Siemens CNC Mixing MM with Inches

Siemens CNC Mixing MM with Inches

Category : Siemens

Siemens CNC Inches and Millimeters (Do you switch?)

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I remember a long time ago we used to run metric programs and imperial (inch programs). So if the drawing was in inches then that’s how we programmed it. Seemed like a good idea at the time no converting figures and no need for writing on drawings.

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You had an inch drawing, inch micrometer so why not have a program that’s in inches too?

One day someone (not me, honest) changed an X figure by 1 inch thinking it was a 1 mm.

The difference between one inch and 1 mm is what we call a country mile or a shit load. Anyway the part flew out the chuck on the impact of hitting the tool.

G20 was assigned to our history book. Oh and the boss shoved the broken tool up my arse.

I often work in Scotland where there are a lot of engineering companies specialising in Oil and Gas type work. As a result most of them work exclusively in inches. To be honest to me it’s a pain in the arse and I never quite get used to it. Although when I did my apprenticeship at Rolls Royce in 1971 things were just changing over so in theory I should have a brain measured in inches (it wouldn’t be many by the way).

Metrification (it’s not a new sexual identity)

I need to be careful here, because I’m now at that age where you can bang on for hours about all this kind of innocuous shit (it’s what old men do apart from pissing themselves if there’s not a toilet nearby).

If You Must

To change to inches on a Fanuc control select MDI and go to your setting screen, press the offset/settings button.

Just change from 0 to 1.

Siemens CNC

Siemens CNC

Shut your machine off and back on again then zero return it if required. Everything will change to inches (you’ll now have four decimal places)

The only thing that won’t change is your MACHINE position display. This is the one that tells you where you are from zero return. Most people don’t bother to change it.

On Fanuc 10t and 31i parameter 3104 bit #0 changes machine position screen to inches 0 is mm 1 is inch personally I wouldn’t bother if you intend to change the machine back.

G20 and G21 in your program will not normally change over to inches it is just a check and should give you an alarm if you didn’t first change the parameter above.

Bad News

Unfortunately when you swap to inches on a Fanuc or a Haas control it does not convert your figures. It just changes the display to four places. So x20.001 (millimetres) will change to X20.0010 (inches).

On a Haas control your tool length and work offsets will be converted so an offset of 25.4mm will change to 1. inch.

Siemens CNC to the Rescue

On the Sinumerik Siemens control it’s easy you just use G70/G71 or G700/G710.

G70/G71 just changes position values from inches to metric or vise versa.

With G700/G710 feed rates are changed as well.

Everything converts on this control so there’s nothing to do except enjoy your new units.

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It Gets More Exciting (Siemens CNC)

There are some things I really look forward to telling people when I’m training them on Siemens Sinumerik 840D controls.

Well this is one of them. It has a sad ending though because most people say “ohhh I don’t fancy doing that”.

You can mix n match!! yes you can actually mix the two. So if you wanted to you could do something like this


The G70 switches the control to inches then when you are fed up with inches just program G71 and you are back in millimetres.

Thanks for watching and reading

(Make sure you watch the video above)

If you have been affected by any of the issues in this post or need CNC Counselling then contact me.

Siemens 828 840 Sinumerik Training

Or call us

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