Author Topic: Lapping (and honing) - some techniques for discussion  (Read 69198 times)

Offline Jasonb

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Re: Lapping (and honing) - some techniques for discussion
« Reply #75 on: May 17, 2013, 07:40:08 AM »
Quote
turn up a accurate plug gauge about 0.005 down on required bore diameter

Just for any of out US friends it may be worth pointing out thats 0.005mm not 0.005"  :facepalm:

Offline Ramon Wilson

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Re: Lapping (and honing) - some techniques for discussion
« Reply #76 on: May 17, 2013, 08:32:58 AM »
Well spotted Jason - glad to see the text is read too  ;)

I try to ensure all measurements are noted as metric as that's what I work in most of the time only refering to imperial where necessary - this one slipped the net but you have to agree the 'pic' was in metric.. ;D


I'll modify the post - thanks

Ramon
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Offline b.lindsey

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Re: Lapping (and honing) - some techniques for discussion
« Reply #77 on: May 17, 2013, 12:56:33 PM »
Ramon, just a question...from the pictures it is obvious that both the cylinder as wall as the lap itself are being abraded away albeit slowly...hence the matte finish on both surfaces.  I am curious if you have ever done any measurements on how much the copper diameter changes and if a harder cylinder material (say steel or cast iron as opposed to brass) tends to wear the copper away at a faster rate? Certainly the diameter of the copper part can be increased as needed both to take up for abrasion to the copper and to achieve the desired cylinder bore and I understand that, i am just wondering if there is any feel for the relative amounts of abrasion to the lap and the cylinder and how that changes based on cylinder material?  Hope that makes some sense.

Bill

Offline Ramon Wilson

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Re: Lapping (and honing) - some techniques for discussion
« Reply #78 on: May 17, 2013, 01:26:15 PM »
Hi Bill, that's a good question. I assume you are meaning the amount the copper wears relative to it's use - the actual diameter varies very little,  the annular gap getting smaller as the grit size diminishes.

If it's wear have to admit I don't know - I've never ever thought to see how much the lap actually loses, I guess by being totally focussed on the liner bore measurement. The lap is just expanded as required. I do 'measure' it occasionally as used to keep tabs on any out of roundness - which so far has not been found - but not in reference to it's actual thickness/wear. Never really given it a thought before  ::) I haven't used 'copper in brass' so can't say but in cast iron and En1a (12L14) the wear appears about the same - in high tensile steel a little more. As you might expect the real wear comes in the initial lapping to remove the tool marks - it's surprising how little is required to refine the finish after that unless inducing a taper which takes a little more time.

I would say these are very much 'make and use' tools - they are not suited for sustained runs or long term continuous use - depending on the turned finish in bore to begin with I would say 6-10 bores would be a good average in mild steel.

Hope that helps some - Ramon
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Offline b.lindsey

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Re: Lapping (and honing) - some techniques for discussion
« Reply #79 on: May 17, 2013, 01:34:07 PM »
Thanks Ramon...that does help!

Bill

Offline Ramon Wilson

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Re: Lapping (and honing) - some techniques for discussion
« Reply #80 on: May 17, 2013, 10:41:34 PM »
You're welcome Bill  :)

Following on from the previous post the technique used to create the conditions for a good piston/liner seal in a compression ignition model engine - usually mis-referred to as a 'diesel' is fundamentally more of the same but using a finer grit - in this case F600 - to smooth out the bore and induce a very slow taper from the bottom of the liner to just about where Top Dead Centre would be preferably slightly lower than higher. Technically it isn't neccessary to produce a taper at all but by doing so it provides a the means to lap the piston exactly to the dimension required and also once finished and in place reduces friction in the lower part of the bore. The taper is very small dimensionaly - certainly not measurable with basic kit but again the plug gauge will give an indication.

The lap is thoroughly cleaned of all traces of the 320 grit and a similar paste mixed using F600. The procedure is exactly as with the 320 but this time the lap is allowed to dwell at the bottom end and the lap is not taken all the way up the cylinder. Ideally the area taken up by the contra piston should remain as previously lapped but allowing the lap to go through this area every so often will not affect this too much - certainly not enough to cause concern. It does not take long to produce a situation where the plug gauge, with everything nice and clean, will now enter the bottom of the bore with ease but beginning to tighten at around the exhaust position. As soon as this state is reached any further lapping is really to 'polish'. Lots of lubricant but no more paste. just expanding the lap to provide a slight contact if it gets too loose.

Previously a further lapping using F1000 powder has been carried out but on this latest batch it was felt that the finish acheived with 600 was sufficient for the next stage which is to lap the piston to the bore. The resultant seals are just as good if not slightly better than previous.
These are the liners done for the Racers  - the first of the scaled up engines as you can see the 1000 gives a high degree of polish

on the left is cast iron the right is high tensile steel which appears to polish to a higher finish than En1a

These are the two Tigre liners as shown previously having been lapped with 600 - very smooth but no sheen.



Before the piston can be lapped to a perfect fit with the liner the piston itself requires lapping. For this an external lap requires making. Whilst all that was available was a home made lap something that was considered to be very desirable was an external hone. After along wait for the right kind I was finally able to secure two sets at very reasonable cost off E-Bay. Much to my surprise when they arrived they were absolutely untouched - just one, easily replaced, screw missing


Desirable yes but not exactly neccessary - Used on the Tigre engines they do work well but appear to have no advantage at all over this....


Easily made, this piston lap works extremely well and removes metal just as easily as the hone - the hones were left in the box on this latest batch - the lap you see has done all seven Etas and the original gap cut to allow closing just short of nipping up on the last piston

The procedure is the same  for the liner - begining with 320 the piston is coated and a smear applied to the lap


Constantly checking the dimension, the diameter is reduced until the top of the piston will just enter the bore by about 3mm or so then the lap is cleaned and the grit changed to 600.


The lapping continues until the piston will enter the lower bore becoming tight as the crown reaches just below the exhaust area. Lapping on the lathe stops at this point and the remaining lapping of piston to liner is carried out by hand


Once again the piston and liner is thoroughly cleaned of all traces of the previous compound. A tee handle driver is required and this has an end that will fit inside the piston with a cross hole that will take a reasonably tight fitting brass 'wrist pin'


The piston is attached to the end of the rod and a very small amount of F1000 compound previously mixed applied to the piston and smeared around


The piston is then inserted into the liner and with a very slow wringing and in and out action the piston/liner are gradually lapped together.


By it's very nature this is a slow process and one that cannot be rushed if a good result is to be achieved. If the piston begins to tighten and stick it shouldn't be forced - it can tend to pull itself in - tapping it gently from the other end using a wooden dowel or similar as a driver will soon release it. Wipe some of the paste off and apply plenty of lube and continue. This op does not require much compound. It  can take as much as 15-20 mins for this op - but once the piston is fully inside and about halfway up the bore past the exhaust it's removed and wiped clean - no more - and relubed.


All the lapping from this point uses the residual compound and lots of lube. When it begins to squeak it's getting dry so it's removed, more lube applied and the process continued


The piston is lapped in this fashion until the skirt is just level with the bottom of the exhaust....


... then removed and thoroughly cleaned in solvent. If an ultrasonic cleaner is available then a quick rinse in that is ideal after cleaning.

That's enough for tonight - sailing day tomorrow   :) and an early start

regards - Ramon
"I ain't here for the long time but I am here for a good time"
(a very apt phrase - thanks to a well meaning MEM friend)

Offline Ramon Wilson

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Re: Lapping (and honing) - some techniques for discussion
« Reply #81 on: May 22, 2013, 08:35:08 PM »
Hi again 'Guys'  - apologies for the delay but family problems got in the way. I think we should be able to bring my input on this to a close tonight - hopefully others will add their experiences to the pot  ;)

Once the piston is lapped to the liner it should be removed and stored safely awaiting assembly. The contra piston can now be tackled. Like the prop driver this small item can be a bit awkward to hold to get all the ops done - Previously I have bored the back (top face) out to a strong push fit on an expanding mandrel with the screw removed. The spring in the mandrel providing the drive. This has proved more than sufficient grip to turn the outer diameter and to turn in any concavity required as well as for the lapping.

This can be carried out using the piston lap but the fit required is finely toleranced - too tight and the comp screw is hard to turn and the contra piston difficult to 'fire' back up the liner if over compressed. Too loose and the feel on the screw is marginal at best and the comp screw can easily back off when running though this can of course be compensated for by a locking bar. The lap however is much wider than the CP and can induce 'barrelling' by wringing easily.

After trying several methods however I have settled on using two fine flat india stones held by hand which gives excellent control, is easily rendered and stock removal is quickly judged......







As you can see the two stones are nipped gently together at the lower end holding them in contact at the top end with slight fingure pressure. With the lathe running the stones are moved in a slight arc back and forth across the CP pivoting the stones in the lower fingers. With plenty of lube this gives very good surface finish with no loss of roundness (A single stone can soon induce that BTW) and as said good dimensional control though constant measuring is the order of the day. It needs to be a tight but easy (if that makes sense) push fit in the liner. It's better to be tighter - more can come off, but looser - well the solution to that has already been covered on the Eta build  ;)

A couple of follow up things that should be of interest ....

You may recall the query that kicked this all of was the difference between lapping and using sprung arm cylinder hones to 'true' a bore.
You may also recall that when the cylinder fins were shrunk onto the liner for the Mk2 Elite that they weren't quite in line. Well, in my haste to try to put that right I quickly set the liner in the soft jaws gripping on the exhaust flange in the hope that I might get sufficient leverage to twist the fins slightly. What I didn't notice was that the part of the jaws behind the cut out gripping the flange was just - fractionally just - too small and consequently distorted the liner below the exhaust flange. First thought was that it was a 'bin-ner' but on measuring the distortion appeared to be only about 0.04mm and had not affected the bore above the flange. Lapping took this anomaly out without any difficulties and produced a nice round lower bore albeit slightly larger than that planned.  I took this pic to try to show the distortion accentuated by the lapping. Though not that brilliant you can however just see an un-lapped area (horizontal tool marks) between the two matt areas directly below the top most cut out.


I think it would be fair to say this would never have come out with a sprung loaded hone.
The piston lapped to the bore as well as the others and the engine appears at the first running to have a distinct 'edge' to the others. Less friction at the bottom end? who knows.


The other small thing of interest was something tried for the first time. When the second Elite was assembled it was obvious the piston had not lapped itself in quite far enough up the bore, the piston becoming very tight about 10 degrees before TDC. I was about to re-lap with a very tiny amount of F1000 but remembered that I had been given some 'Diamontine' - at least that's what I think it's called. Despite it's name I'm pretty sure it's not actually a diamond based product however it's a very fine white powder. I had not tried it before so decided to give it a go.

It quickly lapped the high spot out and in so doing  produced an exceptionally polished surface at the tight spot

Again not a very good image but you can see the top band around the piston and the shiner part in the bore. Pleasingly the piston seal on this engine is the best yet.


Well Guy's I think that's about as far as my input can go on this subject. I'm positive there's more to come however so please add to it.

Thank you all for your encouraging words on the way which as always have been much appreciated. I have enjoyed doing it and it should go without saying but if just one bore get's improved it will make it all worthwhile  :).

Regards - Ramon

"I ain't here for the long time but I am here for a good time"
(a very apt phrase - thanks to a well meaning MEM friend)

Offline tangler

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Re: Lapping (and honing) - some techniques for discussion
« Reply #82 on: May 22, 2013, 10:28:48 PM »
Ramon,

Thank you very much indeed.  Very instructive.  You've clearly developed a successful technique and I, for one, have learned a lot.

Cheers,

Rod

Offline b.lindsey

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Re: Lapping (and honing) - some techniques for discussion
« Reply #83 on: May 23, 2013, 12:33:41 AM »
Ramon, I know how much time this took to put together and write up with the great photos and such. Thanks for doing this, I also learned a lot and appreciate all your effort in putting it together in such a clear and instructive way.

Bill

Offline Maryak

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Re: Lapping (and honing) - some techniques for discussion
« Reply #84 on: May 23, 2013, 12:42:32 AM »
Ramon, I know how much time this took to put together and write up with the great photos and such. Thanks for doing this, I also learned a lot and appreciate all your effort in putting it together in such a clear and instructive way.

Bill

Me 2, Thanks Ramon  :NotWorthy:

Best Regards
Bob
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Offline Don1966

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Re: Lapping (and honing) - some techniques for discussion
« Reply #85 on: May 23, 2013, 01:50:10 AM »
Thank you again Ramon for you time and efforts. I have learned a lot about lapping and believe me I knew very little when you started. It is good to have such knowledge posted here for all of us to learn from. I await your next lesson with great interest.

Don

Offline bp

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Re: Lapping (and honing) - some techniques for discussion
« Reply #86 on: May 23, 2013, 01:59:31 AM »
Terrific stuff Ramon.  Lapping/honing has always been a bit of a black art.  You have illuminated the darkness!!
cheers
Bill

Offline steamer

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Re: Lapping (and honing) - some techniques for discussion
« Reply #87 on: May 23, 2013, 02:05:37 AM »
Thanks Ramon!....per our discussion, I'll be lapping the bore and quill on my SB 9   Tailstock.   It is slightly different because of bore depth...but I'll tell ya...you have set the standard for the subject...and I'll be hard pressed to do the subject any justice at all!

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Offline AussieJimG

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Re: Lapping (and honing) - some techniques for discussion
« Reply #88 on: May 23, 2013, 06:04:58 AM »
Thank you Ramon, I could not put it better than Bill has already done.

I can't help feeling that lots of us will be doing a bit more lapping and getting better results.

Jim

Offline vcutajar

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Re: Lapping (and honing) - some techniques for discussion
« Reply #89 on: May 23, 2013, 07:03:41 AM »
Thanks Ramon for a very instructive thread.

Vince

 

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