Holding piston

I want to hold a piston on. a mandrel so I can machine the OD to size I want to hold it in place with some type of glue hot melt and hold it with a centre on the crown as well any ideas on the type of glue

Thanks Oliver

Best one I've come across is for holding carpets down BUT you need an industrial glue gun to get it hot enough. It then becomes EXTREMELY RUNNY but sets like rock when cold. It is highly fragile though so couldn't handle an intermittent cut.

You'll have to take very gentle cuts so as not to generate enough heat to remelt the glue.

I think you might be better off with loctite and then warm it up afterwards to break the bond. Take care as fumes as loctite breaks down are not good for you !

Ian

Oliver, I agree with Ian regarding loctite, you don't say what size your piston is but regardless of size I would also suggest you use a support with your running centre.

To do this make an extension to go inside the piston with a large 60deg cone to mate with the running centre, no need to fix to the centre just hold it in place until you have applied a reasonable pressure and the lock the tailstock barrel.

In fact you may well find you can do the job without any glue providing you have a large enough "driver" in your chuck Do the 60 deg cone with either a large centre drill or set the compound to 30deg and turn one in the bar -- use a collet or 4 jaw to ensure concentricity.

Presumably you could put a gudgeon pin in and then use an eye bolt to hold the thing against a faceplate or into a short mandrel. Might be more secure than adhesive, less bu99ering about and less chance of splitting the blessed thing on a cone?

Muzzer

hi oliver,

my method (for miniature loco work) is to turn the piston a bit over size (say 10 thou) then fit the piston rod and either hold the piston rod in the 4-jaw clocking it with a DTI or hold in a collet, then turn to size. i dont see the point of trying to turn the piston to OD size without the piston rod being fitted as there is the possibility of error creeping in there, plus i usually hold the piston in the chuck whilst screwing in the piston rod hard with loctite added which you wouldnt be able to do if you had already finished the piston to size coz the jaws will mark the finished piston.

hope this makes sense!

cheers,

julian

G'day.

A decent piston holder for turning and lapping etc. may be of use based on the design at **LINK** Tech Tip Of The Month for July 2007. Different collars may be made to suit various bores.

Regards from the land of the kangaroo

* Danny M *

Oliver,

Watchmakers traditionally use a "wax chuck" for this sort of work [albeit usually on a small scale], where the "wax" is Shellac.

... but, I agree that Loctite should do the job nicely.

MichaelG.

Assuming its an IC engine then I yse the gudgeon pin and a tapped mandrel

The pistons for my hot air engines are made with a threaded hole in the crown, this in the finished piston takes the forked piece for the con rod. To turn the out side of the piston, I use a short rod that fits neatly in side the piston, with a threaded bit on the end that screws into the crown of the piston. Using this method I can take the skirt down to about 1mm thick (some times I have gone a bit too far, Oh well start again), basically I'm out to get minimum weight from the moving parts. Ian S C

In the high volume commercial production of pistons for engines, (Cars, Lorries, Tractors,Machines etc) the inside of the skirt is first turned with a short precise register.

This is then used to locate the piston on a dedicated bung, pulled onto it by a pin in the Gudgeon Pin bore, and a drawbolt arrangement.

The Ring Belt, Lands, and Skirt, are then turned to Barrel and Oval form, and any Combustion Chamber, using this one set up.

I would suggest that you follow this route, so that you do not have to worry about adhesives softening or failing, and can be sure that Ring Grooves and Skirt are concentric.

Howard

Thanks for the replies, the pistons are for my 2.5cc tether car engines, it has an ABC set up Aluminium piston Brass liner Chrome plated and ground to a highly polished finish. The pistons have a short life and the liners are resuable after a little work a decoke basically. These engines work hard and have an rpm of about 39500. By best run with this car has been 260kph.

The pistons are made from high silicon content about 35% aluminium and they fit in the liners tapered bore and are an interference fit at the top of the stroke. The OD of the piston is 15mm.

The plan was to hold the piston on the madrel will a centre with a cup to protect the piston crown and glue it in place so I could move the centre to check the fit to the liner.

I had thought about loctite but no sure how hot I would have to get it to unstick it.

This is my first go at making opistons for such high performance pistons I have been told how to do it by one of the top Swedish racers and egine builders but the glue part is lost in translation he suggests that its the type used in holding glass for grinding but have been unsuccesful at finding any

Thanks

Oliver

Oliver,

For info. Loctite degrades completely at somewhere round 200°C, but softens before that.

However: Given what you are doing, I think Shellac might be the best answer.

MichaelG.

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Edit: There is an excellent article about using Wax Chucks, here.

Edited By Michael Gilligan on 07/11/2013 15:31:19

Super Glue. An American, W R Smith, wrote a very interesting chapter in his book "Clockmaking & Modelmaking - Tools and Techniques" on using Super Glue instead of Shellac on wax chucks. I have used Super Glue to hold small difficult to hold parts when machining. I have not had the glue fail during any operation but I have kept machining loads low. After machining the part can be separated from the mounting, or chuck, by a light tap with a mallet. If that does not do the trick boil the assembly in water for about five minutes and repeat.

Super Glue is amazing stuff but not really a very good general glue!

JA

JA,

Agreed ... Super Glue [Cyanoacrylate] is excellent for many of these jobs.

It does, however, lack the very useful feature of softening with heat. The beauty of Shellac is that you can "adjust" the position of the component, to make an existing surface run concentric.

MichaelG.

Oliver, if your Swedish mates talk about ,' Glue' for gluing glass then possibly they are talking about optical cement that is cured by Ultra Violet light. Much like the dentist now uses UV to cure the tooth filling. What the UV does is accelerate the curing time. This cement is available from optical glass manufacturers, the name of a company I know is, 'Edmundson' who can supply all optical things.

As an aside the old optical cement (Balsam) was made from tree resin and refined until it was clear, this matched the lenses glued together and it was cured by heating.

This was superceded by a soft plastic glue that again was cured by heating but had a longer life and better adhesion to the glass.

This again has been superceded by the resin which is cured by UV. and is very strong and gives time to adjust the position of the glass parts and cures almost instantly. No heat being required.

I have had a further thought, perhaps they refer to the glue used to stick glass to glass on cars, like the blocks on car windows. The sliding type.

Clive

Edited By Clive Hartland on 07/11/2013 22:28:52

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Oliver,

Sorry, I had missed that part of your post, until Clive commented.

The "glue" used for holding lenses whilst grinding the edges is, again, Shellac [sometimes mixed with Pitch, to make it less brittle] ... This version is sometimes known as Dop Wax, and is also used in Gemstone Cutting.

In traditionally-built camera and microscope objectives, the lens elements are made slightly oversize on the diameter. They are then centred, on a Wax Chuck, and the [now probably slightly eccentric] edge is ground to be (a) concentric with the optical axis, and (b) the correct size.

Hope that helps

MichaelG.

Michael, things have changed a bit over the years, as you say the lens is polished to a satisfactory quality and what they do now is fix it in the lens holder, and the brass or Alu with lens is then centered as a complete unit and the metal body machined to take out the eccentricity. It was a bit of a surprise to receive a new replacement lens and notice the glass was off to one side in its mount. They are centered using a Laser device and it takes only seconds to complete. The errors are usually very small.

We have used shellak and cellulose to fix circles in instruments but it is all the UV type glue now, they come centered and are put straight into the instrument and software takes up any errors in face to face errors in centricity. Prisms are held in position by grubscrews, then cemented and the grubscrews pressure released to stop pressure marks in the glass. Shellak is used to lock screws and plates so that they hold position.

Clive

Edited By Clive Hartland on 08/11/2013 07:37:55

Clive,

Thanks for the additional info.

Yes, I realise that things have changed for production work ... but I think the traditional adhesives are more relevant to what Oliver wants to do.

Just found this supplier of vaious waxes.

MichaelG.

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P.S.  This page from Nikon might be of interest.

Edited By Michael Gilligan on 08/11/2013 07:53:53

Loctite make Glass Bond, a UV setting adhesive, it comes in a 3 ml plastic tube, I got some years ago to repair a fish tank, just sat the tank out in the sun light, it worked very well, but it's only useful where the UV light can cure it. Ian S C

Here is a piston I am trimming for a friends model diesel engine, 2.5cc

I set the compound over to about 0.75 deg, it is actually at an angle so that
every 0.04mm that the compound moves backwards is 0.001mm diameter cut.
With a dti it is set at 0.05mm per 40 mm travel.
One picture shows the colour change with a 0.003mm diameter cut and the other shows the liner at it's
fitted position and the oil colour change for the amount taken off to fit the piston 0.5 mm more up the bore.
The bore taper is about 0.00061mm per 1mm of travel, so about Ø 0.0012 per 1mm travel.
Measured taper is Ø 0.012 for 10mm