Author Topic: After a fixturing suggestion...  (Read 4695 times)

Offline Allen Smithee

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After a fixturing suggestion...
« on: July 01, 2024, 12:12:14 PM »
I need to make a couple of these prop drivers for Axi 41xx series motors, because they are no longer made and the current versions are incompatible. For reference, the overall diameter is 26mm, the shaft thread is M8, the rebated holes are for M3 allen-caps and the resulting part will transmit 1.3kW (1.7bhp) into a 15-10 propeller at around 8,000rpm.



Most of it is very straightforward turning, drilling and tapping in some 1-1/8" H15 bar.  The main shape and most features I can do in my Myford S7. I have a modified Seig X2p benchtop milling machine and I have a 6" rotary table, so the three rebated attachment holes I can easily & accurately achieve in the mill. But the side features are inlet ducts blended for the cooling ports of the motor. It's a 15mm diameter cut but I'm struggling to think of a simple way to fixture it to be able to cut all three accurately (so the part remains balanced) using the tooling I actually have. I down' own a tilting table that would allow me to mount the rotary table at 56degrees, nor do I have any kind of indexer.

Is there a simple solution or am I going to have to make something?

All thoughts appreciated!

AS
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Offline Jasonb

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Re: After a fixturing suggestion...
« Reply #1 on: July 01, 2024, 12:29:32 PM »
Drill or preferably ream an 8mm hole into a block of something you have to hand. Also drill and tap the three M3 holes to match the pattern of the part

Set block at the required angle in your mill vice and insert the turned and drilled part into it, fasten with screws. You can now plunge cut with a 15mm dia cutter or boring head.

When done unscrew and move round to the next position, refix with screws which will take car eof teh indexing and cut the next and so on.

Could also be done with a hex collet block if you can rig something up so it goes back into the vice at the same angle/position.

PS the drawing suggests it is more than just a 15mm dia at an angle due to the two lines that radiate out.

Offline cnr6400

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Re: After a fixturing suggestion...
« Reply #2 on: July 01, 2024, 12:33:38 PM »
Is there a reason the air slots have to be angled and straight? It occurs to me that if the design was changed to straight slots cut with a ball nose end mill, you could machine them in the mill with the cutter parallel to the other holes. The flange under the prop would remain, to support it, and cut with a ball nose endmill would relieve the same or more weight and leave even more room for airflow than the straight slots. Just food for thought. Good luck.
"I've cut that stock three times, and it's still too short!"

Offline Jasonb

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Re: After a fixturing suggestion...
« Reply #3 on: July 01, 2024, 01:13:34 PM »
If done straight you would lose area to clamp the prob against which may result in slipping.

regarding the machining the block could also be made to fit a 4-jaw or faceplate if you don't have a boring head or want to buy a 15mm cutter.

Offline Allen Smithee

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Re: After a fixturing suggestion...
« Reply #4 on: July 01, 2024, 01:49:12 PM »
Drill or preferably ream an 8mm hole into a block of something you have to hand. Also drill and tap the three M3 holes to match the pattern of the part

Set block at the required angle in your mill vice and insert the turned and drilled part into it, fasten with screws. You can now plunge cut with a 15mm dia cutter or boring head.

When done unscrew and move round to the next position, refix with screws which will take car eof teh indexing and cut the next and so on.

Could also be done with a hex collet block if you can rig something up so it goes back into the vice at the same angle/position.

I was hoping for something that didn't involve manual positioning of the 120 degree iterations. I'm looking around the workshop to see if I can find a way to hold the rotary table at the 56degree angle firmly enough so I can use that. I considered using a collet block, but I'd need to make a lot of other fixturing to be able to remove and reposition it precisely enough.

Quote
PS the drawing suggests it is more than just a 15mm dia at an angle due to the two lines that radiate out.

That's just a visual artefact of the way I built the part in fusion using a lofted cut. I've 3d printed the part to confirm the geometry and hole positions, and those lines don't appear in the STL file (or the resulting part).

AS
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Offline Allen Smithee

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Re: After a fixturing suggestion...
« Reply #5 on: July 01, 2024, 02:04:15 PM »
Is there a reason the air slots have to be angled and straight? It occurs to me that if the design was changed to straight slots cut with a ball nose end mill, you could machine them in the mill with the cutter parallel to the other holes. The flange under the prop would remain, to support it, and cut with a ball nose endmill would relieve the same or more weight and leave even more room for airflow than the straight slots. Just food for thought. Good luck.

Well I don't have a 15mm ball-nose cutter, so the point is moot(!), but I'm copying the geometry of the OEM part as far as possible for a motor of this size to ensure I don't compromise the cooling. The angled slots essentially project directly from the cooling ports on the motor. As Jason said, the front face needs to be essentially circular to grab the propeller boss securely.

These are relatively powerful motors - roughly equivalent to a good 10cc 2-stroke or 15cc 4-stroke (the 2-stroke would need to be geared around 1.8:1 to produce the same torque).  I have several in this class and they are very useful, but the Axi brand are the "quality" ones which are normally very expensive (over £120 each). I happened across a chap who had a consignment of a special marine variant called the "Axi 4122/22WAVE" as new-old stock - these have some small water-resisting adaptions and a special fixed-throttle speed controller (I think they were intended for Straight-Runner competitions or similar), but are otherwise pretty well the same and the Axi 4120/18v1 (a motor I use and like a lot) but they have none of the propeller-driving hardware. I was able to by a pair of these for less than a tenth of the price of the new Axi, so it's worth my while spending some effort making prop-drivers!

AS
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Offline Jasonb

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Re: After a fixturing suggestion...
« Reply #6 on: July 01, 2024, 04:14:02 PM »
It is always possible to cobble something together to hold the R/T


Offline petertha

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Re: After a fixturing suggestion...
« Reply #7 on: July 01, 2024, 05:47:34 PM »
I think a mill fixture held in vise would be better. That way you can (I presume) orient and clamp the part using its existing bolt holes. That just leaves the task of cutting the circular recesses with end mill or boring head. I've shows 2 red dots which represent alignment pins to rest on your vise jaw. A handy way of setting oddball angles by X,Y position to the square fixture in the absence of protractor like devises.

Offline Allen Smithee

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Re: After a fixturing suggestion...
« Reply #8 on: July 02, 2024, 10:30:40 AM »
Thanks for that thought, Jason - I'll have a look through my assorted stuff and see if I might do something similar. But Peter's post has also made me wonder if I'm overly fixated on the rotary table when I might be able to achieve something far more simply using the three screw holes as indexing holes.

Thanks all - I believe I can do it now!

AS
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Offline Jasonb

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Re: After a fixturing suggestion...
« Reply #9 on: July 02, 2024, 11:49:22 AM »
Peter's is the same as I first suggested so to the two of us it is the logical way.

With the rotary table at an angle you will have to find a way to index the part as you mount it to the table in order that the screw holes fall mid way between the cut outs so that is likely to need a fixture anyway if more than one to make. It would not be practical to drill the holes with the table flat on its back and then leaving the part in place set up the table for angled cuts unless it were a one off part.

Offline petertha

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Re: After a fixturing suggestion...
« Reply #10 on: July 03, 2024, 02:15:29 AM »
Actually this fixture could be improved with the part shaft facing up & just using the SHCS bolts to both retain & locate into clocked position. Bolts aren't really proper for accurate indexing because of the threaded OD acting on the surface of the hole so you'll have to make a judgement call there. Is the circular relief notch just a continuation of the cooling vent hole in the motor can or does it need to accurately mate something & becomes important for concentric prop shaft running? For better accuracy you also use 2 close fitting dowel pins to index the 2 bolt holes & then some kind of strap clamp to secure the part. Its the old fixture time vs. part making time HaHa. Hey, if its a rare & desirable part, make a few more while you're at it more & flog them on Ebay or RC forums!

Offline petertha

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Re: After a fixturing suggestion...
« Reply #11 on: July 03, 2024, 02:18:06 AM »
I'm guessing 'out of stock' is indicative of your predicament that its an older gen motor & spare are getting hard to find?

https://www.modelsports.com.au/product/axi-model-motors-radial-mount-set-for-axi-41xx-xx-v1-series/

Offline Allen Smithee

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Re: After a fixturing suggestion...
« Reply #12 on: July 03, 2024, 07:47:43 AM »
I'm guessing 'out of stock' is indicative of your predicament that its an older gen motor & spare are getting hard to find?

https://www.modelsports.com.au/product/axi-model-motors-radial-mount-set-for-axi-41xx-xx-v1-series/

Yes. I contacted the manufacturer and they confirmed they will not be making any more (they regard it as an obsolete part). There are still a couple in one shop in Germany, but they will not ship to the UK due to post-brexit bureaucracy, and there's one in a shop in Belgium that want so much to ship to the UK that it would almost be cheaper to jump on the Eurostar and go there to get it (they want £80+VAT+duty just for the shipping for a £11 part!). In both cases I've phone the shops to see if there's scope for negotiation or pleading, but to no avail.

AS
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Offline Allen Smithee

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Re: After a fixturing suggestion...
« Reply #13 on: July 03, 2024, 07:53:23 AM »
Actually this fixture could be improved with the part shaft facing up & just using the SHCS bolts to both retain & locate into clocked position. Bolts aren't really proper for accurate indexing because of the threaded OD acting on the surface of the hole so you'll have to make a judgement call there. Is the circular relief notch just a continuation of the cooling vent hole in the motor can or does it need to accurately mate something & becomes important for concentric prop shaft running? For better accuracy you also use 2 close fitting dowel pins to index the 2 bolt holes & then some kind of strap clamp to secure the part. Its the old fixture time vs. part making time HaHa. Hey, if its a rare & desirable part, make a few more while you're at it more & flog them on Ebay or RC forums!

The size & location is only important for balance purposes. My current thought is a variation on this fixture, but with a central spigot that locates in the central rebate. This rebate will be turned accurately because it's actually the location feature on the motor, so that for location and three screws for retention should probably do it.

AS
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