Lapping (and honing) - some techniques for discussion

[Bearcar1]

Keep those 'cards and letters' coming Ramon, this is proving to be a very stimulating subject and has many of us interested. 


Bc1
Jim

[Ramon Wilson]

Hi again 'guys' - time for a bit more ?

Couple of things first -

Don I have never given a great deal of thought to the actual length required - I don't actually know if there is a recommended length to diameter ratio.  Some of the commercial laps I've seen pics of seem to be quite long relative to their diameter but all the laps made so far have been around one and three quarters to twice the diameter. All the bores treated so far, though longer than the lap have responded well to this ratio without bell mouthing. They can also be used to lap a blind bore to within a small distance from the end.

Most of the small I/C engines dealt with  have had ''tapered' bores - that's in inverted commas because the degree of taper is not really measurable but more a 'feel' thing. This method of treating such a bore will get covered later but basically it's created by dwelling in the area required larger. It occurred to me after your query that possibly this type of lap could have a taper induced into it by providing a secondary screw in line with the first. Worth a thought perhaps.

On the last engines built a different form of lap was used - based on one made previously in aluminium and to all intents something that would open radially. A steel shaft was turned at a slow taper (5 degree inc.) and a D bit type tapered reamer made at the same time so that matching blanks could be made in the future. Incidentally using a more shallow taper makes for easier control but requires much more longitudinal movement for a give expansion. Too coarse and it will become extremely difficult to move using a reasonably small screw.
Although relatively easy to make it's 'shortcomings' soon transpired in that not only did it require considerable torque to expand but that once expanded it could not easily be released due to the slow taper.


On the Super Tigre build there was the requirement to lap the phosphor bronze bearing sleeves in the front and drum housings. These were relatively small bores 10.0 and 9.5mm diameter and aluminium was preferred because of lapping bronze...


This time a mod was made - the shaft was reduced in diameter and a sleeve made to slide over it (front left) this way the collet and expanding screw could be slackened and the lap released by pushing on the screw using the tailstock. Not precise but it meant some control over over expansion



It worked extremely well however, lapping the bores to a very precise fit to accept their respective shafts - this is the front housing being lapped.


When it came to doing the bores a larger lap was turned and this had radial grooves cut in to ease expansion - because these are not parallel to the taper but to the OD the amount of torque required to expand this was quite excessive.


Though a reasonably good finish was achieved it was felt that the aluminium did not give quite as good a finish on the En1a (12L14) as the copper laps do but comparatively it was quicker and perhaps easier to make. Personally however I would not go down this route again except in smaller sizes - say 10 mm and below. The control on expansion that the 'D bit' type lap gives is far far better.

I'm sure there are others who go about this in other ways - it would be good to hear from you if you do as there's always another way of doing something

With the (internal) laps basically covered it's time to move on to the lapping itself. To do this a variety of grades of abrasive compound are required. There are several types of abrasive that can be used - Aluminium Oxide, Silicon Carbide, Diamond etc.
Except for the Diamond - which usually comes in paste form the difficulty is in getting these compounds in small amounts but there are sources to be found on the 'net. The use of the readily available diamond paste however is something to be treated with a degree of caution. It is really designed for lapping hardened steel components and as such it will embed itself in anything soft - lap and work - which will not come out with the usual solvent based methods of cleaning. To ensure all traces of compound are removed the work (and lap) require cleaning in an ultra-sonic cleaner. Obviously failure to remove particles of diamond compound will soon lead to the work lapping itself in further before probably lapping itself out again 


Personal choice is Silicon Carbide - 'Carborundum' - but the variety does not have to be vast  - about three to four grades at the most. As with most things though it will depend on what's to be done - the material, amount to remove and the surface quality and/or any 'distortion'.
Valve grinding paste can be used at a push thought the coarsest is probably too coarse for the kind of work likely to be encountered. I have no idea what the actually grades are but I would think that the 'coarse' is much coarser than 320 grit which is about ideal for the initial lapping of steel/cast iron.

These small jars contain Silicon Carbide powder mixed with a small amount of light oil to a thin and runny, paste like constituency


The three larger jars contain cellulose (gun cleaner thinner) The part is popped into each in turn to clean it - After an initial wipe with paper towel a good swirl in the first to remove the worst followed by the next two in turn produces a pretty clean part ready for checking.

To give some idea of the effectiveness these two cylinders are after the initial lapping with the ali lap above using 320 grit - the one on the right still showing traces of tool marks requiring just another 'lick'


Hope that's enough for tonight - just had that late night usual 'reminder' "Bathroom's free'' 

Next we'll get to grips with the lapping process itself.

Regards for now - Ramon

[Don1966]

Ramon, thanks for your response to my question. I have again enjoyed your very informative class and look forward to more. Again thank you very much for taking of your time to give us these lessons.

Don

[pgp001]

Nice one Ramon, excellent write up.

Where do you get your lapping grit from by the way, I assume it is no different to blasting media.
I have access to around 100 tons of various grades of the stuff at work, so no problem there.

Phil

[Jo]

Oh nice. Ramon those look really useful. Which would you recommend for the Crosskill which has a 7/16" bore? the Aluminium or the copper? And where do you buy your Silicon Carbide powder for doing the lapping?

Thanks again Ramon, these posts are really useful  .

Jo

[sco]

G H Thomas talks about using 'horological powder' for lapping dovetails.

Simon.

[capin]

  Mr Ramon, thanks for sharing your knowledge and mastery of the trades with us ! I as so many here wait daily for the class lesson. Thanks again, Brian

[swilliams]

Great stuff Ramon

I've got to make one of those D-bit type copper laps, they look the business.

Steve

[tangler]

Lapidary suppliers seem to supply small quantities of SiC powder.  I've just remembered that we have a small stone tumbler in the loft - I must go and investigate, it could be useful for all sorts of model engineering stuff!

Thanks to Ramon for taking the trouble to expound on his techniques.

cheers,
Rod

[Ramon Wilson]

Hi guys - thanks as usual 

I was intending to carry on with a some more tonight but had overlooked the fact that it's 'change of cap' time - Plastic Model Group meeting tonight 

If I'm back in reasonable time maybe but failing that tomorrow for sure.

Have checked out the lapidiary suppliers since the last post - just be aware that they supply powders at the very coarse end - F320 grit is about the coarsest that's needed for our use.

Regards - Ramon

[gbritnell]

Hi Ramon,
Plastic models too! I started building plastic when I was about 9 and still do to this day. Our club meets once a month. We have auctions, shows, contests and just plain bull sessions. I just finished an F-86 Sabre decked out in the Canadian show team colors of the Sky Lancers.
gbritnell

[pgp001]

I used to think blokes who were into making plastic models were not "real" model engineers.

That is until I saw one of Ramons early posts showing the jet fighter he has built and detailed, now I consider myself to be a reasonably competent model engineer, but I honestly could not see myself managing to do anything like that.

It deserves a second look
http://www.modelenginemaker.com/index.php/topic,1344.msg17986.html#msg17986

Phil

[Ramon Wilson]

Hi Guy's - a good night last night but a bit too late back

First off a digression OT

George - I'm pleased to hear that you too enjoy the plastic world along with others on here. I returned to it about ten years ago when I made a model of a Lancaster for my cousin in law's eigthieth birthday. It was the time of the build up to the Gulf War and having made military models in the early seventies couldn't help but think what ideal subjects some of the armour would make festooned as they were with all kinds of kit. 'Fate?' decreed seeing a Tamiya magazine for the first time which had a stunning feature on a Russian T55 tank all heavily weathered - I was hooked 

Phil - Thanks for your kind comment but personally, though I often use lathe or mill to create parts, I don't consider plastic modelling 'model engineering' either. It is however IMHO a true modelling form in it's own right. Gone are the days when all there was was 'Airfix' - today the hobby has an unbelievably vast amount to offer for anyones creativity and some of the results are just simply outstanding. With todays modern machining methods however the tooling that produces these incredibly shaped plastic parts must be something truly special to see.

Forgive the digression - back to the laps


One of the first things done even if it's a one off is to turn up a accurate plug gauge about 0.005mm down on required bore diameter. Given the fact that most have very limited means to accurately measure a bore using a plug gauge will not only get you very close to the limits required but will give you a very good idea of whats 'going on' inside the bore - high spots or taper are soon detected if the fit is good.  Usually made (for me) from En1a (12L14) and polished to a high degree the only thing to be wary of in a liner made from similar steel is galling or 'pick up' The bore must be thoroughly cleaned of any trace of lapping compound before trying.




A note before we go further - this is a very messy process.  I have tried using gloves but the lack of feel with work gloves is quickly noticeable, Nitrile gloves on the other hand constantly get nipped between lap and part and get ripped if you're lucky and wind round the lap if you're not  Theres only one thing for it - lots of barrier cream and reluctantly accept to offer to do the dishes for a week   I think I've said this somewhere before but it's worth repeating

The lap should be set such that the liner hits the chuck when about 25-30% of the lap protrudes - any more and bell mouthing can begin to occur. A card or plywood, plastic or fibre washer is place over the shaft to prevent the liner galling on the chuck jaws as can be seen below. The limit of travel at the other end can either be by a matter of judgement or a stop can be set in the toolpost. One made from a piece of 6mm ply works well.

The premixed 320 grit compound then is wiped over the lap with a finger ...


With the lathe stationary, the liner is pushed on and the paste worked about. The liner should not be forced on - the grit will offer a resistance and this is something that must be learnt - it wants to be a tightish fit without excessive force but if it is loose then remove and expand the lap a little. Obviously if too tight then wipe a fair amount of the grit off and try again. If the lap was made with the clearance as recommended then there's probably too much compound.  Taking a good grip on the liner the lathe is set running at about 200 rpm and the liner immediately moved back and forth to spread the paste uniformly throughout the liner and lap.
Stop at this point and apply plenty of lubricant. It should look like this... (note this is withdrawn this far for the pic !)


Paraffin (kerosene) with a small amount of oil (about 90:10) is an ideal lubricant but others may prefer to use something else.


This is the kind of wetness ...


The liner is moved back and forth reasonably quickly - not excessively - until the lap goes loose. Sometimes more lubricant has the effect of re-arranging the grit which gives a bit more lapping resistance but eventually - in a reasonably short space of time the lap is obviously no longer working. Clean up and check the bore using the plug gauge then if required repeat the process, expanding the lap a little further. Continue this until the gauge feels as if it will just enter.

The following  pics give a better idea of the amount of protrusion of the lap. Although a slightly tapered bore is beneficial in these small diesel engines, at this stage the aim is to produce a parallel bore with a smooth matt finish devoid of all tool marks - preferably throughout the bore but particularly above the exhaust line.






Everything should be 'smoothing out' by now and the lap should look like this


Unfortunately this is not a good pic but the liners require a nice uniform matt finish at this stage with the plug gauge just able to enter - on no account should it be forced if tight but if it does get stuck tap it out gently from the other side using a short piece of wood as a drift. Obviously if the bore should get scored doing this the only option is to lap it further to remove the scoring.


If this was a steam engine bore personally I would leave it at this stage. If there was the desire to cross hatch it this would be the point to use a cylinder hone. I have no experience of fitting piston rings as yet but this is the stage I would leave a bore if I was - as shown before, the Bentley liners are such.

Model aircraft 'Diesels' on the other hand require a much better piston and liner fit to obtain the compression seal required and further work is required to bring the liner to that stage - more on that next time.

I'm trying to keep the momentum up on this so my apologies for any 'gaps'.  However as mentioned a while back there's the need to get this article finished for Model Engineer - guess what stage we're up to on that today  Sues on holiday this week coming too so that may just have a bearing 

Bear with it 

Regards for now - Ramon

[steamer]

Ya doin fine man!

Keep it coming Ramon!   this is good!

Dave

[Don1966]

Another great lesson, just keep it coming Ramon. I really appreciate your efforts and time. Thanks again.

Don