# How Do You Measure Groove Depth When Depth>width



## tomw (Sep 13, 2015)

Dear All,

I am cutting some piston grooves on a steam engine model. The are to be 0.054 +0 -.005 in depth, and 0.050 +/-.002 in width.

So, I cannot use my usual trick of measuring across something round of known size that sits in the groove (the groove is deeper than it is wide). 

How would you go about confirming the depth of these grooves?

Thanks,

Tom


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## Tony Wells (Sep 13, 2015)

The standard way is to use a blade micrometer. Do you have a depth micrometer? Hard to say without knowing what you have on hand. Gage blocks? Need more info.

Width is easier with pins or blocks. Set up a go/no-go pair. Of course, there are instruments for that also.

In the "olden days" a pair of spring calipers and verniers would be used to get the "depth", or actually the groove diameter.


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## tomw (Sep 13, 2015)

Tony,

Thank you for the rapid response. I have a 1" mic, 6" digital calipers, a set of gauge (or gage) blocks (but not two), height gauge, digital height gauge, and a surface block.

I had not heard of a blade micrometer. Neat and expensive. I guess I could machine a go/no go gauge on my mill, Say .049 by .053. If I take an outside mic reading and it is larger than the original diameter of the piston, I know I need to go deeper on the groove.

I wouldn't be so worried about this except for the tolerance call out on the plans. My original plan was to rely on my crossed dial.

Cheers,

Tom

p.s Here is a picture of me making a trial piston. It is .508 dia x .250. The ring grooves are .05 by close to .054.


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## John Hasler (Sep 13, 2015)

Will the blades on your calipers reach to the bottom of the groove?  If so put one blade in the groove and the other on the o.d. next to the groove on the opposite side of the piston and set zero.  Then put both blades on the o.d. on opposite sides of the piston.  The reading is the groove depth.  The small error due to the blades not being quite opposite will be negligible.


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## Andy Rafferty (Sep 13, 2015)

On steam engines the depth you are searching for is only critical in considering the expansion of the ring material and the diameter of the piston in relation to the bore. You have the perfect set up in your picture, indicate the outside of the piston with your tool and trust the dial on your cross feed plus the allowed error of the print better to be deeper than too shallow. Good Luck.  Andy


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## Billh50 (Sep 13, 2015)

one way would be to turn up 2 parts that fit the micrometer anvil snug and having a .060 long diameter to fit grooves. Close micrometer to get initial reading with them on micrometer so you know how much to subtract from reading in groves.

another way would be to use a dial indicator with a tip turned down to .048 and set the indicator as close to center height of part as possible and check depth.


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## tomw (Sep 14, 2015)

Andy, Thanks. That is the approach I have decided on. For future projects I am still interesting in hearing how folks to this.
Billh50, I like your idea of creating an blade micrometer. I might just try that!

Thanks,

Tom


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## Silverbullet (Sep 15, 2015)

You could use thread wires, or drill bits to ck also .


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## Billh50 (Sep 15, 2015)

you could also buy these to put on micrometer anvils........http://www.mscdirect.com/product/details/06235006  or http://www.mscdirect.com/product/details/76404102


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## tomw (Sep 16, 2015)

Silverbullet, thread wire or other round things won't work because the depth of the groove is greater than the width.

Billh50, thank you for the links!


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## Badspellar (Sep 16, 2015)

You can always grind a flat on anything round to make it fit the groove.


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## Bill Gruby (Sep 16, 2015)

If I am going to measure a lot of grooves I make a simple set of go/no go gages. Just like snap gages.

 "Billy G"


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## chips&more (Sep 16, 2015)

Tom, you can turn a diameter test piece with the groove tool and note on the cross dial the size needed using a regular mike. Then put the piston in your chuck and make your grooves turning the dial down to that referenced mark you just made on the cross feed. You can check the width with pins…Good Luck, Dave.


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## kwoodhands (Sep 29, 2015)

tomw said:


> Dear All,
> 
> I am cutting some piston grooves on a steam engine model. The are to be 0.054 +0 -.005 in depth, and 0.050 +/-.002 in width.
> 
> ...



Lap a  1/16" T-type parting tool to .050. Then locate the first groove and when the tool starts to cut , set the dial to zero.
Cut .050 deep. If you will use a viton seal ,have it on hand to check the fit.
mike


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## Holescreek (Sep 29, 2015)

Why not just use the height gauge you already have? Find the high spot on the OD cylinder and set zero then put the tip in the groove and measure down to the high spot of the groove diameter. Most indicator tips (assuming you keep an indicator on your height gauge of course) are less than .03" around.


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## Arrak Thumrs (Sep 29, 2015)

Or a dial indicator.  The plunger type--not the lever, test indicator type.  Replace the screw on tip with a long, thin one.  For example:  tapered contact point, 4-48


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## kd4gij (Oct 1, 2015)

With the tools you have, if the tips of the calipers won't fit into the grove. The back of your calipers is a depth gauge.


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## Paul in OKC (Oct 1, 2015)

I like the idea of grinding flats on a drill bit shank, like a 1/16 bit to just fir the grooves and measure across.


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## tomw (Oct 3, 2015)

Wow, there are a lot ideas to digest. I am going to be machining a new piston for the first PMR model I made, and I will definitely try many of these options. I love measuring things both accurately and precisely. It's a sickness.

Thank you all for your help.

Tom


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## Bill Gruby (Oct 3, 2015)

Nope, it's not  a sickness of any type, it is the way of life for a machinist.

"Billy G"


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