Announcement

Collapse
No announcement yet.

Finishing piston o.d. with toolpost grinder

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • J Tiers
    replied
    With the TPG, you have to NOT apply much pressure, and you have to spark out a lot.

    But the job can get done.

    The Yugo will get you where you are going. It's just not as much fun to do it with the Yugo, and it goes slower.

    Leave a comment:


  • Doozer
    replied
    I have a Brown Sharpe #13 grinder, a Covel 512 grinder and a Heald #7 ID grinder.
    I can say the difference between a toolpost grinder and a cylindrical grinder is like
    a Yugo and a Mercedes.
    I have a 1933 Van Norman Piston Grinder for sale cheap.

    ---Doozer
    Last edited by Doozer; 01-17-2022, 11:13 AM.

    Leave a comment:


  • Mcgyver
    replied
    Originally posted by eKretz View Post

    A toolpost grinder can do easily 90% as well as a "real" grinder,
    A real grinder is a world of difference. Much heavier and better balanced. Its going to have finer infeed and the biggy, the adjustable top table. That's what lets you fairly readily get things with a tenth the same diameter over there length. About impossible on a lathe. I regularly cylindrically grind for finish and accuracy but rarely grind on the lathe, its imo 10% as good not 90%.

    I agree grinding imposes a low pressure but its not zero. At the same time, setting of for really accurate turning (super sharp tool and 10ths infeed indicator), how much force do you think a 2 tenth DOC imposes? not much, and even then, whether this at all matters will vary part to part. I did a bore yesterday, wanted 5 micron clearance between it and its mate, got 7 turning it. Grinding would be a better finish, agreed, but I doubt it would have made it any more accurate and possibly less so given the shake rattle and roll of the typical TPG (and dumore and wolf, not exactly junk, but still brings way more vibration than a proper grinder) adds and the lack of flood on the lathe (in my case).

    Where I do sometimes grind on the lathe is internal stuff. But its still dicey compared to a grinder. Even went so far as to build a 2 plane dynamic balancer to get vibration out of the system. That improved it but its still the really poor cousin to cylindrical grinding (which btw I do on a tool grinder which is the poor cousin of a proper grinder...it is however scraped into perfection, ground up, so does really accurate work). The last internal grind I did I used an air pencil grinder which actually work quite well.
    Last edited by Mcgyver; 01-17-2022, 07:03 AM.

    Leave a comment:


  • luthor
    replied
    A TPG is a very handy piece of equipment to have in a home shop, I purchased one for around $300 Australian dollars a few years ago to grind the #40 taper in one of my milling machine spindles. I have not used it since but I am sure it will get used again in the future.
    Click image for larger version  Name:	Beaver Spindle (2).jpg Views:	0 Size:	61.0 KB ID:	1981212
    Attached Files
    Last edited by luthor; 01-17-2022, 03:39 AM.

    Leave a comment:


  • Rich Carlstedt
    replied
    Brian
    Cover the Lathe ways and carriage with Plastic film/sheet and then lay wet paper towels on the plastic.
    The abrasive dust will stick to the wet paper and cleanup is a breeze as you just fold up the plastic and discard
    Rich

    Leave a comment:


  • eKretz
    replied
    The reason it is so easy to hit size with the grinder is because of the lack of tool pressure. If you use it properly for finishing it takes off what you move in, every time. On a finish cut at the grinder, you don't have to worry about tearing, lines in the finish, or any of that jazz. You get better bearing with interference fits, etc. Long list.

    If you use the grinder only for finishing, with the proper wheel and a good dress, the part does not need to be flood cooled, and there won't be much heat generated. If you're hogging off a lot of material and/or using the wrong grit/grade/porosity of wheel, running with a poor wheel dress, etc. that's no fault of the grinder.

    A toolpost grinder can do easily 90% as well as a "real" grinder, but it requires a little more know-how to get around things like chatter and needs good dressing etc. because it won't be as rigid. The nice thing about the low tool pressure is that you don't really need every last bit of rigidity to do a good job. It just makes things easier. The infeed can be got around by kicking the compound or by the use of fine infeed mechanisms that some toolpost grinders have built-in. I've got one that can feed in tenths, that is built-in as part of an eccentric adjustment mechanism. I generally use the cross silde for roughing-in, then the fine adjustment mechanism for finishing.
    Last edited by eKretz; 01-17-2022, 01:24 AM.

    Leave a comment:


  • mikey553
    replied
    I am somewhat familiar with cylindrical grinding and would like to add one more reason as to why cylindrical grinding is being done by the industry. This is the accuracy which can be achieved on a consistent basis. It is relatively easy to produce 3/4" diameter journals on a grinder with .0003" tolerance. Try to do it on a lathe in production quantities.

    In addition most of the cylindrical grinding is done between dead centers, which is the most accurate way of doing it. So grinder does not suffer from the typical lathe problems like spindle bearings and live center inaccuracies. To be fair there are multiple issues with grinder accuracy too, but they can be solved.

    Leave a comment:


  • Mcgyver
    replied
    Originally posted by eKretz View Post
    You can't turn a diameter to as true and round a surface as you can grind. If that were so, nobody would bother to grind anything. .
    Grinding on what?

    When people bother with grinding (the 99.9% of machining that isn't home shop machining) its done a cylindrical grinder. While it has considerable advantages (and I regularly take advantage of them) its not really comparable to grinding vs turning on the lathe. Outside of using a cylindrical grinder, just who is bother to grind anything? I don't think many or very often, outside of what I said - if you need an improved finish or the work is hardened.

    I think it an erroneous conclusion that grinding in the lathe is going to produce a more true and round surface than turning. The those will depend on on the accuracy of the ways and spindle rotating the work (same as cylindrical grinder). You do with grinding get a better finish, or less deviation of highs and lows on the surface, however even that's not always the case with a tool post grinder; they're light, vibrate, don't always have the best spindles etc.

    Tool pressure with a grinder is as near to zero as you're going to get.
    yes and no. There is definitely some force when grinding, and how much is there with a few tenths DOC turning? Long skinny work can be tough to turn, and bloody tough to grind.

    Then there is the heat grinding generates; you pretty much need flood when cylindrical grinding but most guys lathes aren't going to have that. Remember, temp where the cut is happening is a function of tool speed and without flood the part will grow a lot more with grinding than it will with slow sub thou lathe cuts with a very sharp tool
    Last edited by Mcgyver; 01-16-2022, 06:06 PM.

    Leave a comment:


  • J Tiers
    replied
    Grinding is way superior.... here's my reasoning.....

    The grinding wheel has a high SFM. And it is covered in thousands of cutting edges that are self-sharpening to an extent.

    So, yes, the high SFM allows removing meaningful material in a short time, with a great finish. And, you do not need a lathe that turns at 25,000 rpm.

    With a grinding wheel only 3" diameter, turning at say, 3450 rpm, your SFM is 2700FPM. If you want that SFM on a 3/16" diameter workpiece to turn it, your lathe must spin at almost 58,000 rpm. Maybe yours does, mine does not.

    Leave a comment:


  • BCRider
    replied
    To each their own but I found it was easier than trying to bump the cross slide dial by that small an amount. And with how I like the compound gib to be set a hair on the stiff side at least on my lathe the final results worked out according to the mic readings. It's worth a try in any case.

    And yeah, it's certainly the time where a nicely honed HSS cutter will shine for removing scant tenths on demand too.

    Leave a comment:


  • eKretz
    replied
    You can't turn a diameter to as true and round a surface as you can grind. If that were so, nobody would bother to grind anything. Tool pressure with a grinder is as near to zero as you're going to get. Can you get pretty close with proper and careful turning? Yes.

    Leave a comment:


  • Mcgyver
    replied
    Originally posted by Ringo View Post
    You can turn your compound to such an angle that the thou on the dial relates to tenths in reality diameter
    Yeah, in theory a bit, but it only sort of works. The reason is there has to be clearance in the compound slide for it to work, and further it is away from a high quality perfectly scraped new condition, the worse its going to be. and you now you've made that clearance almost perpendicular to the axis you want to have really exact control over. Lock or remove the compound and set up a tenths indicator on the cross slide when need a lot of infeed accuracy. That and razor sharp tools. And hold your tonque just so.

    Leave a comment:


  • BCRider
    replied
    Ringo, agreed on all counts.

    When I did my 5C holder which is just mild steel I turned it then lightly sanded the taper with a suitable size backing bar. The idea being to just flatten the crests of the surface texture from the cutter so there was a smoother riding of the collets in the taper The actual "valleys" were left visible.

    Mind you... the siren's call of a new tool.... that's hard to ignore....

    Leave a comment:


  • Ringo
    replied
    I had the similar question when I turned my collet holder.
    The answers from that came out to:
    you can turn an OD as accurate as you can grind
    grinding leaves a mess
    grinding is suited when you have to turn a heat-treatable part oversize, then heat treat it, then grind to final size
    You can turn your compound to such an angle that the thou on the dial relates to tenths in reality diameter

    Leave a comment:


  • BCRider
    replied
    Not to upset the applecart on this but why can't your lathe turn to better than +or- .001 just with regular tools? Or is it more about the final surface finish? Or just because you want to play with the new toys?

    From the bit of grinding I did to true up the 5C shanked ER32 collet chuck I got a while back using a Foredom clone as my "tool post grinder" I found that just taking off a half thou was a fair bit of work for the small low power and small stones. So if the goal is 0.876 then I'd suggest starting with 0.878 and just use the grinder for that last thou of material off the surface. Even with that it'll be something you likely find needs a few passes along with multiple final "dusting off" passes

    And then there's that other clever trick for more control over the DOC for the grinding passes if you haven't run across it already. Set the top slide at 6° to the axis and use the top slide's control to have a reduced feed for the cuts you make. The sin of 6° (well, technically 5.7° actually) is 0.1 so each thou of movement on the top slide control is 0.0001 feed on the grinder's DOC. Makes it easier to sneak up on and nail the final size.

    With the ER chuck I had the top slide set to the angle for the collets so I couldn't do it that way. But I used the angle to my advantage by putting a dial gauge on the carriage. So each .001 of advance towards the chuck resulted in .001 x sin 8° or 0.001 x 0.14= .00014. And even that caused some obvious stress to the flex shaft tool and multiple passes to ease the load before taking another .00014 bite. So it was pretty slow going just removing the roughly 5 to 7 tenths of material to true up the taper.

    Leave a comment:

Working...
X