I've recently bought a College Engineering Gear Hobber and so that I have total control over the tooth form (for small clock wheels) I need to make my own hobs.
Standard hobs will have equal tooth and space thickness but I need 48% tooth and 52% space so 'off the shelf' hobs are not an option - unless someone here knows better and can offer a method of achieving this with standard hobs.
I have a customer who re-grinds all manner of cutters who has given me an offcut of M2 HSS (unhardened) to try out and my first trials have not been good. It seems that the slightest let up in feed causes immediate work-hardening.
I've tried HSS and carbide at various speeds - 350 - 600 rpm (32mm dia) with no real success.
My biggest 'fear' is the fact that I need to screw-cut this (ACME form) and the single point tool will have a tip width of 0.83mm.
One would normally use Cycloidal form gears for clocks but creating a hob to generate a cycloidal form is very complex. Using unmodified Involute form riscs too much friction so a slightly thiner tooth can be used to eliminate this. Normal machine gearing is designed to maximise the 'power' transmitted but with clocks the object is 'motion'.
Surely sharpening hobs is only ever done on the face of the flute (gash?). I read your comment as suggesting that the 'form' is re-ground.
This also re-inforces the suggestion that the form is reground so I must be mis-understanding something.
After sleeping on this, I think that even re-grinding the form wouldn't change the tooth/space ratio - it would simply make the hob a smaller diameter which ultimately only reduces the hob/blank centre distance.
So I still hve no idea how shapening could affect the tooth/space ratio.
JG, I think you will have to do a bit more homework on the construction and use of hobs as you are not getting the overall picture, this is meant politely.
First off a hob is full form relief ground with a radial cutting edge. In English this means the cutting face is vertical from the centre and provided that on regrind's it remains vertical the form stays true.
Because it's form relieved only the front cutting edge does the work, all the 'new unused' tooth behind is in fresh air as it's smaller so friction doesn't come into the equation. A new cutter will cut the same tooth form as a part worn cutter - fact.
Re grinding will change the tooth space ratio as the space gets bigger but see above it doesn't matter. It is true that the hob will reduce in diameter because of the radial grind being applied to a relief ground shape, in English as it backs off it gets smaller.
However this has no bearing on the tooth shape, it will reduce the hob / blank centre distance as you say but no one EVER takes this into account because hobs come in all diameters. What does matter is not the hob / blank centre distance but the depth of cut applied to get the tooth shape. This is constant no matter what diameter the hob is.
Find a gear cutter of the Brown and Sharpe type, the single cutter either in the metal or a decent picture and have a look at the geometry of the tooth form. A hob is exactly the same construction except that it isn't shaped but rack sided and all should become clear.
To be honest I think you are worrying about a problem that doesn't exist.
I wouldn't take any advice from you in any other way John :)
Of course! The brain is now in gear.
The point that I had missed was the 'relief ground' issue.
Because my prime concern is cutting brass and generally less than 2mm thick (on occasion some pinions might be as much as 5mm) I had intended to discount 'refief' and, when gashing, leave less than 1.5mm land. I realize that this reduces the potential life of the hob (less space/land to regrind) but I'm not in a commercial production environment so that is not a real issue.
If I'm not mistaken the relief is applied by moving the effective centre of the hob but the tooth form is constant.
I'm still ot totally convinced.
If the tooth form is constant (as I suggest above) then sharpening will not affect the tooth/space ratio at all.
If I am totally wrong in this and the hob tooth thickness is also reduced, (a very complex problem) then in effect the gear tooth thickness produced would surely be greater once hob had been re-ground. I can certainly understand that the amount of relief applied would make virtually no difference to the tooth form (within commercial tollerances) but taking the extreme of a 10/90 tooth/space ratio surely would have a marked effect.
I'll do some involute generations to get a better understanding.
That much at least I understand.
Again fully understood.
That is a strong possibility and this discussion is very beneficial to my understanding but actually my original question was more to do with machining M2 and the recommendations of the group with regard to tool material and speeds.
In the light of the difficulties I have found with M2 would I be better off using Silver Steel - would I find similar problems?
I think the problem arises in the description of the tooth space ratio, something I have never had to think about before and we may be discussing a different animal.
To me tooth space ratio is looking sideways at the cutter and the space being the gap between the teeth and the tooth is the depth of the tooth. On a new cutter this is usually 50 / 50 or even greater as the more tooth, the longer life.
When worn say 10 / 90 there is a longer wait until the next tooth comes round.
If this is what you think it is we are on the same wavelength.
Even this description isn't geometrically correct. Take one tooth of the cutter [ hob or B&S type- doesn't matter ] The tooth behind the cutting edge is only there for strength and future use, once the cutting edge and the following 10 thou or so have entered the blank because of the relief the rest is doing nothing other that stopping that 10 thou that's cutting, breaking off. So in fact your cutter, even at new is running at something like 95 / 5 space to tooth.
Yes Silver Steel will machine better than M2. Unless you can get the revs up high enough carbide won't come into it's own and you would be better off with HSS and plenty of proper cutting oil, not motor oil. For cutting brass the difference in hardness won't matter, in fact for brass harden the hob fully out and don't temper down. You will find that the heat generated in the tips of the tooth when grinding will be enough to just drop the temper that bit.
Years ago I had to make special form cutters for complex wood mounding's, we used 3" Silver Steel, 12 teeth and hardened and temper to light straw. Cutter life wasn't what we expected even in wood. The cutters were re hardened, not tempered and face ground. Cutter life went up 4 fold.
I did mean deeper (too early in the year for me to be thinking straight!)..
Running the hob a bit deeper would give you the same effect as increasing the centre distance, that was my point. Remember that you are now dealing with involute gears and not cycloidal. They will still mesh properly if the "centre distance" is too great, however you adjust that parameter. What you are looking for seems to be more clearance or backlash. With the involute form and unidirectional load, as on a clock, then cutting the teeth a bit too deep should work as well as increasing the centre distance by a tad.
Absolutely right - just as Brian has mentioned - I'm talking about the tooth/spce ratio on the cut gear and you are talking about the hob tooth/gash ratio - at last!
They're right when they say that a picture tells a 1000 words!
One small mis-understanding extends the discussion exponentially.
I think Mark understood where I was coming from and his suggestion may well be the way to go.
You've been peeking in my workshop :)
Thanks John, that's the voice of experience I was looking for.
Few people realize that wood is often more difficult to cut than metal (especially Keruing) and there are special grades of carbide desgned specifically for wood - they are harder than those for CI or steel but not as 'tough'.
I think a lot of confusion has been created by somewhat wooly terminology in the original post, viz:
"Standard hobs will have equal tooth and space thickness but I need
48% tooth and 52% space"...
Describing the problem more accurately*, this might be better phrased as:
"Standard hobs cut gears with equal tooth thickness and tooth space, the sum of the two being the circular pitch, but I need gears with
48%CP tooth thickness and 52% CP tooth space".
As I read it, the OP is asking about how to hob gears with, effectively, large backlash (clocks working in one direction etc.).
If we go back to Mr Law's book, figure 29 'tooth proportions', the circular pitch is shown as twice the tooth thickness, t. Circular pitch is 50% fresh air, 50% material. We are reminded that we are cutting not the teeth but the spaces between the teeth. Clearly, there has to be some clearance (other than that in the D+f direction), and Law says that some books quote tooth thickness as 0.48 CP (hence tooth space 0.52 CP).
I believe the OP was asking how to make a hob to cut gears to such proportions.
I'm not for a minute suggesting that accuracy equates with correctness here as I simply don't know if standard hobs do cut as surmised by the OP.
I agree, but at some point one has to put public duty before personal pleasure?
I once had a boss (Area Service Manager) who was so good starting these sort of "discussions" and spreading false rumours that he was banned from head office! He was obviously required to attend some meetings but was escorted by someone from HR to and from the meetings!
The problem of not being able to see past the end of ones nose John :)
Actually I was asking for comments regarding machining M2 but added the information about the hobs to give interested parties a fuller picture of the potential problems.
Had I simply asked what tool material and speed/feed to use, then 'Carbide and High' would have been a reasonable answer but screwcutting an ACME form hob cannot be done at high speed . . . . . or can it? :)