Match Drilling/Marking a Large Flange Retainer Assembly

[DRAGON64]

I have started on a large project; 5.5" with a 4" motor mount. I am adding an Aeropack "flanged" motor retainer, and I am getting a bit antsy; I have my rings prepped and ready to go, and then I noticed that the screw holes in the flange are barely larger than the screw mounting through them. To make things a little hairier, the match marked and drilled holes have zero room for error. if one hole is drilled a little off, the hole flange mount is wasted.

What is the best regarded method for insuring proper alignment for drilling holes that all 12 will mount without issue? Pictured below is the final ring assembly, and I have marked the hole centers with a Sharpie. I have one chance to get it right, lest I have to scrap the assembly and start over...

 

[RainierWolfcastle]

Assuming using basic hand tools:
I’d use the flange itself clamped to the ring, spot drill with a drill close to the diameter of the screw hole and once all are marked remove the flange and drill through required size. It will scratch the anodising inside the hole, but the screws probably will anyway and they cover it up.

Better would be make a jig from a scrap of MDF or G10 board, drill that and confirm it is correct to the flange. Then use the jig to drill through the rear ring. Worth it if you’ll be doing future rockets with the same retainer and will use it again.

 

[PBic]

I’ve never done it, but you may be able to hot glue the parts together, drill the holes, then separate them with a heat gun.

 

[OverTheTop]

I’ve never done it, but you may be able to hot glue the parts together, drill the holes, then separate them with a heat gun.

Good idea. Or use some decent double-sided tape (eg VHB). The spot drill through the flange as RW suggests. You could even drill the holes completely through.

You could also fix it in location with two holes and bolts, then drill and bolt the rest.

It is possible that the flange will only mount in one position if you do this by hand. I would suggest putting a subtle alignment mark on the two parts so you can line them up once they are disassembled.

Another alternative is to 3D print a drill guide if you have a printer.

 

[DRAGON64]

The ring assembly was glued together with JB Weld, as the rings had to remain centered on the motor mount for re-assembly after fin mounting. The retainer was also slipped over the mount, with motor case in place, to get the center achieved. The retainer was then clamped in place, in order to get the marks as shown.

After pondering this all night, I have figured it best to model up the ring and hole pattern of the retainer, and then 3D print the flat flange part to use as a drill guide. I did consider using the flange as a drill guide, but was afraid I would screw up the retainer threads.

As @OverTheTop offered, I had considered mounting the retainer one hole at a time and adding the threaded inserts and screws as I progressed, but I still had concerns of the tolerance stack. The screw holes on the retainer flange left very little room for error...

Thanks all, I will post a follow up, should I find success through the 3D printing of a hole guide.

 

[Flyfalcons]

If one of your holes ends up slightly off then simply enlarge the hole on the retainer or drag it with a cutting bit. No need to scrap it.

I mount mine by securing them in place, drilling the exact size hole through the retainer and CR, then remove and use a step bit to drill to final size. My next Patriot build video will show this.

 

[heada]

When I do this on 75mm, I put the ring on the mmt, load a case into the flange and slide that into the mmt. Now the flange is perfectly centered. I clamp the ring to the flange and I then spot drill all the holes.

The one time I didn't use a case to center things up it was off by a bit and I had to sand the inside of the mmt for the case to fit.

 

[_kestrel_]

I’ll second putting the CR on the MMT. No good having perfectly spaced holes if the won’t let the motor into the MMT. A set of transfer punches are handy for this type of thing - exactly why they exist.

 

[DRAGON64]

Adding the centering rings and retainer base on the motor mount is the first part of the instructions from Aeropac, to include putting tape on the motor case to aid in centering the retainer about the motor case.



Due to the retainer designed to fit inside of a 5" airframe, there is little room to use a center punch sue to the close proximity of the retainer threads. Earlier today I modeled the base of the retainer, and 3D printed a down and dirty print to use as a jig.



I mounted everything back on the motor mount to check the fit, then I used AquaNet hair spray to mount the jig to the centering assembly.



Tomorrow I will set up the assembly on my small drill press and drill through the jig to start the threaded insert mounting.

Note, the centering ring assembly consists of (2) 1/4" wood rings, and (1) /18" G10 ring.

I am contemplating drilling the G10 ring to fit the #8 hardware, and turn the assembly over to drill through the wood only to mount the insert. My thought process is, since I chose to mount the inserts into wood (with CA thread re-enforcement), that should the inserts fail, they will not pull through the G10 ring with the smaller #8 screw hole...

 

[OverTheTop]

Sounds like a well thought out plan.

 

[RainierWolfcastle]

I am contemplating drilling the G10 ring to fit the #8 hardware, and turn the assembly over to drill through the wood only to mount the insert. My thought process is, since I chose to mount the inserts into wood (with CA thread re-enforcement), that should the inserts fail, they will not pull through the G10 ring with the smaller #8 screw hole...

That sounds like an excellent way to do it, I can’t see any non-catastrophic load that could cause issues with that set up. I’ve had a 54mm partially pull the inserts out of a plywood ring after a pretty warm and rough ride, your way would totally eliminate that possibility.

 

[user 30299]

Adding the centering rings and retainer base on the motor mount is the first part of the instructions from Aeropac, to include putting tape on the motor case to aid in centering the retainer about the motor case.

View attachment 432490

Due to the retainer designed to fit inside of a 5" airframe, there is little room to use a center punch sue to the close proximity of the retainer threads. Earlier today I modeled the base of the retainer, and 3D printed a down and dirty print to use as a jig.

View attachment 432491

I mounted everything back on the motor mount to check the fit, then I used AquaNet hair spray to mount the jig to the centering assembly.

View attachment 432492

Tomorrow I will set up the assembly on my small drill press and drill through the jig to start the threaded insert mounting.

Note, the centering ring assembly consists of (2) 1/4" wood rings, and (1) /18" G10 ring.

I am contemplating drilling the G10 ring to fit the #8 hardware, and turn the assembly over to drill through the wood only to mount the insert. My thought process is, since I chose to mount the inserts into wood (with CA thread re-enforcement), that should the inserts fail, they will not pull through the G10 ring with the smaller #8 screw hole...

I ended up with a very tight alignment on my 75mm retainer. So tight I had to add "witness marks" to the retainer flange and thrust plate. If you took the retainer off and rotated it a hole or two the cap screws would go in a bit tight, or misaligned.

 

[OverTheTop]

I ended up with a very tight alignment on my 75mm retainer. So tight I had to add "witness marks" to the retainer flange and thrust plate.

Situation normal for this sort of thing usually .

 

[DRAGON64]

So, these large retainers no walk in the park, but I managed to get mine completed;



I built as I had planned to do, except that I did not CA re-enforce the threads in the wood centering rings.



The reason for this is, most of the threaded inserts split while being inserted, and became permanent feature of the assembly. All not as planned, the end result was the same:



Lastly, I did have to step drill all of the holes from .170 (#8 screw clearance) to .203 (#10 screw clearance) in order to ensure the retainer remained centered about the motor tube, and motor case. Now on to the next task.

Thanks all for the input!

 

[user 30299]

So, these large retainers no walk in the park, but I managed to get mine completed;

View attachment 432624

I built as I had planned to do, except that I did not CA re-enforce the threads in the wood centering rings.

View attachment 432627

The reason for this is, most of the threaded inserts split while being inserted, and became permanent feature of the assembly. All not as planned, the end result was the same:

View attachment 432628

Lastly, I did have to step drill all of the holes from .170 (#8 screw clearance) to .203 (#10 screw clearance) in order to ensure the retainer remained centered about the motor tube, and motor case. Now on to the next task.

Thanks all for the input!

Did the insert split while you were screwing it into the plywood?

 

[DRAGON64]

Sure did, the metal was soft. I tried employing the technique when cutting threads, to back out half a turn, and the returning and progressing another 1/4 turn (rinse/repeat), and the inserts still split. To complete the full setting of the inert, I threaded the screw in, and used the screw to turn the insert until flush.

 

[Flyfalcons]

That is what I have to do as well to thread in the inserts. They are way too soft and the hex head rounds almost immediately otherwise.

 

[David Schwantz]

Drill one at a time. Put a bolt in it and do it until all are done. that is keep putting bolts in as you go. It will come out perfect.

 

[DRAGON64]

That is what I have to do as well to thread in the inserts. They are way too soft and the hex head rounds almost immediately otherwise.

My thoughts exactly; it was almost like the inserts were made of pot metal.

Drill one at a time. Put a bolt in it and do it until all are done. that is keep putting bolts in as you go. It will come out perfect.

Very similar to the process I employed for this assembly, definitely a good exercise in patience...

 

[heada]

I've seen some low quality ones but the solid brass ones I got from McMaster are pretty good.

https://www.mcmaster.com/90016A007/

 

[Zbench]

I would have used T nuts for that application. Since you are drawing the retainer tight to the plywood they would work perfect for that application. You could use a little rocketpoxy if you are ever worried about them coming loose.