M20 x 1.5 tap in backplate

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baldkev

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Hi,
Ive got a chuck that doesnt fit my lathe. I took it to a few engineers locally, but so far none have an m20 x 1.5 tap.... looking on ebay, a tap appears to be available for 20 quid. I guess thats probably a cheap one ( 2 engineers said they are about 70 quid )

If i get a 20 quid tap, is this something i can do myself? The backplate hole is 5/8. I believe it needs drilling out to 17mm and then tapped, but is this something i can do? Or do i get a tap and take it to an engineer?

Ive got a basicvtap and die set which ive used a few times, but of course this is for a lathe, so needs to be fully centred..... probably best to use an engineer!
 

Yorkieguy

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I think rather than using a tap, an engineer with a lathe with metric screw-cutting facilities, having first enlarged the hole with the backplate in the headstock, would then cut the thread. Alternatively, using a tap, for the thread to be perfectly in line and concentric, the backplate would need to be in the headstock and the tap in the tail stock.

I doubt that you could use the tap in a tap-wrench and hold it perfectly perpendicular while tapping the thread, and of course you don’t get a second chance if you mess up.

Just my thoughts - others may see things differently, but it seems to me that unless it’s spot on and perfectly concentric, you’ll get an unacceptable amount of ‘runout’

Good luck with it.

David.
 

TFrench

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Are you sure it's 1.5 pitch? That's normally an electrical component thread. Don't think I've seen a lathe with that spindle before. Yorkieguy is right though, that's a job for single point threading on a lathe to get it bang on.
 

Sideways

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A job for the lathe. A thread is nowhere near precise enough to get a chuck centred and square on a lathe. Normally there is a close fitting straight section inside the bore that matches a corrsponding part of the spindle. That has to be bang on square and minimal clearance otherwise the chuck will wobble. All the thread is doing is holding the chuck tight on the real registration surfaces. The machinist needs to know the exact dimensions of the registration surfaces on the spindle to make the chuck backplate match it. 1/100th, even 1/1000th mm accuracy ideally. If you can't measure it, buy one ready made to fit.

Note, I was thinking metal lathe when I wrote this. The same principle applies with wood lathes but less demanding on tolerances. I did once play with a bought thread adapter for a wood chuck. Sold as suitable for a few brands of chuck, it wasn't. The registration surfaces didn't meet, so alignment was only via the thread. It wobbled badly and after a couple of replacements all the same the retailer refunded me.
 
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baldkev

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Hi guys, thanks for your replies and the clarification with regards lathe, not tap. Id better look for a better engineer!

Are you sure it's 1.5 pitch?
Yep, pretty sure, thats what the internet has it listed as.... i left the live centre with them as it has the correct thread within it. I need to go collect the backplate and centre.

Unfortunately they dont do a backplate with the thread i need, so i bought a blank. Its either that or get a whole new chuck, although im told this is a good'un so worth trying.....

The machinist needs to know the exact dimensions of the registration surfaces on the spindle to make the chuck backplate match it. 1/100th, even 1/1000th mm accuracy ideally. If you can't measure it, buy one ready made to fit.

That makes a lot of sense... i have a faceplate for the lathe, so i can drop that off with the backplate and live centre, hopefully that'll be ok

Thanks for the help 🙂 👍
 

Lons

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I had a Tyme Cub lathe many moons ago with a 20 x 2mm thread and I bought a good quality tap, never used it on metal faceplates but used plenty of wood chucks and face plates without issue. I still have the tap when I think about it.
 

Mick p

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I had a Tyme Cub lathe many moons ago with a 20 x 2mm thread and I bought a good quality tap, never used it on metal faceplates but used plenty of wood chucks and face plates without issue. I still have the tap when I think about it.
Sorry to jump in I have a cub I’d be interested in your tap if it’s for sale thanks MickP
 

Lons

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When I think back I seem to remember there was a Multico lathe with 2.0 x 1.5 thread and I think Electra Beckum as well, might even have been the same lathe rebadged. I can't be 100% certain so you would need to check.
 

baldkev

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Its a multico junior.... its got a reasonable capacity etc, but mostly i bought it on a whim one night on ebay 😆 i disnt even think about threads and accessories. Mind you, it was £100 ish.....
 

Lons

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Hi Mick
This is the tap and as you can probably see is in perfect as new condition never used to cut metal.
I bough it a long time ago and remember wincing at the cost but can't remember how much I paid. Hall doesn't list this size now at least on their website so I've no idea really what I should ask for it and will have to have a think, you could send me an offer by pm if you want to but I don't want to breach forum rules.
Bob

tap 1.JPG
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