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Old 04-25-2012, 04:56 PM   #1
rhitee93
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Default Cutting small threads on a big lathe

I was trying to cut some 5-40 threads on a rod the other night. The threaded section was about 0.5" long so I really needed to support the part with a live center. However the part diameter was so small I couldn't get a decent center hole.

Is there an old machinists trick for doing this, or do I just need to find a smaller center and center drill combo?

I tired to cut the threads without support, but there was too much spring. In the end I just ran a die up on the shaft.



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Old 04-25-2012, 05:01 PM   #2
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Default Re: Cutting small threads on a big lathe

Something I've done in the past. It worked though I still consider it a dodge.

Make a bushing with a hole that's a good running fit on your part. Mount the bushing in the tailstock chuck. Support the outboard end of the part to be threaded with the bushing.

Personally, I use dies for stuff as small as 5-40.



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Old 04-25-2012, 06:58 PM   #3
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Default Re: Cutting small threads on a big lathe

Spring cuts with a dead sharp tool that is on center and aligned properly are needed for small threads.


I did a little write up about cutting small threads here:

http://www.homemodelenginemachinist.com/index.php?topic=14637.msg150812#msg150812

In the thread some ideas came up about stopping deflection.

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Old 04-25-2012, 08:28 PM   #4
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Default Re: Cutting small threads on a big lathe

Cool post. In hind sight, I don't think the tool I was using was really sharp enough. I couldn't take enough spring passes to ever get rid of the tool pressure.

I should have been pretty close to dead on center, but with a piece this small, any error in setup is significant. I'll check all this and try again. You have inspired me to not give up on this

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Old 04-25-2012, 11:29 PM   #5
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Default Re: Cutting small threads on a big lathe

Hello Brian,

I have attached a PDF of a diagram that I just drew to illustrate how I go about just such a job. The trick is to add some material on the end of the part in order to allow the use of a center for support. For a job of this size, I would allow 1/4" for the center and a 1/4" length of a diameter reduced to the minor diameter of the thread. Please see the attached diagram. In the diagram, the stock size is .250" diameter and the threading tool is 1/4" square HSS for reference, everything else is to your description -- a 5-40 NC external thread x 1/2" long. The 60 degree angle is not critical, just consider that a maximum as it is parallel to the angle of the center. I would typically just use the 30 degree end cutting edge angle of my RH turning tool for the angular relief as it matches the angle of the threading tool.

There is one disadvantage to this method and that is that you can't "cut and try' with your mating part. You must measure the pitch diameter of the thread either with thread measuring wires or a thread pitch micrometer. There is a way around it by accurately turning the relief area immediately to the right of your part to the thread's minor diameter and using the correct flat on the threading tool (.003" for 5-40 NC 3A). This will indicate when the thread is at the proper depth of cut as the tool flat touches the minor diameter provided that the threading tool has a 60 degree angle and the proper width flat on the tip . Thread measuring wires are not expensive and they are not difficult to use, certainly a worthwhile addition to the toolbox.

I am not trying to over complicate this problem, just show a possible solution. This is the way I have done this type of job many, many times. If you must single point a very small thread on a fragile part, this method works great. It may be easier to cut the thread with an adjustable die holder that is supported by your tailstock.

If you would like additional information on threading formulas and measuring the pitch diameter with wires, I will be happy to provide that information.

I hope someone finds this method useful.

Regards,
Mike

Attached Files
File Type: pdf Addition Of Material For Center Support.pdf (74.3 KB, 178 views)
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Old 04-26-2012, 12:59 AM   #6
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Default Re: Cutting small threads on a big lathe

Mike,

Thank you for taking the time to make that diagram. That is another one to file away in my notebook. I have never used wires to measure thread depth, but I have seen it done. I have never tried to be precise when I cut threads, and just use the cut and check method. Some day I should try to do it by the numbers to prove to myself that it works.

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Old 04-26-2012, 11:28 AM   #7
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Default Re: Cutting small threads on a big lathe

Quote:
Originally Posted by rhitee93
I was trying to cut some 5-40 threads on a rod the other night. The threaded section was about 0.5" long so I really needed to support the part with a live center. However the part diameter was so small I couldn't get a decent center hole.

Is there an old machinists trick for doing this, or do I just need to find a smaller center and center drill combo?

I tired to cut the threads without support, but there was too much spring. In the end I just ran a die up on the shaft.
Hi,

My suggestion is that you start the thread on the lathe using really light cuts with a super sharp tool and lots of lubrication, once threads start forming but before you get in to the heavy pressure stage of the cutting you change over to a die . I am not a fan of solely using dies on small diameter stock for threads if the concentricity is critical as a whole lot depends on the alignment of the tailstock-die holder combination and the way the die sits in the holder ( has anyone indicated a small die in a holder? ), so I think the above method is perhaps a good compromise. As you may know you could also angle the topslide to 29.5 degrees to cut the thread to persuade the tool to cut on the front side and reduce the cutting pressure.

Regards,

A.G
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Old 04-26-2012, 12:04 PM   #8
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Default Re: Cutting small threads on a big lathe

Thanks A.G.,

Your concerns about starting a small die square is what motivated me to try to single point cut the threads on this part. To be honest I was just hoping to get enough of a start that I could chase a die up on them later.

I always set my compound at 29.5 deg when threading. I didn't know there was any other option (Although I did know that this was to keep the pressure on the front side of the tool)

I don't have a die holder for my lathe. My solution was to hold the die in a 1/2" collet in the headstock, and hold the rod in the drill chuck in the tailstock. Looked goofy, but it worked.

All in all, this was a quick and dirty piston rod to check the alignment of the running gear in my PMR #1 build. I plan to build a nicer one from better looking stock so I'll get a chance to do this all over again soon.

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Old 04-26-2012, 11:22 PM   #9
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Default Re: Cutting small threads on a big lathe

Quote:
Originally Posted by rhitee93
I was trying to cut some 5-40 threads on a rod the other night. The threaded section was about 0.5" long so I really needed to support the part with a live center. However the part diameter was so small I couldn't get a decent center hole.

Is there an old machinists trick for doing this, or do I just need to find a smaller center and center drill combo?

I tired to cut the threads without support, but there was too much spring. In the end I just ran a die up on the shaft.
Turn a smaller diameter at the end ,say .060 x .187 long. Bore a hole in round scrap 1/16"x 1/4" long.Chuck this in the tailstock. A drop of oil on the end of your part and you are ready to go. You can part off the excess or leave it on if it does not interfere.
Might be better to cut the excess off with side cutters and sand to dimension.Parting this screw off might bend it.
mike
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Old 04-26-2012, 11:27 PM   #10
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Default Re: Cutting small threads on a big lathe

You may find this thread helpful.

http://www.homemodelenginemachinist.com/index.php?topic=4220.0

Rick



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