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23rd Feb 2019, 08:51 PM #1Philomath in training
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- Norwood-ish, Adelaide
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Replacement bandsaw bracket - the importance of sequence
Andy posted a 'want to buy' at the end of January for a bracket, to fit his Grizzly bandsaw. I volunteered to make up another one if no one closer was willing and as I was the only volunteer, he sent it over. This is what arrived. It's a zinc pot metal and the loose piece down the bottom of the picture is a threaded boss. Someone tried to weld/ braze/ solder it back in but all that did was destroy the bracket further. To my mind, pot metal is very difficult to repair, can be of questionable quality/ strength and not really worth the effort. Far better not to try and make a new part. According to the internet, this boss is a known weakness of this model saw but parts are not available as it is a superseded machine. As the boss is part of the blade tracking set up, rather important...
P1040408.JPG
Eric kindly got me a piece of 5083 aluminium from Capral. If you are chasing Al, worth trying your nearest as they cut larger bits to length and there is usually a rack of offcuts available. The starting size for this bracket was 120x100x50mm (around 1.65kg). I used some Di-chem that Eskimo gave me a few years back for marking out. Usually I use a thick texta, but occasionally the mood takes me for something different. The top was done with Red, the bottom with Blue. Not sure there was any advantage in using one colour over another for this job.
The machining is not particularly difficult, but is one of those jobs where the sequence of operations is important. I think if I was asked what the most vital thing I've learnt about machining is, I would say the sequence of operations as this dictates the work holding. For those who have not tackled parts like this, I'll try and explain the approach I used and why.
So after squaring up the block and marking out the top, the first thing was to cut to depth the tops of the tongues. Typically this is done in steps to avoid loading up the cutter too much (rule of thumb DOC is less than half the cutter diameter), so rather than cut to the line on the top surface for each step, a better way is to cut to within say 0.5 to 1mm of the finished (horizontal) dimension, cut down to depth and then remove that small amount of material left on in one pass - gives a much cleaner vertical surface (no steps).
P1040404.JPG
After doing that, I roughed away the material either side of the central ridge, again leaving a little material to clean up later. Because there is a boss on one side of that feature, I left a step on my roughing to allow me to invert the part while giving myself two parallel surfaces to hang onto in the vice. (So I still roughed around the boss but with less depth).
So having roughed out the top, the next thing to do after marking out was to drill the side holes. The other side of the tongue cuts through half the hole for a portion of it's length. Easier to drill before making that cut. When designing parts that need to be machined I will either make the surface the part sits on flat (so it is easier to get the hole square) or (as in this case) allow some material to make drilling easier then remove it. The pocket (apart from the shape) was simple enough. I used a four flute cutter to clear the area out (and a vacuum to suck up the chips), so prior to milling I used a slot drill to put in a hole, allowing me to step down to depth. As needed. Again, I did not go to the line with the steps but stayed back and then did a finish pass. I actually used a different cutter for finishing, as I had one with a small corner radii on the flute tips. This allows a small radius (in this case 1mm) to be put in the corner, lessening the stress point so giving a stronger bracket. The photo shows the completed pocket while machining the other side of the tongue.
Matching cuts when swapping cutters can be tricky, but I have some 0.5mm shim strip that I use. I pull it back and forth while winding the stationary cutter down on to it. I know when the shim is pinched, I'm 0.5mm off the surface. I also occasionally use it for establishing where the cutter is in relation to the side of the work too. Quicker than swapping out for a edge finder.
P1040405.JPG
To get the boss to blend into the central ridge (see last photo), I mounted the part on a rotary table, noting that the centre of the boss has to be on the centre of rotation. That way, the milling cutter can cut straight along the side of the ridge, step out a little, the rotary table used for the circular feature then a couple of straight movements to finish. Notice too that the part is clamped to parallels. This is so I can use the tongues as datum surfaces as well as clamp on them (as they are already machined) without stressing or bending the part. The clamps are set back a little so the cutter does not nick them. My rounded tip cutter was also used for the finishing cuts here.
P1040407.JPG
This is the part so far. It weighs 411g but still needs a central hole and tension indicator, as opposed to the existing zinc allow bracket that is 604g.
P1040409.JPG
Michael
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23rd Feb 2019, 09:32 PM #2Golden Member
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- Jul 2011
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Michael,
That looks great. Thanks for the photos.
Eric
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23rd Feb 2019, 11:31 PM #3Most Valued Member
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Thanks for the write up and pics Michael, looks easy when you know what to do.
KrynTo grow old is mandatory, growing up is optional.
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24th Feb 2019, 07:05 PM #4Philomath in training
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At the risk of being boring...
To finish up, I did the bore (95mm deep with a flat bottom - thank goodness for Bob, Kaiser and the boring extensions). For any bored hole a pilot is required so I drilled one 12mm in diameter then enlarged that to 22mm before boring to 24mm. However, once in around 3 or 4 diameters twist drills don't pass chips out as well, so a good plan is to drill down with the smaller drill then when it starts to clog, drill with the larger drill to effectively reduce the distance the smaller drill has to push the ships out.
P1040411.JPG
Then the slot and engraving on the top. The slot was cut with a 6mm slot drill. The chamfer used a wood router bit - carbide router bits work well for processing aluminium.
P1040412.JPG
All up, the part is now 292g, so roughly 80% of the original material has been converted to swarf...
(I think my personal best is converting 95% to swarf - see //metalworkforums.com/f65/t1989...86#post1903386)
Michael
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24th Feb 2019, 09:24 PM #5
Hi Michael,
That looks great ! I do envy your ability to produce nice work.
With regard to carbide wood router bits, yes they are good on aluminium and brasses because the don't have any rake on them. I've also used them on mild steel with good results, but they do need high speed and fine cuts.
I used one to put rounded edges on the Norman tool holder that I made. I should make another to suit a boring bar.
Thanks for the write up and pictures:Best Regards:
Baron J.
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24th Feb 2019, 09:54 PM #6Most Valued Member
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What BJ said, goes for me too.
Top shelf stuff, THANKS.
KrynTo grow old is mandatory, growing up is optional.
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24th Feb 2019, 11:59 PM #7
Put me down for a me too.
It looks great work work Michael!
Grahame
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25th Feb 2019, 08:36 AM #8Most Valued Member
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how did you do the numbering..or didnt I read that bit
(for Grahame's info only..went blank again after I clicked submit)
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25th Feb 2019, 07:12 PM #9Philomath in training
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The lines were done with a scriber held in the mill chuck and indexed along. I do have a pantograph engraver so could have done that, but I also have a set of 3/32" (2.4mm) number punches that come in handy for things like this.
Michael
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25th Feb 2019, 09:11 PM #10Most Valued Member
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- Nov 2007
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- melbourne australia
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Nice work.
When are we going to get "Likes" back?Chris
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25th Feb 2019, 10:01 PM #11
Ot
Hi Guys,
(for Grahame's info only..went blank again after I clicked submit)
It has been doing this for a few days now !Best Regards:
Baron J.
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26th Feb 2019, 07:18 AM #12Most Valued Member
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- Aug 2011
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- Melbourne
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Nice work Michael,
I agree about the sequence of operations, it's been one of the biggest learning areas for me too.
The tensioning block looks bloody great. Looks like an original part for a quality built machine. Numbering looks mint!
SimonGirl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.
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26th Feb 2019, 08:55 AM #13Most Valued Member
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