# timing belt gears



## firebird (Jan 14, 2011)

Hi

Can somebody help me out with a bit of maths (not my strong point)

I need to cut a couple of gears to suit a toothed belt I have. The belt has a tooth pitch of 5mm. Lets say i want to cut a gear with 30 teeth. Is this the correct formula?

Number of teeth = 30
Pitch = 5mm

30 x 5 = 150
150 ÷ 3.1415927 = 47.746482
Diameter of pulley blank = 47.746

Cheers

Rich


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## agmachado (Jan 14, 2011)

Hi Rich,

I'm not sure, but I will confirm in the CAD... Soon I post of the result !

Cheers for you too,

Alexandre


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## shred (Jan 14, 2011)

Timing belt pulleys (they look a bit like gears but aren't) are likely to require a somewhat different formula since the tooth shape is much different.

My Machinery's Handbook doesn't seem to have that topic, but I'd hit SDP-SI.com and if they make one like you want, they'll have full specs on it, like say this one: https://sdp-si.com/eStore/PartDetail.asp?Opener=Group&PartID=8499&GroupID=218 (to pick a random 5mm belt one with 30 grooves in it)


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## Jasonb (Jan 15, 2011)

The size you have come up with is the PCD, the actual dia of the pully over the teeth is less, can't tell you how to calculate by how much but HPC give the OD as 46.95mm

http://www.hpcgears.com/newpdf/timing_pulleys_5.0mod.pdf

http://www.hpcgears.com/newpdf/technical_timing.pdf

J


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## firebird (Jan 15, 2011)

Hi

Thanks for the replies.

This is where I got the formula from. I copied and printed this section but I can't find the web page I got it from.

There is usually ample tooth space on commercially made timing pulleys, that I don't think that that is critical. Smooth corners with nice little radii would be good, of course, to prevent chewing the belts up.

The really critical dimension of a timing pulley is its OD. Actually, its OD is most commonly referenced as the pitch diameter, because that is where the belt's true pitch circle is created. The belts are manufactured with very exact and accurate tooth spacing. If this pitch diameter of the pulley is not exactly correct, then the belt is going to tend to 'crowd the teeth' on the pulley, and cause wear. There is a fair amount of frictional grip of the belt on the OD of the pulley, and the belt does not readily fall back into the gaps created for the belt teeth. So, "hack out" the tooth space however, but make sure the OD of the pulley is an exact multiple of the belt pitch.

eg.: 3/8 belt pitch, 10 tooth, OD of pulley is (10 * .375) / 3.1415927

I've machined a few timing pulleys both with form cutters and with slitting saws. I typically would gang a pair of slitting saws far enough apart to straddle two teeth on the blank, as this creates a bit of a natural angle on each side of the tooth face. That leaves the work of filing or sanding a bit of a fillet radius on the top corner of the tooth before the pulley is put into service. 


Cheers

Rich


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## jim216 (Jan 16, 2011)

Rich, 

My first and only foray into timing belts was converting my X3 mill to belt drive. I used a 5mm x 300mm HTD belt.  I started will 3mm drill bits but wound up using an 1/8" end mill. In spite of my lack of skills it works well. 

I used this calculator 

http://www.sdp-si.com/Cd/default.htm






More pics 
http://profile.imageshack.us/user/stovebolt

Uploaded with ImageShack.us


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## bob ward (Jan 17, 2011)

These guys http://www.hobbyparts.com.au sell lengths of toothed pulley extrusion for a variety of timing belt profiles and tooth counts. More to the point for this thread is that they also show the OD of the various extrusions, eliminating the need for mathematics.


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## firebird (Jan 17, 2011)

Hi

Thanks Jim and Bob, very useful

Cheers

Rich


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## Simon0362 (Jan 19, 2011)

And for use after you have created those pulleys.....

http://www.davall.co.uk/downloadlibrary/BELT CENTRES.xls 

which is a very lightweight Excel programme to determine the centre distances - or can be used to determine the length of the timing belt if the centres are known.
Regards,
Simon


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## firebird (Jan 19, 2011)

Hi Simon

Thankyou, also very useful

Cheers

Rich


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## firebird (Jan 31, 2011)

Hi

I have pursued this a little further. Let me show you what I have done so far. I would appreciate any comments/help/advice.

I have a couple of belts, both are from the X3 milling machine. The earlier type (lower left in the photo) is a thinner 1/4 inch pitch square tooth belt and the later type (upper right in the photo) is a heavier 5mm pitch belt. I decided to have a go at the 5mm pitch.





Using this link very kindly supplied by Bob Ward 

http://www.hobbyparts.com.au/store/item/pg5024x200alu/pulleystiming5000mmpitchgtstock/

I came up with the following info for a 24 tooth pulley





Category: Pulleys - Timing - 5.000mm Pitch - GT - STOCK
Grooves (No.)::	24
Min. Usable Length (L) (mm)::	200.00 (7.8740 inch)
Pitch Diameter (PD) (mm)::	38.20 (1.5039 inch)
Outside Diameter (OD) (mm)::	37.05 (1.4587 inch)
Material::	Aluminum Alloy
Shank Diameter (D1) (mm)::	12.70 (0.5000 inch)
Shank Length (Sl) (mm)::	19.00 (0.7480 inch)

Turn a piece of ally bar to 37.05mm. I am going to need 2 gears of this size so I have turned enough to get them plus enough for a spare or 2.





With the rotary table set up in the mill I set a 2.5mm slitting saw to centre height.





Then aligned the tail stock.





The work now mounted between rotary table and tail stock





Cutting in progress. I'm cutting to a depth of 2mm and advancing the rotary table 15° at a time to give 24 teeth.





All the teeth cut.





It looks pretty good but the belt doesn't fit too well.





On closer examination the belt has rounded teeth so maybe I need to round over the top of the teeth???
Have I started with the correct diameter of bar???

Any ideas 

Cheers

Rich


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## Admiral_dk (Jan 31, 2011)

I'm no expert on this subject, but you used a 2.5 mm slitting saw and it has parallel sides. Looking at the "teeth" you made, they are trapezoid in shape and the "wrong way around". This suggest to me that your cutter shouldn't have parallel sides, but should be a trapezoid with rounded corners, so to speak and since the belt has the shape it has, I'll guess that the angle should be a little bigger that you would think.

It's also obvious to me that the correct cutter will only be totally correct for one diameter gear.


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## jim216 (Jan 31, 2011)

Hi Rich, 
 There is a good article on converting the X3 to belt drive, he uses a 5mm HTD belt, the author recommends using 3mm drills I wound up using 1/8" drills and an 1/8" end mill, you will have to join the the Yahoo group to get the PDF or you can email me, this is John Stevenson's group,   "Dick Stevens spindle drive conversion" 

http://tech.groups.yahoo.com/group/X_Series_Mills/?yguid=391225240


Basically you drill on the PD then turn down the excess to the PD. ( I think I said the correctly).  Or you can use an end mill and mill from the side on a blank that is already the correct diameter. 








Caution if you decide to convert your X3 the authors center distance was different than mine I had to use larger gears for the same size belt, didn't want to wait for a smaller belt.


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## firebird (Feb 1, 2011)

Hi

Interesting observations Admiral_dk some food for thought there.

Hi Jim, that's a clever way of getting the round bottom to the teeth. I'm not converting my mill though at the moment I'm just using the belts for another project. More food for thought though.

Thanks guys

Rich


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## firebird (Feb 4, 2011)

Hi

A quick question

On a timing belt programme I'm looking at for a 24 tooth gear it gives these sizes

 Pitch Circle Diameter= 9.549 mm

 Outside Diameter= 8.999 mm

What does that mean??

Do I turn the blank to 9.549 or 8.999??

Cheers#

Rich


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## Brian Rupnow (Feb 4, 2011)

According to the dimensions in reply #11, the pitch diameter is larger than the outside diameter. The FINISHED OUTSIDE diameter of the blank is going to be 8.999mm.


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## tel (Feb 4, 2011)

????????????

24 teeth on an od of 8.999mm (0.3543")? That comes out at a tooth pitch of something around 1mm - never seen a belt that small.


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## firebird (Feb 4, 2011)

Hi 

sorry Tel

I forgot to put the belt pitch in

This is for 5mm pitch


 Pitch Circle Diameter= 38.197 mm

 Outside Diameter= 37.057 mm

Cheers

Rich


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## HS93 (Feb 5, 2011)

this may interest you, the tooth shape that is.

Peter

[ame]http://www.youtube.com/watch?v=ViN4t4YI_T4&feature=related[/ame]


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## firebird (Feb 5, 2011)

Hi

Thanks Brian

Thanks Peter, that's really good. I was trying to work out how to make a form cutter just this afternoon.

I examined the belt a little closer today with some magnification. It's a 5mm pitch so I used a 2.5mm slitting saw assuming that the teeth and the hollows (hollows not the right word but you know what I mean) were the same size but the are not. The teeth are slightly larger so I re cut them with a 1/8 slitting saw (3.2mm)





The belt is now a much better fit. Far from right I know but at least the belt fits.





Cheers

Rich


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## firebird (Feb 6, 2011)

Hi

The wife went shopping this morning which gave me an hour or so in the workshop. I have decided to have a go at making a form cutter. I have started with a piece of 5/16 silver steel (drill rod) and machined it down on opposite sides to 2.69mm (.106"). Note I am using the square collet block. I'll let you know how it works.





Cheers

Rich


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## HS93 (Feb 25, 2011)

Spotted this , it may be off some help, he seems to get a nice finnish even with a small machine.


http://www.deansphotographica.com/machining/projects/gearcutting/gearcutting.html


Peter


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## Majorstrain (Feb 25, 2011)

Hi Rich,
I don't want to take the fun and satisfaction away from you in making a form tool, but I would probably use a 3mm or 1/8" ball mill to cut the profile.

Then again I might of missed something in my speed reading of the thread. If I have then disregard the above comment.
Cheers,
Phil


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## HS93 (Feb 26, 2011)

again this may be of interest forgot the cnc part it just saves you winding the handels, but he used a ball end tool 2 passes and then a counter sink tool to form the tooth top so no tool making

[ame]http://www.youtube.com/user/hossmachine#p/u/57/a5hgHx7irqk[/ame]

http://www.cnczone.com/forums/761841-post232.html
hope it helps , I only want you to perfect it so I can try  

Peter


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## firebird (Feb 26, 2011)

Hi Peter

very interesting, thank you. Oh what I would give for CNC

Phil, a 1/8 or 3mm cutter will be too big. You haven't missed anything its me that's missed the target. I'm suffering from serious brain fade. :wall: :wall: :wall: A form cutter like the one I have shown being made won't work. It will still cut a square sided slot! I tried making a tapered D bit type cutter but that didn't work too well either. Trying to find technical info on belts and gears is harder than you might imagine but I have at last found some. I have a couple of belts which have the T5 tooth profile. They are still 5mm pitch but slightly different to the belts shown earlier. This is what the teeth look like.





this is the spec

Pitch (P) - 5.000mm.
Tooth Length (T) - 1.40mm. 
Tooth Height (H) - 1.20mm. 
Belt Height (W) - 2.20mm
Corner Radii ( r ) - 0.40mm. 
Material:olyurethane Reinforcement::Fibreglass 
Appearance::Translucent 
Breaking Strength (N)::1190 
Min Temperature (Deg C)::-18 
Max Temperature (Deg C)::82 
Tooth Angle (A) (Deg.)::40


Crucially the specs give the tooth form and the angle. I still like the slitting saw idea as several gears can be made at once in one length then parted off. I am currently working on a jig to grind the sides of a 2.5mm slitting saw to 40° to give the correct tooth form. I'll report back when more progress has been made

Cheers

Rich


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## cfellows (Feb 26, 2011)

I made a 5mm pitch cutter out of a slitting saw to make some of these gears. I just bevelled each side of the cutter to the correct angle and the correct width at the tip for cutting the goove with the proper bottom width. I didn't try to put any side relief on the cutter. It worked fine in aluminum. I also made a cutter for a 3/8" pitch timing gear. The gear is about 5.5" in diameter and I've been using it on the spindle of my Logan 11.5" lathe for about 5 or 6 years now. Unfortunately, I don't remember how I made the cutter. Hmmm, wonder where that got off to???

Chuck


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## tel (Feb 26, 2011)

You were on the right track with that form cutter - what you need to do is taper it, back off the two trailing edges, form two teeth on the end and away you go! use it same as an end mill. I've made several cutters loke the over the years (I think I originally got the idea from LBSC's Tich book) and they work remarkably well


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## firebird (Feb 27, 2011)

Hi Chuck

That's encouraging to know someone else has done it with a slitting saw.

Tel 
th_wwp th_wwp th_wwp th_wwp

Cheers

Rich


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## Ken I (Feb 27, 2011)

Attatched short *.pdf "handbook" for Synchroflex belts.

Maybe of help.

Regards
      Ken 

View attachment synchroflex.pdf


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## firebird (Feb 27, 2011)

Hi

Thanks Ken

Cheers

Rich


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## Foozer (Feb 27, 2011)

Been reading this thread and something I suppose is so simple that it completely escapes me. The "P" dimension as shown on the flat belt drawing. Doesnt that change with pulley diameter?

I'm sitting here staring at a small toothed belt and pulley bending it around and the light will just not come on.

Robert


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## Ken I (Feb 27, 2011)

Perhaps it would be less confusing if you visualise "P" at the pitch centre line at 1/2 "H"

(The neutral axis of the belt - otherwise you can envisage "P" being smaller on the inside of the PCD and larger on the outside of the PCD - don't sweat it.)

So for a 5mm pitch belt x 20 teeth you have a 100mm circumference PCD or (÷¶) 31.83 PCD plus "H" for the OD.

Sure the shape changes slightly with curvature but this is taken up in some slight clearance in the profile as well as the elasticity of the belt.

Regards,
      Ken


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## Foozer (Feb 27, 2011)

Ken I  said:
			
		

> Perhaps it would be less confusing if you visualise "P" at the pitch centre line at 1/2 "H"
> 
> Regards,
> Ken



The pencil and paper computer coming up, a few draw outs and I'll get it.

Thanks for the explain

Robert


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## tel (Feb 28, 2011)

> Tel
> th_wwp th_wwp th_wwp th_wwp
> 
> Cheers
> ...



OK, first put a bit of silver steel in the lathe. Turn down a short section at the end to a diameter equal to the inner width of the gap, then turn a section to the wide (outer) width of the gap, then generate the taper. You should end up with something that looks like the secnd pic. Next time we'll take it to the mill, then back to the lathe.


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## tel (Feb 28, 2011)

OK, now, as I said, over to the mill and slab down two sides to about the smaller diameter. You can see now that that little pip we machined has served two purposes - it has established the small diameter for the cone and it has acted as a depth gauge for milling the sides. But it is no longer of use - back to the lath and turn it away.

Now you should have something that looks like the last pic.


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## tel (Feb 28, 2011)

All that's left to do is file some relief behind the cutting edges, harden it and try 'er out!

Works pretty good! I must stress, however, that I did not take a lot of care with this one - just a roughie to illustrate the process, the widths and angles were purely arbitrary sizes blah blah .....


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## tel (Mar 1, 2011)

Well, it looks like that was a waste of time!


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## ozzie46 (Mar 1, 2011)

Not so Tel. I enjoyed it a lot. 

 I don't have any plans on making a tooth pulley yet but if I do you can bet this will be the reason I tried or did as I had no idea how to go about it before.

 Ron


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## Ken I (Mar 1, 2011)

Applause for going to the trouble.

Feel better now ?

I enjoy your threads also.

Ken


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## Foozer (Mar 1, 2011)

tel  said:
			
		

> Well, it looks like that was a waste of time!




Not at all, you just made it look so embarrassing simple. 

Robert


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## firebird (Mar 1, 2011)

Hi

Tel, definitely not a waste of time. Sorry I didn't reply sooner. Many thanks for taking the time to show us.
:bow: :bow: :bow:

Cheers

Rich


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## tel (Mar 1, 2011)

Thanks blokes - I probably seemed grumpier than I actually was - having some problems here (unrelated).


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## Ken I (Mar 2, 2011)

We all have days like those.

According to my wife I'm that way most days.

Ken


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## tel (Mar 2, 2011)

Ooooh yes! And having kids (who are now all in their 30's) makes it worse - should have stuck to breeding goldfish!


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## Brian Rupnow (Mar 2, 2011)

Tel---Thank you for that very informative post. That was great, and I have filed it away for future reference. As far as kids go----If I had known 30 years ago how great grandchildren were, I'd have had them first and just skipped the kids!!!---Brian


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## Lew Hartswick (Mar 2, 2011)

Brian Rupnow  said:
			
		

> As far as kids go----If I had known 30 years ago how great grandchildren were, I'd have had them first and just skipped the kids!!!---Brian


BUT even they grow up and become 20s . :-(
  ...lew...


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## Brian Rupnow (Mar 2, 2011)

Lew Hartswick  said:
			
		

> BUT even they grow up and become 20s . :-(
> ...lew...


Yes, but I'll be dead by then!!!


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## firebird (Mar 3, 2011)

Hi Tel and Brian and Lew

I just got my first grand daughter, she's 6 month old now and absolutely brilliant ;D ;D ;D

Just one thing though, how did we get from timing belt gears to grandchildren :big: :big: :big:

Bit like me in my shed I suppose, I start off making one thing and end up somewhere completely different but hey ho its great fun.

Cheers

Rich


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## firebird (Mar 17, 2011)

Hi

Well I have been playing about with this a little more and I'm now getting somewhere. I am going to use belts with the T5 profile, they are readily available and reasonably priced. I will have to make quite a few pulleys for a few projects I have in mind so I decided to make a bit of kit that I will be able to use many times. Basically what I have done is to use the slitting saw idea but regrind the blade. First of all here's a sketch of the T5 tooth profile.







Next job was to make a gadget that fits into the lathe tool post and a grinding wheel mounted in the chuck on an arbor. I have used the insides of an old wiper motor to control the tool.





Here's a video of the tool in action. Note I have the lath covered with newspaper to protect it from grinding dust.

[ame]http://www.youtube.com/watch?v=W-SId6h5ans[/ame]

The slitting saw after re grinding





Setting up in the mill.





About half way round cutting a 24 tooth pulley. The blank is actually 2 pieces of 6mm aluminium clamped and turned together as a test piece.





Finished cutting





The belt is a good fit in the pulley.





It needs the tops of the teeth rounding over slightly but I can do that easily with a small file.
I think I have it cracked now. I have a lot more photos if anyone is interested in more details.

Cheers

Rich


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## Brian Rupnow (Mar 17, 2011)

Very nice work Firebird. I like your converted wiper mechanism.---Brian


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## Groomengineering (Mar 17, 2011)

Looks like you got 'er whipped Rich, great job! I'll have to remember this one.

BTW, congratz on the little one 

Cheers

Jeff


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## firebird (Mar 18, 2011)

Hi

Thanks Brian, the wiper motors have a very nice worm and gear in them with almost zero backlash. 

Thanks Jeff

Cheers

Rich


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## Twmaster (Mar 18, 2011)

Rich,

What sort of auto are you getting those wiper drives from?


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## firebird (Mar 19, 2011)

Hi Mike

I work in the motor trade so any car we scrap I try to get the wiper motor and the radiator fan motor. Ford and Vauxhall are the most common ones I get and seem to be a standard motor fitted across the range.

Cheers

Rich


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## Chitownmachine (Mar 19, 2011)

Wiper gear train?? THATS AWESOME!!! I was actually looking for something like this, because I like to fool around with tool making! THANKS!!


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## firebird (Mar 19, 2011)

Hi

They have a very useful mechanism inside





Uploaded with ImageShack.us





Uploaded with ImageShack.us

Cheers

Rich


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## Twmaster (Mar 19, 2011)

Excellent. I'll keep my eyes open for a used assembly like that.


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## Chitownmachine (Mar 20, 2011)

I second that! Great reuse!!


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