Problems Form tapping 303 galling and rough threads

[Ianagos]

Hey there guys I’ve had a heck of a time tapping some 303 round bar.

Parts just need .8” deep 3/8-16 tapped hole.

I broke my last few spiral flute taps due to a dumb mistake. Chucking pressure was very low because I was doing tubing before and the part rotated taking out the tap. Weird how it did a few then instantly took out 2 taps in a row

So I was out of decent cut taps.

Decided to try some Balax form taps I have on hand.

First one galled like crazy and actually stalled my spindle but nothing broke.

Now running about 10sfm I got it working but the threads look pretty rough and I still feel like things are galling up a bit. I did try putting oil in it but didn’t seem to make much difference. Otherwise the machine runs qualichem 251c

What to can you guys recommend for tapping this reliably?

By the was Im drilling for the form tap with a 8.9mm drill for about a 60% thread engagement from the chart I saw.

 

[Milland]

It may be the specific batch of 303 that's being a pain - like 304, there's not a lot of tight control on the mix that leaves the mill, so you never know exactly what you're getting.

It may also partly be 303's nature - the additives to make it "free" machining may also make it more brittle and less conducive to form tapping.

A nitrided tap may help, due to the nitride tending to reduce galling, but looking for a high-pressure lube/paste may also help. Maybe something like this:

Tapping and Cutting Paste - PLCS USA

Or this:

TAP - Molyslip

Or this:

Cimcool Cimtap Pink Tapping Paste Compound 16 oz B00401

may help.

 

[Strostkovy]

You aren't by any chance trying to run a forming tap in a hole drilled for a cutting tap, right?

 

[Milland]

You aren't by any chance trying to run a forming tap in a hole drilled for a cutting tap, right?

Op does say this: "Im drilling for the form tap with a 8.9mm drill for about a 60% thread engagement from the chart I saw."

 

[Ianagos]

Core size correct and a nice round hole?
Tap clocked in so will be running true?
Coolant strength @ 10%?

Holes is nice before tapping right at .350 or so which is about biggest allowable for 3/8-16

I Did not check the runout on the tap but I doubt it’s terrible.

Coolant is around 9-10% concentration. Had to raise it as I normally keep it around 6%



Currently using balax 3/8-16 bh6 uncoated taps

The top of the thread looks half decent but they bottom is where it gets rougher.

 

[TeachMePlease]

Disclaimer: I've never used a form tap. But isn't 10SFM kinda low?

 

[Ianagos]

Disclaimer: I've never used a form tap. But isn't 10SFM kinda low?

Balax the tap manufacturer recommended about 20sfm but they put a disclaimers for blind holes reduce by 25-50% so I tried 20sfm and it was ok but 15 sfm had less tearing and 10sfm seemed about right aswell.

60sfm galled and seized tearing out all the threads. And sending an expensive tap to the trash because I can’t remove the galling.

Im looking for y’all’s experiences in this. How would you run a 3/8-16 form to in 303 down a blind hole to get .78” good thread?

 

[Ianagos]

Well machine ran 250 parts today and the roll tap actually got some chips in the flutes. What’s funny though is that the threads started looking a little better towards the middle/end

Not sure why I had so much trouble.

 

[dgfoster]

How were you holding the part? I assume collet in a lathe or soft jaws in a mill. A three-jaw or a vee-jaw/flat-jaw could deform the part easily to cause problems.

Added: I took a look at Regal Cutting Tools site concerning form tap use. One thing that stood out was their comment on tool rotational speed:

“Second, form taps are typically run at speeds 1-1/2 to 2 times the speeds of a cut thread tap. Form tapping needs to be accomplished in a timely manner before the material can work harden from the forming action and create premature tool failure. “

This comment seems especially relevant since you are forming stainless.

Denis

 

[rklopp]

“Form tapping needs to be accomplished in a timely manner before the material can work harden from the forming action... “

This is not right. If there is any time-delay associated with work hardening, it is orders of magnitude faster than the forming action. As far as form tapping stainless goes, the work hardening might as well be instantaneous. There may be reasons the speed needs to be high, but work hardening is not one of them. The reasons are more likely associated with reduced deformation resistance at high deformation rates (e.g., thermal softening), coating performance, and lube behavior.

 

[dgfoster]

This is not right. If there is any time-delay associated with work hardening, it is orders of magnitude faster than the forming action. As far as form tapping stainless goes, the work hardening might as well be instantaneous. There may be reasons the speed needs to be high, but work hardening is not one of them. The reasons are more likely associated with reduced deformation resistance at high deformation rates (e.g., thermal softening), coating performance, and lube behavior.

Regardless of the mechanism involved it is likely good advice coming from a respected tool manufacturer and that is the most important reason for quoting it.

I guess they are not specifying the time frame involved or the microscopic mechanism though crystal lattice disruption is believed to be the fundamental cause of work hardening. I guess I would tend to believe that the metallurgists that must be involved in tool development at Regal tool must have some input into this statement. Maybe not. But this does make me think of the fascinating slow-motion micro-level videos most of us have seen demonstrating metal movement/melting at the very cutting edge of an insert.

I think it is fine to challenge the assertion. Do you have some good evidence concerning the rate of development of work hardening and at what tool speed it becomes significant and at what speed it diminishes? Obviously drilling operations in stainless (especially the more cantankerous alloys) are good examples of rapid hardening occurring if metal flow is interrupted but it also demonstrates how it can be avoided by maintaining active and continuous cutting.

Denis

 

[Strostkovy]

I do know for some materials high bending speeds will reduce cracking due to work hardening, so it makes sense to me

 

[rklopp]

Obviously drilling operations in stainless (especially the more cantankerous alloys) are good examples of rapid hardening occurring if metal flow is interrupted but it also demonstrates how it can be avoided by maintaining active and continuous cutting.

Continuous cutting (which is moot for form taps) does not avoid the work hardening, it avoids having to push the cutting edge through a pre-work-hardened layer. Continuous cutting keeps the edge buried in softer, less-work-hardened material below the surface hardened by the prior edge passing. The work-hardened surface layer goes out with the chip.

One piece of evidence that work hardening is not time-dependent at form tapping timescales is that the work hardening parameters in the Johnson-Cook material model are constants and not time-dependent. The Johnson-Cook model is a very widely used material strength model for metals at high deformation rates. The model does have parameters for dependence of the strength on deformation rate, but not rate-dependence or time delay of the work hardening.

 

[MaxPrairie]

Due to the lack of ability to form 303 compared to 304 or 316, I have always used cut taps on 303. We had some 1/4" rods that needed a 45 deg bend in the a couple years ago and as a test piece to make sure everything was good I ran one out of 303. Everything was good until I went to bend it, then cracks formed on the tensile side of the bar. When I ran the actual parts out of 304, never saw an issue. Maybe....maybeee a thru coolant form tap could fix the galling.

 

[david n]

303 turns, drills, bores, reams, mills great...............but in my experience, it seems to tap like crap. Maybe it's me?

I make a few thousand parts outa 303 every few months and after fightin' tappin', I drill and thread mill(2nd op on a turned part).................form tap, cut tap, different coatings, etc........ and I just seemed to break taps or produce crummy threads...................

 

[Ianagos]

Well I’ll tell you this I followed the people who makes the taps recommendation and I got through the job.

When I followed the roll taps need to go fast theory I destroyed a tap in 1 second flat. I read that at first so I sent it at 60sfm with cutting oil. Tap galled and just ripped the threads out of the part.

Maybe with another brand of tap that would have worked but I’ve had very good luck with Balax taps and like I said their recommendations worked although not well. The people you quoted recommendation did not.

 

[dgfoster]

Continuous cutting (which is moot for form taps) does not avoid the work hardening, it avoids having to push the cutting edge through a pre-work-hardened layer. Continuous cutting keeps the edge buried in softer, less-work-hardened material below the surface hardened by the prior edge passing. The work-hardened surface layer goes out with the chip.

One piece of evidence that work hardening is not time-dependent at form tapping timescales is that the work hardening parameters in the Johnson-Cook material model are constants and not time-dependent. The Johnson-Cook model is a very widely used material strength model for metals at high deformation rates. The model does have parameters for dependence of the strength on deformation rate, but not rate-dependence or time delay of the work hardening.

Maybe not so moot. Certainly material goes from a soft to a work hardened state quickly in the case of stainless steel cutting and deformation. Clearly a tool encounters soft material and deforms/cuts the steel and very soon thereafter the material near the cut/deformation is harder. How quickly that occurs and whether the rate and amount of deformation can be influenced by tap rotation is the question. I would hazard a guess that the folks at Regal have some sound reason for their recommendation. Or, maybe they are just blowing smoke. I'll take them at their word until I know better.

The OP evidently tried to increase rotation speed using an unknown setup and lube and evidently had poor results. I am sorry to hear that and figure that raises questions about the recommendation.

Denis

 

[dgfoster]

I thought it might be interesting to see what Emuge had to say about roll form taps. What I found was they have developed an array of coatings and geometries of form taps specific to several material types. They list specific designs for stainless, cast aluminum, another for extruded aluminum, for low alloy steels, and another for ductile materials. The form taps I have used have all been USA- made, but I suspect, generic designs. Evidently, there is a lot more to form tap geometry and coating than I ever guessed. Which loops back around to making it not so surprising that use recommendations from one manufacturer may easily differ from another. And also suggests (not at all surprising) that some very capable engineers and technical folks have put quite a bit of work into optimizing form taps.

Here is an Emuge brochure on form taps:

https://www.emuge.com/sites/default/files/ZP10019_GB_RevB%20Form%20Taps.pdf



Now multiply that times however many quality tap manufacturers there are and it looks like there may be some real interesting tapping solutions out there.


Added: all the tapping speeds for form taps in the Emuge brochure are greater by 30 to 100% than cut tapping speeds listed side-by-side in this brochure:

https://www.emuge.com/sites/default/files/MultiTAP-8PG-FULL_LINE-2015 v10b.pdf

Interesting stuff.

Denis

 

[dian]

Continuous cutting (which is moot for form taps) does not avoid the work hardening, it avoids having to push the cutting edge through a pre-work-hardened layer. Continuous cutting keeps the edge buried in softer, less-work-hardened material below the surface hardened by the prior edge passing. The work-hardened surface layer goes out with the chip.

One piece of evidence that work hardening is not time-dependent at form tapping timescales is that the work hardening parameters in the Johnson-Cook material model are constants and not time-dependent. The Johnson-Cook model is a very widely used material strength model for metals at high deformation rates. The model does have parameters for dependence of the strength on deformation rate, but not rate-dependence or time delay of the work hardening.

as far as i know the parameters are strain, strain rate and temperature. the stress/strain rate relationship is "exponential" for many materials. thats why you go slow when drilling stainless, no?