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Request to do a small machining job

N

nyulpista

New Member
Hello,
I am building a variable inductor for an electronics project. I need a contact wheel that will tap the inductor where needed. Please see a picture showing the general arrangement and the drawing of the piece I need machining.

I would need 2 pieces, material is brass. General tolerance is +/- 0.1 mm, but please try to maintain the groove parameters as drawn. Let me know please if you could make it happen. Being new on this forum, I am not sure how we could arrange pickup and payment. Thanks in advance!
Regards,
Steve
20250604_101740[1].jpg
1749052575102.png
 
Hi,

I am located in Courtland, just 8 minutes from Tillsonburg, South-West Ontario.

I am a HAM Radio operator and the project is an antenna tuner. It requires two identical variable inductors, the drawing is shown in my original post. It has to be robust to withstand a kilowatt of RF power. I have already designed and 3D-printed the supporting mechanical components, all I need is contact wheel as shown on the drawing. The inductor will be rotated with a stepper motor and the contact wheel will roll on the turns of the inductor. The contact wheel will slide on a 4.75mm stainless steel bar and will act as a contact point with the wheel. As the inductor turns, the contact wheel will roll on the coil and change the inductance.

The material is 'brass' but I am not familiar with different brass alloys, I am just a retired electronics engineer. :) I wound the coil using a 2mm copper wire and the groove on the contact wheel has the same radius to maximize the contact surface.

Thank you for your interest, appreciate any help from the Forum.

Regards,
Steve - VE3USP
Courtland, Ontario
20250604_221740[1].jpg
 
Welcome to the Forums!

My last Antenna "design", was for my Dad' Seven Tube Hallicrafters SW radio, circa late 1940's early 1950's. I was 10 & I simply *ran a wire* out my window screen, to our detached garage! (years later I learned a thing called Antenna Theory) !!!

I listened to BBC, BERLIN, VOA, etc. depending on how long I could stay up!

1969: US ARMY 35L20, Avionics Equipment Repair, Trouble Shooter ONEFIVE, I fix your Shorts, NO Sparks Included !!!!

philip, from the Great Pacific NorthWET, Oregon, USA

"The Truth spoken softly, is LOUDER . . . .

. . . . . . . . than the Lies that are SHOUTED."
 
PeterT said:
If I use your R=2.0 mm and 0.5mm offset on both edges, I get a resultant distance of 9.73mm from the center of groove to axis. So 2 * 9.73 = 19.46mm (vs your 18.8mm)
Where are our methods different?

View attachment 65523View attachment 65524
Click to expand...
I agree with @PeterT The dimensions don't make sense. The radius of the groove can't be 2mm. If it was 1mm (2mm diameter) then we get the 0.5mm from each edge of the wheel but now the ID of the groove is 18mm not 18.8mm.

1749109132526.png


Assuming the above are the real dimensions the this is how I'd make it.

1. First cut an arbour with say 12.7mm (1/2") and 4.5mm and a threaded section for a nut to clamp the raw material.
2. Chuck the raw material in the chuck and drill/bore to 4.5mm.
3. Press the 4.5mm arbour into the hole and clamp down with the nut. At this point all turning is relative to the arbour which is held in a 0.5" 5C collet.
4. The various diameters and faces are turned.
5. The part is moved over to the rotary table/indexer/4th Axis on the mill held in a collet.
6. A 4mm end mill is used to cut the groove in the edge as the part is rotated around the 4th axis.

It's not a difficult turning/milling job once the dimensions are cleared up.
 
I assume that the 4.75mm rod is actually 3/16. I have a 3/16 reamer to ensure a proper fit of disk to rod but a 4.75mm one is rather uncommon in my experience.

Is the wire actually #14 which is 0.080 rather than 2mm diameter? Or perhaps #8 which is 0.160?
 
@grease monkey is located fairly close to you. I believe he has the complementary skills to help make what you need. If not, there are many members in the oakville, burlington, hamilton area East of you.

All great guys with the machines and skills needed to make that for you. Before you know it, you will be buying a machine and working metal yourself!
 
@jcdammeyer and PeterT: Yes, it is my mistake. The radius is indeed 1mm, not 2mm as on the drawing.

@kstrauss: The rod is 3/16 at 4.7mm diameter. I think if the hole is 0.005mm or even 0.01mm larger than the rod, then it would roll freely. It should not bind on the rod; rather it should roll on the wire.
The wire is #12AWG, part #: Southwire 12/1 T90 SIMpull Solid Electrical Wire at HomeDepot.ca. SKU# 1000803450

@Susquatch: I would be more than happy to buy a small lathe and do it myself, or just rent a 1/2 hour machine time. I used to work as a Lathe operator before I became electronics engineer some 50 years ago. I think BusyBee used to rent machine time, but not anymore...

@Philip: Thanks for the story and hope to see you on the bands. 73!

​

 
I think there are about 12 members making this in their shop today! What about the material is anything brass like suitable or are there certain properties required?
 
PeterT said:
Ok that makes sense now. Let me check shop supplies because I think I have R1.0 carbide insert grooving tool (but I already know I don't have an R2.0). The overall turning isn't difficult but a custom grooving tool would consume some time if you wanted any degree of accuracy.

View attachment 65531
Click to expand...
Peter is placing the bottom of the groove at 9.4mm from the axle but I think you may actually want a full half circle in the part? Implying the bottom of the groove is 9mm from the axis?
 
Janger said:
think there are about 12 members making this in their shop today!
Click to expand...
I hope not. I PM'd @nyulpista but lets not trip over ourselves. I mentioned I was happy to step aside if someone was closer. Shipping a box of air across town is 20 bucks these days. I (barely) have enough material in the right size. Since its kind of an open discussion & a good example of things we may not think about, I'll paste in my longwinded comment about the bore FWIW.

One think you should get a better handle on is the shaft bore. You should measure the shaft you are using with a micrometer along several positions/orientations along the shaft so you know exactly what you have & base the bore on that with appropriate running clearance fit. When you say 0.005mm (=2 tenths), that is actually a pretty precise dimensional fit, normally requiring a reamer. I don't think you want to buy a reamer & its an odd duck size. I'm confident the closest numbered drill would work OK in this application. I also have metric drills in 0.10 increments. But the trick is which one. Brass is a funny material. Typically brass drills edges are 'dubbed' so they cut properly, but which also means they cut a smidgen oversize the drill itself. I don't think it will be an issue in this small diameter range, but my advice is to err on the slightly tighter size if splitting the difference between one drill size or another. We only have one shot at the bore. It would be a bit of a shame to have to open the diameter on a drill press with unpredictable results. But maybe another option is to lap your shaft a bit?
 
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To me, the most important takeaway is understanding the problems that result from roughly converting dimensions to metric when using North American manufactured supplies. Of course, Chinese sourced components could be metric, imperial or something else!

We learned that the wire is actually #12 rather than 2mm. The shaft is 3/16 rather than 4.75mm. Many of us have 3/16 over/under reamers in our tooling stashes and they would provide a nice running fit. I suspect that rather few of us have 4.75mm over/under reamers.

Just the comments of a grouchy old man working on his second morning coffee...
 
PeterT said:
I hope not. I PM'd @nyulpista but lets not trip over ourselves. I mentioned I was happy to step aside if someone was closer. Shipping a box of air across town is 20 bucks these days. I (barely) have enough material in the right size. Since its kind of an open discussion & a good example of things we may not think about, I'll paste in my longwinded comment about the bore FWIW.

One think you should get a better handle on is the shaft bore. You should measure the shaft you are using with a micrometer along several positions/orientations along the shaft so you know exactly what you have & base the bore on that with appropriate running clearance fit. When you say 0.005mm (=2 tenths), that is actually a pretty precise dimensional fit, normally requiring a reamer. I don't think you want to buy a reamer & its an odd duck size. I'm confident the closest numbered drill would work OK in this application. I also have metric drills in 0.10 increments. But the trick is which one. Brass is a funny material. Typically brass drills edges are 'dubbed' so they cut properly, but which also means they cut a smidgen oversize the drill itself. I don't think it will be an issue in this small diameter range, but my advice is to err on the slightly tighter size if splitting the difference between one drill size or another. We only have one shot at the bore. It would be a bit of a shame to have to open the diameter on a drill press with unpredictable results. But maybe another option is to lap your shaft a bit?
Click to expand...

Hi PeterT et al,
Yes, I have read your PM yesterday evening. I knew my reply was going to be a bit long, so I was going to get to it this morning.

The only concrete offer I received was from you, which I very much appreciate!

Reading your note about the 0.005 tolerance, I realized I made a mistake by mixing up Imperial and Metric as @kstrauss suggests. The 0.005 tolerance in millimeters is indeed very tight as you point out in your PM. The application needs this contact roller run freely on the shaft, so the '0.005' should be in inches which would translate to 0.127mm. Your proposition to simply use a drill bit is the right way of making this part. I did not know that drill bits came in 0.1mm increments, so a 4.8mm bit would be perfect.

The suggestion from @kstrauss to move the tool a bit left and right to make the groove just a tad wider than the AWG12 wire of 2.05mm is good. The depth of the groove on the coil former is 0.5mm, so the groove on the contact roller needs to be less than 1.5mm.

I wanted to identify the errors I made in dimensions to avoid unnecessary high accuracy tooling as the Forum is trying to find the best way to make this little piece. There is no critical tolerance on this part and as long as the contact wheel rides on the wire, life will be good :).

Thanks again for the expert help and suggestions.
 
kstrauss said:
To me, the most important takeaway is understanding the problems that result from roughly converting dimensions to metric when using North American manufactured supplies. Of course, Chinese sourced components could be metric, imperial or something else!

We learned that the wire is actually #12 rather than 2mm. The shaft is 3/16 rather than 4.75mm. Many of us have 3/16 over/under reamers in our tooling stashes and they would provide a nice running fit. I suspect that rather few of us have 4.75mm over/under reamers.

Just the comments of a grouchy old man working on his second morning coffee...
Click to expand...
KSrauss, it hurts my eyes to even READ, without my second cuppa Joe in the morning!

Indeed, as a student of the University of TUBE, There are a few people I know, who have Mastered the Imperial "decimal inches" to METRICS, by switchiing to an olde, but viable Re-ReConversion Unit, that of a Furlong . . . .

(slowly hobbleing away on his Cane, which nearly needs to be WHITE, with a Red handle, to indicate I CAN hardly SEE without my Glasses on . . . Whilst they are resting on my ForeHead . . . .)

Now about Coffee, my friend, a former *US NAVY Mess Cook*, tells me their SECRET, to the Development of MAKING a good cuppa Coffee, IS ALSO within the given topics of INTERNATIONAL MEASUREMENT:

>>>DO NOT USE TO MUCH WATER<<<, In fact, an entirely unsatasfactory liquid is produced with even a SMIDGEON of to much WATER !!!!

philip, from the Great Pacific NorthWET, Oregon, USA

"The Truth spoken softly, is LOUDER . . . .

. . . . . . . . than the Lies that are SHOUTED."

Postal Scriptus Might I add the Dimension of the CONTAINER, for a Good CUPPA Joe Mine has s "25 Fluid ounces" LINE, ANYTHING below that Amount, is hardly a Cup At ALL !!!
 
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