M300 Gauge Clone

[George S.]

Got the gauge measured.
The plate is slightly undersized, and the 38.1 mm long feeler gauges slightly oversized in thickness, perfect length.
Stacked for use, they're approximately .001 mm oversized, if I use averaged plate thickness. But this will slightly vary across the plate.
Looks like they're aware of the tolerances and matching the pieces, or perhaps just luck.
The plate is nonmagnetic stainless, the feelers are aluminum.
Definitely not commercial/industrial quality, but it'll work very well for what I'm doing.

 

[George S.]

I'm not sure what the brass screws are for.
I put dabs of grease on the screw heads and inserted the plate into the deck.
The screw heads don't touch the deck. Maybe they are contact points for another type of deck?
No marks for head insertion depth are engraved on the plate.
That's OK in this case. I made a pencil mark as a reference.

 

[J!m]

I don't see any tolerance on the first drawing... That's the problem with reverse-engineering: you don't know if what you are referencing (copying) is at mid-tolerance, or top/bottom of allowable tolerance.

 

[George S.]

They talked about tolerances in threads on TH. I'll have to go back and look, but thought the optimum thickness was 2.445 mm, which is given for dimension A.
Then it states a flatness of 0.04 mm that I assume is + or -.
I do remember seeing a post about the final dimensions everyone agreed on for producing a reproduction, but can't find it.
I figure Abex of Japan had a good handle on this at the time, so that's the piece the Chinese would clone. Hence my comparing the clone to the Abex.
Being that this is old analog audio, with huge tolerances in cassette shells, wells, and transports, I see the M300 gauge as a rather crude device for initial setup.
Nakamichi gauges and jigs I've read about are a different matter.

 

[J!m]

Flatness is usually total, not +/-. Like TIR.

Some shell dimensions are not critical but others are critical. They have different tolerancing on different areas of the same part.

 

[Chris Cables]

I sought to have a batch made back around the beginning of summer.
However, I couldn't find a machine shop who would commit to the specs in my RFQ which used the original IT specs to replicate the M300 gauge. Not even from my old neighbour Henk-Jan who makes stuff for the aerospace industry. He obvs has bigger fish to fry, the selfish fkr.

I posted about it on TH and ofc the thread got closed out uneccessarily. I could have updated it if I'd found a shop eventually but fkd if I'll bother posting about it there if I ever do.

I think the biggest challenge for any machine shop is to not have the gauge go out of spec during machining, which is why the original is made in a two-piece configuration.
It should be relatively easier to fab now than it was back in the 70's for any competent shop. Understanding how steel can change when machining is essential knowledge to have in order to keep tolerances in check.
One important thing I learned from the episode was that it's probably a lot more convenient to source pre-flatted steel rather than pay for expensive milling/machining time. Pre-hardened/flat steel plates would be ideal for this application and will just need 3 holes milling out in both parts and forming and finishing.

In any case that's one of the better Chinesium gauges I've seen. Better than the ones I bought to compare with the IT specs - they were completely usueless.
How much did it cost and where from ?

Flatness is usually total, not +/-. Like TIR.
.

There's usually a tolerance for everything @J!m. In the case of the M300 it's +/- 0.002".
Going further, that also becomes a cumulative tolerance when used in concert with the feeler gauge as it too has its' own dimensional tolerances.

 

[George S.]

Chris, ordered this one before I left for vacation.
https://a.aliexpress.com/_mru6R7K

 

[Chris Cables]

Chris, ordered this one before I left for vacation.
https://a.aliexpress.com/_mru6R7K

(2 sold)
One of them was you. lol.

Well, I haven't seen the Chinese gauges with 2 brass screws before. What purpose do they serve?
The 3 screws on the IT-M300 are to fasten the 2 component parts togther. I can't see what purpose the 2 screws on this one perform, apart from making it look like it's made from 2 parts, which it isn't.


I had to chuckle at this typical Chinese word-salad:
'Magnetic head is engraved on the measuring plate for reliable measurement'
Yeah, that's gonna make it really reliable, lol. And just exactly which measurement are we referring to there??

To verify flatness George, you won't be able to do it with calipers - you would need a decent height-table and measure accross 8/9 points. to get a half-reliable measurement.

I'd propose this one is probably as un-trustworthy as the ones I bought previously.
Use/trust at your peril!



EDIT:
Hang on a minute, is it made of 2 parts or not? I can't really tell.
Looking a tthe dimensional diagram they supplied makes it look like it might be when looking at that sectional view. However, there's no A-A section indication anywhere on the plan view.

Can you clarify? Photo of the backside would help...?



EDIT No.2:
Oh Jesus, it gets worse - they just re-printed the ABEX tape drawing (including the ABEX name!!) and didn't even bother re-hashing it for their own purposes.
It has totally different details around the capstan/head cutouts and holes.
Drrrp

Edit No.3.
Ok, I get it now - re-reading, you compared the original ABEX drawing to your clone. Dangerous. What datum/s did you choose?

 

[J!m]

I’m sure the main piece could be made from aluminum jig plate. And it could be Blanchard ground if needed. That small part would be flat within a few tenths easily.

 

[George S.]

One piece of plate and two brass screws.
I see no function for the screws on this particular deck. They have been machined for height.
Plate and screw head thickness would correlate to measurement (b).

 

[20tajk7]

The screws are used to align the "Nagar" cassette decks, the method is different in these very rare decks

 

[Chris Cables]

I’m sure the main piece could be made from aluminum jig plate. And it could be Blanchard ground if needed. That small part would be flat within a few tenths easily.

You could, but it would more than likely go out of spec during either machining or finishing. The flatness spec being the most susceptible imo.

 

[J!m]

A good machinist is mindful of the order of operations…

But jig plate is very flat and very stable. If thickness was right it could probably be used off the shelf and meet .002 across the corners of such a small piece. I think it is sold at .003/foot max deviation.

Bronze is another option. Easy to work and holds tolerance but honestly thousandths is not high precision. Another order of magnitude is. Four places not three.

I have done lapping on proper critical parts two helium light bands flatness. That’s high precision.