Finishing piston o.d. with toolpost grinder

[Brian Rupnow]

This is something I haven't done before, and I'm soliciting information from anybody that has. I have a new toolpost grinder from Little Machine Shop. (It doesn't actually fit on the toolpost but bolts to the topslide---Go figure???). I want to make a new cast iron piston, and hold the o.d. to +/- 0.001". Can I use the toolpost grinder to grind the o.d. of my piston to finished diameter? I would leave the piston attached to it's parent metal and turn it from piece of 1" diameter stock, then turn it with conventional mill tooling to 0.880" diameter, then without changing my set-up, grind the o.d. of the piston down to the size I require. ---About 0.876". It sounds logical to me, but again, it's something I haven't done before.---Brian

 

[Ghosty]

Brian,
It would be the same as grinding a shaft, you are just finishing to a size, just doing it different to normal at the moment. All my pistons have been alloy so never ground any. I have ground piston ring blanks to size, but can be done quicker with different grades of emery strips.
Cheers
Andrew

 

[abby]

Brian why can't you just turn the piston to size ?

 

[Brian Rupnow]

I have a hard time holding +/- 0.001 accuracy when turning. I thought maybe if I used the grinder I could sneak up on my final dimension 0.001" at a time. That plus the fact that the surface finish on a piston is rather important.

 

[JLaning427]

If you have a toolpost grinder, yes, you should be able to use it to "sneak up" on your final dimensions.

I am a little confused as to how it mounts. Does your lathe have a compound slide on it, and does the grinder bolt to it or to the cross slide? If you have a compound, you can swing it to some angle (pick something small, say 20°) and then as you advance the compound 0.001", you get a significantly smaller amount of travel (depending on your choice of angle). And the grinding wheel can and should take off small amounts at a time. Don't want to plunge cut 0.005" in a pass.... More like 0.0002"

James

 

[ShopShoe]

I'm intrigued by this part of this thread. I've seen that grinder, and various home-built ones. I'll be watching closely to see if it works.

Brian, I wish you the best with this.

--ShopShoe

 

[awake]

... turn it with conventional mill tooling to 0.880" diameter, then without changing my set-up, grind the o.d. of the piston down to the size I require. ---About 0.876".

I have a hard time holding +/- 0.001 accuracy when turning. I thought maybe if I used the grinder I could sneak up on my final dimension 0.001" at a time. That plus the fact that the surface finish on a piston is rather important.

Brian, I have done very little grinding, so take this with a large grain of salt, but ... I believe you are off by an order of magnitude. As I understand it, with grinding you want to be thinking in terms of taking off a tenth (.0001") at a time, not a thou (.001"). Grinding all the way down from .880" to .876" is just a waste of grinding time and media.

On a lathe, the typical way to achieve this is to have the grinder mounted on the compound, with the compound set at 84° or 6°, depending on how your compound is marked - nearly parallel with the axis of the lathe. The sin(6) = .1, so a setting of .001" on the compound will result in an inward movement of only .0001".

Keep in mind that if you get a taper when turning conventionally - if your tailstock needs adjustment (if turning between ways) or your bed is twisted / out of alignment or you have excessive slop or wear - you're going to get the same taper when grinding.

Also important is the fact that, when you are working with tenths, heat is a huge factor ... and grinding can create a lot of heat. That is why grinding is typically done with coolant - and another reason to think light passes at a time.

Finally, don't forget the mess that comes with grinding; you will need to do everything you can to contain the ways-destroying grit that is produced. Yet another another reason not to try to take off .004" with grinding if you can possibly help it.

Again, please apply copious amounts of salt to the above ... maybe someone with extensive grinding experience will either confirm or correct what I have said!

 

[JLaning427]

Taking 0.004" off of the OD of a part by grinding isn't too excessive.

I used to work with a commercial tool and die shop, and we left 0.010" on pretty much everything as a minimum, to account for stuff moving during heat treatment. Saw a couple of pieces of D2 that turned into Pringles (ok, they only moved 20 or 30 thou, but they were still junk). All that material was surface ground off, usually a tenth or 2 at a time. We left more than 0.010 on parts over 6" long.

Anyway, grinding that much off shouldn't take too long. Yes, it's a number of passes, but still shouldn't take long.

As mentioned, grinding grit is murder on ways, it's basically course lapping compound. You need to cover everything you care about. I'd even say tape the gaps in the chuck.... And then clean clean clean afterwards. But it can produce a nice clean finish, assuming the bearings in your headstock are decent.

James

 

[awake]

James, sounds like you know whereof you speak. I am just trying not to put too much of my foot in my mouth! But it sounds like I was at least in the ball park on how much to try to grind in a pass (a tenth or two rather than a thou).

I did think about larger grinding allowances in the situation where something has been hardened and therefore has moved, but if I understand Brian's situation, he is not planning to heat treat. Thus I was thinking he could reduce grinding (and accompanying scattered grit) by only allowing a thou or two. Is that a good / reasonable idea, or is it better to go ahead and leave 4 thou or 10 thou or ... ?

 

[Badhippie]

Brian
If the piston is aluminum the stone will just plug up. Any way you can do it on a mill with a adjustable bore head

 

[Badhippie]

Let me explain myself you would be using the boring head to turn the OD of the piston.
Also holding a piston to .001 is not bad at all this would pass any tolerance spec of any piston

 

[abby]

I believe Brian said the piston was cast iron , and it will not require heat treatment so , if it must be ground then I would leave just a couple of thou on the diameter and grind dry.
I really believe that a good enough job could be done by turning and polishing with emery cloth on a flat backing pad.
I can recommend the use of a flat diamond hone to take the last 1/2 a thou off something like a hard steel mandrel.
The fine ones leave an almost polished surface.
Just as a point of interest a recommended method of fitting model steam locomotive piston valves , this is for red brass valves in cast iron cylinders , is to make them a very tight fit and then hammer them through their bores a few times , probably sounds drastic to IC engine builders but it works.
Dan.

 

[Jasonb]

I'd be looking at why you can't turn to 0.001" rather than using the grinder.

Am I right that you tend to use TCMT inserts most of the time? if so I think I have said to you before possibly in the piston ring thread get some TCGT inserts with 0.2mm corner radius or grind up a sharp HSS bit and use that with the lathe set to the finest feed rate. It works for me on the same machine as yours and I can hit tenths that way so you should be able to as well. The standard **MT inserts are not so good at sneaking up on a size.

 

[stragenmitsuko]

Brian do a google search for "vertical shear tool " .
It's a simple hss tool , but it works flawless .

Lock the crosslide , set your compound to 3° and feed in with the compound .
That will get you the accuracy needed .
I do it all the time when machining shaft ends for bearings .

Pat

 

[Noel Gordon]

I just have 1 comment to make, WHY grind a piston to size when you can simply turn it to size
Piston to bore clearance is NOT important as the rings if made correctly do all the work. All of my rings have a 2 degree angle with the ring face that is in contact with the bore and the reason for this is to always helps compression as the rings wear, contact of the ring faces up. A simple trick taken from building racing car engines for the last 60 years. Good luck, just try it and see what happens. Noel.

 

[Jasonb]

That is unless you go ring less where fit it quite important, in which case I will lap the piston to the previously lapped bore

 

[JLaning427]

WHY grind a piston to size when you can simply turn it to size
Piston to bore clearance is NOT important as the rings if made correctly. Noel.

I believe Brian has made numerous sets of rings and is continuing to have issues with ring sealing. Hence he is thinking about other alternatives, such as having a ringless piston, or a tight fitting piston with rings.

James

 

[Bentwings]

Taking 0.004" off of the OD of a part by grinding isn't too excessive.

I used to work with a commercial tool and die shop, and we left 0.010" on pretty much everything as a minimum, to account for stuff moving during heat treatment. Saw a couple of pieces of D2 that turned into Pringles (ok, they only moved 20 or 30 thou, but they were still junk). All that material was surface ground off, usually a tenth or 2 at a time. We left more than 0.010 on parts over 6" long.

Anyway, grinding that much off shouldn't take too long. Yes, it's a number of passes, but still shouldn't take long.

As mentioned, grinding grit is murder on ways, it's basically course lapping compound. You need to cover everything you care about. I'd even say tape the gaps in the chuck.... And then clean clean clean afterwards. But it can produce a nice clean finish, assuming the bearings in your headstock are decent.

James

I also did a huge amount of very close tolerance I’d an of grinding much of it on carbide ring dies excellent notes on stock and temp you would be surprised at what temps do when talking .0001 or tenths best have very accurate mics and surface plates height gage gage block set tenth or better indicators clean work areas. Few lathes can reliably cut tenths we had brand new hardinge tool room lathes that you could get retry close if you were careful it took ceramic tools to get the really good finishes . Small belt sanders were ok but even they took finessing to get the right finishes if yours wiring with aluminum I like hss tools carefully ground with nice honed radius everything ha to be just right n lathes a loose carriage ain’t going to get it loose or worn spindle gearing ain’t getting it either the big I’d grinder had ai bearings with accurate pressure gages and very good air filters rpm was outta sight back then the big of Grinder had ver specific wheels often used on one job only tossed when done the wheel dressers cost was unbelievable. You can do pretty well at home it it’s hard to beat factory production stuff designed for the job cylinder bore finish is very important. I I’d precision honing that you rest need to examine what goes on here generally there as about .003 stock to come ou honing .002 got the size in range with .001 to get very near final then a few tenths to do the final plateau and cross hatch and sizing . Sumner is the big name in honing . They can get you he right stuff but it ain’t cheap the home shop probably can’t actually test or inspect s it is don commercially . Sumner can get you the right tool it will be upbtobouvtobuse it correctly I’ve done a lot of car eng blocks ans race car stuff you need a bore gage that reds tenths micrometer that’s can read tenths or better and finessability to set these tools up then you have rings to either purchase or make more tech here too. It’s really no hard one you understand what is going on again I’ve had good luck with sunnen products the dingleberry hone is a staple for cylinder finishing . A littl honing oil and dingo berry won’t tak stock out t leaves h nic finish needed.
Byrn

 

[kiwi2]

Hi Brian,
I don't know if you are following Tinbeerwah's glow plug model aircraft engine build, but his latest episode shows what seems to be a good way of sneaking up onto the correct piston diameter.

Regards,
Alan C.

 

[Richard Hed]

I'd be looking at why you can't turn to 0.001" rather than using the grinder.

Am I right that you tend to use TCMT inserts most of the time? if so I think I have said to you before possibly in the piston ring thread get some TCGT inserts with 0.2mm corner radius or grind up a sharp HSS bit and use that with the lathe set to the finest feed rate. It works for me on the same machine as yours and I can hit tenths that way so you should be able to as well. The standard **MT inserts are not so good at sneaking up on a size.

I'm glad you wrote this, as I have a hard time understanding the nomenclature of the inserts, even tho' I have studied them, examined them and used them, it is still very difficult for me to understand it.