Making a large internal lap for one off job?

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Making a large internal lap for one off job?

This topic has 10 replies, 6 voices, and was last updated 28 December 2010 at 09:43 by Peter Bell.

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20 December 2010 at 20:04 #60923
Peter Bell
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@peterbell11509
Hi,

Just boring out an Arborga head casting but need to lap it to get a decent finish before we sort the new spindle size out. .

Seen plenty of articles about making nice precise smaller laps and made some previously with great success but has anyone got any ideas about making a largish (60mm) lap for a one off job, is it possible to make one from close grained wood for example?

Thanks

Peter
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20 December 2010 at 20:04 #11726
Peter Bell
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@peterbell11509
20 December 2010 at 21:06 #60927
Engine Builder
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@enginebuilder
Wood works fine Peter. I have used it many times for lapping a cylinder. You need to make it so it can be expanded. On method I have used which works well is start with 2 pieces of wood say 2 x1 and about twice as long as the cylinder .Form a groove across the centre of one piece. Glue them together at the ends only to make a 2×2 blank. This can be turned to the required diameter between centers and a wedge pushed between to expand the lap.

David
20 December 2010 at 21:31 #60930
Dusty
Participant
@dusty
I take it this casting is cast iron, if this is the case why lap it? The danger with lapping is that particles of the lapping paste will embed themselves into the cast iron. I hope you see where I am going with this?. You will turn the head casting into an outside lap and the spindle will wear, more to the point the harder the spindle the more it will wear.

I would use a hone (Polly sell them) this if used properly will give you a nice figure of eight finish which will hold the oil. This in turn will prolong the life of the head and spindle. If you do decide to lap, the lap must be of a softer material than the component being lapped. You must make sure that every last speck of lapping compound is removed from the casting, not an easy task especially with cast iron and its porous nature. Lapping has its uses, but to my mind this is not one of them.

Edited By Dusty on 20/12/2010 21:32:51
20 December 2010 at 22:18 #60935
Versaboss
Participant
@versaboss
Dusty, using Timesaver lapping compound would prevent the problem you mention. This stuff breaks completely down and leaves no grit in the cast iron. Afaik Timesaver is available in England (and USA, naturally). Google may help.

Greetings, Hansrudolf
20 December 2010 at 22:31 #60936
Peter Bell
Participant
@peterbell11509
Thanks for the advice, I like the idea of wood, however, since posting I have been rummaging in the materials box and have a piece of suitable aluminium which I could adapt to fit a previous taper lapping arbour. So it looks like that is a possible direction. But I also need an external lap and wood seems attractive at this size

Regarding the lapping paste. I also use Timesaver which does not embed itself like carborundum especially if the right grade is used. I have had a lot of sucess with this ever since seeing a demo at a ME exhibition ages ago.

The downside on hones is in my experience it is possible to make things “bell” shaped, but perhaps thats just my (lack of) experience!

Peter
21 December 2010 at 03:01 #60943
Ian S C
Participant
@iansc
   Hi Peter, I use wood as a lap when making cylinders for my hot air engines. I turn the wood a tight fit in the bore, make a saw cutalong the length of it, and put a screw (wood or self tapping) to expand the lap. Then settle down to a long process of lapping out the last few 1/10ths of a thou. ‘Fraid I don’t have any Timesaver, but it would help no end, as I’v just got some very fine grade grinding paste (a 500gm pot).

   As far as bell mouthing goes, the trick is not to allow the hone to come out of the bore too far. Don’t let it come out further than half way between the tip of the stone and the hinge point. Ian S C

               PS   My cylinders are steel, rather than cast iron.
21 December 2010 at 08:15 #60946
Peter Bell
Participant
@peterbell11509
Hi,

Thanks for the comprehensive instructions and it looks like wood wins! I found that Timesaver really lives up to its name a long time ago and it is well worth going to the trouble in getting some, the yellow grade works well of softer materials including brass.

I also want to lap the spindle, anyone have any hints on an external lap for this, a wooden tube springs to mind any suggestions on applying adjustable pressure/tension?

Peter
21 December 2010 at 08:34 #60948
John Olsen
Participant
@johnolsen79199
Maybe a piece of large round wood, bored down the middle to the right size, then a couple of hose clips around the outside to apply the pressure? I have done this sort of thing in a smaller size with a brass lap. Hose clips are also sometime handy for improvised dogs for driving jobs between centres, they will sometime fit the job when the proper ones won’t. (Does anyone else still do jobs between centres? )

regards

John
21 December 2010 at 13:49 #60970
Ian S C
Participant
@iansc
For an external lap, 2 sticks of wood, say 6 to 8″ long, hinged together at one end. Near the hinged end make a groove to accept the shaft. Put it over the shaft, which is mounted in the lathe, apply the grinding paste and start the lathe bring the two sticks together, and thats about that. Ian S C
28 December 2010 at 09:43 #61316
Peter Bell
Participant
@peterbell11509
Thanks for the advice, over the festive season we have been working on the job and it is now finished and we are very pleased with the results.

We had bored the head casting previously, this was not withour delay as the Abene mill we were using suffered a gearbox failure just as we got started and it took a week to overhaul it and replace 3off 3mm dia needle rollers in a clutch which were causing the problem

After finding a piece of aluminium the right size we made an internal expanding lap. This worked well and we soon had this nicely lapped out using medium and then very fine Timesaver compound.

The old spindle was turned down assembled so we could use the bearings to support it in a large collet chuck in the lathe. The spindle turned out to be a cast material and was cracked where someone had missed drifting a drill out. We sleeved this with EN8 locitied on and turned to almost size. In classic tradition the final cut produced grooves and a slightly wrong size which was worked on with fine wet & dry.

A wooden lap hinged at the top was made based on from some of the suggstions, the centre wood was made of Jarra and promptly split on the first use. However the loose piece was held in place with a cable tie and all was well.

Measuring the fit of it it all was quite awkward and we managed to get one end to just about fit and worked down the spindle. We could use the spindle to keep checking but eventually resorted to a an ancient micrometer more as a comparator on the shaft to see where the errors were.

Eventually we got it to be a nice press fit along all its length so we were very pleased with the outcome. A bit more fiddling and working resulted in a smotth “push” type fit. We think that the aluminium lap produced a more accurate finish but the wooden one was certainly easier to make and use.

Thanks to “Timesaver” and everyones helpful suggestions we got there

Peter
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