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Wire EDM and nitriding?
5

Wire EDM and nitriding?

Wire EDM and nitriding?

(OP)
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

RE: Wire EDM and nitriding?

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

RE: Wire EDM and nitriding?

(OP)
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?

RE: Wire EDM and nitriding?

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

RE: Wire EDM and nitriding?

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.

RE: Wire EDM and nitriding?

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.

RE: Wire EDM and nitriding?

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.

RE: Wire EDM and nitriding?

(OP)
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.

RE: Wire EDM and nitriding?

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.

RE: Wire EDM and nitriding?

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

RE: Wire EDM and nitriding?

The tempering process is sometimes referred to as a draw.

RE: Wire EDM and nitriding?

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

RE: Wire EDM and nitriding?

(OP)
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?

RE: Wire EDM and nitriding?

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.

RE: Wire EDM and nitriding?

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.

RE: Wire EDM and nitriding?

(OP)
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.

RE: Wire EDM and nitriding?

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-...

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

RE: Wire EDM and nitriding?

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

RE: Wire EDM and nitriding?

(OP)
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.

RE: Wire EDM and nitriding?

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

RE: Wire EDM and nitriding?

724Napier

there is more than one way to process this Job
all the information given is valuable information and is
well given. however these are my suggestions.
since we have not seen any drawings it's tough to say.
I manufacture aircraft/aerospace applications.

heat treat the material rough stock, Bar Stock
semi finish turn & mill
stress relieve 50 degrees below the tempering temp.
wire edm spline. hold low limit to allow for stock removal of .0005 max per surface.
request that re-cast layer is not more .0002 per surface.
a very light bead blast spline area
deburr. pre-nitride is a must.
2 stage Nitride
chemical etch to remove white layer ( I do this all the time (speak with the heat treater) simple and easy, use gages to make sure it does not blow over if available)
NDT to insure white layer is removed.
inspect
finish grind if required for bearing surfaces.
stress relieve after grinding
mag particle inspect
final Inspect

thats it

HTH

Mfgeng

RE: Wire EDM and nitriding?

forgot to add do a bake after chem etch. but the heat treater knows that. :)

RE: Wire EDM and nitriding?

Quote (724napier)

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.

Whoa there! I would not consider a turboshaft engine output shaft spline to be a "semi-critical" application. They are usually highly engineered splines, and the OEM typically provides very specific requirements for the spline material properties, fits/tolerances, and operating environment in terms of installation misalignments, lubrication, dynamic balance, etc. The spline joints of turboshaft outputs usually use crowned external teeth which helps minimize radial loads on the engine shaft bearings due to installation or operating misalignments. These crowned splines have high flank contact stresses, so they typically use case hardened teeth. Since you are nitriding the internal spline teeth of your adapter, I'm assuming that the external teeth of the mating part are probably crowned. The crowned spline joints used on turboshaft outputs typically are major diameter fit type splines. The major diameter fit constraint minimizes radial movements between the mating halves of the spline joint from dynamic effects at high rpm.

Since this application is for a turboshaft engine output, I'd highly recommend shaping the spline rather than using wire EDM. I don't think you will obtain the accuracy this particular internal spline requires using wire EDM. If you would provide more specific details about the spline (ideally the ICD from the engine OEM), I'd be happy to provide more assistance.

Best regards,
Terry

RE: Wire EDM and nitriding?


tbuelna - you raise some very important points. Thanks for putting us back on-track.

FYI - Given the correct geometry/programming is used; wire EDM is of such high accuracy that parts manufactured this way can be used as gauges.

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia
http://www.aussieweb.com.au/email.aspx?id=1194181

RE: Wire EDM and nitriding?

(OP)
Tbuelna,

You are correct that it is critical, but not flight critical. The male spline is on the engine dyno not on the turbine. It mates to the turbine engine by four bolts. The male shaft on the dyno is made from 15-5 PH and is not crowned. There is currently another shaft being used to connect the dyno to the turbine. It is made from nitralloy and is a fillet root side fit. There's been no issues with it and it has many hours on it. I am out of town at the moment, so that is all the info I have off the top of my head. Thanks for your help. You to gearcutter.

RE: Wire EDM and nitriding?

gearcutter-

I agree that the profile of a wire EDM surface by itself can be extremely accurate, but the problem with using wire EDM for an application like this is ensuring that the finished wire EDM surface is accurately located relative to other critical features of the part. Where I work we have a very nice Mitsubishi wire EDM machine, and one issue we have is how to accurately locate the start of an internal cut (like an internal spline) relative to the part after feeding the wire through the start hole.

RE: Wire EDM and nitriding?

Terry

the main concern with EDM many companies don't allow it. far as accuracy it has been my experience there has been no issues holding P.D. runout to the parts datums.
the other problem from what have seen is the creation of the spline geometry. it has to be exact. normally it is best to very with gages or a mating part.

724napier

Gear cutter is correct that if plasma nitride is used. if done correctly there will not be any white layer. however hard tooling has to be used for masking. which gets more difficult. as whole EDM is a very reliable source of making splines. but you need to start 3 parts to make 1. because you need a part that they can use as a setup to verify the programing. and other tooling issues.

HTH

RE: Wire EDM and nitriding?

mfgenggear-

I agree that it is possible to produce an accurate internal spline using a good quality wire EDM machine. However, I would argue that you can usually get a better quality internal spline with less trouble using a conventional shaping process. You can usually find an aerospace gear vendor that has existing tooling for the splines used on aircraft drivetrains, so non-recurring costs should not be an issue. Personally, if given a choice between using wire EDM or shaping to produce this spline, I would choose shaping every time.

I would also agree that the best way to inspect splines is with master gauges. For most of the aircraft parts with splines that I have designed a set of go/no-go gauges was typically made, primarily because the parts with the mating halves of the spline joint were manufactured by different vendors. So the only practical way to ensure the mating halves of the spline had a proper fit was a set of master gauges that we could send to each manufacturer to validate their parts.

RE: Wire EDM and nitriding?

Terry

hahahaha yes we could go back and forth on this one. shaping is a better route to go because it is assured there is the correct spline geometry. however when splines have small numbers of teeth, and are fine pitch, and are less than 1.00 Pitch Diameter.
there is a tendency for the cutter to deflect badly. dependending on the quantity of parts if it's a blind hole I go with punch broach. if it's a through hole I go with with a pull broach. if it's only 1-2 parts I will use wire edm or sinker if it's blind hole & if it's allowed by the customer to use ether. I will shape it if that is my last resort. and yes it does work. but remember it has to be tooled. the cutter has to have the correct whole depth or D+F, the cutter has to be the correct number of teeth so it will not side trim. it has to have the correct length so it will clear the tooling and the part.

plus depending the configuration if off the self tooling does not work it has to have a special pot to hold it. see it starts to add up.so now we are specking of dollars for one part. so if it is acceptable to wire cut by the customer it is an another tool in the bag. hahahaha

Yes gages are usually a must for mass production but for one or two parts a mating part will work. it just has to fit. and a 2 or 3 between wire inspection check is sufficient to make sure it's in the correct size. I can and do design spline gages. when it is necessary. or I purchase them from another spline gage manufacture.

the gears and spline I work with are and can be master gear quality or master spline quality from the old spec AGMA Q15 and even tighter.

sorry for the long post but just given my 2 cent.

Best Regards

MfgEng

RE: Wire EDM and nitriding?

Quote (mfgenggear)

hahahaha yes we could go back and forth on this one

But isn't the purpose of this forum to provide a place where gear nerds like you and I can endlessly debate an insignificant topic like this?

I'm pretty sure we lost 724napier long ago, but I hope we provided some useful advice before he checked out!

RE: Wire EDM and nitriding?

(OP)
tbuelna,

I am still here. I have just had a hectic week and haven't gotten much time to respond. I am waiting to hear back from some vendors on having the spline shaped, since we do not currently have a shaper only an EDM. I have been soaking all the advice up from you mfgenggear and gearcutter. I actually had some more questions for you guys as well.

-When is it worth it to use a VAC-ARC material vs. regular air melt?
-As far as Go/No-Go gages, do you recommend making them on an EDM or is it better to buy a precision ground set? This is only a prototype part and will be very low production.
-Also in reference to the Go/No-Go gages. Is the Go gage typically made with the minimum effective circ space width while the No-Go is made to the max effective? It seems like it would be better to have more than just these two gages.

Thanks again to all you guys for the help. It's pretty amazing how deep this topic can get.

RE: Wire EDM and nitriding?

(OP)
Also,

When using an EDM to wire cut an internal spline. Do you typically set the circular space width between the min/max effective or the min/max actual?

RE: Wire EDM and nitriding?

724 Napier

Vac Arc is worth the extra cost.

Buy the set of gages gages that are certified.
For an Internal Spline the MIn Effective is the Go Gage
the max actual is the No Go Gage. is what I recommend. ANSI B92.1-1996.

Use the min actual for the EDM programing then offset to allow for the overburn
during the programing. use the gages to verify during cut.
and after heat treat.


RE: Wire EDM and nitriding?

724napier-

Unless you know the pedigree of the external spline your adapter will mate with, and/or have detailed documentation describing the interface requirements of your adapter spline, then as mfgenggear suggested for just two or three parts it would be best to simply match your adapter spline to the existing shaft spline. Having a certified pair of go no-go spline plug gauges made might cost $1000 or more, and it doesn't sound like you have the budget for this.

As for the cost/benefit of using vacuum melt quality materials, for the small number of pieces involved the higher raw material cost will not be a significant factor in the overall cost of the parts, so it is definitely worthwhile. Vacuum melt quality steels are very beneficial where fatigue life or reliability (fracture control) are concerns.

Good luck with your project.
Terry

RE: Wire EDM and nitriding?

(OP)
Thanks mfgenggear and tbuelna, now if I could only find a supplier with vacuum melt material in stock I feel like I will be in good shape.

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