Thread size on xd 3420 motor

I got a chinese made xd 3420 motor, 12v 30w, on the dimensions it says the shaft is 8mm so i just assumed it would have an m8 thread, it won't take an m8 nut, i'v measured the thread with a micrometer at 7.79mm, is there a thread that is 7.79mm.

I wanted to attach an 8mm 303 stainless steel shaft to the motor, in preparation for the motors arrival i'd already cut an m8 thread on 303 round bar with a hand tap/die set, the m8 nut i was going to use was type used for joining threaded steel rod.

This is the motor below, would it possible to go over the motor thread with my m8 die, i only have small standard metric tap and die set, what tap and die do i need to cut a new thread and make a connector, Thanks.

**LINK**

7.79mm would probably be within tolerance for an m8 thread - they are always smaller than the nominal diameter. What is its pitch? Maybe it's M8 fine?

Bypass the thread conundrum entirely and Loctite the shaft extension on.

From the seller's pictures, I guess you are using it in a lawnmower or an abrasive chop saw. Those eBay Watts sure have increased in size since I was at school. Quite the opposite to a packet of polos.

Could be 5/16 UNC or UNF. Spec diameter for both is is 7.938, so 7.79 would be understandable.

UNC is 18tpi 1.411mm pitch, UNF is 24tpi 1.058 mm pitch vs M8 Coarse at 20.3tpi (1.25mm) or Fine 24.5tpi 1mm pitch.

Given that several vendors sell the same motor with a drill chuck option, and 5/16-24 UNF is a standard drill chuck mount, that would be my best guess.

Correction of UNF pitch - M8 Fine might fit - both have 60 degree flank angles.

Edited By Peter Cook 6 on 18/08/2023 14:04:47

I'm not sure what pitch it is, how would i be able to tell or measure, i only have basic hand tools, i will see if i can get an m8 fine nut online and see if it fits.

Measure the pitch (distance between crests) over as many turns of the thread as you can. Simple division will give you a good idea. UNF at 1.058 mm pitch should be easily distinguished from the 1.25 of M8. but not M8 Fine which is almost the same.

The alternative is to screw the nut that came with it on as far as it will go. Measure the distance from the nut to the end of the thread. Unscrew the nut a few full turns, and measure the distance again. Math will again give you the pitch.

Edited By Peter Cook 6 on 18/08/2023 14:05:57

I've seen the motor with drill chuck option available, I will get nuts in 5/16-24 to see if they fit, if they do i'll look for an imperial tap and die set.

I wrongly assumed it would be an m8 thread, i should have gone for the unthreaded version and threaded it my self, thanks for different thread sizes i will try each one.

Thanks, i've just measured the threads on the motor shaft, i count 12 threads on half inch, there is only half inch of threads.

I will get both an 5/16-24 unf and m8 fine nut and see which one fits.

Um, 12 threads on half an inch is 24tpi. but how accurate was the measurement? As M8 x 1.0 (fine) is equivalent to 25.4tpi and M8 x 1.25 (coarse) is 20.32 tpi, the measurement has to be good.

This motor comes in many variations, and although I found two specs saying M8, yours could be UNF for the USA market. If M8 it's more likely to be coarse than fine.

How sure are you of the test nut? Perhaps it's not M8!

Check both motor and nut threads for damage - dinged threads struggle to mate.

I'd try gently running the M8 coarse die down the motor spindle. Don't force the die in case it's a poor fit or the spindle is hardened. If the thread is only damaged the die will clean it up. With more energy it might also convert 24tpi into a usable metric M8, but the thread will be mangled and weak - too much of a bodge for me.

Dave

I've looked at the motor shaft thread under a magnifying lamp the thread is finer compared to a couple of m8 bolts and the m8 course thread i put on the 303 stainless rod.

I've ordered both 5/16 x 24 and m8 x1 fine nuts to see which one will fit as you say they are close, i can't see any damage on thread as the nut that came with it screws on ok, the nut that came with the motor won't screw on a m8 course bolt, at least i'm sure it's not m8 course, once i'm sure what thread it is i'll get a tap and die to make a shaft connector.

Decent thread gauges settle the what thread is this debate, it is very satisfying when the correct thread gauge is offered to the perfect matching thread. My thread gauges are all pre owned Moore & Wright so I don’t know if the cheaper sets provide the same satisfaction. It is unlikely that anyone will wear out a set of thread gauges so they will last a lifetime even if they are used items.
Mike

Thread gauges are cheap enough and very handy in situations like this. Most suppliers advertising on this site offer an Imperial/Metric set for less than a tenner. Personally, I have sets for Imperial, Metric, BA and American UNC/UNF

You could see if it is M8 x 1 by offering up an M6 bolt or screw if you have one, makes a good poor man's thread gauge.

I assume the motors come with a right hand thread?

M8 x 1mm is a common enough thread ! more so in electrical stuff. As jason says use an M6 x 1mm bolt as a thread gauge. Noel.

Fixed that for you

DEFINITELY get sets of thread gaugaes, so that you don't have htis problem in the future.

You can measure the OD of the external thread - it will give a good idea of the actual size (Often measured as slightly under size due to truncation / rounding ) and the thread gauges will tell you the pitch."

Note the word "sets", having Whit, Unf / Unc, Metric, (and even BA can be usefiul at times )

These should allow you to identify a thread fairly quickly, and avoid problrms with attempting to mate two different thread forms or pitches.

A relatively small, but worthwhile investment.

Howard

Not thought about contacting the supplier and asking??????

T'internet and e-mail is a wonderful tool.

Regards Ian

I messaged the seller and a couple of other who sell the same motor, upto now no response, i think they are more online sellers without customer service.

I''ve ordered a set of 3 thread gauges after reading Mike poole's post this morning, i didn't even know such a thing existed until reading mike's reply, i'm a newbie at engineering with basic tools and knowledge.

Hi Nick W The qestion was not mine ! I have thread gauges in Imperial, Metric and BA ! They are invaluable for me and often used when a thread of unknown type is need of identifying ! Noel.

The thread turned out to be M8 x 1mm, the 5/16 x 24 nuts came did not fit, the set of thread gauges turned up and the 1mm fitted perfect, it's good job i oredered the gauges, some sellers have fast despatch & delivery, still waiting for the M8x1 nuts.

The guages will come in handy in the future i won't be stuck again, thanks everyone.