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21st Dec 2020, 09:44 PM #1Golden Member
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Rotary Table - Lathe Gear Blank Arbour
The first project on my rotary table with the dividing mechanism is to cut a 63 T gear for my lathe on the CNC turret mill.
The photo shows the workshop ideas sketch for an arbour to hold the gear blank on the RT:
- a commercial MT2 blank arbour to fit the RT centre taper,
- a piece of bright mild steel bar to be glued to the MT2 arbour,
- the BMS part to be shaped to mount the gear blank,
- a MS rod to be glued across the BMS part to provide two drive pins to engage the gear blank spline,
- an ER16 nut to clamp the assembly up.
A CAD drawing will be done to allow cool reflective time indoors to check the fits etc.
The critical fit of a gear on a shaft is the inner diameter of the spline, easy to finish the arbour and check the fit whilst still on the lathe
The cross rod is envisaged as dia 8 mm milled down to the nominal 6.35 mm size to fit the spline, easy to to fit and mill while the arbour is on the RT.
The spline is a nominal 1" x 6 way bottom fitting shallow straight sided to BS 2059.
The spline in my blank was made using a 1/4" broach, so deeper than necessary.
The blank is a nominal 10 mm thick, mine is 9.8 mm in cast iron.
Lathe Gear Blank Arbour Ideas compr.JPG
Comments welcomed.
Keep well and enjoy the season (as best we can),
John.
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22nd Dec 2020, 05:47 AM #2Most Valued Member
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Hello John,
Is there any reason why you don't/didn't make your own arbor rather than bits and pieces?
How far away from the face of the rotary table will the blank be sitting?
What are you referring to in regards glue?
You won't need the drive pins the gear blank would be retained by the nut.
If you are turning a thread for the ER16 nut, make it 5/8" UNF instead.
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22nd Dec 2020, 09:30 AM #3Golden Member
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pipeclay,
Just avoiding the MT2 for the moment.
The glue will be Loctite so that I could reclaim the commercial arbour if I decide later to do something different.
I have made MT2 arbours in the past, but none on the current lathe, and I don't have a taper-turning attachment.
The clearance between cutter and table at end-of-cut is adequate, I will use CNC moves so overtravel won't be a problem
I included the the ER16 nut simply because I happen to have one and it is a convenient size.
The alternative for me would be M16.
You are probably correct about not worrying about the spline dogs - that was a flush of inspiration !
I have done numerous jobs on the lathe with just a nut on an arbour without any problems.
The actual gear-cutting will be sometime about 29Dec with a good friend participating.
Keep well all and enjoy the season,
John.
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22nd Dec 2020, 10:10 AM #4Most Valued Member
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If your lathe has a compound you can cut the taper.
Be mindful you may get vibration if the arbor is too long, unless supported.
How are you holding your cutter?
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22nd Dec 2020, 11:41 AM #5Most Valued Member
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Have you checked that the MT2 arbor actually fits in your RT?
Only asking as I found on mine that while the taper is MT2, the table doesn't have a full depth taper, and a standard arbor doesn't go in far enough.
Steve
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22nd Dec 2020, 07:12 PM #6
Hi Guys,
Yes my HV6 is the same. I actually made a plug that fits into the centre and can be used to locate the RT and a work plate that fastens on top. I have a MT2 mandrel with a Myford chuck screwed end, when the chuck is fitted there is a 3/4" inch gap under the chuck, making using the chuck this way very poor.
The bore in the RT is also a few thou out of square as measured from the mill spindle.Best Regards:
Baron J.
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23rd Dec 2020, 09:07 AM #7Golden Member
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I have cut MT2 before with the topslide, just a pain, best avoided if you can.
The cutter is a new module 1.5, dia 55 mm, bore 22 mm, held on a NT30 mandrel in the vertical mill.
I have a Spin Indexer Tailstock that I will jury rig to support the arbour.
Currently on the mill table getting its base machined square to the spindle.
The MT2 taper checks out Ok, and its depth is good to go.
I also have made MT2 plugs to enable centre alignment.
My chuck is backless with 3 holes drilled through letting me clamp it directly to the slots in the table.
A bit of tapping is still necessary to get it perfectly on centre.
Perhaps time to make an alignment spigot.
Hmm, squareness of the table/taper is something not checked.
Will investigate after this job, together with flatness.
Made the arbour extension last night and glued with Loctite 620.
Keep well,
John.
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29th Dec 2020, 10:45 AM #8Golden Member
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Made the arbour to the earlier descriptions, but with a M12 thread as that looked neater on the short stub available.
The photo shows the mill setup, ready to install the cutter and do the 63 Tooth gear.
63 T mill setup compr.JPG
But, an anomaly has arisen, the data on a the few Harrison gear drawings I have show an interesting variation in the Base Circle diameter (BD) of the gear.
Teeth --- OD --- BD ----- Difference
30 --- 47.95 --- 42.286 --- 5.664
66 --- 101.95 --- 93.030 --- 8.92
80 --- 122.95 --- 112.76 --- 10.19
88 --- 134.95 --- 124.04 --- 10.91
92 --- 140.95 --- 129.68 --- 11.27
99 --- 151.45 --- 139.544 --- 11.906
As used for these gears, the BD is not the bottom of the gear profile, it is somewhat less than that.
I will research this today as the TAFE textbook and Ivan Laws book show the BD as either at the bottom of the profile, or a little above it.
There may be differences in the way the BD is defined, but Harrison may have deliberately varied it to achieve some operational advantage.
This would also mean that they used special cutters.
The cutter I have is #7 in the metric series, 55 T to 134 T, and is marked "3.3", 20 degrees and M1.5.
I take the 3.3 value to be the full cut depth of the tooth, as measured from the blank OD.
Various references quote full depth as "generally 2.25 x M", which would be 3.375 mm for M1.5.
I have a 88 T (in the cutter range) available to compare with the cut as it progresses.
If it looks Ok, it will be assumed Ok !
Keep well,
John.
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1st Jan 2021, 02:11 PM #9Golden Member
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Job done.
Photo 1 shows final setup and completed gear, the eagle eyes will note that the cutter moved to the front for convenience.
Photo 2 the gear after de-burring.
63 T mill final compr.JPG 63 T gear compr.JPG
Used a couple of simple cnc code lines selected alternately to feed forward to a stop, then rapid back to a stop, operate divider, repeat 63 times.
Cut was 3.3 mm deep, 10 mm/min at 100 RPM.
Will now be able to lathe screw cut 8, 10, 16, 20 and 40 tpi.
Note that the set on the banjo is different to that shown in the lathe handbook.
Theoretically possible are 32, 40 and 80 tpi, but these cannot be set on the banjo.
A second jury-rig banjo would be necessary.
I will now spend some time investigating the next gear to be cut.
Probably end up cutting all the gears listed by Harrison for imperial threads on a metric machine !
Keep well,
John.Last edited by electrosteam; 1st Jan 2021 at 02:14 PM. Reason: sorry, old image crept in, cannot erase it
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