How to Make a Mandrel? - (Help)

I need to make a small cast iron pulley - 1/4inch wide, 1/1-4 diameter, with a through hole of 7/16 inch as a finished shaft size.

My blank is just 12mm wide, so a mandrel seems the only way to machine this.

Would appreciate a quick tutorial on making a mandrel for this job!

Not done this before.

TIA

Steve

Steve is there a boss on the pully or will you be removing 1/4" material right across? Flat edged pully or recesses for a belt?

J

Steve,

The easiest thing is just to turn a small spigot on the end of a larger bar; drill and tap the end, and secure the flywheel with a heavy washer and cap-screw.

You can then do it all in the 3 Jaw chuck ... Provided that you don't disturb the original bar, it's all concentric.

When you've finished, the bar goes back in the stockpile ... for re-machining next time.

MichaelG.

Michael that would not work if the stock needs to be taken down to 1/4" thickness as the washer and screw head are in the way, thats why I asked. It would need an expanding or tapered one.

J

Hi
Start the similar to Micheal G but leave 25 thou on also counter sink the end where the hole was tapped I would use about 1" diameter so it give you a nice true shoulder to keep it nice a true
Remove from chuck make a saw cut so when you insert and tighten countersunk screw it will expand . Put back in chuck . turn down to the required diameter . put job holding square against the shoulder tighten screw the mandrel will expand . Even at 1/4" thickness this should work
Nobby

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O.K.

Tapered is easy ... as is Shellac

Loctite can be awkward, because it really needs some space.

MichaelG.

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Edit:  Alternatively, do as I first suggested, which would leave the central boss over-size. ... When the pulley is finished the boss can be skimmed down in the lathe, by chucking on the outside diameter.

Edited By Michael Gilligan on 22/06/2014 13:18:38

Hi Steve

Step 1:
I would put the pulley casting in the three jaw then face the pulley boss and skim the rest of the side until it is flat
then ream or lap the 7/16 hole to size in one setting.

Step 2:
Reverse it in the three jaw, making sure the freshly turned surfaces are running true against the chuck jaw faces, then face the boss in the centre. You are only turning the face of the boss therefore the three jaw will be fine as long as it faces parallel even if the bore is not concentric with the spindle.

Step 3
Put a small end of scrap steel in the lathe say 20mm diameter (Or a diameter a little less than your pulley boss diameter) and turn a section down to 7/16" wide enough for the pulley and nut, testing it against the bored pulley for a nice firm fit, than use a die or the lathe to thread it for a 7/16 Whitworth nut. Alternatively reduce the to be threaded section of shaft down and use a M10 die and then use an M10 nut for clamping. The threading does not have to be super accurate as long as the diameter and the mounting face has a nice finish. The nut will take up the slack I would just use a die for speed.

Step 4
Set the finished boss on the pulley against the turned face on the mandrel in the three jaw, tighten the nut against it and face the two sides and periphery in one setting.

Depending on the features of the pulley you may need to put a thin collar between the nut and the pulley to allow access for the cutting tool. Also make sure the stub mandrill sticking out of the chuck leaves enough room for you to face the back side of the pulley when doing step 4.

I think in the time I have taken to write this the job would be nearly done. I have a tin full of these little mandrels, However it is usually quicker to make a new one than bother trying to set one up dead true in the 4 jaw chuck.

Regards
John

Thanks for that. The pulley has no boss and it does have a 60 degree "V" to take a round belt. The pulley is secured to the crankshaft by a grub screw in the bottom of the "V". It's not a casting, just the last fraction of a piece of CI bar I had left in the box.

I had in my head that the mandrel would be tapered and the blank tapped on for some reason, but the use of a parallel sided stub mandrel seems plain enough now so thanks!

Steve

Don't think you even need to make a mandrel.

Hold it in the 3 jaw with about 5/16" sticking out, face, clean up OD and cut the Vee. Drill and then bore/ream the hole. Transfer to mill and drill & tap for the grubscrew.

Then just use the grub screw to hold it to a bit of 7/16" bar with the faced side against the chuck jaws while you reduce the thickness to 1/4"

All good suggestions. MichaelG's original is my favoured method. I make my mandrels out of PGMS - then I can use them again in a collet chuck.

As far as the Loctite is concerned I think that leaving a gap is a bit of a myth - it isn't like solder that needs to be able to flow into a gap through capilliary attraction. I've read somewhere recently from a Loctite rep that if you can assemble it then it will stick, the greater the thickness of Loctite that you have the weaker the joint. My experience is that precision ground mild steel in a reamed hole works very well. My Wyvern crankshaft is assembled in this fashion with no additional pinning and it's worked so far (though it's only had about 15 minutes running- time will tell).

HTH,

Rod

A method I have used with good success is to turn the mandrel to a slip fit in the flywheel. Mill a flat on the mandrel approximately 10% of the diameter. use a wire or pin, that is slightly less in diameter than the amount milled off the mandrel. slip the wire in the gap, rotate the flywheel in the opposite direction of the rotation and it will lock lock it tight on the mandrel. When finished, reverse the rotation and it will slip right off. a good benefit of this is that you can make the mandrel a bit shorter than the flywheel hub and you will also be able to face off the hub without interfering with a locking bolt. I have also used this method to face the ends of cylinders after boring.

Rick

If you need to turn right to the centre, make the mandrel slightly oversize, tap it for as large a screw as you can get it, but only use the tip of a taper tap. Split the end with a hacksaw. Turn it down to a good fit in place, slip on the pulley blank and fita screw which will expand the mandrel.

Neil

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Rod,

Thanks or your support

Regarding the use of Loctite on a close fit ... I would be happy to use it myself [especially on a Brass mandrel], but I thought it best to advise Steve "by the book". Both Loctite and PermaBond advised us, 25+ years ago, to use a few thou' gap. ... Obviously, therefore, there is some small risk that the groove in the pulley might end-up a couple of thou' offset from the hole [whereas by using Shellac it is easy to float the blank into centration on a revolving mandrel].

MichaelG.

Recent advice from Loctite appears to be that it works under shear conditions so we should be less concerned about those 1 thou gaps.

Anybody interested in doing some trials?

Neil

All the Loctite products have differing properties depending on the one you select, I use 620 as a retainer!

A retainer is what you need to hold a component on a shaft not a thread locking product.

e.g. LOCTITE 222 Low strength, general purpose Threadlocker, Breakaway torque 6Nm

Loctite 620 **LINK**

Diametrical clearance: Up to 0.2 mm
Tensile shear strength: >24.1 N/mm²
Service temperature range: -55°C to +230°C
Fixture time on steel: 80 min.
Pack sizes: 250ml

or

Loctite 638 http://tds.henkel.com/tds5/docs/638%20(OLD)-EN.PDF

Diametrical clearance: Up to 0.25mm
Tensile shear strength: >25 N/mm²
Service temperature range: -55°C to +150°C
Fixture time on steel: 4 min.

Do not assume there needs to be a gap as it can be used on press fit parts, also it can take over 24hrs to achieve full strength, one of the errors is to assume it has bonded to full strength when you can not move the part, it is probably ok to handle but probably not machine or impose any real loads. Note: 0.05mm gap will cure to full strength faster than 0.1mm and so forth,

On top of that the materials have an effect on the time and strength.

Bob

Edited By Bob Brown 1 on 22/06/2014 22:28:29

Edited By Bob Brown 1 on 22/06/2014 22:35:35

Edited By Bob Brown 1 on 22/06/2014 22:41:11

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Neil,

That's an interesting piece of advice ...

What exactly do you [or they] mean by "it works under shear conditions" ? ... and, is the statement "so we should be less concerned about those 1 thou gaps." part of their advice, or is this your interpretation ?

... also, are they refering to a specific product, or to all the anaerobics ?

My understanding [from about 30 years ago] was that anaerobics are essentially gap-fillers rather than adhesives in the true sense ... and therefore the joint characteristics are broadly similar to a soft soldered one. i.e. poor peel strength and an "optimal" layer thickness.

The classic screw-locking, or cylindrical fitting, applications; I believe, rely upon the fact that there is a contrained layer of material ... as that layer is subjected to shear forces, it swells "radially" within the joint and so locks the components harder together. [*]  ... Hence my first question !!

N.B. ... I am very happy to be proved wrong, or out of date.

MichaelG.

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[*] ergo ... when working conventionally it is working under shear conditions.

Edited By Michael Gilligan on 22/06/2014 22:51:50

Edited By Michael Gilligan on 22/06/2014 23:16:25

Update: ... This very useful document appears to be the current guidance from Henkel, covering Loctite Retainers.

Factor 3 (Page 10), and Factor 4 (Page 11) are particularly relevant to close-fitting joints.

It seems that the advice that we were given remains conceptually valid, but is now presented at the microscopic scale. ... This is good, because it encourages the use of simple "light press fits".

MichaelG.