1973 Suzuki GT250 'Robyn': cafe racing commuter

Ordered some round bar stock of my favourite metal.

I might need to study this up

I'm big kev excited.

 

your link is broken

I might need to study this up

seems the [ url ] tags automatically add the http:// prefix...

 

No knurling this bad boy... just some machining to make fork caps and wheel spacers .

 

I'm sitting at my desk right now stroking it.

 

Runs about $150/foot for 35mm diamater of grade5/6al4v. Commercially pure (grade 2/what you use for jewellery) is a bit cheaper, about $70/foot.

 

It's similar to 316 (which is the worst stainless for machining) but you do bigger cuts, you do it at slower RPMs and you use more coolant. The coolant is partly cause it has far worse thermal properties (so it heats the shit out of the tool) and the chips also capable of catching on fire and staying on fire. It's also a shitheap to tap threads in, but that's not on the cards; cutting external threads is not too bad.

Oh and you get to worry about the metallurgy of the tooling, because Ti likes to react. High cobalt HSS or tungsten carbide is the stuff to use.

No point going CP Ti except to say it's titanium; might as well just machine it out of 303/304 stainless for the same strength profile. I want to learn how to machine it so I can make stuff where the extra strength actually matters (then you can gain lightness in two ways: one, the lower density of Ti, and two, the item can be made smaller e.g. thinner walls on spacers etc).

 

Yeah, you have the added fun of a hard, reacted surface so you also need to cut beneath that. Still, that's why I bought it: might as well learn on a non critical part. Plus, yolo.

 

Yeah you get an oxide layer on the top of titanium too, but you get a bunch of other funky stuff like titanium chloride and a few other fun things. That's why the metallurgy of the tooling matters (plus need low chlorine coolant).

 

Long day was long.

Battery sorted, to allow fabbing of oil tank/electronics thingo.



Yes, I could have gone to some uber lithium thing... but the charging system on a GT250 sucks arse and I don't have the >$500 to upgrade it to a modern regulator/rectifier setup etc. That will come in the future... till now, SLA is the business.



Made up spacers so the brake caliper bolts don't eat into the disc, and also aren't the same as the factory ones that chew into my sexy, polished forks. Need to drill the corners of the bolts so I can lockwire them so they don't come loose now they don't have lock washers.



Made a sexy pattern in some aluminium while mocking up something (more on this in a minute).



Clutch lever, clutch cable, LH grip, LH switchgear sorted. Need to make up a little bracket for a kill switch also on the LHS and we're sorted.



Repro switchgear out of the UK is nice. Better than I could do restoring the factory switches.

Awaiting all of the wiring terminals & some wire so I can make a new wiring harness. I have an OEM harness that is unused but it seems a waste to butcher that to make this a cafe racer. Will do a bunch of rewiring so it'll basically be a factory harness but all nicely modified to suit the different headlight, brake switch etc.



Tapped a new thread for the footpeg to mount to.



@Charlie started to knock up an oil tank/battery holder etc. Nice fabbing skills, though we will need to make mk2 before welding.

But here's where the day got interesting.... (next post).

 

Spun up my titanium in the 4 jaw chuck. Used the full length rather than lopping some off as a) I don't want to waste any and b) it makes holding the titanium possible in the Z-slide to do some milling.



Turned down one end to start making up a fork cap. Lots of little annoying changes in diameter to get this done. Machines relatively nicely but you have to spin it slowly and soak it with coolant. Also, the swarf is tenacious and vicious.



Checking I'd got a few basic dimensions close to perfect.





Speaking of dimensions, without direct readout this was the only way to measure the longitudinal feed. This dimension is damned critical for this cut, as it sets the preload on the fork.



Quite a lot of the work done. Cut more than I needed to as a tail on the material as a) I have a plan for the tail and b) I wanted to give myself lots of room when cutting the thread. The titanium really did make a nice thread though.



Out comes the Z slide and a 10mm diameter end mill to cut some flats onto the end. This means it will screw in just like a factory one, even down to the size of the socket needed to put it in.

Speaking of the size, it took a bit of maths to get the stuff right and some hilarious metric to imperial to metric to imperial conversions and guesswork buuuuut...



I got the number I was shooting for.



It still needs a final clean up where I had to just file the burrs off the flats on the hex head.



Dimensionally identical. One weighs half as much and looks four hundred times sexier.



Centre bored. Not actually a requirement but hey the material isn't needed.



Needs some final finishing work and polishing up, but damn, not bad for homemade on a 42 year old lathe.





Sex.

P.S. the best part of the day was my bacon roll that I cooked while doing some various other machining tasks.



Good day in the shed with @MickLC and @Charlie. Thanks!

 

Where there's a will there's a way.