Drawing and Machine tolernces

[dchallender]

Hi folks

I've just started to build a marine triple expansion engine to the JP Bertinat design after having the castings lie around for 7 years (I had kids and was busy! ;D).

Anyway I now have time and I have built a number of Stuart model engines in the past and given that the triple is a more complex engine I was wondering what tolerances should I be using for the machining of the castings etc. I was thinking 0.1mm / 0.004" Is this acceptable? as you can see from a section of the drawings there are no visible tolerances on the dimensioning or on the drawing itself. My previous engines I have fitted the parts together as they were relatively simple engines and parts, this one is different and want to minimise build errors acumulating.

I had a search on the forum and cannot see anything specifically around typical ME tolerances that are used / recommended.

Any advice greatly appreciated, I will also start to document the build as I go from now on and try to get it on here as a thread - just realised I have taken any photos yet in my eagerness

rgds

Darren

[jasonb]

You really need to take a good look at each part and determine how critical it is and that will give you a guide to teh accuracy needed.

For example a couple of tapped holes for a non essential item could be placed +-0.005 but if all your bearings and housings were out by that much you would have a serious problem so these should be 0.001".

Sometimes the way the dimension is given will also indicate the accuracy but it will depend on the draughtsman but the standard is

2 decimal places +-0.01
3 decimal places +-0.001
4 decimal places +-0.0001

I generally try to work to 0.001" which is not that hard to achcieve and I feel is what you will need to be aiming for on teh marine engine but you will need to be marking out with a height gauge not steel rule and a DRO on the mill makes accurate placement of holes and faces a lot easier.

Look forward to seing some photos but can't see the drawing you refer to.

J

[dchallender]

Hi Jason

Thanks for the advice I suspected that I needed to go for a greater accuracy and you have confirmed that now. I also took delivery yesterday of a DRO for my mill which i am fitting on Monday which should help me improve the accuracy of the milling of the various elements.

Best rgds

Darren

[jasonb]

With the DRO you should actually be able to eliminate a lot of marking out, just use an edge or centre finder edepending on the item, zero the readout and then just do all your work from that point. Though on a complicated part it is a good safety method to do some basic marking out as a double check when you come to machine.

Is it just a digital caliper scale type or proper scales and readout with all the functions?

J

[Tel]

Assigning tolerances can become very confusing - just make the parts to fit to the best of your ability and don't worry about 'em.

[dchallender]

Hi Jason

the DRO are a series of digital scales with remote readouts so nothing too fancy as I couldn't really justify the expense on a high end DRO for the mill they will go down to 0.0005" in measurement though. I think I will end up doing a combination of both using a height guage to mark out and do coordinate drilling / milling.

I'm working to 0.001" as a general rule on the tolerancing and managing to hit this at the moment so things are looking ok.

Tel - to your point I do think I will be fitting some of the parts as well though.

The good thing the whole thing is making me think and getting me back to some real engineering which has been a while. ;D

again many thanks

Darren

[abby]

Darren , you must be very careful with cheap DRO's as they not are as accurate as you might believe.
Although the current crop of remote readout devices may read out to 1 thou or even 1/2 a thou they are only accurate to + or - 4 thou, and practically useless for anything other than woodwork.
Dan.

[davebreeze]

Apr 20, 2012 20:19:32 GMT abby said:

Darren , you must be very careful with cheap DRO's as they not are as accurate as you might believe.
Although the current crop of remote readout devices may read out to 1 thou or even 1/2 a thou they are only accurate to + or - 4 thou, and practically useless for anything other than woodwork.
Dan.

The ones Arc Euro Trade are selling are quoted at around + or - 0.001" (between 0.02 and 0.04mm).

[Deleted]

Apr 21, 2012 16:09:08 GMT davebreeze said:

Apr 20, 2012 20:19:32 GMT abby said:

Darren , you must be very careful with cheap DRO's as they not are as accurate as you might believe.
Although the current crop of remote readout devices may read out to 1 thou or even 1/2 a thou they are only accurate to + or - 4 thou, and practically useless for anything other than woodwork.
Dan.

The ones Arc Euro Trade are selling are quoted at around + or - 0.001" (between 0.02 and 0.04mm).

Agreed. Alright for banging clearance holes in plate work on a mill, but certainly not accurate enough for precision machining.... You should be able - with sharp tools set properly - to put on a cut of less than half a thou, and rely on it.
JB