Screw size

[Grierson]

A naive question! I’m looking to attach a ZWO EAF to my focuser. The only mounting hole available is the one currently occupied but the focus lock screw which has a 4mm diameter thread size. I assumed wrongly that an M4 screw would fit. The M4 screw looks and measures the same as the focus locking screw which came out but it will only screw in to the hole a couple of turns. Clearly the threads are different. Can anyone suggest an alternative. Thanks - John

[RT65CB-SWL]

4mm = 5/32" is the closest imperial size I have found, if that helps. Finding a supplier maybe 'hit & miss' though, assuming that is the size.

Edited June 11, 2019 by Philip R

[Tomatobro]

M4 is available in two pitch sizes.

0.7  and

0.5

[Grierson]

Thanks for the responses. I've done a bit more investigation by counting the threads. As far as my old eyes can determine the M4 screw has aprox. 33 threads per inch whereas the screw that came out has aprox. 28 per inch. I wonder if that makes any sense to the any engineers out there. I should also have said that the focuser is American made not Chinese. 

[DaveL59]

is there a decent tools/hardware place near you, not the likes of BnQ but a more specialist place? They may be able to check it against a size guide and determine the correct thread pitch and size you need. I do have a metric set of tap+dies and gauge but might not help if the US have used their UNC or whatever sizes, not to mention York is a long way from me.

[symmetal]

According to this thread sizes chart the only screw with 28 TPI is UNF 12-28 or 1/4"-28 but they are both wider than M4.

M4 Course (standard M4) has 36.3 TPI.

UNC 8-32 is similar width to M4 though has 32 TPI.

UNC and UNF are most commonly used in the USA.

Alan

 

Edited June 11, 2019 by symmetal

[Grierson]

Again many thanks for taking the trouble to respond. Unfortunately we have no (old fashioned) hardware stores here but we do have Screwfix although according to their website they only have metric screws. I see a number of forum members recommend ModelFixings and their website has massive listings so I’ll give them a call in the morning, as they appear to have UNC/UNF screws, and see what they say. - John

[JamesF]

Ebay can often be a useful source for small numbers of screws in sizes that the likes of Screwfix wouldn't stock.

I'm not sure if I have a UNC thread gauge.  If so I should be able to check the size of the focus lock thread on my 80ED.

James

[Grierson]

Many thanks James, but unfortunately I changed the focuser on my 80ED to a Feather Touch a while ago which has added to the problem of getting the correct bits! Having said that Alan indicates that UNC screws are American as well so I might be heading towards the answer and giving me a whole new area of knowledge around this simple question in the process. 😄

Edited June 11, 2019 by Grierson

[JamesF]

In that case I can't help you as I don't have a Feathertouch, only a Moonlite :)

For what it's worth, the stock Skywatcher single-speed crayford and R&P focusers appear to use different size locking screws.  The crayford looks like it's M4 -- none of my UNC/UNF thread gauges would fit, anyhow.  The R&P however looks closer to M5.  It did match up with a UNC/UNF thread gauge, but that might just have been coincidence.

James

[Oldfort]

The world of threads and screw sizes is a bit of a nightmare, as I recenly found when trying to get the right screw for an oldish bit of scientific kit.  It turned out to be 0BA.

For most astro gear my rule of thumb is that if the manufacturer/brand is US then the the thread will be UNC (occasionally UNF), and otherwise metric (coarse).

[Grierson]

Ah BA nuts and bolts. All my amateur radio gear was held together with 4 and 6 BA nut and bolts. Those were the days. 😉

[Skipper Billy]

Ba nuts and bolts are still widely used in the model aircraft world.

https://www.gwr-fasteners.co.uk/ba-screws-nuts--washers-1-c.asp

[Chriske]

4 hours ago, Tomatobro said:

M4 is available in two pitch sizes.

0.7  and

0.5

The standard M4 pitch is 0.7mm
If you find M4 with a 0.5mm pitch it is called FM4(Fine Metric)

FM  is very often used in model engineering.

[symmetal]

As James says ebay is useful for small numbers of screws. KaysFasteners are very good, giving quick free delivery. Scroll down the left column to see the UNC and UNF fixings (and any other type )

Alan

[JamesF]

Yes, I've used Kays a fair bit.  Always been good.

James

[Grierson]

Thanks Alan I’ve ordered a little selection for £4 from KayFasteners so hopefully one of the will be the answer.

Billy, when my lad was young and in to model flying it was PVA glue that held things together but then again we only flew fixed wing models!!

[RT65CB-SWL]

1 hour ago, Grierson said:

...when my lad was young and in to model flying it was PVA glue that held things together but then again we only flew fixed wing models!!

When I was a lad, they were held together with a wing and a prayer!

Edited June 12, 2019 by Philip R

[Grierson]

1 hour ago, Philip R said:

When I was a lad, they were held together with a wing and a prayer!

😁

[Bukko]

On 11/06/2019 at 21:25, Grierson said:

Many thanks James, but unfortunately I changed the focuser on my 80ED to a Feather Touch a while ago which has added to the problem of getting the correct bits! Having said that Alan indicates that UNC screws are American as well so I might be heading towards the answer and giving me a whole new area of knowledge around this simple question in the process. 😄

If you have a feathertouch, then email Starlight Instruments in the USA, they will certainly help you identify what you are after.

I have always found them to be extremely helpful.

Good luck.

Gordon

[Grierson]

Thanks Gordon, I have e-mailed Wayne but have not had any response yet. They perhaps don't take kindly to ZWO equipment being bolted on to their focusers. 😀 

[Grierson]

For the odd member of the forum who is not totally bored with this screw size saga I can now report that Wayne at Starlight Instruments came up with the information. I now know much more about Metric, ENC and UNF screw sizes than I ever did before! Particular mention must be made of KayFast who replaced the wrong screws within 24 hours at no extra cost even returning the postage cost which was included when the wrong screws were sent back. (For what it's worth the screw size is UNC 8-32) Thanks to all who tried to help. - John

[symmetal]

Glad you're sorted out now John.  Yes, KayFast have always given great service when I've used them.

Alan

[Gina]

Same here.