Cutting non standard (low tooth count) gears

[Jasonb]

What gears do you actually want for the pump, you just said 12T corresponds to them in which case why ask about shift as a standard cutter does down to 12T?

As I said simple way to check is to rough out a couple of gears and measure the ctr distance with the increased diameter, quicker than trawling the net.

Thats the free Gearotic I'm using

[petertha]

What gears do you actually want for the pump, you just said 12T corresponds to them in which case why ask about shift as a standard cutter does down to 12T?

As I said simple way to check is to rough out a couple of gears and measure the ctr distance with the increased diameter, quicker than trawling the net.

Thats the free Gearotic I'm using

That is the point of the whole post, but maybe that got lost in discussion. To recap, yes cutter#1 is clearly stamped 12-13T, 12 is obviously compliant on that basis. My understanding is these cutters make a mostly/pseudo-involute curve optimized for that tooth count range. Great, we have a tooth curve. Now open the Ondrive app (or consult gear formulas, same thing). Enter the exact same thing: 12T & Mod 0.8. it calculates all the resultant parameters on a simplistic basis with x1 = 0 default. But a red warning pops up relating to undercut. I take this to mean if you make the gear blank OD & DOC as though it was a normal gear, like a large diameter gear, you will encounter issues. In other words a matching 12T cutter does not MAKE it a normal gear, it is still considered an albeit mildly non-standard gear. The higher the suggested x1 compensation factor, the more non-standard it is. So they provide you an x1 input cell & underlying calculations to compensate & remedy the warning flag. Which results is enlarged blank OD from the basic OD calculated by PD & module. You sill use the same cutter & same DOC but its applied to a different OD. THAT was question & point of the post. If you happened to enter a larger tooth count gear & no warning pops up, that's because no compensation required, make gear blank OD as nominal & same DOC.

Yes, I fully intend to make gears & test this, but having a plan based on engineering fundamentals makes sense, at least to me.

[Jasonb]

 here is a thought, just simple maths not too worried about all the figures the calculator gives.

If the shifted OD is 12mm = 6mm radius

The whole depth of cut 1.8mm ( (1.0 + 1.25) x 0.8 )

So the diameter at the root of the tooth is 12 - 3.6  (2x 1.8 ) = 8.4mm  = 4.2mm radius

6mm plus 4.2mm = 10.2mm which is the closest the two gear centres can be

Gearotic gives 10.3mm so a little clearance between tip and root

Trying to mesh the gears at the non shifted 9.6mm ctr distance could see a lot of swarf in the oil

[Jasonb]

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[petertha]

Good observation Jason, thanks for sticking with me. Perfectly timed with my almost simultaneous WTF moment when I laid out the profile shifted gear OD against what it was calculating for smallest root diameter = 8.2846mm. Swarfy oil for sure. Strangely the calculator does not provide a corresponding profile shifted shaft spacing indicator even though they allow input of a second gear. Leaving foolish people like me to misinterpret that pitch diameters always intersect on their tangents. (Well I had some coaching from Gemini The Fibber so I'll blame it on Human/AI miscommunication because I was quite adamant about the question).

Back to the drawing board. Here is what it said. I either have to build this into a spreadsheet or give Gearotic a go because it seems to be more complete. Sorry for the screen grabs, pasting text makes it go all wonky.

[Jasonb]

I still don't think you need to apply any shift.

The red warning is coming up as the bottom of the tooth is narrower than the thickest part approx where they make contact. This is the type of "undercyt" Charles was talking about. Ye sit can weaken a gear but does not stop it being cut with an off the shelf cutter. Your gears are not transmitting any real power so not a problem

You actually need to get above 21T on that calculator for the warning to go without using shift. Plenty a model timing gear made with smaller tooth counts and no shift applied

It would only become a problem if you needed say 10T gears which you would not be able to cut with an involute cutter due to the type of undercut I was talking of where the bottom of the root is wider than the gap further up. These gears get even weaker but if you take it to extremes of say 2T the you are into roots blower type shape which move plenty of air or liquid with their narrow middle and large lobed ends.

That is the good thing about Gearotic it does all the calcs for each gear plus the pait at the bottom of the left side. and you can see the profiles and ctr distance change as parameters are altered

[Charles Lamont]

I would have supposed that a 12-13T cutter would be designed to cut a pair of 12T gears that will mesh at nominal pitch. If generating the profile with a hob or rack then addendum modification (which is different from profile shift) would be advisable.

[petertha]

OK, it appears I have come full circle & misinterpreted the error popup. I guess what its saying is reduced involute contact, potential strength reduction due to undercut etc. But interference is not the issue, at least at this particular 12T size. If I run the default x1=0 it gives root depth of 7.5396, blank OD = 11.2mm & tangent pitch circles of 9.6mm. Clearance exists (not considering root but fillet effects which I'm sure the cutters incorporate).