Chinese Depth micrometer rods

I bought what i thought was a good priced second hand chinese (IP54 branded... yes that's the name on it!) 0 to 150mm digital depth mic.

Turns out the 0 to 75mm rods are missing,

Before i open a dispute and return it, i'm wondering if anyone knows if i can buy the 3 rods i need on their own,

Or if someone has one of these chinese mics that the main head has died.broken, and has some rods they'd sell me (seems most chinese mics use the same type of rods, but the big branded mics that actuall sell spare rods have different ones)

The rods measure 4.46mm diameter, and the bit that holds them in the mic body is a tiny circlip type thing (the type that's like a piece of wire wrapped around a groove)

I did toy with making some rods up, would need to buy some 4.46mm ground steel rods / drill rods, but it seems i'm looking at gauge pins for that exact diameter, might get the 0-25mm one out of one of them,

Of course they'd never be any where near accurate, no way i could get the ends ground perfectly flat at 90 degrees to the shaft,

If you can get some 4.5 mm Silver Steel, that might be a good starting point. A 13" length may be sufficient to make what you need, presumably 0-25, 25 - 50 and 50 - 75 mm?

Once cut to a little over length, the ends can be carefully faced, with a sharp HSS tool and fine feed to the required length. The lathe should ensure that the ends are square to the axis of the rod.

You could make the required snap rings out of piano wire possibly.

You would need to grind up a tool, possibly a 90 degree Vee form, to produce the groove in the rod, unless you fancy your chances of grinding a tool with a radius small enough to match the diameter of the snap ring.

Position of the groove relative to the end of the rod may be the vital dimension, requiring some trial facing cuts. and measurement.

Howard

measured the rods diameter with a micrometer (used calipers before) and they are between 4.489 and 4.493mm

So 4.5mm rod should fit,

I 'think' the grove with the snap ring in it is just to hold the rod up the snout of the tool, if i insert a rod backwards, once i've got the air that gets trapped in there out, it will move out a little... more a chance to drop the rod out when not measuring that owt else, as the rod will be pressed against the inner end when pushing down on something to measure.

The seller has offered to let me return it, but then there's the thing of getting another one for ~70 quid.

An option would be for me to buy a non digital chinese depth mic that uses the same rods, can get a 0-100mm one for £42.50,

Take the 3 smallest rods, then try and sell the non digital mic on as a 75-100mm one only... can you tell i'm on a tight budget here

But not selling the other mic on means i'd have paid £111 for the digital one with all the rods, that's kinda getting to what a new one costs from china, but then the risk of import taxes bumping the price up.

I know people will say 'don't bother with the digital part' but i'm really bad at maths and mix numbers up, so even reading the digital readout i can screw things up,
i've got no hope with reading lines and adding the numbers together... going to be hard enough for me to remember to add on the length of the rod when measuring over 25mm... kinda hoped a digital mic would allow you to input the rod's length to add to the reading.

Gazz

As Howard says DIY should be practical with a sharp tool.

Getting them exact to length is probably impossible but the circlip style end location isn't tremendously precise so normal practice with these is to zero the display or set a fixed offset by reference to a suitable gauge block or other standard before using. Bit more faff but no great issue once you have suitable references.

The non digital breed have an adjuster on the end of the rod, usually apair of threaded rings locked together.

Clive

I have a couple of mechanical Mitotoyo depth mics and during my work with aircraft parts, I had to measure the stroke of a hydraulic landing gear lock for the Airbus A320. The movement was within the mic range, but I still needed two of the rods. I got the toolmakers to make me a custom rod which was halfway between and then could measure the stroke in one operation. The rod was silver steel with a pressed on end. You could easily make rods which would not have to be exact lengths if you also have a set of slip gauges. The digital can be set to zero at any point, so the exact rod length does not matter. Turning the ends of each rod with a tool set exactly on centre will give a rod that is better than 0.0001" at the end which is plenty for real life work.

I don't understand the above statement, can you please explain. Thanks

Tony

The original rods should have been ground and lapped square at the bottom end, but you can get pretty good results with facing off in a lathe.

If the lathe doesn't face EXACTLY square, (Many lathes face VERY slightly concave ) the out of flatness over 2.25 mm (radius of a 4.5 mm rod ) is barely going to be measureable with the kit that most hobbyists have.

So 0.0001" should be well within capability.

Within a measuring instrument containing all original parts, there will be a number of tolerances, on each component, albeit fine limit, which build up to depart from ABSOLUTE accuracy under controlled conditions.

We think of slip gauges, in good condition, as being accurate to a millionth of an inch. Stack up several slips, with the protective ones, and you can have a variation of several millionths of an inch from one box of "perfect" slips to another from the same manufacturer, even under the closely controlled conditions of temperature and humidity of a Calibration Room. .

In high volume production industry, a measuring device that is in error by 0.01 % of its operating range (SAY 0.002" in 18" ) will be condemned

And there will be very few hobbyists who work under Standards Room conditions, with measuring equipment that is calibrated on a regular basis.

Working consistently to less than 0.0001" is going to be difficult with hobby machines that are.either built down to a price or used and, therefore, worn.

In the hobby world, most of the time working to less than 0.0001" is going to stretch machine, measuring equipment and operator skill to their combined limits.

Being realistic, many of the parts in our car engines will have been made to a tolerance of 0.001". Not much requires closer than that, except some surface finishes, and fuel injection equipment which does require single micron accuracy.

Howard

Dead right, Howard, I am pleased to get to +- 0.001" ,0.025mm for all but exceptional jobs. I worked many years in the manufacturing of aircraft components and rarely saw tolerances tighter than +- 0.0005".

Amazingly i have managed to locate someone who was throwing one of these out as the electronics had died (The one thing opposers of digital mic's always point out, electronics die that's it, game over unless it's the type with the standard micrometer barrel fitted as well as the electronics head, then you have a heavy and clunky mic with parts sticking out that just get in the way now)

The dead mic set i'm getting is missing the 0-25mm rod, but has the other 5, so i'll be chopping one of them down,
Just measured the 2 rods that i can measure with my calipers, and they are:

75-100mm = 120mm
100-125mm = 145mm

So i deduce the 0-25mm rod should be 45mm long in total.

At least that is the one rod i can get a little out and be able to set zero easily without a gauge block (looking for a set of them after i get the 4 jaw chuck for the mini lathe, as i want to make an eccentric bushing thing for my 2 month old (chinese of course) mill to improve the quill rack and pinion mesh and reduce the 0.47mm of up n down play it has)

Thinking i'll cut the rod with the ground end intact, and machine the taper and grove for the 'circlip'. and even better if i can do it with the end that will not be used in the lathes jaws.. tho the rods are hardened stainless steel, i doubt my lathes jaws will mark them.

45/256" is 4.46mm

think my measurment is out for the length of the 0-25mm rod,

Should it be 70mm? rod length is lower measuring length plus 45mm, not 20mm. think i was reading the upper range to cock that bit up.

If you are not sure, just fit one of the surplace rods, zero the mic mechanically and measure how much is projecting.

Edited By old mart on 27/04/2021 14:14:34

The 0-25 mm rod should be 70 mm long, by my reckoning (25 + 45 mm ) judged by the 45 mm difference between the rod lengths and the measuring ranges quoted above.

So, it looks like you are almost completely set up and ready to go!

Howard

Gazz, was just digging around and came upon this by chance, it may solve your problem ?

https://www.engineering-gauge.co.uk/products/digital-depth-micrometers

John

I now have the broken micrometer someone on another forum sent me, and i have my measurments wrong for sure.

This is the rod lengths that i have:

125-150mm = 170mm

100-125mm = 145mm

75-100mm = 120mm

50-75mm = 95mm

25-50mm = 70mm

So thats makes the 0-25mm rod 45mm long?