Need some tips on machining my TM400 fork legs

[McGillicutty]

Hello from California, for my TM400 project I would like to turn down the diameter of the fork sliders to mimic the appearance of the early Suzuki Works bikes as well as shave off a little weight. To do this for the entire length of the slider I`ve been told the area with the spade/lug for the brake backing plate needs to be milled using a rotary table, something I don`t have, but Dave Miller of Dave Miller Concepts got around this on his Works Replica RM125 by leaving the OEM diameter for the brake spade/lug area so that there is a ``step`` in the diameter which actually looks pretty cool, so that problem was resolved. My only remaining obstacle is the raised area at the bottom of the leg where the oil drain port/screw lives. I think there may be enough material left if I were to turn down the slider to mill a flat on the slider so the screw would still be able to have a flat surface to seal against with just enough threads remaining to work. The bottom of the slider is the smallest diameter at *USD*1.818 and it grows to 1.890 at the top of the slider. The I.D. is 1.379 meaning the stock wall thickness is 0.220 at the bottom and 0.256 at the top of the slider. I`ve been told the minimum safe wall thickness would be 0.150 so I was planning on turning the O.D. down to 1.680 and was wondering if anyone else has done this and if so how much material did they remove and what problems if any, did the oil drain port present. Thanks much.

[Lozza]

First ,most of us from Australia and the rest of the world works in metric. The machining could all be completed on a lathe, and as the old machinst's rule is it is easier to take it away than add it on. Aluminium swarf doesn't weigh much at all so the net weight saving would be minute. For the oil drain hole make a copper washer follow the radius of the OD of the fork. should seal OK. Best bet is to have it CNCed on a trunion table. 

[tmman]

i think the machining is more for aesthetics than a weight saving measure,, the DMC bikes McGill talkes about are a nice lil work of art.. n look quite the part.. i have done a set in the lathe.. grip the end where fork boot goes live cntr on other end n off ya go.. there a lil outta round but come up fine..as for the drain hole when as the last time you ever saw anyone use it?? i welded mine up but a set screw could be used with loctite or the brass washer method..

[brent j]

I machined my TM125 forks down many years ago. I wouldn’t worry about the weight saving part. No offence but if weight was that much of an issue you wouldn’t be riding a TM400.
I trimmed mine to just the main dia of the fork leg, I just removed the minimum amount of material to get the “factory” look I wanted. I didn’t worry about machining past the brake anchor or below it. Machining away the long taper to the fork wipers and putting in a step in looks the part and once the original fender mounts were gone and the surface was polished to a dull finish and lacquered it had the desired effect.

[McGillicutty]

Thanks guys for your input, I appreciate it.  If I decide to remove material at the bottom of the sliders I will weld the oil port closed but I may just leave it and machine only the upper portion of the legs to match the smaller diameter at the bottom for aesthetics as you say.  I am doing a massive amount of work to make this TM as light as I can afford to make it so while I know the aluminum that I can safely remove from the forks won't amount to more than a few grams, I am trying to make the grams add up to 40+ lbs, sorry I mean 18+ kilograms!     A 30 - 40 lb lighter bike will have some effect on a rider's performance, it's going to have an equally large effect as eye candy to those who love vintage machinery so those are the reasons I'm doing all this.  That and I'm having a blast fabricating, learning and searching for information and rare part!  Thanks again.

[lukeb1961]

That and I'm having a blast fabricating, learning and searching for information and rare part!  Thanks again.

Learn to think in metric and life will be complete. Sounds like a brilliant project! oh and I bet Lozza just wants an excuse to use CNC in a sentence    Photos! without photos it isn't happening 

[Lozza]

That and I'm having a blast fabricating, learning and searching for information and rare part!  Thanks again.

Learn to think in metric and life will be complete. Sounds like a brilliant project! oh and I bet Lozza just wants an excuse to use CNC in a sentence    Photos! without photos it isn't happening 

Guilty as charged I suppose(but the word was trunion). CNC would leave any lugs in place and put a raduis instead of a step which would not only look better be a fair bit safer also. Most of the mass is in the engine.

[topari]

Slightly off topic, but would anyone know the type of aluminium is used in fork legs ? Given it is weldable, maybe a 6000 series like the ubiquitous 6061 ? 

[Lozza]

Casting grade 5083

[topari]

Casting grade 5083

Thank you. I will be relocating my brake lug on the kdx200. Looks like I need to match like for like.

[McGillicutty]

That and I'm having a blast fabricating, learning and searching for information and rare part!  Thanks again.

Learn to think in metric and life will be complete. Sounds like a brilliant project! oh and I bet Lozza just wants an excuse to use CNC in a sentence    Photos! without photos it isn't happening 

Actually I have been measuring everything with my metric scale but then I have to convert it to inches since my lathe/mill digital readout doesn't do metric!  A CNC would be awesome but I'm having enough on my table just learning how to use the tools I do have, can't imagine having to learn to use the CNC as well.  This old dog is finding I don't like learning new tricks as much as I used to, I just want to know now without having to learn!    I'm using an aluminum 78/79 KX250 aluminum swingarm (it fits perfectly in the frame) with a magnesium RM rear hub so I had to align the sprockets, determine the proper wheel spacer dimensions and fabricate them, then I had to design and fabricate a chain guide as nothing OEM works because of the mismatched parts.  I'm very much looking forward to the project's completion so I can see how much weight I've managed to save.  All my hardware is Chinese titanium including the screws that hold the center cases together so I'm trying to save weight, no matter how little the amount at every step.  Not really considering this any kind of works replica, just a modified TM400.  I have a few photos of the bike on the stand with just the swingarm and wheel on as I'm trying to layout the chain guide but there's nothing remarkable about that.  Surely I will be proud to distribute photos of the end result when I get there and really consider this a culmination of the VMX community's knowledge since it really is the result of advice, assistance and knowledge I've had generously been provided by all my fellow VMX enthusiasts.  Thanks to all who've helped me along the way.  Photos to come for sure.

[Lozza]

You don't have to 'learn' CNC at all, just send the leg to Chris

  http://moto2-usa.blogspot.com.au/2012/02/finally-someone-hired-me-to-make.html

[McGillicutty]

Seriously?  Does Chris take small jobs like turning fork legs and if so has anyone used him?  How are his rates?  I didn't see any contact information for him on the article in the link, does anyone have his contact info?  Thanks.

[Tahitian_Red]

Mike,

I was gonna reply to your MX Swapmeet post, but I'll just do it here.  You might want to get a hold of J.P. Morgan, in S.F. and ask him about his TM DeCoster replica forks.  There is also a guy in So Cal that has a 175 lbs TM400 and if you search The Swapmeet Discussion Board you may find a list of some of the tricks he used.  Even if you don't get to 175 lbs any sub 200 lb TM400 would be super cool!

 

[McGillicutty]

Thanks for the heads up Tahitian, I've actually been in contact with both and Roger Granger who has the 172 lb TM400 has traded about two dozen emails with me divulging all that he did to his bike.  He as well as just about anyone else I've contacted have been completely gracious and forthcoming in helping me with my project.  I am in awe of to quality of people in our sport!