Broken part of quickchange toolpost

[Martin DowingParticipant @martindowing58466]

Hi,

Below is a photo of quickchange toolpost, which I have recently broken.

Broken part works together with a spanner driven eccentric and secures tollholder in given position.

Such a part is very easy to make but I have few questions.

1. Why it was made from such material that breaks so easy? (I suspect it is spheroidal cast iron).

Is there any virtue in such design or is it just a poor design in terms of material choice?

2. Should I make this part from such an inadequate material again (so it can break up again as well), or a part made of lets say BDMS or cold drawn stainless of reasonable tensile strenght if a bit harder surface is needed would serve better?

Thanks for any comments.

Martin

Edited By Martin Dowing on 09/11/2016 21:31:50

[Martin DowingParticipant @martindowing58466]

[Martin DowingParticipant @martindowing58466]

For some reason initially an image have failed to download but I tried again and now it is in original post above.

Edited By Martin Dowing on 09/11/2016 21:13:35

Edited By Martin Dowing on 09/11/2016 21:34:26

[David Standing 1Participant @davidstanding1]

I can answer 1)

It was probably made in China, and built down to a price, and not up to a quality.

[MWParticipant @mw27036]

Cast iron doesn't sound like it should belong there nor a long term solution for clamping the tool holder, precisely because of whats happened. It's brittle and it could crack eventually.

The advantages of cast iron are it's general rigidity and heftiness but should it be used on thin parts exposed to repeated wear? I dont think so. 

The dickson clone part in question i have (which i'm feeling is probably china/indian steel, i'm not a man of prestigious means) is made of BDMS and seems to be going well other than a few scuffs around edges.

I honestly think it was made of something else or if they really did use cast iron, unfortunately the only reason i can think of it was cost skimming to reduce machining times.

Or they just used it because it was there and they could? who knows. Maybe being a disimilar material to the steel it is less likely to gall or cold weld? 

However your one looks more like an original? 

Michael W 

Edited By Michael Walters on 09/11/2016 21:59:38

[Martin DowingParticipant @martindowing58466]

Here is another photo:

Please look on crystalline structure of broken part.

I am pretty sure that it is cast iron, most likely spheroidal one because grey cast iron should break under stress involved in no time at all and this part was still serving for a while.

However it got cracked with no warning.

Anyway it is nice to hear that replacement made from BDMS is serving well its purpose.

I am surely going to use it for a replacement part.

Edited By Martin Dowing on 09/11/2016 22:18:13

[SillyOldDufferModerator @sillyoldduffer]

Faulty heat treatment? A more likely cause of brittle failure than being made of cast-iron I think.

[MWParticipant @mw27036]

Yeah i can see it now definitely the crumbly stuff some of the less well made stuff on my lathe comes from. I had a hex form dog clutch shatter to pieces on my machine that looks similar to that. I replaced that with BDMS too. puzzled me as well, why do it if you could see it coming?

I think that should be a good replacement because it will bend before it will break. I'd imagine the worst that could throw at you is burr up (over a seriously long time) but it wouldn't break. You could case harden it as well if you wanted to go to town. Be as tough as a decent screw driver then. But avoid leaded MS if you're gonna do that. All other grades should work.  

Michael W

Edited By Michael Walters on 09/11/2016 22:25:57

[Martin DowingParticipant @martindowing58466]

Yes, perhaps case hardened BDMS would be the best idea.

F/C leaded steel is not really often used in my shop, so no, I won't use that.

I have red that there are grades of F/C steel suitable for carburization and hardening (or for silver soldering and welding) but I do not know what these exactly are. Anyway I don't have any of those and ordinary BDMS is cheap and I have plenty.

[MWParticipant @mw27036]

Yeah, F/C isn't the same as leaded as you pointed out so it would work EN1A can have a propensity to rusting readily but if i do use it then i never wash the parts because you can't get all the moisture out of it,

so i only wipe it clean. EN3B is a little harder but will have a better disposition when it comes to rust. But given where its being used i can't see these issues being a problem.

I have a mallet made of EN1A and it doesn't show any signs of rusting, funny that some steel can catch it like the plague but other bits of the same material are completely devoid of it. 

Michael W

Edited By Michael Walters on 09/11/2016 22:51:08

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