Mini grinder.

[Jim Allen]

Terry,

"The piston would be turned straight (no top taper) to about 0.010" oversize, all the internal milling done leaving a step in the bottom, then the wrist pin hole would be put in with a V block type mandrel in the lathe, lapping to final size.  Circlip grooves would be cut as well at this time.  This is how I did them before, it worked very well.  Or I might do it on the mill this time, set up properly in  a V block would make sure it's right.

It would then be mounted on another mandrel that holds it on the step with an arm to tighten down holding it from the wrist pin on the inside.  Indicate it in the 4 jaw and it's ready to finish grind.

Grind the straight part very close (a couple tenths) to size then grind the top taper.  Fit the sleeve.  Go back and forth between grinding the straight part and taper until the desired fit is had.  Tedious, yes!  But to fit a piston to an existing liner what option do you have? 

There's more than one way to skin a cat, not that the cat would care tho..."

First things first:

I noticed in the video you posted how rapidly your grinding stone was moving in & out. If I remember correctly this movement is a hydraulic system not an electrical system. Also the rotational speed of the stone used was very low. Both the rapid stroking & the slow rotational speed would not be the best procedure for small size OD, ID manual grinding. Your method for setting the compound would certainly work, but it invites errors to be made. Latter I'll explain a fool proof method that involves one tool for each angle desired & a good quality indicator to set the compound accurately. "One thing about building miniature engine parts is to be sure you have made the dimension intended". Do you have a set of 5-C collets & the 5-C spindle adaptor for your lathe? These tools are a tremendous time saver for holding parts. I'll also explain how to hold pistons for machining without the use of a mandrel. This tool would not be necessary for the CNC set up because the material comes through a bar feed. The procedures & the order of the steps required to make any machined piece is important! It will be impossible to get the correct piston to cylinder fit with the method you proposed & if possible, very time consuming. I speak from experience!!

Look at the photos of the pre-machined piston blanks. They are all machined with diamond coated tools to a tolerance of + or - .0005". Can you understand the advantage of finish  machining several different piston sizes & then fit them by hand to your existing cylinders sizes? This is why you see hundreds of finished pistons made in .0001" steps. All you need to do is determine what size you will need for the fit you desire.

To check the roundness of any part to be made for our miniature engines the only tool required is a Sunnen Gage. You do not have to purchase this tool from Sunnen, it can be rented from them.

Jim Allen

 

[BobHorowitz]

Fascinating stuff.

 

[Terry Keeley]

First things first:

I noticed in the video you posted how rapidly your grinding stone was moving in & out. If I remember correctly this movement is a hydraulic system not an electrical system. Also the rotational speed of the stone used was very low. Both the rapid stroking & the slow rotational speed would not be the best procedure for small size OD, ID manual grinding. Your method for setting the compound would certainly work, but it invites errors to be made. Latter I'll explain a fool proof method that involves one tool for each angle desired & a good quality indicator to set the compound accurately. "One thing about building miniature engine parts is to be sure you have made the dimension intended". Do you have a set of 5-C collets & the 5-C spindle adaptor for your lathe? These tools are a tremendous time saver for holding parts. I'll also explain how to hold pistons for machining without the use of a mandrel. This tool would not be necessary for the CNC set up because the material comes through a bar feed. The procedures & the order of the steps required to make any machined piece is important! It will be impossible to get the correct piston to cylinder fit with the method you proposed & if possible, very time consuming. I speak from experience!!

Look at the photos of the pre-machined piston blanks. They are all machined with diamond coated tools to a tolerance of + or - .0005". Can you understand the advantage of finish  machining several different piston sizes & then fit them by hand to your existing cylinders sizes? This is why you see hundreds of finished pistons made in .0001" steps. All you need to do is determine what size you will need for the fit you desire.

To check the roundness of any part to be made for our miniature engines the only tool required is a Sunnen Gage. You do not have to purchase this tool from Sunnen, it can be rented from them.

Jim Allen

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The grinder is all pneumatic, the in and out speed can be adjusted from very slow to quite fast.  I was told to speed it up for the final spark out passes which you can see in the video.  The motor I have is 30K, they have others that run to 120K but finding wheels rated to that would be tough.  Diamond or CBN only I'm sure.

Yes, I have a very nice Royal MT5 to 5C adapter and a selection of collets (not as impressive as yours!).  I find myself using them most of the time for hobby stuff, a few Hardinge but mostly China stuff that came with the machine.  If I need something very close a bit of fiddling gets me tenths.  I'm watching out for a Royal collet closer at a decent price but haven't had much luck so far.

I can certainly understand when making hundreds of engines why finishing the liners first then fitting various sized pistons makes sense.  Chrome is a ***** to finish!  What I can't understand is why can't the manufacturers make the parts to fit without having to hand fit?  I've heard the machinery now making the Nelson's and Nova Rossi's can hold millionths, is this not close enough to make a piston properly fit a liner?  Is it because the liners are still hand honed rather than I/D ground? 

I'm planning to make a few blanks like your picture above, put in the wrist pin hole and hold it on a mandrel with the 4 jaw.  Then finish grind the O/D and top taper, fit the liner and go back between grinding the O/D and top taper until I get what I want.  Tedious?  Yes!  But cutting that top taper with the compound on my lathe will be second rate to grinding as far as I'm concerned.

ps:  what's that fixture in the last photo for?

 

[Terry Keeley]

Hi Terry,

The machine I was referring to earlier was a Mitsui 8x18 surface grinder with a Ded-Tru grinding attachment mounted on it.

This combined with an opptidress wheel dresser is the standard in the mold grinding industry for grinding core pins ect.

It is a very cost effective means of precision I.D. O.D. grinding.  With this attachment you can also through feed and in feed c-less grind.

Before my father purchased a proper C-less grinder for small diameter close tolerance parts I taught myself how to C-less grind with a Ded-Tru. The only drawback is that in order to get true roundness you need a roundness gage which is a costly piece of equipment for the average hobbyists.

this gage is necessary to view the shape of the O.D. On a polar chart and then to make the necessary blade adjustment to reduce the out of round condition.  Simply using a mic or an indicating mic is unacceptable as it only checking between two points and cannot determine lobbing. The roundness Gages that I used are made by rank Taylor Hobson and use an air bearing for the spindle and electronic gage heads with adjustable pressure. I believe the ones I used were guaranteed to be accurate within  .000002 inch.

on the other hand I can see Jim's way of manufacturing the piston as a very cost effective way of sizing the O.D. Of the piston.

many years ago me and my father visited Moore Special Tool Company to purchase a Jig grinder. During this visit we got the grand tour of the facility.  One of the projects  that caught my eye was the diamond turning of aluminum on an air bearing for an optical application.

i believe if you go to their website you can find the very impressive videos of this process.

With this process you can achieve incredibly low roundness and micro finish. It might be possible to mount an air bearing in your lathe as a cost effective solution to rebuilding you're lathe spindle..... Maybe Jim would have an idea on this as I have never turned this type of aluminum in a lathe and I don't know how easy it is to cut.  It maybe possible in the future for my coworker to make some custom PCD cutting tools on our laser for testing purposes.

Looked up that DedTru attachment, very nice:  

 

[Jim Allen]

Who said this, "If you always do what you always did,


you'll always get what you always got..." Nothing will turn out to be more true if you select the wrong techniques, procedures, tooling & basic machining practices when building parts for a high performance miniature two cycle engine!!!

The grinder is all pneumatic, the in and out speed can be adjusted from very slow to quite fast.  I was told to speed it up for the final spark out passes which you can see in the video.  The motor I have is 30K, they have others that run to 120K but finding wheels rated to that would be tough.  Diamond or CBN only I'm sure.

Yes, I have a very nice Royal MT5 to 5C adapter and a selection of collets (not as impressive as yours!).  I find myself using them most of the time for hobby stuff, a few Hardinge but mostly China that came with the machine.  If I need something very close a bit of fiddling gets me tenths.  I'm watching out for a Royal collet closer at a decent price but haven't had much luck so far.

I can certainly understand when making hundreds of engines why finishing the liners first then fitting various sized pistons makes sense.  Chrome is a ***** to finish!  What I can't understand is why can't the manufacturers make the parts to fit without having to hand fit?  I've heard the machinery now making the Nelson's and Nova Rossi's can hold millionths, is this not close enough to make a piston properly fit a liner?  Is it because the liners are still hand honed rather than I/D ground? 

I'm planning to make a few blanks like your picture above, put in the wrist pin hole and hold it on a mandrel with the 4 jaw.  Then finish grind the O/D and top taper, fit the liner and go back between grinding the O/D and top taper until I get what I want.  Tedious?  Yes!  But cutting that top taper with the compound on my lathe will be second rate to grinding as far as I'm concerned.

ps:  what's that fixture in the last photo for?


The rotational speed of your grinder is good for ID grinding, but the in out speed must always be very slow! In fact, the slower the better! Think about the size of the wheel in relation to the size of the hole to be ground whenever you do ID grinding. Then think about what the size of the wheel could be when doing OD grinding. Do get the picture?

The run out accuracy of the royal #5-MT to 5-C adaptor can easily be greatly improved! Both the 5-C adaptor & the #5-MT spindle are hardened. Make sure both pieces are absolutely clean & drive the adaptor into the spindle in several different positions. Observe that the adaptor will show some runout no matter where it is placed into the spindle. However, if you were to finish grind (.00005" to .0001"/side) the adaptor's seat for the 5-C collets & mark the position of both pieces in relationship to one another; as long as both pieces are in the same position, the runout will be .00000". Always remember this, even Hardinge precision grade collets will have some runout 1.000" to 2.000" from the collets face.

Hard industrial chrome IS NOT DIFFICULT TO GRIND OR HONE!! Proper tools, holding fixtures, stones, wheel speed, correct rotation speed, stroking speed, stone dressing, lubricant, the amount of cut/pass & "practice" will allow any hard chromed cylinder to be finished honed or ground in less than 15 minutes, including the wash up time. Again, having the necessary tool (Sunnen Gage) to measure the amount of taper & the roundness of that taper is essential. As far as the ID grinding of chromed cylinders is concerned, the key part is the holding fixture & how easy it is to zero the piece to be ground. Keep in mind that if either of the two pieces does not have a super finish, proper hand fitting becomes IMPOSSIBLE!

The CNC machines at Aero Precision Machine can easily make parts to + or - .00005 in dimension & roundness. You won't be able to do that on your Super 11 manual lathe. However, you should be able to machine parts to + or - .0001" in dimension & roundness. 

You have mentioned using a mandrel several times. FORGET THIS IDEA!! Go back to the posted photos of the internal expanding collet made by Royal. THIS IS THE ONLY CORRECT METHOD FOR HOLDING A MACHINED PISTON FOR FINAL MACHINING OF THE OD. Did you notice the pin, which is a push fit in the wrist pin hole. You will put an indicator on this pin, as the piston is rotated, to insure the machined OD is dead square with the wrist pin's axis.

I will not discuss grinding or lapping a piston again. "YOU WILL NOT GET A BETTER FINISH OR A ROUNDER PIECE THAN MACHINING WITH A GOOD GRADE DIAMOND COATED CARBIDE LATHE BIT!! 

The fixture in the last photo is used to put a pre-machined hole in a piston. The hole will be square to the machined piston bosses inside & square to the outside at the same time. It also places the wrist pin hole in the center of a piston once the center of the "V" has been zeroed with the milling machines spindle. Notice how the extended foot holds the piston securely down in the "V" & allows the wrist pin hole to be drilled through it's middle section. I posted a photo of the wrist pin being pressed into the piston boss. Why did this become necessary on high HP, high RPM pylon racing engines turning RPM's above 30,000? Aero Precision Machine did this for the 35,000 RPM engine (MB-40) of Rob Metkemeijer to solve piston & connecting rod failure problems. You might consider this as a possible solution to your piston breaking problem. You will need a honing machine & some practice to hone the wrist pin hole smaller on the back side. One more thing about the wrist pin. It should not be moving back & forth between the "C" clips when it is full floating.

 

[Terry Keeley]

Hey Jim:

Thanks again for all the great info, as stated you are increasing my success rate many times before I even get started.

I don't want to upset you because I'm going to finish grind my pistons.  I have no doubt that your method of turning them with PCD or diamond tooling will produce a great result it's just that for the equipment I have I don't trust using the compound on my lathe to turn that top taper as well I don't see how I'm going to be able to remove 0.00005" (fifty millionths) on the O/D to fit an existing liner.  I have the greatest confidence my grinder can do all that.  You even state that my Super 11 is a tenths machine, my grinder is way better than that.

Does your external expanding collet hold the piston by pushing out on the skirt?  The mandrel I have in mind does not hold the piston this way, it has an internal arm that pulls the piston down to the bosses and transmits the rotational force via the wrist pin.  I guess it could be made with a 5C emergency collet and if I had quantities to do that would definitely be the way to go but for a couple I'm happy to zero it with the 4 jaw.  When I last made pistons I used a barrel lap to finish the wrist pin holes, just a nice light "snug" fit, it worked very well.  How are they done now for the Nelsons?

So the sleeves used in the Nelsons are still finished by hand on a Sunnen hone?  I wonder what Nova Rossi, CMB, Picco and OPS use?  Maybe I'll shoot Al Hobbs a note, he's visited some of their factories. 

Nice fixture for drilling the wrist pin hole.  

 

[Jack ODonnell]

Hi Terry;  How much taper do you use on the top of the piston and how far down from the top ?         J.

 

[Jim Allen]

No Problem Terry, 

"Does your external expanding collet hold the piston by pushing out on the skirt?  The mandrel I have in mind does not hold the piston this way, it has an internal arm that pulls the piston down to the bosses and transmits the rotational force via the wrist pin.  I guess it could be made with a 5C emergency collet and if I had quantities to do that would definitely be the way to go but for a couple I'm happy to zero it with the 4 jaw.  When I last made pistons I used a barrel lap to finish the wrist pin holes, just a nice light "snug" fit, it worked very well.  How are they done now for the Nelsons?

So the sleeves used in the Nelsons are still finished by hand on a Sunnen hone?"

The Royal 5-C internal expanding collet works best because of several important things. All surfaces that need to be machined, the major OD, the very important tapered top area which is approximately .100" in length for .45 to 1.00 cu in engines & the piston crown are completely exposed. The question you need to ask yourself is this; If you are holding the piston with a pin in the wrist pin hole, HOW WILL YOU ZERO THE PISTON'S OD TO THAT WRIST PIN HOLE??? Remember the photo of the piston mounted on the Royal 5-C internal expanding collet with the long (3.000") pin gage sticking out on both sides. FAILURE TO ZERO YOUR PISTON WILL RESULT IN ALL TYPES OF UNSOLVABLE PROBLEMS!!! Pushing out on the full piston skirt with a moderate force creates no problems, except that the piston will be slightly smaller at the bottom by .0002" or .0003". SO WHAT. With this holding method, nothing happens to the critical area from the wrist pin hole to the piston crown.

I don't think you can easily make an internal expanding collet from a machinable collect. The threaded back end of the collet is a seperate piece from the collet body. The draw bar pulls on a tapered shaft, notice the nut on the back end which is fastened to the tapered shaft,  which opens the serrated front end. The collet's front end  can be machined to any size which means it will be dead concentric with the lathe spindle.

Wrist pin holes can be finished with a hone or diamond coated boring bars. If you bore the wrist pin hole when the piston is in the holding fixture, nothing else needs to be done except to machine the retaining ring groove. I should mention that measuring of small holes is never done with any type of mechanical measuring device; " IT IS DONE WITH THE USE OF PIN GAGES." I bore my wrist pin holes with diamond coated carbide, in the fixture shown, for a .0001" interference fit. This process also illustrates why a good fixture can save a great amount of time; because once the fixture has been zeroed, you're only moving the pieces, in & out, for the same operations. All wrist pin holes in Nelson pistons are honed on a Sunnen honing machine. All cylinder sleeves in Nelson engines are honed. This includes any cylinder that needs to be re-honed.

I should also mention that the shop has a like new Kellenberger UR 175-1000 fully automatic OD - ID grinder with all the possible tooling & a filtered coolant system. Could this machine be set up to ID grind toy engine chromed cylinders? Absolutely. What would be gained? Better roundness or a better finish?

The 25 cylinders that I ground in this machine DID NOT run any faster than the hand honed cylinders done in the 40 year old Sunnen Honing machine!!

Jim Allen

 

[Terry Keeley]

Hi Terry;  How much taper do you use on the top of the piston and how far down from the top ?         J.

Ha!  You must be a mind reader, I was wondering that very thing.  A stock CMB 101 piston has about 0.002" per side that ends just above the wrist pin hole:



I haven't measured the CMB 67 piston yet but IIRC the old Picco 67's had about 0.001" per side.  They all seem to end just above the wrist pin hole.  This is what I was shooting for 20 years ago but it wasn't done that well with my little C5 lathe. 

I had one of the CMB castings I have analyzed and they were very close to 390 aluminum (17% Si).  Matweb lists the CTE for 390 at 22.5 um/*C:

http://www.matweb.com/search/DataSheet.aspx?MatGUID=24292258c5a54077bf749db43cad1037

RSP lists the CTE for their RSA-444 at 14.6:  

http://www.rsp-technology.com/site-media/user-uploads/rsp_alloys_microengines_2018lr.pdf

About 35% less.  So should the taper be reduced by this amount?  Got any insight into this?  Wonder what the commercial engines that use this have for top taper? (Nelson, MB).

Also, CMB pistons all have a taper on the bottom as well, I don't think Jim is doing this to the Nelson pistons, any idea what this is for?  Jim?

edit:  I seem to remember A390 had 12% Si, that's what Jim also mentions below but I could only find data for 17% material.  IIRC the blanks I got were 12%.

 

[Terry Keeley]

The question you need to ask yourself is this; If you are holding the piston with a pin in the wrist pin hole, HOW WILL YOU ZERO THE PISTON'S OD TO THAT WRIST PIN HOLE??? Remember the photo of the piston mounted on the Royal 5-C internal expanding collet with the long (3.000") pin gage sticking out on both sides. FAILURE TO ZERO YOUR PISTON WILL RESULT IN ALL TYPES OF UNSOLVABLE PROBLEMS!!! Pushing out on the full piston skirt with a moderate force creates no problems, except that the piston will be slightly smaller at the bottom by .0002" or .0003". SO WHAT. With this holding method, nothing happens to the critical area from the wrist pin hole to the piston crown.

I don't think you can easily make an internal expanding collet from a machinable collect.

I plan to drill/ream(possibly bore)/lap the wrist pin hole in a V block on my mill.  If the mill is straight and the set up is good why wouldn't the wrist pin bore be perpendicular the the piston's O/D?  When the blank is mounted on the lathe if it's zeroed in accurately along it's length the wrist pin hole should still be perpendicular, no?

I didn't mean I'd make an expanding collet from an emergency collet, but that I could make my piston holding fixture from one.  But I'll probably just use 12L14 round and mount it in the 4 jaw. 

ps:  Al Hobbs says as far as he knows Rossi, Picco and CMB all send out their liners for chrome and finishing.   

 

[Jim Allen]

 "A stock CMB 101 piston has about 0.002" per side that ends just above the wrist pin hole." WOW!! I HAVE NEVER SEEN A PISTON MACHINED THIS WAY FOR USE IN ANY ABC OR AAC ENGINE! When we discovered the very important thing about the taper on the top outer edge of the piston, we were really looking for a solution to the piston scuffing problem. At first some engine modifiers & builders thought that a higher amount of Silicon in the piston's alloy would eliminate the scuffing problem. IT DID NOT! The piston crown is exposed to the combustion heat & it will expand much more than the sides of the piston. We decided to cut a taper (.002"/side) on the top outer edge of the piston for a distance of approximately .100". Moving the sealing point from the top outer edge to a place down on the sides solved all scuffing problems & allowed the use of lower Silicon alloys such as A-390 (12% Silicon), Malhe 138 (18% Silicon). Higher silicon alloys such as Dispal 250 (21% Silicon), RSA 431(30% Silicon), RSA 444 (30% Silicon) increased the wear resistance, but could also cause a permanent increase in the pistons dimensions ( sometimes termed "growth"). The more silicon an Aluminum alloy contains, the lower its thermal expansion coefficient. Again, notice where the piston's side ("wear band") is rubbing against the cylinders wall. It is clearly visible on new as well as used pistons. It is the key indicator that shows that both parts are round & there is no contact with the top outer edge of the piston or the bottom outer edge of the skirt.

"I plan to drill/ream(possibly bore)/lap the wrist pin hole in a V block on my mill.  If the mill is straight and the set up is good why wouldn't the wrist pin bore be perpendicular the the piston's O/D?  When the blank is mounted on the lathe if it's zeroed in accurately along it's length the wrist pin hole should still be perpendicular, no?" You want to ensure that the wrist pin hole is "dead square" to the machined outside diameter of the piston, especially after you have moved it & you are remounting it! I'm aware that one Royal internal expanding collet may not be cost effective for making a few pistons. However, there are many cost effective internal expanding collets available on Amazon. Think of the valuable time you will save & you will also be assured that the part is made correctly!!

I'm again posting photos of new (first three photos) & used (last three photos), pistons that were machined as I explained above. Show me any piston, new or used, that looks similar to these photos??

Jim Allen

 

[Terry Keeley]

 "A stock CMB 101 piston has about 0.002" per side that ends just above the wrist pin hole." WOW!! I HAVE NEVER SEEN A PISTON MACHINED THIS WAY FOR USE IN ANY ABC OR AAC ENGINE! When we discovered the very important thing about the taper on the top outer edge of the piston, we were really looking for a solution to the piston scuffing problem. At first some engine modifiers & builders thought that a higher amount of Silicon in the piston's alloy would eliminate the scuffing problem. IT DID NOT! The piston crown is exposed to the combustion heat & it will expand much more than the sides of the piston. We decided to cut a taper (.002"/side) on the top outer edge of the piston for a distance of approximately .100".

I measured a new CMB 67 piston and found the top taper was about 0.0015" per side down about 0.230" to just above the wrist pin hole like the 101.



It looks to me that they are trying to match the taper to the expected expansion rate so that the crown comes out to meet the sleeve at operating temperature.

Here's a 67 piston and liner with 100+ runs on it, you can see the wear band goes right to the crown.  A little hard to see but the wear band in the chrome matches it.  You can also faintly see the cracks I'm getting in the corner of the skirt.

 

[Jim Allen]

"It looks to me that they are trying to match the taper to the expected expansion rate so that the crown comes out to meet the sleeve at operating temperature.

Here's a 67 piston and liner with 100+ runs on it, you can see the wear band goes right to the crown.  A little hard to see but the wear band in the chrome matches it."

From the photos you posted I would agree & this is exactly what YOU DO NOT WANT!! Think about how much more area will be involved when the wear band is not at the top most outer edge of the piston. Once the edge is worn away, there will be no more seal between the piston & cylinder. The engine will run, but it will be down on power. The only way for you to see the necessity of moving the wear band away from the top edge of the piston is to do it properly, with the correct alloys, with the correct tooling & with the correct tooling procedures. 

I don't understand why the piston in your photos look so grainy after machining. If this is what other pistons in manufactured engines look like, I would have to ask what alloys are being used to make the piston & how are they made?? I have listed several alloys that are suitable for machined pistons. None of these alloys, when done with diamond coated tooling, at the proper speed & feed, would give a finish similar to what I see in the photos you posted.

The second photo I posted is a .0001" bore gage with a moveable sleeve added to the tools shank. The sleeve sits on top of the cylinder & allows a measurement to be taken at any point as it is rotated. It is used to determine the exact taper amount from the top of the  exhaust to the top of the cylinder.

Jim Allen 

 

[Terry Keeley]

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Great minds think alike.  I added a 20 TPI bronze tube to mine so I can check it every 0.050".