be converted to the other by arithmetic, but obviously involves Pi.

MichaelG.

I think 50% of th eposts in this thread have pointed out the various limitations of your method but if there are even more it makes it less of a viable alternative.

To list a few

Limited to easily cut materials

Exact number of teeth an unknown

PCD of resulting gears unknown

Need a tap to start with

Limited tooth size as large threads are screwcut not cut with taps

Poor tooth form so limited load/speed

Not that quick if your brass gear was anything to go by

Etc

J

PS I'm also a hobby CNC user, not even got an engineering background.

Edited By JasonB on 20/09/2021 07:01:32

Ok JB, I agree with all your comments AND

the fundamental limitation is the choice of screw thread pitches available for practical use

To keep it simple, and assume that the CP is approx = to screw thread pitch then you could realistically go from 2mm say 1/2 UNC (or M14 x 2mm) to M24 x 3mm to M36 x 4mm

This is equivalent of Mod 0.64, 0.95 and 1.27 so wont go far, only for modellers (stand by for incoming)

My plastic gear looked good enough to model, quick, cheap and easy to make

But fear not for I have a cunning plan which, if parts arrive, I will post a vid in a week

Rest a while in the SCR comfy club chair

Posted by brian jones 11 19/09/2021 22:54:39

Piccie of the armchair, big trough in the squab is there?

Cant really see what happening at the business end, the
spindle is clamped and you motor the y slide in and out to
cut the tooth, then index along x wise for the next tooth is it?

5 hours phew, were you compensated by the hour?

Could this duty not have been done with a motorbike chain?

---------------------------------
Here is another view of the cutter and the rack .

Not really as bad as it sounds at 159 tooth and 2
minutes a tooth ,that is to cut the tooth retract set to
next position . 2 x 159 = 318 minutes 5 hours 9 minutes.
It is only a hobby machine ,i have 7 other machines
that run as part cnc that use the same system ,both Doreen and
Maureen have been Cnc'd for about 16 years.
It would of course be possible to have the all running at the same
time i guess it would be like

but much better to have just the one running at a time so you can
enjoy the

The bike chain idea has some appeal ,easy enough to nip into
town with a pair of bolt cutters ,plenty of choices there ,still
leaves the problem of cutting the sprockets ,back to one
tooth at a time ,another plan foiled.

I'm just worn out writing all of this,where's that armchair.

John

Not sure I understand that? Do you mean it'll cut gears of different tooth count for a given DP or Mod value, or that one cutter will work for multiple values of DP or Mod?

Andrew