I do lots of 4 lead threads, but I now need to do a 3 lead 36 TPI
thread on the Atlas 6" lathe. I set up the gearing for 12 TPI,
but I'm having trouble figuring out how to do the different starts.
The gearing calls for the 32 gear at the spindle through an idler to a
20/40 pair (or 24/48, 32/64) to a 48 on the leadscrew. The leadscrew
is 16 TPI.
It can't be done with the thread dial alone, so I'm trying to figure
out if I could slip some gears for the second and third leads. If the
gear on the spindle was divisible by 3, there'd be no problem.
Anyone have any thoughts?
-Bruno
Set your cross slide at 0 deg, machine the first thread as normal., then
crank the cross slide .0277" for the next one, and another .0277 for the
last one.
I've never actually cut a multiple lead thread but I've heard of this
method. Also if you can turn the part between centers, use a faceplate
with holes (or whatever) 120 degrees apart to drive the dog which
rotates the part for each new cut. I've also seen lathe chucks with 360
degrees marked around the perimeter. I'm guessing that you'd just loosen
the chuck somehow, turn it the required distance and re-tighten in the
new place.
My question is how deep to cut the threads? Can you still use thread
wires? Do you have to cut all three threads once then measure?
Randy
Turn between centers using the 3 jaw chuck to hold a scrap of bar stock
turned to a 60 deg. angle for a center. Put the dog against each jaw in turn
to maintain spacing. I often use the 3 jaw with a turned center in it for
turning between centers. I have a piece of stock marked so it goes in the
chuck the same way and I don't halve to take a cut on it unless I'm doing
close work. I can take a cut on the center quicker than I can change the
chuck to a drive plate and back again.
Tom
I don't understand what you mean by this. The degree settings are on
the compound (not cross slide) and that's set to 29 degrees to cut the
thread. Are you suggesting I cut the thread at 0 degrees going
straight in?
But using your logic (I think), I just ran an experiment. I set the
compound to 30 degress and cut a lead as normal. I then reset the tool
by bringing the cross slide in by .0481" ( 1/36 * sqrt(3) ) and the
compound out by .0555" ( 1/36 * 2 ) for the next pass and again for
the third pass. Under magnification and with a thread gage, it looks
just right.
THANKS!
-Bruno
Spindle/Idler * Idler/Lead
32/24 * 48/48 is one suggested gearing.
This is the same as 48/24 * 32/48
and now you have a spindle gear which is a multiple of 3.
Will this or some other equivilant fit?
Bruno wrote:
Another route that works is to set the compound at 30 degrees. Set your
compound and cross slide to zero for just touching the thread surface. Cut
your first thread. The back out the cross slide. Advance your compound by
twice the thread width (sin30=.5). In your case 2/12=0.1666. Set your cross
slide so its .866 (tan30=.866) times this value further out , in your case
.1666/12= .01389. You're now just touching the bar one thread further
ahead. This works for any number of thread leads.
====================One way is to use the compound to offset the thread by a given
amount, but this prevents you from using the compound [set at 30
degrees] to control the depth of cut of the thread. The offset
would be one thread pitch or 1/36 or 0.027777778 with the
compound set exactly parallel to the spindle. If you do this
don't rely on the compound/top slide graduations, but use an
indicator and test bar between centers.
IMNSHO, a better way is to slip a gear.
What gears do you have available?
All the intermediate compound gear combinations you list do is a
change of 2:1 between the spindle gear and the lead screw gear.
Any combination that will physically fit will be OK. This is
important because it may make the gear you require available for
the spindle/leadscrew.
To get 12 TPI with a 16 TPI lead screw requires when the spindle
makes 12 revolutions the lead screw makes 16 revolutions [to
advance 1 inch] thus you need an overall ratio of 12:16 or 3:4.
The problem then is to select a gear set from the ones you have
with a spindle or lead screw gear that has a factor of 3:4. You
can use the intermediate compound gears to allow the use of
spindle/lead screw gears with even (sub) multiples of teeth. A
spreadsheet program is a great help here. Assuming your gear set
starts at 20 and goes through 100 you would have:
Spindle Lead screw gear
gear for 4:3 ratio
20 26.66666667
21 28
22 29.33333333
23 30.66666667
24 32
25 33.33333333
26 34.66666667
27 36
28 37.33333333
29 38.66666667
30 40
31 41.33333333
32 42.66666667
33 44
34 45.33333333
35 46.66666667
36 48
37 49.33333333
38 50.66666667
39 52
40 53.33333333
41 54.66666667
42 56
43 57.33333333
44 58.66666667
45 60
46 61.33333333
47 62.66666667
48 64
49 65.33333333
50 66.66666667
51 68
52 69.33333333
53 70.66666667
54 72
55 73.33333333
56 74.66666667
57 76
58 77.33333333
59 78.66666667
60 80
61 81.33333333
62 82.66666667
63 84
64 85.33333333
65 86.66666667
66 88
67 89.33333333
68 90.66666667
69 92
70 93.33333333
71 94.66666667
72 96
73 97.33333333
74 98.66666667
75 100
Of course only the ones with whole numbers of teeth are
physically possible. Remember you can use intermediate compound
gears to divide or multiply the teeth required by 2/3/4 etc.
Thus you could use a 75:50 gear with a 30:60 [1:2] compound to
get the same affect 75:100 = 3:4 75/50X30/60 = 3:4
If you frequently do multiple start threads, an indexing
faceplate can prove very helpful [I am assuming your are
threading between centers.]
The older machining books frequently have examples of these shop
made tools.
"Machine Shop Methods" by Milne has an example on pages 131-132.
He also has a short section on multiple start threads on pages
130-131 which may be helpful.
Milne is available from Lindsey books as is a reprint of change
gear calculations.
see:
http://lindsaybks.com/bks/milne/index.htmlhttp://lindsaybks.com/bks7/sscut/index.html
What are you building -- this sounds interesting.
BTW you may wish to double check the set-up information. It
should be 36:48 with no compound gears for 12 TPI with a 16 TPI
Lead screw, just series transmission gears which won't affect the
ratio. As 48 is 3 X 16 slipping 16 teeth would seem to be the
easiest method to get a triple lead.
Unka' George (George McDuffee)
..............................
Only in Britain could it be thought
a defect to be "too clever by half."
The probability is that too many people
are too stupid by three-quarters.
John Major (b. 1943),
British Conservative politician, prime minister.
Quoted in: Observer (London, 7 July 1991).
One way to do it is to orient the compound parallel to the bed
(and don't use it for infeed as is usually done), and cut the first
thread, then advance the compound 1/36" (0.0278")and cut the second.
When that one is complete, advance another 1/36" (to a total of 0.0556")
and cut the third pass.
It means having to do the infeed with the cross slide instead of
the compound, but apparently the UK normally cuts threads with direct
infeed instead of the angled infeed which the compound gives you.
Now to see what others have suggested.
Good Luck,
DoN.
--
Email: < snipped-for-privacy@d-and-d.com> | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.