1. Plastic
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## Mulitple Start Thread Programming Question

What am I doing wrong I am turning the following thread internally:
5 TPI
6 Start

I have the following code:
G00 T909
M24
G97 S100 M03
G00 G54 X3.06 Z0.2
G76 X3.5251 Z-0.5 K0.1365 D0.04 F1.2
G00 X3.06 Z0.4
G76 X3.5251 Z-0.5 K0.1365 D0.04 F1.2
G00 X3.06 Z0.6
G76 X3.5251 Z-0.5 K0.1365 D0.04 F1.2
G00 X3.06 Z0.8
G76 X3.5251 Z-0.5 K0.1365 D0.04 F1.2
G00 X3.06 Z1.
G76 X3.5251 Z-0.5 K0.1365 D0.04 F1.2
G00 X3.06 Z1.2
G76 X3.5251 Z-0.5 K0.1365 D0.04 F1.2
G00 X3.06

From research I thought feedrate = # of starts/TPI
Z stepover = pitch

But when I try to screw in (Plastic bottle) into these female threads it "DON'T SCREW!!"

I have even tried threading with just one start with feedrate of F0.2 and they thing screws in perfectly.
I have tried slower RPMs thinking that 120ipm is too fast but this does nothing.
Any help sure would be appreciated.
Brian

2. Diamond
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The visualization problem, as I see it, is that you counted 5 intervals in 1 inch, but none of these intervals are yet 360° of rotation. To determine a true lead, you need to know the length one start traverses in 360°

Each start is necessarily 60° apart.

Therefore the 5th start is 5/6th of the lead because it is behind the 6th start, and is only 5/6 of a turn completed at the 1 inch mark. So the real lead (feed/rev) is:
1/(5/6*5/6) = 1.44"

The distance between Z start positions is 1.44/6 = .240

I don't believe it myself, so if I'm wrong, I am prepared to eat crow

3. If you are trying to make 5 threads per inch, shouldn't the feed rate be 1.0?

1 inch divided by 5 is .2

The six starts determine the six different starting positions of Z, which you seem to have correct.

4. Plastic
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Thanks guys for your input, but unfortunately, neither worked. I tried the 1ipr feedrate and 1.44. The 1" gave me a five start thread as the sixth pass just chased an existing thread.

I am at my witts end. Any more suggestions?

5. Plastic
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okay, time to ask a stupid question.

If you have a female multistart thread, will a male single start thread screw into the part?

I thought you can screw say a 3/4-16 single start screw into a 4 start female 3/4-16 threaded hole.

6. Diamond
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Originally Posted by pmcbh
Thanks guys for your input, but unfortunately, neither worked. I tried the 1ipr feedrate and 1.44. The 1" gave me a five start thread as the sixth pass just chased an existing thread.

I am at my witts end. Any more suggestions?
Did you adjust the Z start points to a .240 interval in the F1.44 test? Just checking What did you actually see happen in this test?

Are you certain that your thread data is correct? Are you working from a blueprint spec or reverse engineering from a part?

7. Aluminum
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Brian,

I posted an example of multistart threading to the macro programming sticky
a while back.

http://www.practicalmachinist.com/vb...62-post83.html

That one is for OD threading. I have a matching example at:

You can look over the macro math to see how to figure your pitch and
your Z offset for each start. Basically, your pitch for the G76 cycle is
#starts * basic thread pitch. So for your 5TPI six start, you want 6/5 = 1.2 .
You are going to need to start waaay off the end of your part to get accelerated
up to the feed you need for that coarse thread. Your start position in Z
should be basic pitch * start # + 3 * basic pitch * #starts, or for your case
0.2*start # + 3.6 .

-- Ralph

8. Stainless
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Feed rate should be correct at 1.2"

Start spacing I thought I had figured out but now that I think on it more it's wrong. (I thought a start spacing of .1714" should do it, 1.2"/7)

JP

9. Originally Posted by pmcbh
okay, time to ask a stupid question.

If you have a female multistart thread, will a male single start thread screw into the part?

I thought you can screw say a 3/4-16 single start screw into a 4 start female 3/4-16 threaded hole.
No, the helix angle changes with each lead. I visualize a 4 lead 3/4"-16 as 4 separate 3/4"-4 threads.

10. Diamond
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Originally Posted by JP Machining
Feed rate should be correct at 1.2"

Start spacing I thought I had figured out but now that I think on it more it's wrong. (I thought a start spacing of .1714" should do it, 1.2"/7)

JP
The way I figure it, if it is a 6 start thread, but only 5 intervals fit in the first inch, then the 5th start has not yet advanced a full revolution. The fact that it happens to fall on the inch mark is misleading.

Obviously, if a 5 start thread had 5 intervals in the first inch, the Z spacing would be .2

If a 6 start thread ends at 1.2, then it's spacing is also .2 which leads to a logical impossibility, that 6 start has the same lead as 5 starts.

For the love of god, will someone who is familiar with metric solve this for us?

11. Stainless
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There's no way to program thread starts in polar angles is there????????

12. Plastic
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G0 z.4 x3.625
g76 p010000 q30 r10
g76 x3.2751 z-.56 p1249 q30 f1.2
g0 z.6
x3.625
g76 p010000 q30 r10
g76 x3.2751 z-.56 p1249 q30 f1.2
g0 z1.
X3.625
g76 p010000 q30 r10
g76 x3.2751 z-.56 p1249 q30 f1.2
g0 z1.6
x3.625
g76 p010000 q30 r10
g76 x3.2751 z-.56 p1249 q30 f1.2
g0 z2.4
x3.625
g76 p010000 q30 r10
g76 x3.2751 z-.56 p1249 q30 f1.2
g0 z3.4
x3.625
g76 p010000 q30 r10
g76 x3.2751 z-.56 p1249 q30 f1.2
x2.5
z2.5 m9

13. Are you sure the numbers are correct. The pitch necessary to cut the thread is 0.8333. You must then cut that 6 times, but you run out of room. The number of leads must be less than the number of threads per inch, I may have to think about that for a minute. This is a 4 lead 5 TPI thread. The distance from peak to peak is 0.200.

14. Plastic
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okay, problem solved, actually adverted. My customer says when I call him "HELL I just using this as a prototype so yeah a single start thread is just fine"

So we are making some chips on single start parts.

But concerning the 6 start 5 tpi, I thank all of you for your responses

One thing I think we will all agree upon is that Lead = pitch * # of starts.

So my lead was 1.2" and pitch was definately 0.2" But where else I went wrong I have no idea.
I even tried the suggestion above to move starting Z points way off part and still no luck.

15. Diamond
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It would still be interesting to know where the error lies. I wonder if there was an upper limit to the permitted feedrate, and that was being exceeded and so it was 'clamped' ,as they say, at some limit? One would think it would throw an alarm of some sort.......but maybe not.

16. No idea where the "metric" part is in this equassion. ???

I have never made a multi-start thread (that I can remember enyway), but my best guess would be along the lines of a lead of 1.2 (I agree) but with a Z offset of .166666666 diff each time.

No idea how a 6 start thread would thread into a 1 start thread - but whatever...

---------------

Think Snow Eh!
Ox

17. Cast Iron
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Depending on the rpm that was run, it is possible to max out the encoder and it will give the best it can. I had to run an insane helix on a part that was 21 inch pitch, ran a quick program to cut air and see if any alarms popped up. All was good, quoted it, got it, ran first piece and all I could get was a 14 inch pitch. Called the HAAS apps engineer and he explained that most controls have no high end alarm, in theory it was possible, just no encoders known to man could keep count with the rpm running so low and the screw going so fast.
Terry
P.S. I did complete the part in a manual lathe with a few modifications, then they kept changing the pitch, I think the last one was in the 36" range

18. Originally Posted by Bellindustries
Depending on the rpm that was run, it is possible to max out the encoder and it will give the best it can. I had to run an insane helix on a part that was 21 inch pitch, ran a quick program to cut air and see if any alarms popped up. All was good, quoted it, got it, ran first piece and all I could get was a 14 inch pitch. Called the HAAS apps engineer and he explained that most controls have no high end alarm, in theory it was possible, just no encoders known to man could keep count with the rpm running so low and the screw going so fast.
Terry
P.S. I did complete the part in a manual lathe with a few modifications, then they kept changing the pitch, I think the last one was in the 36" range

V shaped slitting saw type blade in Z axis live tool holder interping in C would git where you wanted to go.

----------------

Think Snow Eh!
Ox

19. Cast Iron
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Unfortunately wasn't an option. Biggest problem was internal helix on a 13" long 3/8 wall tube 6" id. not much wiggle room. Actually the bigger problem was no live tooling.
Terry

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