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#3346591 - 04/18/14 02:08 PM Q for Machinist
sleddriver Offline


Registered: 02/06/10
Posts: 4150
Loc: Central Texas
I've been trying to find a replacement "driver" for an electric yard tool I've owned for years. (For more story, click here. Evidently it's no longer available individually, so I decided to make one.

Here's the original part. The larger end contains splines and mates with the electric motor shaft. No problems there. The opposite end had a square shaped insert into which a steel, square shaped flexible shaft was inserted. Worked fine for 23 yrs until heat and fatigue caused the plastic corners to round over.




I inserted some set-screws in an attempt to prevent the shaft from spinning and this held for awhile. However, drilling the plastic further weakened it:


So I set up some 1" delrin (I think) rod in my Taig to turn one:


Here's the result. My taper is a bit steep at 45. Wasn't sure how to cut a different angle. The 45 was done with a champhering tool.


So my Q's are:

  • How to create splines inside the large end? Two ideas came to mind: (1) Remove the motor, position it with the spline end up, and tap the new piece with a hammer to get a 'force fit". (2) Heat the motor shaft with a torch and plunge it into the hole.
  • How is a round hole turned into a square one on the opposite end? Don't have any chisels that small. I could go with set-screws again, perhaps smaller ones. Or heat the flex shaft and plunge it into the round hole.


I went with plastic rod as it's what I had on hand. However, I could make the piece again from either brass or aluminum, which might hold set screws better.

I mainly use the Taig for turning pens and simple jobs. Haven't gotten into cutting tapers or dealing with splines and square holes yet. Any suggestions you could provide would be appreciated! Thanks.
_________________________
1998 Volvo V70 T5 220,623 mi. Original Owner.

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#3347056 - 04/19/14 12:41 AM Re: Q for Machinist [Re: sleddriver]
sleddriver Offline


Registered: 02/06/10
Posts: 4150
Loc: Central Texas
Wow...they really are an endangered species...
_________________________
1998 Volvo V70 T5 220,623 mi. Original Owner.

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#3347376 - 04/19/14 11:54 AM Re: Q for Machinist [Re: sleddriver]
expat Offline


Registered: 05/12/09
Posts: 5267
Loc: Canada
Are splines in the Big end of the old piece in good condition?

I'm thinking to cut off the big end, turn down the diameter then insert the old splined section into a drilled hole in the new Delrin.
But I'm not sure what you could glue it with.

Alternatively you could grease the shaft of the motor, then build up layers of Devcon or other reinforced epoxy over the splines . Again, you would then turn it down and insert it in a drilled hole in the new piece.

I don't think I would have used Derlin, Brass may have been better.

For the Square end, I would have forced and hammered a section of copper tube over a similar Diamiter square shaft, until it formed a square tube.
Turn the square tube to make it as round as possible. Then insert it in a drilled hole in the new piece with glue, or if it's made from Brass, solder it.

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#3347382 - 04/19/14 12:02 PM Re: Q for Machinist [Re: sleddriver]
tom slick Offline


Registered: 05/26/03
Posts: 9189
Loc: Central Coast, Calif.
In my world we'd broach the inside profiles or have the part cast with all of the details.

The chamfer would be cut with a taper attachment or setting up the cutting tool's edge at the desired angle and plunging it into the part.

I think your idea of press-fitting the part onto the shaft is worth a try.
_________________________
You get what you pay for...
So keep in mind how much you paid for this advice.

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#3347423 - 04/19/14 01:50 PM Re: Q for Machinist [Re: sleddriver]
expat Offline


Registered: 05/12/09
Posts: 5267
Loc: Canada
A Broach would be ideal, but this is a blind hole which just makes life more difficult.

Is the Taper critical?


Edited by expat (04/19/14 01:52 PM)

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#3347448 - 04/19/14 02:31 PM Re: Q for Machinist [Re: expat]
sleddriver Offline


Registered: 02/06/10
Posts: 4150
Loc: Central Texas
Originally Posted By: expat
Are splines in the Big end of the old piece in good condition?

They appear to be. I don't believe they've ever slipped as the other joints have failed first.

Originally Posted By: expat
I'm thinking to cut off the big end, turn down the diameter then insert the old splined section into a drilled hole in the new Delrin.
But I'm not sure what you could glue it with.

Yeah..what glue holds well with plastic in a blind insert like that? Interesting idea of yours..never thought of that.

Originally Posted By: expat
Alternatively you could grease the shaft of the motor, then build up layers of Devcon or other reinforced epoxy over the splines . Again, you would then turn it down and insert it in a drilled hole in the new piece.

I don't think I would have used Derlin, Brass may have been better.

I thought about something similiar using JB weld. However, we're back to having it adhere to plastic. Adhesion would probably be better in brass or Al.

Originally Posted By: expat
For the Square end, I would have forced and hammered a section of copper tube over a similar Diamiter square shaft, until it formed a square tube.
Turn the square tube to make it as round as possible. Then insert it in a drilled hole in the new piece with glue, or if it's made from Brass, solder it.

Interesting idea..I do have some similiar dia. copper pipe laying around.

During research, I found a video of a coupler being machined of aluminum to connect a model boat engine to a flex. shaft driving the prop. I need something similiar. Got me to thinking about not turning down the square shaft receiver section at all. I need enough diameter for setscrews, which is what the video showed.

It may take several attempts to get it right, but I don't mind. I'll be learning along the way.
_________________________
1998 Volvo V70 T5 220,623 mi. Original Owner.

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#3347450 - 04/19/14 02:34 PM Re: Q for Machinist [Re: tom slick]
sleddriver Offline


Registered: 02/06/10
Posts: 4150
Loc: Central Texas
Originally Posted By: tom slick
In my world we'd broach the inside profiles or have the part cast with all of the details.

The chamfer would be cut with a taper attachment or setting up the cutting tool's edge at the desired angle and plunging it into the part.

I think your idea of press-fitting the part onto the shaft is worth a try.


I'm rethinking the taper idea. See above reply.

I could still press-fit with brass, aluminum or delrin. Need to try out different approaches and see what works.
_________________________
1998 Volvo V70 T5 220,623 mi. Original Owner.

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#3347451 - 04/19/14 02:36 PM Re: Q for Machinist [Re: expat]
sleddriver Offline


Registered: 02/06/10
Posts: 4150
Loc: Central Texas
Originally Posted By: expat
A Broach would be ideal, but this is a blind hole which just makes life more difficult.

Is the Taper critical?

I'll have to look up broaching, not sure about what it means.

Re: Taper. I'm rethinking that now. It's not necessary for clearance inside the outer tube the flex shaft rides in as there's plenty of room. So I'm thinking of taking advantage of that.
_________________________
1998 Volvo V70 T5 220,623 mi. Original Owner.

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#3347489 - 04/19/14 03:22 PM Re: Q for Machinist [Re: sleddriver]
tom slick Offline


Registered: 05/26/03
Posts: 9189
Loc: Central Coast, Calif.
Broaching info

I'm sure that part is plastic and tapered because it was injection molded. Injection molding is cheap and it can have a lot of details. It it tapered to use less material and makes it easier to get out of the mold.

I'd make it out of an aluminum or brass rod because it'll be easier to machine than plastic. Both would be soft enough to hammer onto the spline shaft and you could use a file to make the square end.
_________________________
You get what you pay for...
So keep in mind how much you paid for this advice.

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#3347550 - 04/19/14 05:14 PM Re: Q for Machinist [Re: tom slick]
sleddriver Offline


Registered: 02/06/10
Posts: 4150
Loc: Central Texas
Found this on the site you ref'd:


Top left rings a bell here. Your explanation of IM makes sense here. Thanks for clarifying. Re: filing, I do have a set of needle files, but nothing that small. I'd need about 1/8. I'll have a look though.
_________________________
1998 Volvo V70 T5 220,623 mi. Original Owner.

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#3347661 - 04/19/14 07:45 PM Re: Q for Machinist [Re: sleddriver]
expat Offline


Registered: 05/12/09
Posts: 5267
Loc: Canada
Can you show a picture of the splined shaft and give some dimensions.
I'm trying to think of a simple way you could make a Broach.

Another way would be, if the Driver were made of Brass or Aluminium, Drill a Hole just large enough in diameter to accept the splined shaft. then drill at 90 degs and Tap for Pointed set screws (or Grub screws) use several screws and stagger them along the inserted section of the shaft.

You could do the same at the other end, but with the flex drive I think you might be better making a square Broach hole than screwing onto the flex drive with Grub screws.

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#3348219 - 04/20/14 03:06 PM Re: Q for Machinist [Re: expat]
sleddriver Offline


Registered: 02/06/10
Posts: 4150
Loc: Central Texas




Outer dia. of splines: 7.7mm/0.3"
Spline length: 9.7mm/0.38"
Spline height: maybe 0.42mm/0.016" emphasis on maybe
Overall shaft length: 15.2mm/0.6"
_________________________
1998 Volvo V70 T5 220,623 mi. Original Owner.

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#3348743 - 04/21/14 01:13 AM Re: Q for Machinist [Re: sleddriver]
expat Offline


Registered: 05/12/09
Posts: 5267
Loc: Canada
That's a pretty fine spline.
It may be a little difficult to make a Broach like that, and using the motor shaft AS a broach, it might not cut too well.

If the Driver were made of Brass you could drill the hole for the motor shaft slightly undersize, heat the brass driver then press it on the shaft.
Heating will give you a little expansion of the driver, and help with you ending up with a tight fit.

On the flex drive end, it should be easy to make some Broaches (start small, get bigger) from Key stock.
Taper slightly the Key stock and cut series of fine grooves in the sides with a fine Hacksaw

https://www.youtube.com/watch?v=7n1r5XfVkyk

Finnish with a Full sized Key stock with a shallow hole drilled in the end. The diameter of the hole should be a little bigger than the width of the Key stock.
It will give you sharp corners on the end of the key stock, so you can Cut-out the taper left in the broached hole you made in the driver.

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#3349248 - 04/21/14 03:40 PM Re: Q for Machinist [Re: expat]
sleddriver Offline


Registered: 02/06/10
Posts: 4150
Loc: Central Texas
Found this interesting. This is similiar to what I want. Using setscrews might simplify things. (He's taken his little Taig a long way.)
_________________________
1998 Volvo V70 T5 220,623 mi. Original Owner.

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#3349350 - 04/21/14 05:53 PM Re: Q for Machinist [Re: sleddriver]
expat Offline


Registered: 05/12/09
Posts: 5267
Loc: Canada
I don't like the idea of the set screws on the Flex drive end very much.

My feeling is, all the torque is taken on the contact between the set screw and it's point of contact on the (Flexible) Flex drive.

I would much prefer the square end of the Flex Drive be seated in a Square hole, so the torque transmitted to the Flex shaft can be spread over as large an area as possible.

Set screws might work better at the other end, but again I would prefer not to rely ONLY on the set screw to transmit the torque.

Unless you file a Flat or drill a Pocket on the splined motor shaft.

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