How to measure internal spindle thread

[Grahame Collins]

Hi Guys,
Someone on Forum , I recall has the same lathe as I do, a 12 x 36 Metal master.

I want to make a spider to fit the outboard end of the spindle which has a 38mm bore.

Pitch gauges are near impossible to read on an internal thread, so I was going to oil the thread and do cast of said threads with some Plasti bond.

Hopefully there's a better way that some kind soul might outline for me.

I have not cut a thread on it yet,so this will be a good exercise.

Thanks
Grahame

[Dave J]

Hi Grahame,
If it like all the rest it would be 42 x 1.5mm measuring 41.66mm outside.

Dave

[Grahame Collins]

Thanks Dave,
The threads appears to have no crowns,rather flats.

Probably just crappy machining on my model

Cheers mate

[RayG]

Hi Graeme, Dave,

Mine's M40x1.5 which is strange that it would be smaller than Dave's since I thought we had pretty much the same model?

Regards
Ray

[Dave J]

Probably best to thread a piece of scrap pipe or something first just to check, as I don't think they have any standards.

Dave

[Grahame Collins]

I have just finished reading that the spindle sizes of the 12 x36 Chinese models is all of the place and there did not appear to be a standard regards to the ob spindle thread.

Mine appears to be around 40mm but the flats on the thread crown bugger the maths up.

I was thinking about screwing some soft timber(maybe balsa) in the thread to make a thread form.
Grahame

[morrisman]

You may have to make a gauge out of aluminium .... that matches the thread dimensions on your spindle. You then use the gauge to check the internal threading as you proceed . I did this while making a back plate for a chuck ( haven't finished that yet ) . In my case it was a tricky job cos the spindle thread is tapered on my 50 year old lathe . MIKE

[Dave J]

Hi Ray,
I just checked when I locked the shed, and my thread looks fully formed and is around 39.5-6 mm inside.

Grahame
If your thread has flats it might be the same as mine.


Dave

[RayG]

Hi Dave,

Yes, mine is the same, the Black Book tables I've got for Metric Fine only go to M30 x2.0, but (ISO) Metric Conduit has M40 x 1.5, so maybe that's the one?

On the other hand, there's no reason for them to choose a standard thread, they could be anything..

Regards
Ray

[Dave J]

Hi Ray,
I am not sure about Grahame's, but it looks like ours is 41x1.5mm thread. Our numbers seem to fall right in between the 40 x 1.5 and 42x1.5mm threads in the chart below.
MARYLAND METRICS -- THREAD DATA CHARTS (4)

If it was 40mm thread it would only have around 0.25 thread depth (40 - 39.50 = 0.5/2 = 0.25.

I have a spider down the shed I made a few years ago but couldn't find it, so I grabbed those numbers on line above. When I made it I just did it by trial and error, but thought it was 42mm for some reason.

Dave

[Grahame Collins]

I have just measured the ID with an internal mike and it is
40.03 from from crown to crown.

I will know more when I check (my) thread depth and am assuming I should get that from my PlastiBond cast.

If the thread depth turns out to be 0.25mm that will give me a male thread diameter of 39 .8 40.03 -2x (0.25mm) or is there a tolerance for the fit between male thread od and the female thread ?

All my threading up to this point has been tap and die stuff so this is new to me.

Thanks
Grahame

[pipeclay]

There are thread tollerances but for what you are making it probably wouldnt matter.
If you like do a search for metric thread specs,this should give you an idea of the tollerance range.

[the fabricator]

i was taught this awesome tip that helped me get some accurate measurments on an internal groove i had to manufacture to a sample i was given at work

get a peice of lead wire, or playdoh or anything that squishes in and holds it shape, press it into the threads, carefully peel it out ( this is where its probably better to use lead wire ) and then you have a good reference for measuring pitch, thread form etc )

[nadroj]

You could try a wooden dowel, pencil or similar to rub on the internal thread.
The witness marks can be measured. OK for coarse threads.

Jordan

[franco]

I have just measured the ID with an internal mike and it is

40.03 from from crown to crown.

If the thread depth turns out to be 0.25mm that will give me a male thread diameter of 39 .8 40.03 -2x (0.25mm) or is there a tolerance for the fit between male thread od and the female thread ?

Grahame

Hi Grahame,

Firstly, 40.03 - 0.50 = 39.53 (not 39.8)

If the internal thread is 40.03 crown to crown, won't the male thread major diameter need to be bigger by twice the thread depth, i.e.40.53 mm (assuming 0.25 mm thread depth)? The 39.53 mm diameter would be a loose sliding fit in the 40.03 mm spindle thread minor diameter.

Frank.