Expanding Arbor Question

[edan]

Hello,
I am trying to help a friend who’s boss gave him a “little work at home”. I cannot be too specific about the part to be machined except this will be to perform second and third operations on a pulley. All the bores are from .5002 to .5050.

I have seen a machinist use a similar expanding arbor but I did not get a good look at how it was actually made.

My questions are as follows:

1. How important is the actual slope for the two end pieces?
For example, would 15 degrees work better than the 30 degrees that I have it now?

2. How important is it that the middle part matches the mating part as far as being the same 30 degrees?
As long as the “chamfer” is concentric with the part it does not appear to me to be too “critical” to me but I do not know for a fact so I wanted to ask.

Depending on the replies I will be giving him the drawing tonight. The material he has on hand will be drill rod for the two end pieces and aluminum for the middle part.

Thank you.

 

[student_Machinist]

Looks alright to me but what do i know
Although to be honest i would make the taper much shallower by extending it to where you have the end of the mandrel. It just seems like as it is the concentric repeatability might be difficult to manage.
Ill draw a pic if you cant work out what im on about ;D

 

[mu38&Bg#]

A shallower angle on the taper will provide more clamping pressure for a given torque on the screw. Too little taper and it might be hard to remove the parts, because the collet won't release. Anything over 16° included angle should be self releasing. The greater the surface of the mating tapers the more evenly the pressure will be spread. The angles of the mating surfaces should be as equal as possible to ensure squareness when clamped. You'll also be relying on the screw to center half of your "collet".

I use 5C expanding arbors. I'd just do the same and use the screw to expand it. If the best accuracy is required, make the screw as well to ensure concentricity. Like this....

http://www.ebay.com/itm/3-Pc-Expand...9?pt=BI_Tool_Work_Holding&hash=item256ae83641

Greg

 

[Lamachina58]

Hi there, I have made boxes of mandrels and your drawing looks wonderful but a lot of work for a one off part. It looks ideal for that much range however, .005 is enough IMHO to warrant the design you have come up with.
On my vise build I have some photos of a low cost 5C expanding mandrel I used on one part and the first page a very crude aluminum one for holding a long piece:
http://www.homemodelenginemachinist.com/index.php?topic=16654.15

Here is a link to some off the shelf ones I have used for production jobs, I have copied this modified bolt in a tapered hole many times with success.:
http://www.miteebite.com/products/id_xpansion_m.html

Instead of a bolt with a taper on it like Mitee Bite uses I often use a pipe plug, that is tapered of course and I like the ones with a hex so I can get an allen wrench on it.

I think you are right about 15 degrees being better than 30 but do some trig to see if it will expand.

I have used these from MSC with good luck as well:
https://www1.mscdirect.com/cgi/NNSR...re=ItemDetail-_-ResultListing-_-SearchResults

Very nice drawing, good luck! And Happy New Year!

 

[moconnor]

Hello edan,

Follow Greg's advice on the taper angle. If you can leave the angle of the compound rest undisturbed between machining all of the mating tapers, that will insure that they are a match.

I would suggest increasing the 1/2" length of the chucking diameter to make sure the fixture is stable. Make it at least as long as the clamping surface of what ever type of collet you will use to hold it or if it will be in a jawed chuck, you could increase the length even more to 1" or more. I think this is important, especially due to the type of part you are machining (pulleys).

Very nice CAD work by the way. Good luck with your project. Hope this will help you and your friend.

Regards,
Mike

 

[steamer]

That'll work fine. The bore range is fairly large, so I think the double expanding design is the right way to go.

For more pedestrian work....like for ONE part . I start by turning a piece of stock .010" oversize, and then drill and tap the bore for a pipe plug. Then I split it radially in the band saw. Then, with the pipe plug in, I chuck it back up and bring the OD to the size of the bore. NOW back out the pipe plug, mount your piece. and run the pipe plug back in.

The pipe plug has an approximately 4 degree angle to the thread so when you drive it in it expands the arbor.

this works well for parts that don't vary too much....but I don't know how well it would work for a .0048"....but it might.

Of course, this arbor is expendable, and is only good for as long as it's in the chuck.
Your design can go into the toolbox and be ready to go when ever.

Dave

 

[Blogwitch]

I use the second one down type on this page, the quick release ones.

http://www.arceurotrade.co.uk/Catalogue/Expanding-Mandrels

If you read a little bit about them, they only expand by 0.5mm (0.020") for each size. That should give you some idea of the taper required. On mine, it is very shallow indeed, at most about 10 degrees. I think that is to give a more stable joint between the two major parts.

The ones above give you a better insight into the shallow taper required.


John

 

[MachineTom]

I believe your taper is far too steep, as designed when you tighten the cap end, you will only distort the very ends of the expanding sleeve, with a crushing type axial force, and the center of the sleeve will deform inwards, as there is nothing but air to support it, this defeats the purpose of the expanding sleeve. The taper needs to be shallow and match the taper in the sleeve, this will expand the sleeve radially. I'm thinking 5° taper.

You did not mention the lenght of these parts, but solid tapered mandrels are cheap to buy and easy to make. Then the type already mentioned with a screw that expands the end.

 

[steamer]

The angles mentioned are a fair point Tom. They could be shallower and cover the bushing.

Dave

 

[edan]

Thank you to all who replied.

After reading the above replies I did the following:

1. Changed the slope to 10 degrees
2. Cut all 10 degree slopes in one setting (so they would match up)
3. Extended the collet area to approx 1 inch (I knew this but was not thinking as is should when I drew this up)

All 32 pieces are now finished and well within specifications. I was incorrect in the different bores sizes as one of them was .5102. So the range was .5002 to .5102 and it held everyone running true. I don’t know if .010 is a good range or not for an expanding arbor this size but I was surprised a little that the one (.010 over) did not wobble.
When I assembled the arbor without a part and tightened the screw it appeared to me that I could have actually decreased the slope but to what I do not know as I am aware that with too shallow of a taper that I would “lock up”. (I probably should have done the 5 degrees mentioned above)

This was a fun little job and I learned something new today, so thanks again. My friend wanted me to thank everyone also as now he can go to work on Tuesday and give the parts to the boss.

 

[stew]

HI ,
Some of You may not be aware, but a Adjustable Reamer can be used to hold a Work Piece to be Machined in a Metal Lathe. It is best when using one to only take light Cuts though otherwise if a jammup occurs the Inner Bore of the Part could be Damaged.

All The Best Stew

 

[mu38&Bg#]

Just a few more comments. I like the single taper design in the link Bogs posted. "MT2 Expanding Mandrels (Screw Locking)" It I was going to make one that would be my choice. Two tapers doubles the chance for error in making the chuck and in use.

I might wait until the parts actually arrived. Chances that the parts use up the entire tolerance range are pretty small, unless the machining operation was pretty crude.

Greg

 

[Springbok]

Cannot the boss do it for himself I hope you charged him

 

[Lamachina58]

I'd love to see the end design! Very informative thread and more resources. Once again, great forum!

 

[edan]

Thanks again for all the suggestions.

Looking at this design I can see where if it was not made right it could throw off the part. I do not know how critical the actual machining is or if I just got lucky.

Anyway, I just thought I would show what I was trying to do. The pulley had to be modified by cutting off the right “hub” flush to the main body of the pulley. (Over the head of the (SHCS)
Then the 1/8 inch pulley grooves had to be modified slightly.
I wanted the pulley to be supported over the area where the part was to be machined.

I guess my lack of experience with the other kind of expanding arbor led me to think of this. My concern was I did not know if just expanding the end would support the full length of the part with that much of a difference in diameter ( .010)? Next time I will try the simple approach and try to make one.

View attachment Arbor.pdf