India screw plate versus antique Swiss Martin et fils

[syfre]

Hi guys,

I need a 0.7mm screw (head 1.2mm) and the corresponding tap so i give a try to the india mini taps and screw plate that you can find on the bay.

The set comes with a 0.7 mm tap and that the one i wanted.

First, i measured the tap and the 0.7mm give 0.66mm and  the 0.8mm give 0.78 on my Mitutoyo (not chinese) caliper. 

I made a test on a brass piece, drill at 0.6 for the 0.7mm tap and at 0.7 for the 0.8 mm. I was able to tap both with out problem. I believe a 0.5mm drill would had been better for the 0.66 tap but drilling at 0.5mm on a bench drill (a proxxon) is tricky.

(I gave a try to the chinese carbide drills you can found on the bay for a couple of dollars. It was a good surprise as they cut really good and don't break on the first time. The set i tried come from THK)

Now the screw. I turn a blue steel rod at 0.8 and try the 0.8 hole of the india screw plate. It was a total failure, the screw plate didn't cut.

As it was my first try at making screw i though i was doing something wrong. May be the steel is too hard ? I decided to try my antique "Martin et fils" screw plate. It is a L model (L means "Latar" thread serie) and i found online the corresponding measurement : the hole 12 must be 0.798mm 0.164mm pitch which is the closer of the 0.78mm india tap (but as the india tap pitch is unknown so there was little chance they match). The "Martin and fils" cut my steel rod like butter. I tried the thread (0.78mm) on the tapped hole and it matched.

(These old screw plates are not consistent, you have to made a test thread and measure it to know the exact values)

I then made a screw using the "Martin et fils" and test it on my test hole : it matched but it was not as tied as you can expect.

[When making a screw the most difficult step is to make the slot. I believe the right way is to use a mill attachment on the lathe and a rotary cutter to cut the slot but without that making the slot "by hand" with a saw is really difficult. After many tries i finally succeed using these steps : first anneal the screw, it's absolutely required, the steel must be softer that your saw. Second, start the slot using a scalpel with a hard blade.  Third, cut the slot with the saw. It's a good idea to made the screw head higher than the final dimension so that you can restart if you mess.]

Not satisfied by the result i decided i needed a tap which exactly match the "Martin et fils". In "The modern watchmakers Lathe and how to  use it"  Archien B Perkins describes how to made a tap from a screw plate. The process is simpler that you can thought. The final tap is a little short because the first one i made break during the thread cutting. I made the second shorten to avoid a new brake. Using the home made tap the screw fits perfectly.

[To made the tap : use an old drill, anneal it, turn it to dimension (0.8mm for my number 12 hole), cut the tread, file the thread three times at 120° using the lathe indexing plate to form the tap and sharpen using an india and arkansas stones, harden then temper ]

I made several tests with the india screw plate and never succeed. The taps are not bad but without the corresponding screw plate there are of little interest. Finally, as you may expect; the antique Swiss screw plate is the great winner.

 

 

 

 

 

[rodabod]

I have an Indian one and it is shite. I did cut a screw with it using annealed rod once. Not very pretty. No chance with blue steel rod!

The mini HSS tap and die sets are good though. Made in China, I think.