How to Calculate Bore Size

[COLIN MARTIN 2Participant @colinmartin2]

Hi,

Am after some advice for a small job (for an expert) but a large job for me!

I have a motorcycle flywheel for which I need to make an extractor for. It is a Spanish Ducati and for some reason the thread for the extractor is different to the Italian one and is unobtainable, so I am attempting to make one. The Italian thread on the flywheel is male and is M62 x 1.00 mm; the Spanish thread is M62 x 1.25, and because I have an Italian extractor, I have decided to make up a sleeve, internally threaded one end to fit the flywheel and externally threaded the other to fit the extractor. I have a piece of suitable round steel bar which I am boring out, but what size should the bore be to fit the M62 thread? And what size should the O/D be for tapping to the M62 thread?

I have never attempted thread cutting before, so I have no idea if I have the skill to do this, but nothing ventured……

 

Thank you,

 

Colin

[bernard towersParticipant @bernardtowers37738]

its 62mm minus thread depth x 2

[FulmenParticipant @fulmen]

Or simply nominal diameter minus thread pitch, i.e. 62-1.25mm.

[SillyOldDufferModerator @sillyoldduffer]

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

[old martParticipant @oldmart]

I usually go to the Motalia website, they have useful thread charts and you can work out the relative diameters based on a thread pitch which is the same as the one you will use. This will be the same whatever diameter the thread is. You should practice with some aluminium rather than going directly to steel. Machine single pointed threads are not cut in one pass like with a die, but in several stages, each increment being a finer depth of cut as the tool cuts more each pass. It can be useful to write down the number and depth of each cut beforehand and then cross them off, and remembering to back off the tool before returning to the start is essential. My first threading was for a backplate for the Smart & Brown model A lathe spindle which is 1 3/4 x 8W and I made a winding handle for the lathe rather than chance it under power.

 

 

 

[bernard towersParticipant @bernardtowers37738]

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

[HopperParticipant @hopper]

One tip for when you get your puller finished and into use. Don’t just tighten up on the extractor bolt in the middle until it strips. Tighten it good and tight then whack it with a hammer, or hammer and steel drift. The impact will often jar the flywheel loose off its taper where steady pressure will not.

I would just make up a whole new extractor from a piece of steel bar, rather than and adaptor that need two threads cutting and makes things more unwieldy to use.

And be sure to grease up the thread on the extractor bolt with never seize etc to get maximum pressure on the job.

[bernard towersParticipant @bernardtowers37738]

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

[COLIN MARTIN 2Participant @colinmartin2]

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

[COLIN MARTIN 2]

On 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 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!)

[COLIN MARTIN 2Participant @colinmartin2]

Would it be possible (after setting up the correct gears for the pitch, and engaging the jamb nuts) to turn the chuck by hand, rather than the lead screw? And I need a bore of 60.75 or 60.83?

 

Thank you,

 

Colin

[COLIN MARTIN 2]

On 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.

[HopperParticipant @hopper]

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.

[rickkrParticipant @rickkr]

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.

[rickkr]

On rickkr Said:

…  If I had no experience single point threading, I’d definitely be doing some practice pieces.  …

Agree!   Plenty of practice.  Also, as internal threads are a fair bit harder to cut than externals, conquer external threads first.

Can Colin tell us more about his lathe?  Depending on what it is, might be possible to simplify the cutting.   For example, often advantageous if the chuck is not of the screw-on type to have the lathe cut threads in reverse away from the headstock, greatly reducing the risk of crashes.

Dave

[derek hall 1Participant @derekhall1]

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!

[COLIN MARTIN 2Participant @colinmartin2]

Hi,

Practice sounds like a good idea, I will look into it. My lathe is a Warco 180, and I have a couple of questions regarding it.

Thinking about making a handle to rotate the lead screw, I tried to turn the lead screw using the locking nut on the end and it would not turn, so how would I rotate the chuck via a handle? Also, can someone tell me why it is necessary to set the compound slide at an angle when cutting threads?

 

Thanks,

 

Colin

[larry phelan 1Participant @larryphelan1]

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.

[rickkrParticipant @rickkr]

In simple terms, if the infeed for cutting threads is done using the crossslide (or compound at 90 deg to the lathe centerling) cutting of the threads is happening on BOTH sides of the cutter.  If the compound is set at 30 deg. (to the lathe centerline) (or preferably at 29.5 deg to give a slight clearance to the none cutting face), thread cutting is happening on only one face of the thread V groove.  I think this makes the cutting easier and smoother.  It also leaves the crossslide free for use to retract the cutting tool at the end of the cutting run.  This rapid retraction and return to a known/fixed position re-establishes a “zero” position of the threading tool.  The in this mode, only the compound is used to advance the cutting depth.

All of this would be detailed in some basic operations manual describing single-point threading, such as South Bend “How to Run a Lathe” or similar.  You should have some sort of instructional reference to guide your practice.  It is not an intuitive process.

[Hopper]

On 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

[Hopper]

On Hopper Said

at 75% thread engagement

Not to derail this topic but afaik there is no such thing in the metric standard. There is something called “engagement length”, but that’s not what you’re talking about. Is it from Unified or one of the older British standards?

[HopperParticipant @hopper]

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.

[COLIN MARTIN 2Participant @colinmartin2]

Thank you everybody for posting, I am learning!

 

Thanks,

 

Colin

[Howard LewisParticipant @howardlewis46836]

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

[Dave HalfordParticipant @davehalford22513]

When you cut infernal threads (because they are) on stuff like this the depth depends on what tool you cut them with full form or part form, for carbide part form they need to be deeper than a 1.25 tapping chart says.

Practise first!

[Author]

Posts