Internal Thread Measurements

[epanzella]

I'm making a tool to pull the propshaft on my boat motor. I needed to make an internal 18 x 1.5mm thread and couldn't bring the mating part to my lathe. I took careful measurements of the male propshaft threads using wires and made a male plug gauge to help me make the internal thread. The female threads came out perfect but this whole process took 5 or 6 hours. I can't use my halfnuts for metric work. Is there a more efficient way to single point internal threads accurately besides making a male gauge?

 

[darkzero]

I can't use my halfnuts for metric work.

Oh but you can. I only use this method now for metric threads & it works great. Post #12
http://www.hobby-machinist.com/threads/cutting-metric-threads.22504/#post-199388

Is there a more efficient way to single point internal threads accurately besides making a male gauge?

I'm not aware of any "affordable" solutions for measuring internal threads. More efficient way is to buy go & no-go plug guages rather than make them. Those suckers are not cheap though depending on the size. I only own like 3 of them but only cause I found em cheap on ebay.

 

[4GSR]

If your lucky and have a QCGB set up to cut 17 TPI or change gear combination to cut 17 TPI. Your lead error per inch is about .0002". I call that good in my book. If your not satisfied with that, Chase the thread near to size and clean out the last thousandth or two with a correct tap.

Edit: Oops, I didn't realize that 17 pitch is not standard with most QCGB's. It can be easy to come up with it if you have some change gears to play around with. Usually change tooth count by two teeth on the input gear into the QCGB will change the pitch by one.

 

[RJSakowski]

It sounds like you took the right approach. Sometimes there just aren't any shortcuts. I have spent hours making the right tool to do a one minute job but it was the right tool for the job and trying to make do can end up with a lot of frustration and possibly a damaged part.

There is a facsimile material which can be used to duplicate a mating part but in your case it would have to be a mate to the nut and you would have to make an assumption about the class of fit. https://www.mscdirect.com/browse/tn...=Casting+&+Facsimile+Compounds&navid=12101772

You possibly could make a test piece by casting solder or other low melting point alloy in the nut to make a replica of the thread.If the OEM nut is a Nyloc nut, I would obtain a spare nut.

Or you could purchase an M18-1.5mm screw from McMaster and compare the thread to your prop shaft and adjust your fit accordingly.

Regarding using the half nuts when cutting metric threads with an Imperial lead screw, I heartily recommend Tom Lipton's (OxTool's) method. It saves a lot of time, especially when cutting long threads.

 

[epanzella]

I was hoping there was some way to measure internal threads without some fancy $250 mike or buying expensive plugs or taps for an oddball thread that I would only use once. I needed to do this yesterday and it sounds like I did what had to be done. Anyway, now I have an 18x1.5mm thread gauge for my gadget drawer. I save all my one-off fixtures, bushings and gizmos and once in a blue moon I get to reuse one of them. Thanks for all the replies.

 

[4GSR]

I was hoping there was some way to measure internal threads without some fancy $250 mike or buying expensive plugs or taps for an oddball thread that I would only use once. I needed to do this yesterday and it sounds like I did what had to be done. Anyway, now I have an 18x1.5mm thread gauge for my gadget drawer. I save all my one-off fixtures, bushings and gizmos and once in a blue moon I get to reuse one of them. Thanks for all the replies.

I've done the exact same thing over the years, make a soft thread gage to gage a thread with. When we cleaned out dad's shop years back, we probably threw out fifty different thread gages along with many fixtures and sort. They were all made for jobs that most were onesies and twosies. I don't recall in the 25 years a single one of them were used ever again except for one! And it was undersize. Finally made another gage to take its place. And for the job it was for, we quit doing about 8 years ago.

 

[epanzella]

It sounds like you took the right approach. Sometimes there just aren't any shortcuts. I have spent hours making the right tool to do a one minute job but it was the right tool for the job and trying to make do can end up with a lot of frustration and possibly a damaged part.

Regarding using the half nuts when cutting metric threads with an Imperial lead screw, I heartily recommend Tom Lipton's (OxTool's) method. It saves a lot of time, especially when cutting long threads.

I'm familiar with the OX Tool method but the way I understand it, it allows you to kick out the halfnuts at the shoulder end, but you need re-engage them and return the tool for the next cut using the lathe reverse which is still a time killer.

 

[RJSakowski]

I'm familiar with the OX Tool method but the way I understand it, it allows you to kick out the halfnuts at the shoulder end, but you need re-engage them and return the tool for the next cut using the lathe reverse which is still a time killer.

Yes, you are correct in that the half nuts must be engaged for reversing, essentially doubling the time to cut a thread.

It's been a while since I cut a metric thread and the old bean gets foggy sometimes.

 

[WoodBee]

Yes, you are correct in that the half nuts must be engaged for reversing, essentially doubling the time to cut a thread.

It's been a while since I cut a metric thread and the old bean gets foggy sometimes.

If you can come up with a way to change the spindle speed quickly (vfd?) You can cut down on this time. Something like a "quick feed" switch. Since thread cutting is usually done on fairly low spindle speeds there should not be many drawbacks I think. I am planning on adding this to my vfd once I get to fitting one to my lathe.
Peter

 

[epanzella]

If you can come up with a way to change the spindle speed quickly (vfd?) You can cut down on this time. Something like a "quick feed" switch. Since thread cutting is usually done on fairly low spindle speeds there should not be many drawbacks I think. I am planning on adding this to my vfd once I get to fitting one to my lathe.
Peter

A VFD would be a good way to speed up the return. My lathe is a gear head so I wan't sure if changing speed would lose the indexing between the spindle and the lead screw.