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Superluminal

Custom parts machining

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Hello, Im after getting some custom parts machined for a Group C / F103 chassis project and was after some recommendations of any workshops people have used for this sort of thing that will have a lathe and milling machine (or a multi axis lathe head)

I have pinged off an enquiry to MinitureManufacturing who already do some bits for Tamiya kits. Anyone else I can try? Its likely to be a one off as I cant see there being a massive market for what im after.

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4 hours ago, Ann3x said:

Ive used these guys in the past:

https://www.protolabs.co.uk/services/cnc-machining/cnc-turning/design-guidelines/

Not super cheap but they do small batch / custom stuff and are fairly sensible with helping out non-pro CAD models.

Just had a look at the link. +/- 0.1mm is a lot a error for turning on a lathe, especially an industrial cnc.... 

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Yeah I noticed that - thats way more than i would hope for a press fit of a bearing into a counter bore or shaft dia to run inside a bearing.

Ive also only got the parts drawn in pdf. They need a 3d model to be able to quote from and I wasnt quite sure how you would model something if you need the finished size to be toleranced for a press fit etc.

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5 hours ago, MadInventor said:

Just had a look at the link. +/- 0.1mm is a lot a error for turning on a lathe, especially an industrial cnc.... 

Thing they are covering themselves. Parts Ive had from them are accurate and Ive used as bearing surfaces.

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If you are after press fits for bearings you need to give the appropriate tolerance rather than say you require a press fit. General machining tolerances are usually 0.1-0.2mm for non essential areas or shape.  Concentric bores with clearance and press fits need to be dimensioned accordingly.  A press fit into steel is generally a H7 fit. H7 being the hole rather than a shaft or bearing. Bearings should be a ground finished size so it’s important you know exactly that size to determine the correct press fit hole size.  The tolerance is roughly -0.00 +0.03mm The H7 fit tolerance will reduce with smaller diameters so you really need good quality bearings that all measure exactly the same.   

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Yes, was going for a G6 sliding fit on the journals of the shafty parts and a H7 for the counterbores in the hubs that have the bearings in.

I have a 25 year old zeus book from when i did my apprenticeship that i dug out the loft which came in handy.

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6 hours ago, Superluminal said:

ve also only got the parts drawn in pdf. They need a 3d model to be able to quote from and I wasnt quite sure how you would model something if you need the finished size to be toleranced for a press fit etc.

If you need help with 2d drawings into 3d models or selecting fit sizes drop me a line.

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11 hours ago, Toolmaker72 said:

If you are after press fits for bearings you need to give the appropriate tolerance rather than say you require a press fit. General machining tolerances are usually 0.1-0.2mm for non essential areas or shape.  Concentric bores with clearance and press fits need to be dimensioned accordingly.  A press fit into steel is generally a H7 fit. H7 being the hole rather than a shaft or bearing. Bearings should be a ground finished size so it’s important you know exactly that size to determine the correct press fit hole size.  The tolerance is roughly -0.00 +0.03mm The H7 fit tolerance will reduce with smaller diameters so you really need good quality bearings that all measure exactly the same.   

 

10 hours ago, Superluminal said:

Yes, was going for a G6 sliding fit on the journals of the shafty parts and a H7 for the counterbores in the hubs that have the bearings in.

I have a 25 year old zeus book from when i did my apprenticeship that i dug out the loft which came in handy.

You guys know far more about it than I do. I just generally try and get most of the stuff I make within +-0.05mm on the mill, and do stuff on the lathe by feel (I'e. keep shaving off a minimal amount until the one part slides onto the other and is a good fit.

I just remember when I was at work and a work colleague and I went to my manager to ask him about buying a DRO for a miniature mill we had. We were keeping the costs down by buying 0.005mm accurate glass slides instead of the 0.001mm accurate ones. When I'd mentioned we just needed to get parts within a few hundreths of a mm he'd laughed at us and tossed a small steel rule across the desk and said use that. He was an ex-machinist....... (We still got the DRO though :) )

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16 hours ago, Superluminal said:

Yeah I noticed that - thats way more than i would hope for a press fit of a bearing into a counter bore or shaft dia to run inside a bearing.

Ive also only got the parts drawn in pdf. They need a 3d model to be able to quote from and I wasnt quite sure how you would model something if you need the finished size to be toleranced for a press fit etc.

When I've built gearbox plates that have to house bearings I've used a boring head. In my shaky old mill I have to run the boring head in an out several times before it stops taking metal off, making it difficult to make an accurate cut 1st time. However it does lend me the opportunity to shave off a little, then a bit more, then a bit more, until the fit is just right. Having said that I've made rejects where I could tap in the bearing with a hammer, but the hole was just small enough to put pressure on the bearing so that it wouldn't turn smoothly, and also holes that the bearings drop out of when you turn the part upside down. That's why I like using larger bearings such as 12x6 and 16x8 in my builds, cutting the bearing holes is a lot easier. That's the thing with build threads on the internet. You guys get to see my finished parts, not the 3 previous rejects I'd made... :unsure:

  • Haha 3

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At least you keep the rag and bone in scrap!

When I did my apprenticeship we used old colchester lathes and bridgeport mills and it was similar trial and error process then. Cut, measure, cut a bit more, measure, cut a bit more, measure.....booger. Start again. Repeat until in tolerance then hand in for inspection and marking.

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To be fair I ran a cheap import boring head in both a bench top mill and a mini Bridgeport clone and always dreaded it until I changed tooling and it's a dream now. Carbide inserts are night and day to the brazed carbide nightmares I was using. I keep meaning to get a better grinding wheel so I can improve my brazed tools.

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12 hours ago, Superluminal said:

Yes, was going for a G6 sliding fit on the journals of the shafty parts and a H7 for the counterbores in the hubs that have the bearings in.

I have a 25 year old zeus book from when i did my apprenticeship that i dug out the loft which came in handy.

Love the Zeus book, you can’t remember everything 😊.   
The bit by bit approach to machining is what we all do. I wizzed up a pair of modified front wheel shafts earlier for a 2wd buggy.  Machined to Diameter 4.95mm for the bearings.  It was too tight. So a bit of emery to get to 4.90mm. Original shafts are 4.8mm and new bearings are loose.  Making parts to suit worn parts is where tolerance goes out the window. Another trick is use heat or cold accordingly to fit bearings or shafts. Works with big stuff (ie not models). 
 

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6 minutes ago, Toolmaker72 said:

Another trick is use heat or cold accordingly to fit bearings or shafts

I was just thinking that before I read this , but I guess seperation of said parts in the future would be difficult ?

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You can try and heat the lot and get it loose that ways, occasionally I have used some localised freezing (air duster turned upside down) too

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1 hour ago, KEV THE REV said:

I was just thinking that before I read this , but I guess seperation of said parts in the future would be difficult ?

Usually if it needs taking apart again the bearing or shaft will worn or knackered.  If it’s a hub assembly you should design in the ability to knock out worn bearings 

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