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  • J Tiers
    replied
    Originally posted by Richard P Wilson View Post

    Indeed, form cutters are only correct in the middle of their marked range, and deviate increasingly towards the end of their range. I agree that a hob, used in the situation where both the hob and the blank are driven can give a better profile than a form cutter used towards the end of its range. The problem is that free hobbing, due to drag might not give such good results, probably not as good as the form cutter at its worst.
    Quite true. The problem with free hobbing is the lack of consistent positive drive, which is sort of inherent, since there is a "no-drive" point as the cutter tooth starts a new cut.

    That brings up the question of whether a hob with staggered cutting edges might work better. That way, there might be better stabilization of the blank as the cut starts. I think overall that there are some issues with that due to relief on the cutting teeth.

    A "hob" without relief would have the best guidance of the blank, if you really want to have the best possible performance. Staggered cutting teeth would help.

    I have not looked at the details of hobs and hobbing for a while, and there is something nagging at me with respect to tooth position. I forget what the issue is, but it has to do with details of the tooth form, the facets produced, and their positions. Someone with more specialized knowledge may be able to explain. Zahnrad would know, but I think he left in disgust.

    At the end of the day, it may be more productive to set up a proper drive gear train than to attempt to figure out how to do hobbing, using an already expensive hob, but trying to do the rest of the process on the cheap.


    Originally posted by Richard P Wilson View Post
    ......................................., and, as I said before, if I've got to use the dividing head to gash the blank prior to free hobbing, I might just as well put a form cutter on, and cut the gear in one operation rather than 2
    Yep. I've done both, actually. When making bevel gears, gashing is a good plan if your holder is not exceedingly stiff and solid. Or if you prefer to have less wear and tear on the cutters. Since you usually make 3 passes anyway, the gashing step is not that terrible an added step.

    It does reduce the work the cutter has to do, if it happens that you do not have proper facilities to sharpen them.



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  • Richard P Wilson
    replied
    Originally posted by J Tiers View Post

    The form cutters for gears are "approximately" the right form, possibly correct for one tooth count, and wrong for all others in the range. So use of a hob gives the potential for a good form at any tooth count, and use of one single cutter for any gear wanted in the particular DP or module.

    The question of how good the form actually is when free hobbing is more-or-less the discussion here.
    Indeed, form cutters are only correct in the middle of their marked range, and deviate increasingly towards the end of their range. I agree that a hob, used in the situation where both the hob and the blank are driven can give a better profile than a form cutter used towards the end of its range. The problem is that free hobbing, due to drag might not give such good results, probably not as good as the form cutter at its worst.
    For the work that I do, its mostly cutting the occasional lathe change gear, 14, 16 or 18 DP, and for these low speed lightly loaded gears, form cutters are plenty good enough. The ones I've done mesh nicely and run quietly. I'm intrigued by free hobbing, and intend to give a trial, but it will be more in the nature, if it works, of having another tool tucked away in the cupboard, to be taken out and used if, for whatever reason, form cutters won't do the job. I can't imagine that it would become my method of first choice. if only for the fact that I don't currently have any hobs, but I do have a decent selection of form cutters, and, as I said before, if I've got to use the dividing head to gash the blank prior to free hobbing, I might just as well put a form cutter on, and cut the gear in one operation rather than 2

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  • davidwdyer
    replied
    Originally posted by DrMike View Post
    There is a limit to how simple you can make something. If it's too simple, it won't do what you want because it's lacking features that you need. Starting with a proper sized blank isn't complicated, it's how you end up with full-depth teeth and pitch circles in the proper place.
    You are absolutely right about the FULL DEPTH TEETH etc! On my first gear with the 24DP hob I couldn't move in to full depth because the top edge of the gear tooth would get too thin.

    This comes from not having the right size to begin with.

    Education is time consuming and expensive.

    Leave a comment:


  • davidwdyer
    replied
    Originally posted by skunkworks View Post
    I thought that the spindle angle (helix angle) of a gear hob was usually printed on them?
    Well, that's probably true, but I have some indecipherable foreign inscriptions. I'll have to have another look.

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  • skunkworks
    replied
    I thought that the spindle angle (helix angle) of a gear hob was usually printed on them?

    Leave a comment:


  • DrMike
    replied
    Originally posted by davidwdyer View Post
    This makes it a little less simple.
    There is a limit to how simple you can make something. If it's too simple, it won't do what you want because it's lacking features that you need. Starting with a proper sized blank isn't complicated, it's how you end up with full-depth teeth and pitch circles in the proper place.

    Leave a comment:


  • davidwdyer
    replied
    Originally posted by DrMike View Post
    All gear calculations are based on the pitch circle.

    Your hobs are marked with diametral pitch (P). The pitch circle diameter is the number of teeth (N) divided by the diametral pitch. So if you want 36 teeth that are of 24 diametral pitch, the pitch circle is N/P = 36/24 = 1.500"

    The portion of the tooth above the pitch circle is the addendum, and for standard teeth is it 1/P high (radius), so the outside diameter of the gear is the pitch diameter plus 2/P. In our case here, we need to start with a gear blank that is N/P + 2/P = (N+2)/P = 38/24 = 1.583" in diameter.

    The portion of the tooth below the pitch circle is the dedendum, and for standard teeth it is 1.25/P deep (it's deeper than the addendum is high so there will be a bit of clearance at the root of the tooth). The total depth of the tooth is therefore the addendum plus the dedendum, 1/P + 1.25/P = 2.25/P = 2.25/24 = 0.09376" = 3/32". This is how much we need to feed the hob into the blank in order to cut the entire tooth, and end with the proper tooth form and have the pitch circle end up at 1.500" from the center.
    That is what I suspected. If you were to want a gear with 1.5 diameter and 36 teeth as in your example, you would need to turn a larger round bar

    down to 1.583 just a little larger than a 1.5" stock piece. This makes it a little less simple.

    Leave a comment:


  • Tom S
    replied
    Originally posted by nickel-city-fab View Post

    They're discussing it on PM now under the same or similar thread title in the "General" section.
    Extra-large bucket popcorn is for sale in the lobby.
    Heh heh heh. I brought the supersize with extra butter.

    Leave a comment:


  • nickel-city-fab
    replied
    Originally posted by J Tiers View Post

    The question of how good the form actually is when free hobbing is more-or-less the discussion here.
    They're discussing it on PM now under the same or similar thread title in the "General" section.
    Extra-large bucket popcorn is for sale in the lobby.

    Leave a comment:


  • J Tiers
    replied
    Originally posted by Richard P Wilson View Post
    Free hobbing intrigues me, so I will give it a try, just for curiousity but as it seems advisable, if not essential to gash the blank first, having set up my dividing head to gash it, why wouldn't I simply use the dividing head to cut the gear with an involute cutter, just like I do now? I don't do that many gears these days, and most of them are spur gears that I've got the cutters for
    The form cutters for gears are "approximately" the right form, possibly correct for one tooth count, and wrong for all others in the range. So use of a hob gives the potential for a good form at any tooth count, and use of one single cutter for any gear wanted in the particular DP or module.

    The question of how good the form actually is when free hobbing is more-or-less the discussion here.

    Leave a comment:


  • DrMike
    replied
    All gear calculations are based on the pitch circle.

    Your hobs are marked with diametral pitch (P). The pitch circle diameter is the number of teeth (N) divided by the diametral pitch. So if you want 36 teeth that are of 24 diametral pitch, the pitch circle is N/P = 36/24 = 1.500"

    The portion of the tooth above the pitch circle is the addendum, and for standard teeth is it 1/P high (radius), so the outside diameter of the gear is the pitch diameter plus 2/P. In our case here, we need to start with a gear blank that is N/P + 2/P = (N+2)/P = 38/24 = 1.583" in diameter.

    The portion of the tooth below the pitch circle is the dedendum, and for standard teeth it is 1.25/P deep (it's deeper than the addendum is high so there will be a bit of clearance at the root of the tooth). The total depth of the tooth is therefore the addendum plus the dedendum, 1/P + 1.25/P = 2.25/P = 2.25/24 = 0.09376" = 3/32". This is how much we need to feed the hob into the blank in order to cut the entire tooth, and end with the proper tooth form and have the pitch circle end up at 1.500" from the center.

    Leave a comment:


  • Richard P Wilson
    replied
    Free hobbing intrigues me, so I will give it a try, just for curiousity but as it seems advisable, if not essential to gash the blank first, having set up my dividing head to gash it, why wouldn't I simply use the dividing head to cut the gear with an involute cutter, just like I do now? I don't do that many gears these days, and most of them are spur gears that I've got the cutters for

    Leave a comment:


  • davidwdyer
    replied
    I forgot to confess all my sins. I am using a bar of 2" aluminum stock. According to Law's book (I think this right, please correct me if I am wrong) I should be

    using the P.C.D. (diametral pitch) to calculate tooth count, etc. instead of the actual diameter.

    It seems to get a 2" P.C.D., I would have to get larger stock and turn it down.

    I was being lazy (a common problem) and so this divergence will probably also affect the tooth shape results.

    I didn't want to waste material and money. Having said that, perhaps if all the gears were cut with this error, they would mesh and work together.

    I should have said all this in the video, but it's too late now.

    Leave a comment:


  • J Tiers
    replied
    That's pretty good. I like that you tried different things, and have found what gives good results.

    I spent all day (9 hours of it anyway) driving, and got back just in time for an hour long meeting on City business, so I have not looked up any of the references I have yet.

    But, good work, and thanks for sharing.

    I have exactly one hob, a 16DP, and a horizontal mill to use it with, so I may give it a whirl also....

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  • chipmaker4130
    replied
    Great video, David! Its something I've wanted to try and your inclusion of what not to do is most helpful. Thanks.

    Leave a comment:

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