its 62mm minus thread depth x 2

The Guhring Tap Drill calculator for M62x1.25 says:  60.83mm diameter (for 70% thread engagement)

The depth of cut will be roughly 0.75mm (1.25×0.6),  not quite deep enough, so cut deeper than this in small steps, after each pass offering the thread up to the male until it screws on not too tight.  A shade loose is probably best for this application, because you don’t want the two jambed together!

This doesn’t quite agree with Bernhard, so wait and see what others say.   I’ve been known to get this stuff wrong!

Assuming you have a screw-cutting lathe?

Dave

+1 for the mandrel handle especially for the larger threads as it gives a much better finish.

that would be copaslip up here then Hopper? and definitely yes to the tap with a copper hammer.

On 15 June 2024 at 11:02 COLIN MARTIN 2 Said:

Thank you for the replies. The winding handle idea sounds good and much safer, I will have a look. The reason why I decided to make an extension to the extractor that I have is because if I make a new one there is a massive amount of machining to do to carve out the I/D of the tool, and I would need to drill and tap a substantial hole in the top for a large bolt. Swings and roundabouts really.

Returning to the bore size, can anyone give me a definitive size?

 

Thanks,

 

Colin

On 15 June 2024 at 08:25 bernard towers Said:

that would be copaslip up here then Hopper? and definitely yes to the tap with a copper hammer.

Indeed it would be. Or black jack would be ok too. (Graphite or moly grease!)

On 15 June 2024 at 11:02 COLIN MARTIN 2 Said:

Thank you for the replies. The winding handle idea sounds good and much safer, I will have a look. The reason why I decided to make an extension to the extractor that I have is because if I make a new one there is a massive amount of machining to do to carve out the I/D of the tool, and I would need to drill and tap a substantial hole in the top for a large bolt. Swings and roundabouts really.

Returning to the bore size, can anyone give me a definitive size?

 

Thanks,

 

Colin

60.5mm would do you. Then keep increasing your depth of cut when screwcutting until you can take the chuck off the lathe and screw the job onto the flywheel thread without disturbing it, so you can resume screw cutting if need be to finish it off.

60.8mm would give you a bit of clearance as SOD explained and at 75% thread engagement, loss of strength would be less than 5 per cent.

I’d suggest you start by cutting the male thread to fit into your existing puller first for practice, and male threads are generally easier. Once you have that sorted, you turn it inside out for the female thread.

Do you have stock of appropriate size to use for practice?  Doesn’t have to be the same material as your final piece.  If I had no experience single point threading, I’d definitely be doing some practice pieces.  Doesn’t have to be the same thread either, if that means doing your practicing on something of a different size or thread pitch.

I agree with Hopper, once you put some load on the extractor give the assembly a thump with a lead or copper faced mallet.

Definitely agree with using Copaslip or similar product on reassembly!

It may not be necessary to set the compound slide at an angle, but Sparey claimed that it made the job easier, since the tool cuts on one side only. I use this method all the time and have found that it has a lot going for it, plus it,s no big deal to set. Since the scale on my machine is not too easy to read, I made two simple templates, one for 60 deg, the other for57deg, to cover the threads I mostly use. This makes setting up simple.

On 15 June 2024 at 13:02 Hopper Said:

No you wont turn the chuck by hand to cut a thread. Unless you take tiny fairy cuts and spend a week doing it. Better to make a crank handle for the lathe spindle, which gives you leverage and control.

Depends on what lathe you have too. If you have backgear or genuine low rpm capability without losing torque, it is easy enough to cut threads at 30 to 100 rpm. Make your adaptor with the hole bored all the way through so you are not running up to a shoulder, or at least a bit deeper than you want the threaded section to be so you have a bit of leeway.

As well as boring all the way through I’d machine an undercut, then you can hear when the tool stops cutting and whack the half nuts open. If you put the tool upside down and cut on the far side the chips fall out better, and you put feed on by turning the handle clockwise, more intuitive. Even cleverer, although I’ve never tried it, is to run the lathe backwards so that the tool starts in the undercut and is running out into fresh air.

If you do angle the topslide (much better than just plunging in my book) remember that the tool advance is now greater because its running down at an angle.

Finally, if you’ve never done screwcutting before, you’d be well advised to do some practising by say cutting male threads on plastic rods, then if anything goes wrong, you can see it coming, and plastic is more forgiving

The crank handle goes in the left hand end of the main headstock spindle, sticking out where the change gears are. Sometimes called a mandrel handle or lathe spindle crank handle, there have been many designs published over the years. In essence it is a round plug that fits in the left hand end of the spindle, with a tapered hole up the middle and a tapered bolt (eg from a concrete anchor bolt etc). The plug is slit lengthways with a hacksaw so it can expand and grip the inside of the spindle. Crank handle bolts onto the plug.

Thread engagement is more of a shop floor operator decision than a specific standard. You can go down to 65 per cent engagement and still retain 95 per cent of strength. Is is the bases of the “pyramids” that bear the load, not the tips. See Tubal Cain’s book Model Engineers Handbook for specifics.

Re iterating what has already been posted, when makingb the internal mthread in the adaptor, mto screw onto the 62 x 1.25 mm flywheel, the bore should be 60.75 mm, but should be very slightly truncated to avoid root/peak interference, so the 60.83 mm for 70% engagement will be the dimension to which I would attempt to work. It will be worth making an undercut at the end of the thread into which the tool can run.

The external thread onto which the extractor will be screwed should be 60 mm x 1 mm pitch, again, very slightly truncated.  Kempe’s Engineers Year book says that the thread should be truncated by H/8, where H= 0.86603 p, so for a 1 mm pitch by 0.1083 mm, to 61.783 mm diameter.

If it needs to be said, start with a light cut, and gradually reduce the depth of cut as the thread progresses.

If you are new to screwcutting, (I am NOT expert!) My preferred method would be to withdraw the tool after every pass, and reverse, without disengaging the half nuts. That should remove backlash problems with the leadscrew, but ensure that you take out the backlash when readvancing the tool for the next cut.

It will take some torque to cutb then thgerad, so a Mandrel Handle will be needed. Unless back gear can be engaged, the lathe may lack nthe power, wnyway, and with such large diameters, the rotational speed needs to be low; another reason for driving the spindle by hand..

As Hopper says, lubricate the forcing screw well, (It should be large diameter, but of fine pitch, 1mm?) to maximise forces.  It might be worth centre drilling the end and inserting a ball bearing, (3 – 5 mm diameter?) to minimise friction.  Once fully tight, as he says, a tap on the end with a mallet is likely to jolt the  flywheel free.

If that doesn’t work, try applying heat from a heat gun to the outer edge of the flywheel. As the heat soaks through to the middle, the flywheel should loosen its grip on the cold shaft.

It might jump some distance once it loses its grip! And will be quite hot.

Howard