Milling a semicircular groove, ball ended cutter, or?

[Ian PParticipant @ianp]

I want to mill corresponding longitudinal grooves on the joint line between two mating parts so that when assembled a loose fitting dowel can act as a locking device.

 

The parts in question are a ‘bolt’ 45mm diameter 3mm pitch made from EN24T, and a nut made from aluminium.

 

It the two parts were the same material then they could be assembled first and a hole drilled, obviously that wont work in this case and as the pin needs to be a loose fit anyway so doing the grooves separately will be OK.

 

I can use an end mill to machine the groove in the nut, probably 1/4″ diameter to take a 6mm pin. I think I could use a ball ended endmill to cut the lengthways grooves in the EN24T threaded part but having never used a ball end cutter I don’t know how it will fare. I imagine it will be slow going and might need a few cutters to cut the grooves as I have to cut 40 in total, each about 25mm long.

 

In the J&L catalogue that show ”radius shank cutter’ that looks like conventional HSS woodruff or tee slot cutters but with a radiused profile. Being in their ‘Value’ range they are not recommended for production work, which I take to mean they may OK for one or two grooves but not stay sharp long enough for my task. I would prefer to use this type of cutter from a finished appearance point of view as the run out at the closed end of the groove will be longer (than half the diameter of the endmill).

 

So, am I hoping for too much from the J&L cutter or will ball ended cutters do the job easily, and cheaper too.

 

Ian P

 

 

 

[Ian PParticipant @ianp]

[Les Jones 1Participant @lesjones1]

Hi Ian,

Could you not just drill the hole using a slot drill which should not have the same tendency to wander. An alternative would be to use the same method but machine each item separately. You could even start with the slot drill taking a small amount off and move the item a little at a time between each down feed until the half hole was complete. This second method would avoid the possibility of the steel swarf cutting into the aluminium.

 

Les

[Ian PParticipant @ianp]

Posted by Les Jones 1 on 20/01/2012 12:21:01:

Hi Ian,

Could you not just drill the hole using a slot drill which should not have the same tendency to wander. An alternative would be to use the same method but machine each item separately. You could even start with the slot drill taking a small amount off and move the item a little at a time between each down feed until the half hole was complete. This second method would avoid the possibility of the steel swarf cutting into the aluminium.

 

Les

 

 

Les

 

I am sure that would normally work but the job is a bit more complicated than I described when I simplified it.

 

Th male thread will actually have 10 equispaced grooves and the nut will have 11. That way wherever the nut stops when it is tight there will always be one pair of grooves that a pin will fit into.

 

There are four pairs of parts so thats actually 88 grooves altogether. For appearance (and to minimise stress raisers) I would like the grooves to run out smoothly rather than with a step that a slot cutter would leave.

 

Ian P

[BazyleParticipant @bazyle]

Could you make additional mating parts in the corresponding material to use during the drilling?

[Ian PParticipant @ianp]

Bazyle

 

To do that I would have to spend almost twice as much on material and then do a lot of machining (internal and external screwcutting) and then half of the parts would just be scrap.

 

I am being considerate to the environment, and my wallet!

 

 

Ian P

 

[GoCreateParticipant @gocreate]

Hi Ian

If the nut is Aluminium and the bolt a much stronger En24 how much load/stress is the bolt going to see that would cause consern for the bolt?

 

I would consider using a carbide round nose endmill, the carbide will give a good cutter life. The end of the groove will have a spherical radius therefore no sharp corners. I would not think this approach will give concern regarding stress concentration. The thread root will be much more of a stress raiser.

 

Not knowing application I hope this helps

 

Nigel

 

 

Edited By tractionengine42 on 20/01/2012 15:56:22

[Ian PParticipant @ianp]

Posted by tractionengine42 on 20/01/2012 15:40:27:

Hi Ian

If the nut is Aluminium and the bolt a much stronger En24 how much load/stress is the bolt going to see that would cause consern for the bolt?

 

I would consider using a carbide round nose endmill, the carbide will give a good cutter life. The end of the groove will have a radius therefore no sharp corners. I would not think this approach will give concern regarding stress concentration. The thread root will be much more of a stress raiser.

 

Hope this helps

 

Nigel

 

 

Nigel

 

I wanted the grooves to have radiused ends more for appearance than neccessity. I am a believer in ‘if it looks right, it is right’ and if these were made by a motor manufacturer I’m sure they would be well radiused.

 

These parts are going to hold the wheels on a car and the thread is much larger than is actually required. The M45x3 (or possibly 2mm pitch) will have a very long thread engagement and will only really be subject to axial tension loads. To reduce weight the male threaded part will be bored out to form a very thick walled tube.

 

Ian P

 

 

 

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Before doing any more do an end view drawing of both screwed end of shaft and nut . Its a bit either way but there is quite a high probability that with parts as described the nut won’t run on the male thread and will jam up .

 

MW

[Ian PParticipant @ianp]

Michael

 

I have already done a CAD drawing of the 10 & 11 hole relation ship to ensure that there was always a position where a pin could be inserted.

 

Whilst there will be a significant portion of both thread profiles cut away it is not over the whole length of the thread engagement. Both the nut and the bolt will have full threads for two thirds of their length and these will be fully engaged at the point where the nut goes tight.

 

In practice the internal and external grooves could be on a smaller diameter but plain parallel spigot. I chose not to design it like that as it entails the internal screwcutting being done up to a shoulder.

 

Ian P

 

 

 

[JasonBModerator @jasonb]

I’d do it with a convex cutter, looks a bit like a involute gear cutter but has a radiused end, it will be far saster than a bull nose milling cutter as they always cut slowly.

 

It will mount on a 1″ arbor in the vertical mill and just index the bolt round though a horizontal mill would be ideal

 

Or I suppose you could grind up a single point tool and use it sticking out the side of a a boring head

 

J

[Ian PParticipant @ianp]

Jason

 

I did suspect a ball endmill might be slow going although its the easiest for me to do with a vertical mill. The cutter in the J&L catalogue looks to be similar to what you describe (A thick slitting saw with convex radius?) but I expect that might be an expensive purchase. Also whilst I could mount it on a mandrel held in a collet its diameter might mean that the groove run out along the part would be quite long.

 

Ian P

[Robert DoddsParticipant @robertdodds43397]

Ian,

From your early description it appears that you are looking to “micro adjust” the tightness of a nut on a thread.

Despite the arguments regarding whether you are retaining the strength of the thread or not you appear to me to be taking a very hard route to achieve the micro adjusting bit.

The use of a “tabbed lock washer” is a well established method of doing this and I would refer you to a whole pile of images of many styles that you can easily see on google “images for tabbed lock washer”

This tabbed washer has been used in both auto and machine tools design

Usually one keyway in the shaft is all that’s needed and then you have a different number of tabs on the washer to grooves on the nut in order to get the vernier effect.

Hope you can find some inspirational ideas off the google images

Bob D

[Ian PParticipant @ianp]

Bob

 

Maybe I did not explain it very well. The grooves are for a loose fitting pin that will act as a safety device for a ‘Knock-on’ type nut holding on a centrelock magnesium road wheel.

 

Although in particular case the wheels will not be taken of and on frequently, a lock washer that needed bending when it was installed is just not suitable. The thread diameter and pitch together with the physics and actual shape of the wheel forging are such that once tightened to the correct torque the nut will be self locking. The (loose fitting) pin is there as a safety measure.

 

Ian P

[Ian PParticipant @ianp]

Posted by GRAHAM Meek on 22/01/2012 18:59:03:

Hi Ian,

 

I would not expect a very high life from a ball ended cutter in HSS, solid carbide might be a better option or even TiCN coated HSS would give you longer cutter life.

 

If I were doing the job I would prefer to use something like a Woodruff cutter but with a radius on the periphery to match your loose fitting dowel, again coated if at all possible.

 

In all cases the thread would be the last thing to cut, burrs thrown up by which ever cutter will mean there will be some serious de-burring to do if these are put in after the thread.

 

Gray,

 

Graham

 

Thanks for your comments, they confirm my original doubts about using a ball ended cutter. The profiled woodruff type cutter is definitely the way to go but the only one I have found is in the J&L catalogue and it has a 3/4″ diameter shank . Unfortunately I have no way of holding it in my milling machine as it only takes ER25 collets and the largest I can get in is 5/8″

 

One thing you have made me think about very seriously is the sequence of making the parts, I originally thought I would cut the grooves last but your comment has made me realise that would have been a nightmare.

 

Screwcutting on what will effectively be a splined bar might be interesting though. I was going to purchase a carbide thread form tool but they are not meant to be used on interrupted cuts so its now a chicken and egg situation.

 

Ian P

 

Ian P

 

[GoCreateParticipant @gocreate]

Hi

Cutting the thread last in this material may not be a good idea.

I had a job where 2 semi-circular threaded parts had to be opened out to a larger internal thread size.

 

When mounted in a fixture in the lathe I had two oposite gaps 4mm wide, the material was En24T and 1-1/4″ 8UN thread.

 

The tip of the threaded insert chipped very quickly, I cut the thread to size, turned or replaced the insert to present a new cutting point, backed the tool off and took a few very light cuts to clean up the thread profile.

 

20 thread tips to do 60 sets.

 

Probably not such a problem in softer materials but in En24T my experiance indicates you wil go through a few inserts.

 

Regarding using ball nose cutters, I mentioned this because I thought you were a woodruf type due to concerns abut stress concentration. Although slower I thought it might be more economical on cutting tools.

 

I wonder why you need a locking pin, My MGB has those konck on/off wire wheel arrangements, the nut face is tapered and engages a corresponding taper in the wheel. Once knocked on with a soft hammer its tight. never come slackened off yet, From memory I think the tapers have a slightly spherical profile

 

I was once shown a wheel nut off a formula one racing car, unbelievably thin in section and extraemly light, this was the same locking arrangement, the nut having a male taper engaging a wheel female taper of what looked like 45 deg.

 

Nigel

 

Edited By tractionengine42 on 23/01/2012 13:20:52

Edited By tractionengine42 on 23/01/2012 13:23:32

[GoCreateParticipant @gocreate]

Sorry

Repeated post by accident

Edited By tractionengine42 on 23/01/2012 13:15:56

[Ian PParticipant @ianp]

Hopefully the locking device will never actually be needed, its just an extra insurance,

 

These wheels sit against a flat flange which has five locating pegs that mate with holes in the wheel. The back face and the nut clamping face of the wheel are both flat.

 

I too have seen racing car chassis components and as you say they are an extremely light weight designed and made to aerospace standards. F1 and other top end racing cars have very sophisticated arrangements to hold the wheels on to allow very fast wheel changes. Some of the club racing cars though that just have a plain central nut to hold the wheel on have the hub cross drilled to take an ‘R’ shaped wire clip. Its there as a precaution which is all intend mine to be, but much less unsightly.

 

I have reread you post but I’m not sure which type of cutter did you thought was going to be the most cost effective.

 

Ian P

[Ian PParticipant @ianp]

With the benefit of all the advice and guidance given by forum members I have changed the design to something a bit more sensible.

 

I no longer intend to cut the grooves in the threaded section (which solves the problem of which to cut first) by putting the grooves in a reduced diameter section that engages with the stepped bore of the nut.

 

The only downside of this method is that the internal thread will have to be screwcut up to a shoulder, this is where an ELS or Grahams ingenious device would come in handy!

 

I will need to mill the groves in the EN24T with a ball ended cutter but I will make them as short as they can be and take my time probably with a carbide or coated HSS bit. I might rough out the grooves by drilling holes with a sacrificial ring of EN24T fitted, finishing with a ball ended cutter for size and appearance.

 

Ian P

[JohnFParticipant @johnf59703]

Hi Ian,

Screw cutting up to shoulder–always difficult ! Why not screwcut out from the shoulder, i.e. presumeing you will have an undercut where the thread ends at the shoulder? — use a left hand screwcutting tool–tool point pointing away from the toolpost– start with the tool inside the component with a bed stop and run the lathe in reverse so that you are “unscewing” the component i.e.carriage travelling away from the headstock. Thus no problem with stopping before the shoulder, your tool exits into fresh air.

 

Since your nut is alloy it is simple enough to grind up a HSS tool for the job but you will possibly have to truncate the thread crest because it will not be full form. Check on this, I am not up to speed on metric thread forms, you certainly would have to truncate imperial or unified threads with this type of tool.

 

I have cut 100’s and maybe 1000’s of threads this way in the 1960’s but do take care if your chuck is on a thread fixing, if it is you need to have a locking method in place with a large thread. If you are on camlock or din spindle no problem.

 

John.

[Ian PParticipant @ianp]

John

 

Yes, I can cut the thread from the shoulder, its slightly more complicated because there are two versions I need, with LH and RH threads, also I would prefer to buy ready made carbide tips with full thread form. I am not sure how many holders and tip types I would need in total though, it might get expensive!

 

My chuck is just a screw on (Boxford AUD) so whilst I have screwcut in reverse before now I would not risk it now especially in EN24T at 45mm diameter.

 

Ian P

[Peter TuckerParticipant @petertucker86088]

Hi Ian,

 

As you are no longer going to put the grooves in the thread but turn to a lesser dia why not just drill holes before turning? The same could be done on the nut blank before boring.

 

Just a thought.

 

Peter.

[Ian PParticipant @ianp]

Peter

It might be just a thought to you, to me it’s Brilliant!!!

Now you have pointed it out, it seems so obvious that I must have been asleep not to realise that the sacrificial metal I was put on to drill, was the same size as what I had just machined off!

Ian P

[Peter TuckerParticipant @petertucker86088]

Hi Ian,

 

Glad to help.

 

Peter.

[Author]

Posts