custom big tap needed

[douglas coatesParticipant @douglascoates40572]

I need to find someone who can make a big custom thread tap for me. I have an ML7 but it's beyond that I think. It's approx 45mm major dia, and around 4 – 5 tpi. It's for cutting wood threads for some vices I make, the reason for flexibility in diameter and pitch is that from a new tap I then make a thread box to match (basically the die part) so I'd need a matching female thread into ali stock to make the outfeed for the die / thread box.
If anyone with a bigger lathe and v coarse pitch gearing will consider this I will pay of course – exact drawings etc will depend on pitch and then I need to check available Forstner bits to get minor and major diameters right – but I would provide a working drawing. Any help appreciated.

[douglas coatesParticipant @douglascoates40572]

[MWParticipant @mw27036]

i don't think my machine could do it either. However if you can't find someone who can turn it, thread milling on cnc (cutter moving in relation to a rotary indexer) would be my best bet.

Michael W

Edited By Michael Walters on 30/04/2016 11:55:54

[HopperParticipant @hopper]

You should be able to cut that thread in a Myford, surely? All you need is a gear on the headstock spindle that is double the size of the gear on the leadscrew, eg a 60T and 30T, or a 40T and 20T. Or if using a QC gearbox, set it to cut 8tpi and introduce the 2:1 ratio in the gears between gearbox and headstock spindle?

You then just have to remember to grind the screwcutting tool with the much extra clearance on the lead side to allow for the helix of the coarse thread.

Or can you clamp the wooden vice components to the faceplate and screwcut the threads straight into the wood in th elathe? Or alternatively, clamp the wooden vice part to the cross slide and screwcut the internal thead using a boring head mounted to the headstock spindle?

Or am I not understanding your question at all?

[MWParticipant @mw27036]

perhaps some lathes have a much better selection of gearing than mine but i think the maximum thread pitch in my lathe is something like 3 or 4mm, a tpi of 4 is 6.35mm pitch so a fair bit beyond.

Michael W

[douglas coatesParticipant @douglascoates40572]

Thanks – Hopper, wood components onto the lathe isn't an option – due to sizes, and the number I need to do, it's very slow and faffy. From my experience the tap and die is the way and you see old sets around (ebay etc) – it just works very well.
I was told by a mate I could do this on the Myford but I'm not a great metal worker and also the volume of steel to come out would take great patience I think, and put a fair load on the little ML7. I really need to pay someone with larger machinery to do this for me, I can then get on and make the vices !! Actually making the thread box is no small task, getting it dead right is a challenge (to me anyway) – i'd post a pic of a screw and nut but not sure I can post pics yet.

[LambtonParticipant @lambton]

It is best not to try to cut a thread on a Myford courser than lead screw (8tpi).

[RainbowsParticipant @rainbows]

Am I reading correctly that you want to use aluminium for a die?

[douglas coatesParticipant @douglascoates40572]

Posted by Rainbows on 30/04/2016 15:13:43:

Am I reading correctly that you want to use aluminium for a die?

no, it's the outfeed guide. These wood taps are totally different to a conventional die, using a vee cutter (or a pair sometimes) to cut the profile and an outfeed female picks the thread up and feeds pitch. A couple of pics would help a lot!

[Martin ConnellyParticipant @martinconnelly55370]

You could mill the threads as stated earlier but with a CNC lathe you can just cut the thread with the required tpi and diameter. A drawing of what is required would help.

Martin

[Martin King 2Participant @martinking2]

Hi Douglas,

I get a lot of Thread Die boxes for wooden threads, lovely items in Boxwood/Rosewood and Mahogany but not in the sizes you require. Biggest I have had is about 1" and 4 or 5tpi. Used for the spindles on Screw Stem planes.

Usually by top makers like Mathieson, Ibbotson and more commonly Marples.

They make an absolute fortune and usually end up in the USA as long as they have the matching tap which tends to be a slight taper.

I probably have about 20 awaiting the discovery of the elusive taps…. In the smallewr sizes one can often find a large Whitworth which will do.

Good luck with your search.

Martin

[douglas coatesParticipant @douglascoates40572]

That's it Martin, I am making a custom one of those. I have a 42mm 4tpi at the moment but the tap (which was kindly made for me) is on its last legs and the flutes were cut wrong. By making the tap and outfeed I have control over everything – diameter, pitch and accuracy of fit. On the current one I get really nice close-fitting screws and nuts, so I am looking to go the next step, a mandrel starter then tapered tap. The mandrel is important to start the thread dead on axis. The original tap[ was cut on an old Barnes, but I'm sure I can find someone up for the challenge … haha.

[Dinosaur EngineerParticipant @dinosaurengineer]

When cutting very course threads it is sometimes better to drive the leadscrew rather than the spindle.A leadscrew handle may be used for this purpose.

[douglas coatesParticipant @douglascoates40572]

Thanks Martin C – I can sketch this and take a pic, but how do I upload a pic? (do I need some min no of posts?). I have to start with 2 diameters (min and major) as I must buy Forstner bits in std sizes. According to what I have at the moment and some rough sums, I think a 35mm minor diameter and 45mm major may do it. So tap hole is 35mm, die blank (turned hardwood dowel) is 45mm.
I took a 4.6TPI as a guess of what would be good, my current is 4TPI but I think a tads finer would be good – in a vice obviously speed and clamping force are traded. So if 4.6tpi, then my reckoning is 9.77mm diameter difference at 60 degree vee form. I can get 35mm and 45mm, and grind a 45 down to about 44.7 so that would be easy.
I could sketch and put it on facebook pge if that helps??

[jimmy bParticipant @jimmyb]

I should be able to do this at work. Could you send me some pics?

Cheers jim

[Max ToleranceParticipant @maxtolerance69251]

Hello Douglas,

I should be able to manage this on my big Colchester. It can go as course as 6mm or 4 TPI. If the tap is for wood what sort of thread form is used? also what material would you be looking to make it from? i.e is it just a high grade steel or does it need heat treatment etc.

Regards Max

[douglas coatesParticipant @douglascoates40572]

Hi Jimmy and Max – i need to draw it properly, will do that tomorow and send or attach here, I see a pic upload button now.
The thread form is not very critical but 60 degree seems good compromise as wood has grain of course and failure will tend to be along the grain. Also it's common to have a small flat on internal and external – the extreme tips of vee do nothing and on the male they just fall off anyway (wood again!)
Re material, a really nice option would be to case harden but all the ones I have used new and old have just been decent steel, not hardened. I'd need to mill flutes on along axis but I can do that (mate with a good milling m/c)
SO, sketch to follow – thanks all for the help, advice etc
Pics of the vices are here if interested:
**LINK**

[Involute CurveParticipant @involutecurve]

I think I can cut ot for you on the CNC Mill but Id need to see the drawing first to make sure.

PM me if you want it doing.

Shaun

Edited By Involute Curve on 30/04/2016 20:52:04

[HopperParticipant @hopper]

Pics of a similar tap and die set up sure would be helpful. Easiest way to post pics is to put the pics in Photobucket or similar then copy and past the Direct URL from Photobucket to the window that opens when you click on the small IMAGE icon at the top of the new post on here. It's the small colored square first to the left of the small camera icon. (Camera icon is for posting pics from an album, a quaint 1980s technology preserved for posterity on the ME site.)

[douglas coatesParticipant @douglascoates40572]

Thanks Hopper – let's try this… resized small in P'shop. Top pic is my custom tap 42mm 4tpi, second tap is a commercial 1" 6tpi that works very well, next pic is the shop-made thread box with ali outfeed and O1 vee cutter. Last pic is box at work into oiled beech. I'm working out the drawing but realised it isn't so simple, if I want small flats (or rads) at the apexes the geometry becomes a bit more complex – to me anyway… drawing soon

[douglas coatesParticipant @douglascoates40572]

here's a rough sketch minus the calcs for precise diameters and pitch

[jimmy bParticipant @jimmyb]

Thank you for putting up the pictures.

[HopperParticipant @hopper]

Are those taps relieved at all, so that the leading cutting edge is contacting the cut thread but the body of the tooth behind it has some clearance, as is done with taps used for tapping metal?

Rads on the threadform for the tap should not be a problem. On a large threadform like that you can easily grind it on freehand on the threadcutting tool used to turn the thread on the tap. Rad on the tops of the threads on the tap can be put on with a file after the thread has been cut. Flats of course are easier and more commonly done for practical purposes. Just grind flat on the nose of the screwcutting tool to get the flat in the "valleys" and simply make the OD of the tap undersize to get the flats on the peaks.

It looks from your sample taps that they are made in the conventional manner as far as the thread itself it a standard parallel thread and the taper is simply applied to the OD. Looks like a bit of fancy milling on the big one to provide chip clearance with the short teeth at the lead-in end.

[Martin ConnellyParticipant @martinconnelly55370]

Douglas, 4.6TPI is very close to 5.5mm pitch. If you did 5,5mm pitch x 42mm diameter in a metric 60° thread form the difference between outside diameter and bore would be 5.5mm. This would give a bore of 36.5mm for your initial hole. I know there will be a slight difference due to the flats you require but it seems a lot different from your calculations of 30 April. To get close to the 10mm difference you have from Ø35mm to Ø45mm you would need a pitch close to 10mm or 2.5 TPI.

I printed off the picture of the thread in your post of 02 May and scaled the dimensions from the 44mm stated on the label. This gives a bore of 37.3mm diameter and a pitch of 6.4mm (approximate of course). This is close to what you would get from a metric thread with Ø44 and pitch of 6.5mm (37.5 bore). This is a little under 4TPI.

Based on these figures I think you should keep the TPI value at 4 or lower.

Martin

[EmgeeParticipant @emgee]

Hi Douglas

The threads on the wooden screw done in the thread-box look to be greater than 60 degrees, they could even be 90 degrees which would be the same as the threads on a tap for tapping wood I made some months ago.

If they are 90 deg that would explain the TPI errors found.

Emgee

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