Best lapping compound for cast iron

I need to lap cast iron cylinder. In the past I was using aluminium oxide for such job but now I got very cheaply few grades of diamond dust from China. I found it very handy for lapping for example hardened spindles.

I wonder if it is suitable for cast iron.

I know that it will work but my concerns are about inbedding lapping diamonds into cylinder wall which will then become to be a lap itself and damage piston during normal work of refurbished machine.

Is it a valid concern or I am a bit paranoic here (copper would be used to make a lap)?

If you are concerned about embedded lapping compound remaining in the cylinder Martin and causing more wear - then why not use Timesaver? That's what it's designed for.

There are several UK sources - this is one of them

Timesaver lapping compound

Regards,

IanT

Edited By IanT on 10/02/2021 09:49:37

I always use Silicon Carbide powder down to 1000g mixed into a paste with oil

When lapping cast iron pistons and cylinders for hot air engines I prefer to use a hone with fixed abrasive. To my mind loose abrasives are for grinding in valves.When working as an instrument maker on spectrometers with adjustable light slits to provide a parallel beam of light.the edges of the slits were lapped flat, flat circular cast iron lapping plates were used, the 3 plates were made perfectly flat by using lapping powder and the Whitworth method to lap each plate in turn with the others. The very fine abrasive in addition to grinding the plates flat,also became embedded in the cast iron ,the edges of the light slits were then held in a fixture and rubbed carefully on the plate,it was surprising how long the abrasive lasted and the number of slits lapped, The company owner was never in favour of using loose abrasive lo lap in circular or flat bearing surfaces,he said just look at how we lap in the slit jaws,just think what that would do to a bearing. The company also had a number of circular surface plates,these were produced in the same way as the lap plates in this case to "kill"the abrasive left in the cast iron ,the plates were rubbed with a flat oil stone .

+1 for Timesaver, get the correct colour though for ferrous parts.

Emgee

Martin,

Sounds as if you are used to the lapping process but if you use diamond paste to lap any unharded material it will embed itself to the extent that conventional solvent based cleaning will not remove it completely. It needs to be done using an ultra sonic cleaner to be as sure as you can be.

I've done a fair bit of lapping of cast - if you have alu oxide then I would use that - that way you can be certain you haven't got diamond paste lurking to wear your liner and piston out as it runs itself in.

I have a range of diamond but would only use it on hardened material.

Jason is using what I supplied him I just mix it with thin oil and use plenty of paraffin as a lubricant

Though I have not used it Time Saver is always highly recommended but is relatively expensive - Silicon Carbide much less so - I've had very good results with it on model diesel piston and liner fits so far

Regards - Tug

Many thanks for all of you for your comments.

I was suspecting that there might be troubles with diamond dust. It is just "too good". This dust done really good jobs on spindles 45-65 HRc but base on what you say it looks like cast iron is too soft for it.

So I will settle with one of 3: Aluminium oxide, Silicon carbide or Timesaver(TM). I have Aluminium oxide at hand, so I will likely use it.

I have also one other idea:

What about brushes with abrasive balls as used in industry?

Here they are: https://www.brushresearch.com/flex-hone.php

Can get one of silicon carbide grain 240.

Did anyone of you tried it and if so what was your result?

Martin

I have used the ball brushes many times on hydraulic cylinder repairs. They work well but as with any other lapping method keep it moving over the whole surface and use plenty of oil.

regards

Martin it will depend on what you are trying to achieve with the lapping, if just to remove machining marks on a perfectly parallel cylinder then the balls may do. However if you are also hoping to correct any slight taper then the balls will simply follow that taper and not be that good at taking it out, brake cylinder hone would do a slightly better job. But a lap is the best option to get a parallel bore.

Also what is going to run in the cylinder a ball hone is ok for an iron piston ring but you may get some wear on other ring packing and definately a bit coarse for a metal to metal cylinder/piston fit.

The abrasive ball type are commercially known as Flexhones. The balls are Silicone Carbide. The manufacturers advice was to clean thoroughly after use, suggesting the use of a nail brush plus solvent such as paraffin (Kerosene ) to remove any trace of abrasive. In this instance the material was centrifugally cast cylinder liners.

If rotated and reciprocated at the right speeds, a good reproduction of the original cross hatch could be produced, which conformed to the drawing requirements.

The raw material used by Wellworthy at Lymington to cast the liners was redundant chairs from railway lines!

Howard

Deleted, sticky fingers double post

Howard

Edited By Howard Lewis on 10/02/2021 16:47:00

Edited By Howard Lewis on 10/02/2021 16:47:17

@Jason,

Lapping is intended for cast iron bearing. Bore is parallel with no tapers. It will work with 45 HRc steel spindle already made, well polished to mirror finish and also parallel.

Bore diameter is now 1/2 thou larger than spindle and lapping process is intended to open it by additinal 1/4 - 1/2 thou so good oil clearance will be provided.

Martin

So if you want a lapped finish why are you looking at (flex)hones which like the stones will leave a coarser finish?

You also mentioned cylinder and piston in your first post?

Edited By JasonB on 10/02/2021 16:59:07

Yes, I thought after I posted that it depends on what kind of bore it is - steam, air I/C etc and whether the lapping was intended to take out any discrepancies like taper or ovality. As Jason says, flex hones though perfectly suitable for their intended purpose will not do that. I have no experience with the ball hones.

Also what is running inside the bore is relevant too, obviously I lap all I/C engines to a fine finsh but rarely bother with a cast cylinder for a steam engine - just a smoothing of the final finish with wet and dry paper or hone it with a flex hone.

If you have some decreasing grades of the AO then that will be fine to produce quite a high finish if required but as said it really dpends on inteneded purpose

Tug

Oops started this before the last posts - A straight forward lapping of the bore with fine grit should be perfect for that application but I too was confused by the original 'cylinder' 

Edited By Ramon Wilson on 10/02/2021 17:07:13

@Jason.

Have called it "piston" but perhaps wrongly.

It is eccentric rotary vacuum pump - there are 2 bores, one serves a a bearing and second larger does the business - "piston" is a couple of spring loaded rotating sliders driven by spindle - maybe you remember when I was asking about rounding edge of such slider few weeks ago.. I have already refurbished such a pump and now I have got another one with other "issues".

Martin

Well in the vain hope that you may find this relevant Martin you might find 240 grit to be much too coarse. I have tried this on I/C engine liners but it locks the lap solid if not enough clearance for the grit - if the lap has to be tapped out the grit can score the bore which sort of defeats the object though maybe not so important on a bearing.

I have found 320 grit to be the coarsest grade to improve a bore from a turned finish - it will be far to coarse to remove the very small amount you say you want to remove. Given the dimensions you state I would have thought 600 grit would be plenty coarse enough.

But then ..........

Tug

Ramon,

OK, I will settle for aluminium oxide "fine" lapping paste and a lap made of copper pipe.

If there are any troubles I will move to fine grades of silicon carbide.