View Full Version : Turning / Boring query...

10-07-2008, 03:48 AM
Hi all,

I need to turn a crank-shaft and flywheel for a large model traction engine im building. Obviously the fit needs to be a good sliding fit but not tight, its held with a large taper key in a keyway.

I am short of bore measuring equipment and dont feel confident about turning both hole and shaft by measurements alone. They are too big to easily use one as a gauge to turn the other.

I have thought of turning up two pieces of scrap, one with a bore and one as a shaft with the correct fits, then using these as a ring or plug gauge to turn the crank and flywheel.

My question is, will this work satisfactorily? or do i risk having a bore thaty is too big or a shaft that is too small?

BTW, the shaft/hole is 1.75" and the the flywheel is 18"dia, crank is 24"oa.

Any suggestions much appreciated.

10-07-2008, 04:20 AM

I would make a plug gauge from 'scrap' which is your journal finished dimension with required finish and tolerance.

As a general rule I always make the shaft first and adjust the hole it has to fit to to suit.

I would therefore machine my journal to size comparing my mic readings and also check with calipers to the guage to ensure it was bang on size.

I would then bore the hole removing material until my plug gauge had the required sliding fit.

You could also make a go - no go type gauge with the go section just under size so you know how close you are to your finished size before you take the final cut to fit.

Hope this helps.


10-07-2008, 04:22 AM
You dont have telescoping gauges?

Yes, you can make plug and ring gauges. Should work fine.

10-07-2008, 04:32 AM
Thanks guys,

DickDastardly, if i read it correct, your method only requires the plug or shaft gauge and the the shaft is then turned by comparison. This sounds good and i do have a 1"-2" micrometer.

Sounds like a plan to me, good to go.

Thanks all.

10-07-2008, 04:34 AM
You dont have telescoping gauges?

Not yet but they are on my shopping list somewhere!

Yes, you can make plug and ring gauges. Should work fine.

Thanks, two answers in the same direction must mean that i was on the right track:)


10-07-2008, 07:47 AM
The original traction engine parts were machined by using ring and plug gauges. Your method will not only work, it will be prototypically correct.

10-07-2008, 08:01 AM
Yes, the more i think of it, the more it makes sense to use a single item for measurements or comparisons.

I'll difinately go with the turned plug gauge and make one end a go/no go by removing a teeny bit more metal.

Thanks all


10-07-2008, 11:43 AM
If you make the crankshaft a bit longer than you require, you can saw a bit off the end to use as a plug gauge, it should be the same diameter?
Regards Ian.

10-08-2008, 04:01 PM

thanks for the advice.

I have now made a nice gauge piece from an offcut of something that turned up nice and gave an almost mirror finish, no idea what it was but it was good to turn.

It's 75mm long and i got it to within 0.01mm of 38.1mm dia and a parallel of 0.01mm. I measured with a 1"-2" micrometer and backed up with my trusty digital caliper, both tools matched readings.

Put a nice 45deg bevel on the end and gave it a light wipe with an oily rag to stop rust. It looks the part and i'm now confident i can do the job.

Thanks again for the advice and tips.

10-08-2008, 09:28 PM
Watch the heat on the flywheel when finishing the hole. Your nicely polished plug gage can become well and truly stuck if the mating part shrinks up on it!
I don't know how much material you need to remove from the hole; but drilling and boring can create enough heat that your plug will slip right in and stay there.


10-09-2008, 08:16 AM
Thats a good point that i was aware of but not thought about.

I'll need to remove about 1/2" dia on the bore, so i'll get it close then let it sit for a while or overnight to cool.