Help with ACME thread

[olympictool1]

Hey all, I have an 8 TPI acme thread that we are putting on to a part that a cap screws on. The customer did not supply a print so i spec'd it out to a 8 TPI acme, everything on the part measures to be a standard 8 acme but no matter what we do while making the the thread the mating part will not fit on. We are threading on the sub spindle of our cnc lathe so we have tried both handed tools and directions, multiple angles of chamfer at the start, dropping the minor and major to the point you would think the cap would just fall on along with tons of other things. I have never had an issue with an acme thread like i am having with this. Any in sight would be greatly appreciated, maybe i am missing something simple and just not realizing it.

 

[dsjansen]

Forgive my inexperience, but are you sure it's an acme thread and not a trapezoidal (metric) thread?

 

[awander]

It's not a multi-start thread, is it?

 

[Bobw]

Have you measured the pitch diameter?

 

[CarbideBob]

Is it a stub?

 

[angelw]

Hey all, I have an 8 TPI acme thread that we are putting on to a part that a cap screws on. The customer did not supply a print so i spec'd it out to a 8 TPI acme, everything on the part measures to be a standard 8 acme but no matter what we do while making the the thread the mating part will not fit on. We are threading on the sub spindle of our cnc lathe so we have tried both handed tools and directions, multiple angles of chamfer at the start, dropping the minor and major to the point you would think the cap would just fall on along with tons of other things. I have never had an issue with an acme thread like i am having with this. Any in sight would be greatly appreciated, maybe i am missing something simple and just not realizing it.

Hello olympictool1,
To get a real handle on the specs of the Internal Thread:

1. Ensure there are no burs and hangups on the Internal Thread.

2. Plug the through hole up to the bottom of the Thread with Plasticine.

3. apply a good release agent to the Internal Thread and cast a mating Male Thread using Epoxy Resin, say Devcon.

4. To ensure you get a good model of the Thread, paint and force the epoxy into the Thread before filling the cavity.

5. Place a short piece of Hex Bar in the centre, with end resting on the Plasticine and pour the epoxy into the cavity, around the Hex Bar. The Hex Bar will provide a means of unscrewing the cast, threaded plug.

6. Also use release agent on the Hex Bar so that it can be removed, allowing the plug to be cut in half to look at the Thread Profile using a Shadowgraph, or other means that may be available to you.

7. Measure the Pitch Diameter and any other aspect you can whilst the plug is in one piece should you decide to have the Thread Profile looked at where the plug will be destroyed.

Regards,

Bill

 

[olympictool1]

There are 6 other threads on part that are all imperial threads so i would be surprised (though not unheard of)if they were metric. Also on our optical the threads are showing 14.56 degrees from center and metric should be 15 It probably couldn't hurt to try a metric insert if all else fails.

 

[olympictool1]

that's pretty awesome and i am going to definitely remember that, that's a great idea. The thing is i already have a male/plug gauge with the original part. Both parts are measuring the same so there has to be something else going on/

 

[olympictool1]

It's not a multi-start thread, is it?

i checked that

 

[olympictool1]

Have you measured the pitch diameter?

checked, ran in to spec, under spec, and identical to mfr part.

 

[olympictool1]

Is it a stub?

also checked, crest width is 1/2 depth

 

[angelw]

checked, ran in to spec, under spec, and identical to mfr part.

Hello olympictool1,
If the Major and Minor diameters of the Male are cut to be a slight clearance fit with the respective diameters of the Female (Cap), by continuing to take side cuts on the Flanks of the Thread, thus decreasing the Pitch Diameter, provided that you're cutting the correct Lead and correct Thread Profile, at some point the two parts must assemble.

Keep going with this process and you will end up with a piece of bar turned parallel at Minor Diameter with no Thread, where the cap will fall on. Accordingly, there will be a point where the parts must assemble as Threaded components. If at that point the Pitch Diameter is way smaller than that of the Plug Gauge that fits the part, then there is something wrong with regards to the Profile, or Lead that you're running.

Regards,

Bill

 

[Stihl290]

Is the wear of the anodize on the minor diameter of the cap normal?

Make me kinda think the material deformed into the thread if you are using the cap pictured to test fit your part.

 

[olympictool1]

It's not a multi-start thread, is it?

Ok so i did miss something. It is a double start ACME, i couldn't see the second start because the starts land on cuts outs of the thread and i didn't even notice it. So that being said with a 8 TPI (please correct me because i think i am wrong) If you were to start the first pass at z0. the second start should be at z.0625?, it all my years i have never done a double start thread, never had a part that needed it so i am a bit rusty on the math.

 

[angelw]

Ok so i did miss something. It is a double start ACME, i couldn't see the second start because the starts land on cuts outs of the thread and i didn't even notice it. So that being said with a 8 TPI (please correct me because i think i am wrong) If you were to start the first pass at z0. the second start should be at z.0625?, it all my years i have never done a double start thread, never had a part that needed it so i am a bit rusty on the math.

Hello olympictool1,
Given that you missed the fact that the Thread is a Two Start Thread, I suspect that you have measured the Pitch of the Thread. In a Single Start Thread, the Pitch is the same as the Lead. Therefore, for a Single Start Thread, the Pitch and the Lead are equal as shown in the following example:

Lead=Pitch X Number of Starts
Lead=0.125 X 1
Lead=0.125

In a Two Start Thread the following would apply:

Lead=Pitch X Number of Starts
Lead=0.125 X 2
Lead=0.25

If you have measured the Pitch of the Thread, you should be cutting a 0.25 Lead for each of the Thread Starts and moving the Z Start Position of the Threading Cycle by 0.125 relative to the Z Start of the First Start to cut the Second Start.

For the move to be only 0.0625, you would have had to measure 0.125 (8TPI) over two Thread Forms.

Regards,

Bill

 

[olympictool1]

Hello olympictool1,
Given that you missed the fact that the Thread is a Two Start Thread, I suspect that you have measured the Pitch of the Thread. In a Single Start Thread, the Pitch is the same as the Lead. Therefore, for a Single Start Thread, the Pitch and the Lead are equal as shown in the following example:

Lead=Pitch X Number of Starts
Lead=0.125 X 1
Lead=0.125

In a Two Start Thread the following would apply:

Lead=Pitch X Number of Starts
Lead=0.125 X 2
Lead=0.25

If you have measured the Pitch of the Thread, you should be cutting a 0.25 Lead for each of the Thread Starts and moving the Z Start Position of the Threading Cycle by 0.125 relative to the Z Start of the First Start to cut the Second Start.

For the move to be only 0.0625, you would have had to measure 0.125 (8TPI) over two Thread Forms.

Regards,

Bill

Ok its coming back to me now, thank you so much. Yes with a physical thread gage the pitch is 8 TPI. (.25 over 2 thread forms)
Thank you again, i am dialing it in now but i think i've got it. I will update

 

[Vancbiker]

With a multi-start thread the helix angle is steeper so you may get some rubbing on your insert since it's clearance angles were likely designed for a single start thread.

 

[olympictool1]

With a multi-start thread the helix angle is steeper so you may get some rubbing on your insert since it's clearance angles were likely designed for a single start thread.

very interesting i will have to compare the new helix angle to this insert. I ordered a different insert last night and we are trying now.

 

[olympictool1]

Somethings just not quite right. We fixed the code for a double start and put in a new insert so we can control the PD (with out cutting the major at the same time) and it seems like it really wants to go, except its like its only catching one start. Every time we screw on the cap its crooked. Any insight? (PICTURE BELOW)



this is a 2 1/8 - 8 ACME
i am using the thread data as follows from a calculator (Original part falls right in the middle of all of these specs)
MAJOR 2.125 - 2.1188
MINOR 1.9800 - 1.9510
P.D. 2.0508 - 2.0315

Here is the code maybe someone sees what I am doing wrong:
(THREAD START #1)
N12
(OD THREAD)
(ACME 2 1/8 -8)
G28V0
M69
G99G18M46
G50S3000
G55
G0T1252
M611
G97S500M203
G0G99G55X2.4Y0.
Z-.125
M8
G92X2.120Z.8F.125
X2.115
X2.110
X2.105
X2.100
X2.095
X2.090
X2.085
X2.080
X2.075
X2.070
X2.065
X2.060
X2.055
X2.050
X2.045
X2.040
X2.035
X2.030
X2.025
X2.020
X2.015
X2.010
X2.005
X2.000
X1.995
X1.990
X1.985
X1.980
X1.975
X1.970
X1.965
G0X2.5
Z-1.0
G28V0
G30U0W0
M610
M01





(THREAD START #2)
N12
(OD THREAD)
(ACME 2 1/8 -8)
G28V0
M69
G99G18M46
G50S3000
G55
G0T1252
M611
G97S500M203
G0G99G55X2.4Y0.
Z-.25
M8
G92X2.120Z.675F.125
X2.115
X2.110
X2.105
X2.100
X2.095
X2.090
X2.085
X2.080
X2.075
X2.070
X2.065
X2.060
X2.055
X2.050
X2.045
X2.040
X2.035
X2.030
X2.025
X2.020
X2.015
X2.010
X2.005
X2.000
X1.995
X1.990
X1.985
X1.980
X1.975
X1.970
X1.965
G0X2.5
Z-1.0
G28V0
G30U0W0
M610
M01

 

[Dan from Oakland]

I doubt that's a true ACME thread and more likely a bottle thread. They are not the same. try this and see if your sample matches up.

cap guide for fitting closures on bottles