Threading again

[morrisman]

Ok I have made progress in my 1 and 3/4" BSF threading .... The tool post is now more rigid and its cutting the thread well . I milled a 1/4' slot in the tool post column and used a bit of HSS as a key . I also milled the column to lock it into the T nut ..PICS explain. I actually removed the piece and tried it on the spindle ..too tight . After fiddling around I managed to pick up the thread again after putting it back in the chuck :oo: What i find difficult is: how do you judge the thread depth ... MIKE

[Dave J]

With taking it out, just leave it in the chuck and turn the whole lot around and it will still be lined up. I do this when doing ones for the D1-4 for my lathe, and it works well.

Dave

[morrisman]

With taking it out, just leave it in the chuck and turn the whole lot around and it will still be lined up. I do this when doing ones for the D1-4 for my lathe, and it works well.

Dave

Ah I never thought of that :doh: Next, I am going to make a test piece, the same size as the spindle male thread - hopefully it will be handy to check the internal threading as I go along. I am very pleased the second attempt at this large internal BSF thread has worked out OK .... learning as you go :2tsup:MIKE

[pipeclay]

What do you mean how do you judge the depth?
Have you calculated the depth for angular approach or for plunge approach?

[Michael G]

A couple of things that I do -


Make enough spring cuts at your maximum depth so that nothing is coming off the tool. As you approach finished depth, do some spring cuts there too, to lessen the work the tool has to do at full depth
Make your part a little longer and have a recess in the front the same diameter as your thread major diameter. When your tool is just kissing that you should be on size
Make up a stub male thread and use that as a gauge - male threads are easier to cut accurately as they are external and the tooling can be rigid. A gauging thread can be brass or something else easy to cut (free machining Al or steel)
use some emery or a cutting tool set up at the minor diameter to remove any burr thrown up while cutting
If you have thread pitch gauges of the correct form (or a tap of the right pitch and form) use that and a torch to see whether there are light gaps between the cut form and the thing you are comparing to. The usual problem is lack of depth - this test may show this.

Michael

[morrisman]

What do you mean how do you judge the depth?
Have you calculated the depth for angular approach or for plunge approach?

Well, if cutting an outside thread , you can use the 3 wire method . But with internal threads its more difficult , as least for me it is . I suppose I should be looking up the correct depth in a chart somewhere ... Where do you find a BSF thread chart showing depth ? The cross feed and compound dials on my lathe are rather hard to read .... they are old and worn , and small too :oo:..Thanks for the tips Michael, very informative

[Michael G]

A Zeus book has BSF information in it

Use a mag base dial indicator to show your cross feed if your dials are not good enough

Michael

[Bryan]

Hi Stuart
Correct
The general rule of thumb is
55 degrees is 0.64 x pitch
60 degrees is 0.61 x pitch
So 55 degrees is deeper.

Phil
.

[pipeclay]

The angular deth/offset method should be .137" or if plunging .091".
At times if plunging you will have to advance your comound to widen the thread slightly.
If using the angular/offset method you would generally on your last cut/s advance your crosslide around .001"/.002" rather than just set it at zero.
When you cut your threads how much depth is your 1st cut and then what are your subsequent depths.

[Dave J]

An easy way for thread depth for either internal or external is to touch the tool on the surface and zero both the cross slide and the compound dials. Then move the tool away from the job and wind the cross slide in or out (depending on whether int or ext thread) to the required depth of the thread (remembering to take out backlash) then re set the cross slide to zero.
You can then wind the compound in or out then start thread cutting working your way to zero on the compound dial. When you withdraw the cross slide at the end of each cut, just re zero it as usual.

Dave
Edit
If you are plunge cutting, just work your way to zero on the cross slide dial and forget about the compound reading.

[Steamwhisperer]

Hi mike,
Have you thought about putting a block of steel under your tool post to take up the gap between it and the topslide. It should stop a lot of flex while screwcutting.

Phil

[morrisman]

thanks for the input ...... Ok I managed to find a set of whitworth thread gauges ...it checked out OK on 7 tpi so I took a chance and removed the piece from the chuck ..it fits perfectly . nice smooth fit . see pic MIKE

[morrisman]

thanks for the input ...... Ok I managed to find a set of whitworth thread gauges ...it checked out OK on 7 tpi so I took a chance and removed the piece from the chuck ..it fits perfectly . nice smooth fit . see pic MIKE

[Dave J]

Always a good day when a thread fits nicely, good work.:2tsup:

Dave

[morrisman]

The general rule of thumb is
55 degrees is 0.64 x pitch
60 degrees is 0.61 x pitch

So for 55 degree at 7tpi it would be about .091" depth ?

[Steamwhisperer]

The general rule of thumb is
55 degrees is 0.64 x pitch
60 degrees is 0.61 x pitch

So for 55 degree at 7tpi it would be about .091" depth ?

Hi Mike,
That would be correct. The only reason this would differ is for the different classes of fit but for general purpose those figures work well. If possible I always do the nut first and use it as a gauge.

Phil

[tanii51]

hmmm what is the class of thread used for a lathe chuck thread and spindle
john

[pipeclay]

More than likely it would be something like 3 or 4 G but it wouldnt really matter that much if it is 6G.
I think each number represents about .01mm clearance.

[Michael G]

The class of thread for a lathe spindle is not as significant as you might first think - Accurate location of the chuck on the spindle depends on a couple of register faces. One is a cylindrical surface adjacent to the thread which is usually the major diameter of the thread or a little larger; the other is a land at 90 degrees to that which is usually just slightly proud of the surface the spindle comes out of - the closer to the bearing the better. These two surfaces define the spindle locating points. The thread is mainly there to hold the chuck in place.

Michael