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  1. #1
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    Aug 2015
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    Default generating gears with a rack cutter - with an existing rack as a template

    Hey all,

    I need to cut two gears. Both 10DP. For this project:

    //metalworkforums.com/f65/t1997...grinder-basket

    The larger gear is an idler that mates with the rack under the grinder table. It is driven by the smaller gear:

    IMAG1255.jpgIMAG1256.jpg

    And said under-table rack (in amazing condition):

    IMAG1254.jpg

    Despite the smaller gear being in pretty bad condition I thought I might try to rebore it and freshen it up. No go - whatever destroyed its host grinder ripped it out of round. Really - is it about 40 though out of round now .. that machined surface 'collar' is not round anymore.

    Id like to experiment with using a rack-type cutter to cut the gears. As described here:

    How a rack cutter can produce gear teeth

    here:

    Gear cutting with a rack form multi-tooth cutter

    and here:

    http://www.geartechnology.com/issues/1084x/miller.pdf

    Here is my dumb question:

    "using the table rack as a template can I make a matching single point cutter to use to create a rack cutter ... (that matches the table rack) ... to cut the gears"?

    Sorry if that needs to be read a second time.

    Can it be that straight forward to get a correct form rack cutter? If so, it means I can skip all the hard maths and trig etc ... which would be tops. )

    Steps would be this:

    1. Create a single point cutter that matches the table rack.
    2. Use that cutter to create a rack cutter that, in turn, matches the table rack across multiple teeth.
    3. Use the rack cutter to cut gears as described in the links above.

    All help and advice appreciated

    Greg.

  2. #2
    Join Date
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    Quote Originally Posted by StrayAlien View Post
    Here is my question:

    "using the table rack as a template can I make a matching single point cutter to use to create a rack cutter ... (that matches the table rack) ... to cut the gears"?
    The profile of a rack will be a straight sided cutter of what ever the pressure angle of the rack is (probably 29 degrees). What you are talking about is basically hobbing a gear.
    Strictly speaking it will not be single point though as there will need to be multiple cutting tips (although all of a straight sided profile)
    The method described will do it but the things that not only will you need to index accurately (as is required when cutting a gear anyway), your hob will need to be cut accurately as well. The reason being that the additional cuts that will turn a straight sided tooth into something with an involute profile will be done by the adjacent cutter teeth, and that will only be a sliver.

    Michael
    Added bit
    gear.pdf
    This sketch shows a 20t 10DP blank hobbed in one position and then rotated 18 degrees to hob the next teeth. As you can see there are some sections that get quite a lot of material removed while there are others that get very little - hence the need for accuracy when making the hob.
    Last edited by Michael G; 15th Apr 2017 at 10:06 PM. Reason: added illustration

  3. #3
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    Upwey VIC
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    Default

    Quote Originally Posted by StrayAlien View Post
    Hey all,

    "using the table rack as a template can I make a matching single point cutter to use to create a rack cutter ... (that matches the table rack) ... to cut the gears"?

    Can it be that straight forward to get a correct form rack cutter?
    Greg,
    Yes, and yes.
    You may need to make the rack form cutter teeth slightly longer (a few thou.) than the rack teeth to ensure clearance at the base circle of the generated gear.

    Andrew.

  4. #4
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    Default

    If you want to experiment, go for it. If you want 2 functional gears in a reasonable time frame then use 10DP gear cutters. As you have existing gears you can use to index from, you don't even need a dividing head.

    I have a fair selection of 10DP cutters. I'm far too lazy to count the number of teeth on the gears in your pictures though so if you tell me what the tooth counts are, I'll tell you if I have the appropriate cutters.

    Or, if you want to experiment and learn something in the process, never mind & have at it. I'll be interested in the results myself though I can't say I'd ever bother doing it. Gear cutters are a lot cheaper than my interest level for jobs like that......

    FWIW I've ground a single point cutter to an involute-ish form using an existing gear and cut a mating gear with it. It was for a slow speed not really high precision job so was satisfactory in practice. There are as usual a number of ways to skin this cat.

    After doing it that way I bought the gear cutters......

    PDW

  5. #5
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    If I remember rightly, Greg, you have a shaper?
    Using a combination of a a shaper, a single tooth cutter made EXACTLY like each of the gears (not the rack!) and Pete's suggestion on using the existing gears as indexers, you could make a replacement set quite easily. I've made a couple of gears like that so far - both successful.
    Alternatively, borrow Pete's cutters and use them on the lathe on an arbour with the gears (old and new blank) clamped to the toolpost.....
    Cheers, Joe
    retired - less energy, more time to contemplate projects and more shed time....

  6. #6
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    Quote Originally Posted by PDW View Post
    I'm far too lazy to count the number of teeth on the gears ...
    14 and 35, not that I take an interest in these things

    Michael

  7. #7
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    Aug 2015
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    Melbourne, Australia
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    Michael: 15 and 35 actually. ) You're at the computer waaay too early! )

    Joe: not even going to mention the time of your reply. )

    Yes - it has to be multi-tooth cutter, not a single point. The single point I was referring to was created using the rack as a template to help make a new multi-tooth cutter. Nice PDF pic to bat the way. Thanks.

    Andrew: thanks. ) I was hoping so. The extra depth seems wise.

    PDW: yes, it is about learning stuff. I'm not in a hurry. But "After doing it that way I bought the gear cutters......" did make me laugh out loud! ) Thanks for the offer of cutters. Very kind. Science doesn't yet have instruments that can measure the scale of my machining ineptitude so I may need help yet.

    The thing I like about this technique is that is cheap, you only need one cutter to do an entire range of gears with that DP and making the cutters should be a good exercise for a newb like me. The downside is time and indexing - for a small gear it looks like you might need to be able to index 2 or 4 times the number of teeth - but as Michael points out, the offset indexing will just be taking shavings so that won't take long hopefully.

    I like the idea of Ivan Law's 'button' technique, but then again, one cutter per gear range per DP. I reckon I'll give it a go sometime though.

    Joe: I like that idea - nice and simple too. I will use that for sure one day. In this case, I don't have good teeth profiles. The small gear is toast. Even between teeth with machining marks still on their tops there can be a different of 20thou depth. Weird .. and the 'good' end profiles are not symmetric. The large gear - although it looks pretty good in the photo - also has asymmetric profiles. Hard to see in pics but one side of the profile 'leans away'.

    I reckon if the profiles were okay I'd be giving this a go to create some cutters. )

    https://www.youtube.com/watch?v=CaBV3Sn9Tkg


    I also like the gear+blank in the toolpost idea. )

    I may have this week off so if I make some progress I post here - or I may chicken out and do something simpler. Thanks all. First challenge will be creating an accurate 29 degree cutter. But I have some acme threads to cut soon-ish so that'll be good to get under my belt.

    Greg.

  8. #8
    Join Date
    May 2009
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    Somerset, UK
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    Quote Originally Posted by StrayAlien View Post
    ...snip...Id like to experiment with using a rack-type cutter to cut the gears. ...snip...Greg.
    Doable if you have a shaper, some form of dividing device, and the inclination.....
    I cut most of the gears for a Geometric Chuck by this method (chapter and verse here..)
    Gear Generating
    If it was just for a couple of gears I would probably buy (borrow?) the appropriate cutters.
    As said previously, you can use the existing gears for dividing.
    Mark
    What you say & what people hear are not always the same thing.
    http://www.remark.me.uk/

  9. #9
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    Model Engineer published an article in the September 14, 1950 edition (Issue 2580) called "Gear Cutting with the Shaper" written by "Base Circle" - a nom de plume, but I don't know whose. I found a copy on the NEME-S site (Index of /Model_Engineer_Files), but I can't remember what NEME-S called it! Send me a PM and I will email you the pdf.

  10. #10
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    Aug 2015
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    Default

    Mark, A J, thanks. Apologies for late reply.

    I have studied that base circle article many times. Mark, I have seen that page and scanned it quite a few times. I have to admit that the simplicity of it and the fact that it generates perfect involutes makes it quite a beautiful thing. It ain't perfect - like you need two blanks for every gear you make and it is likely was slow as a wet week. But ... one day I might even give it a go just to watch it working!

    Though .. when I said above I need to cut some acme threads on some stuff .... this is my shaper cross feed nut.

    20160328_170829.jpg

    Broken and bent. So ... no auto-cross feed on the old Alba 10" at the moment. )

    I'll see how I go. I might get lazy. These are pretty tempting:

    Involute Gear Cutters 8 Pcs DP8 DP10 Dp16 DP24 PA14-1/2 Bore22 HSS 8H Set No1-8 | eBay

    )

    Greg.

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