Grinders More Accurate Than Lathe ?

[BillWood]

Hello,

From the model Engineers Workshop Manual by George H Thomas page 164 (1984 version)

"Today with far better machines at their disposal turners are not except in very special circumstances expected to work much closer than about plus 5 or 10 thousandths because most fine limit work can be done far more readily and cheaply by transferring the job to a grinder"

I've stumbled across this concept a couple of times on the internet and in books.

What sorts of grinders are they referring to ?

My current thoughts are that there would be lots of hobbyists out there (not me yet) who could work repeated pieces to a tolerance of plus minus 2 thousandths quite easily ?

Bill

 

[cjtoombs]

If one is careful, .001" accuracy is readily achievable on a lathe. I believe the notion of grinders being more accurate comes from the fact that you can remove very small amounts (.0001) with a grinder, and thus are able to "sneak up" on a dimension more easily. The amount you can take off with a lathe or mill tool is limited by how sharp you can get it, at the microscopic level. Trying to hold tight tolerances also requires paying close attention to the workpiece temperature, as that can affect the size by several thousandths.

 

[18w]

If you read the quoted remark carefully you will note the author meant that the machinist does not have to hold a tighter tolerance even though the machine is capable. Reason being speed. The part is roughed to within .005 to 010. and then finished to size on a grinder. Typically for parts that are needed to finish at a high tolerance and usually a finer finish. An example might be a transmission shaft for example. The type of grinder for this purpose is a cylindrical grinder.

Darrell

 

[BillWood]

I get the impression that your average hobbyist might have Lathe or lathes, drill press, milling machine, shaper, surface grinder, fancy tool sharpening gadget like a quorn or similar,

............... but that a cylindrical grinder isnt typically in the list of hobbyists gear - is that right ?

How is it more accurate don't the grinding wheels become eroded during the grinding of the workpiece - and wouldn't this be critical for the expected output accuracy ? Whilst I'm at it how about surface grinding wouldn't the wheel erode during surface grinding too ?

Bill

 

[18w]

Bill you are quite right that a cylindrical grinder is not part of the usual home workshop. I was assuming the author was describing the process in a production setting. I should have read a little closer. He perhaps was suggesting that they were using a tool post grinder on the lathe. I certainly do not see the need to leave.010" to clean up with a toolpost grinder. Large amounts of metal removal is not the job for one of those. Cylindrical grinders, and surface grinders provide very accurate means of grinding to dimension and finish because of several reasons. Wheel wear is limited due to wheel material, often times coolant, and the small amount of depth of the grind. We are talking .001" or less of metal removal at a time and many passes unless you are dealing with large machines.. The wheel wears across its face at a even rate and as it wears the wheel is adjusted for more depth. Just like turning in the lathe, the part is measured and the wheel brought in farther as needed. These wheels do not wear like a typical bench grinder. They are capable of many passes with insignificant wear. Hope this makes sense.

Darrell

 

[Holescreek]

I've often wondered why this forum didn't have a "grinding" section, I guess "grinding" is really too vague. I'm pretty decent with a lathe and if I really try hard I might be able to hit a .0002" tolerance with a decent finish if I stopped soon enough to switch to sandpaper to finish the part off but repeatedly? Nope. The reason is that lathes aren't graduated to tenths and frankly the acme screw and nut tolerances were never made to hold tenths.

However, if you have a surface grinder, tenths are childs play even on round stuff with the right tools. Punches are regularly ground to tenths on manual grinders using spin fixtures such as Harrigs, Newboulds, Atco's etc. They do it easily because of the two things above, dials graduated in .0001"(or smaller) increments and better lead screws.

Many years ago I used parts of a $29 5C indexer to make a motorized spin fixture to grind ODs. It's overly massive because the bearings have a 45mm ID ( think).







With the proper wheel (in this case a 60I) I don't have a problem holding tenths on diameters even when they aren't continuous.



You don't necessarily need a motor if you don't mind spinning a handle by hand while you're trying to pay attention to what's happening with the wheel on the part but it gets old quick!

Add a sine plate and the grinder makes short work of putting a taper on straight shank end mill holders:

 

[JimDawson]

How is it more accurate don't the grinding wheels become eroded during the grinding of the workpiece - and wouldn't this be critical for the expected output accuracy ? Whilst I'm at it how about surface grinding wouldn't the wheel erode during surface grinding too ?

Bill

The wheel does not erode all across the surface at once. Much like a lathe tool bit there is a cutting edge, and the rest of the wheel is at the dressed dimension. The following picture is one I snapped of a plate I was working on. As you can see by the sparks, the ''cutting edge'' is about 1/4 inch in from the edge of the wheel, with the rest of the wheel not touching the work because the material has already been removed before the rest to the wheel gets to it. The cross feed is right to left in this picture, and the wheel is about 3/4 inch across the work. I think this is about a 0.005 pass. This wheel has already made several passes without being dressed.

 

[epanzella]

Another reason for turning parts 5 or 10 thou oversize is to allow for dimensional changes during heat treating. Once treated, parts that are too hard to be turned are ground to finish dimension.

 

[coolidge]

I would think a grinder's advantages would include working on harder material that would prove difficult for a lathe, a smoother surface finish, grinding parts too large for a lathe to turn accurately. I used to machine 6 foot long stainless plug screws on a big ass Mori CNC lathe, it had a 24" chuck. But all I did was turn the OD of a few sections, the cork screw section OD was ground. That thing was like turning taffy by the way.

 

[Billh50]

the only time we ground anything at work was when it needed to be held closer than + or - .001 or if it was getting heat treated.