Stock Diameter For Knurling Tools

[mikey]

I wouldn't really call it two methods of thought. It really depends on the material hardness one is working with & the type of knurl they are after.

For softer materials & fully formed knurls you can get away with just feeding in until the knurl is formed properly. For harder materials & partial form knurls calculating is almost always needed to avoid double tracking. For partial form knurls you could get away with just feeding in until a proper knurl is formed & then skim the peaks off to get a partial form knurl. That is if final diamter is not important. And if you are working with a cut knurler you will more than likely need to calculate rather than getting lucky.

Will, when we first try knurling I think most of us probably start off calculating diameters and expect that this will unfailingly lead to good knurls, only to find that we mistrack anyway. This leads to confusion until we figure out that it isn't just about diameter but pressure and other variables, and that isn't in any of the worksheets or calculators I have seen. Moreover, the type of knurl you use makes a difference. Diamond knurls, when used in a scissors knurler anyway, are not as diameter sensitive as straight knurls, and coarser knurls tend to mistrack less than finer knurls. Then there is the fit of the wheels on their axles that can lead to unpredictable movement. There is enough variation in the process that the calculations, while they may help, are not the only determinants of success.

If we just absolutely have to produce a knurl on size and if we can produce a full form or partial knurl properly then I agree that the calculations matter but how often do we do that? Most of the knurling done in my shop is decorative and quite often the OD to be knurled is not machined to a calculated diameter; it just is what it is. What matters to me is that I put good knurl on without wrecking my project.

This is why I suggested that Franko try both ways and see what works for him. I started out doing calculations and found that most of the time it was non-critical for most of what I do. I don't own or use a cut knurler but if I did then my tune would very likely change.

 

[darkzero]

I agree with you Mikey but as I said it depends on the material hardness & type of knurl your after. I use an Eagle Rock K1-44 heavy duty scissor knurler with carbide pins & Accru-Trak convex cobalt wheels. I mostly knurl Ti 6Al-4V & 303 & 304 SS. Just feeding in without calculating diameters don't work here. They work harden very easily. For a full form knurl on Ti 6-4 I have to achieve the full knurl in one pass or I don't get good results. Try that with a bump knurler or cheap import scissor knurler. But then again I'm a bit anal on visual appearance.

 

[mikey]

Anal? Yeah, right! I've seen pics of the knurls you do on those Ti javelins and they are absolutely gorgeous. We should all be more anal!

I absolutely agree that stainless is a bear. I tried 303 and it worked okay but I had to do it in a single pass, too. Then I tried 416SS and that stuff hardens if you look at it - that one took three tries. I haven't calculated a knurl for going on 15 years now but I'll give it another go the next time I do stainless ... or the next Titanium javelin maybe!

 

[Cobra]

Franko
The easiest way I have found for checking the pitch of the knurls is to put a little oil on it and then roll it on a sheet of paper. Then just use a ruler to measure off and inch to count the lines.

 

[Franko]

Will, Mikey, What do you mean by a single pass?

 

[darkzero]

As in traversing the carriage to form the knurl only 1 time.

 

[Franko]

Thanks, Will.

 

[mikey]

Will, Mikey, What do you mean by a single pass?

Let me expand on what Will said. When you make a typical form knurl you are displacing material. Depending on the OD and hardness of the work, how much pressure you apply, the depth and number of the teeth and the phase of the moon you will usually be attempting to get to either a full or partial depth of the knurl pattern. Full depth will have sharp tips on the pattern and partial will have a less than full impression or a flattened tip. There are a number of things that affect this but, in general, you will need to apply a certain amount of pressure to get your pattern to emerge the way you intend. Quite often, and especially if you are not familiar with the material, you will make this pattern happen in stages. That is, you will crank up the pressure to get a pattern and then check to see if you have the impression you want. If not deep enough you will crank it up more until you get what you want.

The problem we were discussing is that some materials like stainless and Ti will work harden on you if you try this multi-pass thing. When that happens you get smearing and galling of the pattern or the knurls just won't cut and it looks like ... well, it looks bad. You also mess up the teeth on the knurls. The approach with these materials is to know how much pressure you need to get the pattern you want and then do the whole section you want to knurl in one single pass instead of multiple passes. This takes experience (as in trial and error).

Nothing to this knurling stuff, right?

 

[Franko]

Thanks, Mikey.

I remember when I got my mill. Nothing to it, I thought. This should be easy. The knobs have micrometers on them.

 

[mikey]

One thing I've always been curious about is the diameter calculations: Since the resulting knurl has appreciable depth (what, about 0.010" per side or 0.020" on the diameter?) why do the formulas ignore this fact and only use the outer dimensions of the stock for calculating?

I suppose that with forming tools the process moves material from the indentations outward, but it always seemed kind of arbitrary to just use the precise stock diameter. I know that knurling usually makes the stock diameter larger as a result — press fits in a too-large hole depend on the fact. So why do the calculations at the original diameter?

Rex

Hey Rex, I was just re-reading this thread tonight and noticed nobody addressed your question.

My understanding is that a knurl with a given pitch will displace metal a known amount when taken to full depth. When we need to have the knurled OD meet a desired diameter we need to start with a known starting OD. Then we take the knurl to full depth and theoretically we should have the final OD we need.

There can be considerable variation in the final OD, not just in overall OD but also in the consistency. Some areas may be larger, some smaller. I'm not absolutely sure why this is but I attribute it to a less than ideal fit of the knurl on the axle because it happens more when I used a junk China knurler and not very often with my shop-made scissors knurler with nice fitting axles or my Aloris 19.

Anyway, hope this helps.