Wire EDM and nitriding? 5

[724napier]

Hello all,

I know some of you guys have a lot of experience when it comes to manufacturing gears and splines. I am looking to get a little help/input on manufacturing an internal involute spline.

To produce the internal spline we plan to use a wire EDM as we do not have the proper equipment to broach it. My fear with using a wire EDM is the re-cast layer that it produces. The adapter is going to be made from nitralloy and the teeth will be nitrided. So I am concerned that the re-cast layer may produce a brittle layer that when nitrided becomes even more brittle. Is it possible to remove this re-cast layer through hand lapping with a male plug?

I was hoping someone with experience could shed some more light on the issue and possible ways of getting around it.

Thank you

 

[tbuelna]

724napier-

It is good practice to always remove the EDM recast layer, even though with modern wire EDM machines the recast layer is quite thin (usually <.0005").

Unless your spline has some unusual geometry it would probably be cost effective to have it shaped. Shaping would eliminate the issue of a recast layer and would produce a better quality spline surface. Check with some local gear houses to see if they have an existing tool for shaping your spline. Also, make sure to round all external corners before nitriding. The min edge break radius should be sufficient to prevent through hardening of tips/corners. Through hardened tips/corners are very brittle and prone to fracture.

If you nitride the spline teeth, you will need to remove the surface white layer after nitriding. Once again, the white layer is quite thin and it can easily be removed using mechanical or chemical processes. Since you are going to the trouble of case hardening the spline flanks you may want to use a honing process to remove the white layer. Honing will improve the flank surface finish and this is very helpful for fretting. For a small qty of parts, I would suggest using a process like ExtrudeHone. The process isn't as precise as conventional honing methods, but it also doesn't require any special tooling and is quick and low cost.

Hope that helps.
Terry

 

[724napier]

tbuelna,

Thank you for the information. I was hoping you would chime in as you seem very knowledgeable in this subject. When you say shaping do you mean manually shaping it on a mill using a shaping attachment? Our shop does have a Bridgeport shaping attachment, although I am not sure when it was last used, if ever. It looks like with this process you are manually indexing the spline and essentially hobbing one tooth at a time.

If we go EDM route would it be worthwhile to ExtrudeHone the teeth after they are cut with the EDM and then again after nitriding?

 

[tbuelna]

I meant cutting the spline teeth using a conventional gear/spline shaper machine. Your Bridgeport shaping attachment is probably designed for producing features like slots/grooves, and not involute splines.

The answer to your last question is yes. Heat treat your blank using a procedure compatible with the nitriding process. Then wire EDM the spline teeth leaving sufficient stock for finish operations. Extrude hone the spline surfaces to remove recast. Perform additional edge rounding if required and mask any surfaces that you don't want case hardened. Nitride the spline flanks. Extrude hone the spline surfaces to remove the white layer and polish them. Also be sure to remove the white layer from any other surfaces that were nitrided.

Good luck to you.
Terry

 

[TVP]

I concur with Terry about extrude hone, both before and after EDM. Another alternative is abrasive micro blasting. This is being used for precision tools, molds, etc. that are hardened tool steel with EDM features.

 

[juergenwt]

We have a lot of experience in cutting dies using wire EDM. At first we had a lot of cracked dies - using A2 tool-steel hardened to 59-61RC - especially in sharp corners due to recast. The solution was to give the die a double minimum draw after wire EDM.
Find out what the min. draw is on the material you are cutting and hardening.
If you are cutting low carbon steel - no problem.

 

[tbuelna]

The nice thing about using Extrude Hone for this internal spline is that it will smooth the flank faces and also round the corners/tips. Adequately rounding the spline corners/tips prior to nitride is important, and it can be difficult to do with a small diameter and/or long length internal spline.

Modern wire EDM machines have the capability to remove most recast by taking a second clean-up pass over the finished surface.

 

[724napier]

tbuelna,

Thank you for all your help. Our EDM machine is a newer machine and can do multiple passes with each pass removing less material. I will have to contact Extrude Hone and see what they recommend. We only need to make two of these, so it sounds like EDM and Extrude Hone is probably the way to go.

Do you recommend shot peening the splines after they are nitrided? I figure this will help with fatigue life, but I would be slightly concerned with distortion.

 

[CoryPad]

Whether the spline can accomodate distortion depends on sizes and tolerances. Small splines with a tight fit will have problems with distortion due to peening.

 

[spigor]

@juergenwt: What do you mean by "minimum draw"?

 

[CoryPad]

The tempering process is sometimes referred to as a draw.

 

[tbuelna]

724napier-

You mentioned in the OP this was an internal spline. Unless the spline has a fairly large diameter and short length it will be difficult to apply shot peen due to the extreme angle of the nozzle to the root surface. Ideally the nozzle is kept normal to the surface being sprayed, and typically only the root fillet areas of gear/spline teeth are shot peened. Shot peening is useful for improving fatigue performance where there is a fillet stress issue, but this is not common with splines. The root fillet stresses of internal involute splines in particular tend to be modest simply due to the truncated profile of the tooth. To minimize root stresses you can use a full fillet root type spline.

When you create the CNC program for wire cutting the internal spline teeth, be sure to add a very small amount of offset to compensate for removal of the recast and nitride white layer. It will save you some work if you add the profile tip radius to the CNC program. The min tip radius should be about 1.5x the nitride case depth. When you machine the adapter blank be sure to leave a generous amount of stock to the inside of the spline minor diameter so that the EDM wire can make a single continuous uninterrupted cut of all the teeth. Also, do whatever you can to prevent uneven force on the EDM wire for the initial cut, because if the wire breaks it can be difficult to align the new cut to the existing cut with the level of accuracy (indexing, runout, etc) your finished spline may require.

As juergenwt noted, wire EDM tends to perform better with harder materials. It would also minimize distortion of the spline teeth if the adapter blank was quenched and tempered before wire cutting them. If you use gas nitriding the process involves temperatures between about 950 and 1050 degF, so you would probably want to temper your nitralloy (135 or N alloy) blank between about 1200 and 1250 degF prior to nitriding for best results.

Hope that helps.
Terry

 

[724napier]

Terry,

Thank you for all the great information. You should write a book on all this some day. Do you have any first hand experience with using Extrude Hone in Paramount, CA? Or are there any other vendors you recommend using for the process?

 

[tbuelna]

724napier-

I've had quite a few parts processed by the guys in Paramount, CA. They have been around for many years. I've never had them process any gears or splines though. I have had them process aluminum castings that had lots of complex small diameter integral pipes that needed their flow surfaces smoothed out, and it worked very well. I also had them process some stainless steel hydraulic manifolds that had lots of cross drilled holes which required deburring/rounding of internal sharp edges, and it worked very well for that.

I live close to Paramount, CA so it was convenient to send my parts there. I don't know where you're located but I believe there are other companies that have the equipment used for this process.

 

[juergenwt]

Spigor- If you look in a book listing the heat treating procedure for different tool-steels you will find the resulting hardness after quenching. Also listed will be a min. required drawing temp. and the resulting hardness as well as other drawing temperatures with the resulting hardness. If you do not give the steel a min. draw after HT than the result will be a cracked part - if not the same day than for sure overnight.

 

[724napier]

Do you guys usually bake the parts after wire EDM to prevent hydrogen embrittlement? I was planning on having the recast layer removed and having the part stress relieved prior to nitriding. I wasn't sure if it also needs baked for hydrogen embrittlement as well.

 

[gearcutter]

Seems like you're going to a lot of trouble for these parts.
Is it a critical application?

For a well designed spline (low - medium stress) coupling and an industrial type of application; we usually don't worry about recast layers as modern EDMs are able to leave the part virtually free of the layer.

For critical high-stress applications, however, here's an excellent thesis on the effects of contaminants on the gas nitriding of Nitralloy-135.

https://www.wpi.edu/Pubs/ETD/Available/etd-112408-113232/unrestricted/WendiThesis.pdf

Unless you re-harden & temper the parts; I wouldn't stress relieve the parts. Nitrided case hardness and depth is a function of the hardness of the part before case hardening. What hardness have you used before the nitriding process?

Wire EDM can & generally will leave a significantly better finish than any gear shaping operation and can also be more accurate, particularly on high alloy materials that are already through hardened & tempered.

What type of nitriding process do you intend to use?
Two-stage gas nitriding or a Plasma/Ion nitriding process is recommended.
Plasma nitriding the parts will remove the need to deal with the so called 'white layer' post heat treatment.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[gearcutter]

Hydrogen embrittlement can be a problem when using an EDM on high alloy, hardened materials. If you're concerned about it; then 'baking' the parts (by holding at a specific temperature) immediately, or within a specified time, after the EDM process will help to significantly reduce the risk of embrittlement.

Most of the material that we manufacture our spline couplings from is 4140. Gas or Plasma nitriding this material will produce case properties with moderate hardness (around 48-52HRC) and high impact strength. It is particularly suited to acquiring deep and consistent nitrided case depths, of up to 0.50mm.
Pre nitriding hardness for this material is usually around 28-32HRC. Hydrogen embrittlement is generally considered not to be a problem on material with hardness values as low as this.

We generally stay clear of using Nitralloy type materials for our spline couplings as the nitrided case will produce too high a hardness for most of the applications that we cater too. With hardness approaching 60-65HRC, while having excellent anti wear properties, this is too high for general industrial applications particularly in the mining and earth-moving fields.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[724napier]

gearcutter,

I would consider the application semi-critical. It's going to be connecting a turbine engine to a dyno. Having all of these processes involved is making it a lot of trouble. My plan is kind of as follows:
1. Rough machine the part.
2. Temper the part to RC 34-38
3. EDM the splines
4. Extrude Hone to remove recast
5. Possibly hydrogen embrittlement bake and then stress relief. The stress relief will be done at a temperature below the tempering temp.
6. Gas nitride using two state gas nitriding process.

I am all for ideas and advice of doing this more efficiently, but I don't want to sacrifice reliability. I haven't found many people with experience in this area other than you and tbuelna.

 

[gearcutter]

I don't think that the trouble that you're going to warrants the application.
Given the application and assuming the geometry of the part helps ensure dimensional stability:

1. Design the spline so as to ensure minimum stress (particularly surface stresses).
2. Rough turn/mill leaving 1-2mm on all diameters/faces.
3. Through harden/temper - 32-35HRC.
4. Finish turn/mill, leaving grinding allowances of around 0.10-0.15mm if required. Features such as keyways, drill/taped holes etc can be finished.
5. Wire EDM ensuring that the geometry you use allows for the addition of edge-break radii and don't forget the wire diameter being used on the machine. Radii smaller than the wire's radius can cause some machines to stall.
6. Plasma/Ion nitride.
7. Finish-grind if required.

We've been manufacturing parts for industrial applications this way for many years now and have never experienced a premature failure.

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181