Knurling question

Hi

Are knurling patterns always taller than the rod itself? For instance

In this photo, the pattern seems to be significantly taller. Are there ways to precisely control the height of the pattern?

Any help appreciated.

Edited By Mark Fry on 29/03/2017 10:30:59

The alternative to pressure knurling is cut knurling. Cut knurling as it says cuts the form so produces within the diameter. There has been an article recently on making a cut knurl.

For regular knurling (forming not cutting) the diameter increases with approximately the depth of the knurl. In the image the smooth part is turned down a little bit.

For more information see this document for example

**LINK**

Conventional knurling displaces material so the top of the knurl will be larger than the original workpiece. Sometimes useful as a putting on tool when that last cut was a bit too much leaving a component that should have been tight in a hole loose. Height of pattern depends on how hard you knurl and starting diameter of the work. For folk like us its really trial and error. Not knurling full depth leaves a shallower rise as does starting out a bit undersize. With a decently hefty lathe and / or strong clamp or nutcracker three wheel style on knurling tool you can force the knurl deeper reducing tip diameter of a fully formed knurl. Not something to try on harder materials. Anything beyond ordinary BMS is chancing your arm. Best done with sharp good quality knurls, may need to buy new. Produces horrible thin, sharp shavings off the sides of the knurl patterns. Need to keep workpiece clean or the pattern will be poor. Something to be done as a last resort in extremis rather than as a normal thing.

Best way to create good smaller diameter or flush with un-knurled surface pattern is to start with a groove in the work. Trial and error again I'm afraid. Forget the "have to exactly the right starting diameter for a clean pattern nonsense" . On anything large enough to be stiff enough to knurl the conventional way, say 5/8" upwards, the diameter change for one knurl pitch is only a few thou's. Much less than the metal displacement so it will self correct if you push hard. As you should.

Cut knurls can produce knurls exactly to stock diameter but many folk find going a couple or three thou under produces a better defined and better looking pattern. Depending on your luck even a few tenths error in the stock can look horrible so using round bar as supplied may be disappointing.

Clive.

PS Benny is quicker on the keyboard.

Edited By Clive Foster on 29/03/2017 11:04:43

When I was a lad, I was a BMC car enthusiast, my friends laughed at me because the Ford cross flow engine was so much better!

However I later learned that the Ford engineering of their engines was far inferior to the BMC A and B series engines. The BMC heads had proper pressed in guides that could be replaced. The Fords had holes bored into the cast iron cylinder head, no guides. When this wore oversize the only option was a new head or internal knurling of the hole to reduce its diameter. Needless to say, this didn't last too long!

Andrew.

If a 2mm stainless steel rod is pressure knurled, could the height of the pattern be 0.025mm? Is a height increase of 1.25% reasonable? Like Clive pointed out, I wonder if the end of a 2mm rod can be made thicker (2.05mm) to give a tight fit.

It depends how much pressure, the deeper the impression the greater in diameter it's likely to be. It's probably going to be more like 0.5 increase for a reasonable pattern placed on it.

Michael W

Michael,

Do you mean a 0.5mm increase? That's quite significant - especially when the rod is only 2mm in diameter.

You will need some mighty fine knurls for 2mm dia rod

Tony

Andrew, we are still putting "K - Line" bronze inserts into cross flow race engines. Size achieved by driving a special type of ball bearing through the guide. I have been fitting bronze valve guides to engines for years and yes I love "A" series BMC stuff.

It's probably going to be 0.2 or so to be more realistic perhaps, maybe you could go on a general rule of thumb of 5-10% dia. increase but it probably wouldn't be too taxing in material to set up a standard pair of knurls and find out.

Why does it need to be so close to that size? Is it to secure a moulded handle or something?

Michael W

Edited By Michael-w on 29/03/2017 15:26:18

this wore oversize the only option was a new head or internal knurling

I was Ford over BMC, too. Guides were of no particular problem as they could be reamed oversize to take oversize valves. I only ever fitted one size up, so not sure how many larger stem sizes were available. Never seemed a particularly good or bad engineering practice at the time - engines were often scrap long before the bodywork disintegrated (not like vauxhalls!) and replacement engines were cheap and easy to swap out.

Benny's link has the proper sums but a handwaving estimate is fairly simple. Assuming the knurl is half in the original material and half raised above it of shape approximating an equilateral pyramid the rise of a fully formed knurl will be about 1/4 of the pitch. So a 20 TPI knurl with 50 thou pitch ought to give 12 thou / 0.33 mm rise if fully formed. I guess 60 TPI or more would be better as you don't really need a full form for taking up clearances. part form is probably better as it won't scratch the bore so badly.

For 2 mm rod you will need a nutcracker style three wheel tool. Nothing else will keep things stable. Not stiff enough for push knurling, although maybe you could get super creative with a steady. A simple clamp knurl will almost certainly squirt it out from under the knurls before you get enough pressure to start forming. Don't know if a nutcracker three wheel type will go down that far. I've done 4 mm or there about on mine but clearance were getting very close. Maybe a special with two knurls one side and a plain pressure roller holding the workpiece in place instead of the standard three knurls.

Clive.

It depends on whether you measure to the tops or the troughs!

Normal knurling is largely a forming, not a cutting, operation so it always increases the overall diameter as material is pushed into shape.

If you imagine a full knurl creates a triangular cross section with roughly an equal amount pushed up as the knurl bites down into the valleys, so the tops would rise by about half the knurl depth. Naturally this is complicated by an element of cutting reducing the amount of material and incomplete forming either exaggerating or reducing it.

In the example you show, although it looks neat at a first look, the knurl points are incompletely formed, it also appears that the surface at right has been turned down below the depth of the knurling - you couldn't extrude the knurl to its full height, plus that bevelled right edge has clearly been tidied up with a tool and shows the shank to the right is smaller in diameter than the depth of the knurl.

It's very hard to have a neat edge to a knurl without skimming the adjacent surface.

In summary, I would estimate the overall diameter increase as two half-depths or about the same as the depth of the knurl.

Neil

Edited By Neil Wyatt on 29/03/2017 18:16:18

Ford used to sell oversize valves to get around this problem.

Many types of stainless steel work harden if you even look at them the wrong way, so (depending on the alloy you have in mind) successful knurling may be difficult or impossible to achieve.

George