Maintenance Monday: Drawbar Pressure

 

We recently started a weekly post on our Instagram and Facebook page for weekly maintenance that you can find @cuddebackmaching on Instagram  and Cuddeback Machining Inc. on Facebook. During this maintenance, we discovered an issue on our HH Roberts Machining Center we wanted to shed more light on that some may overlook.

Drawbar Maintenance

We thought checking our drawbar pressure once a year was going to be sufficient. But after further digging we found it is recommended to check your drawbar pressure every couple of months.  We decided to start keeping a log for each machine so you can track pressure and see if the Belleville washers are starting to break down.

We checked the pressure with this tool from Maritools, CAT40 Drawbar Force Test Gage. Once we put the gage in the spindle, we had 750 pounds of force. Our machine calls for 1,450-2,000 according the manufacturer, HH Roberts. Being half of what our machine called for, it took a few steps to get back into the acceptable range of force needed. We also ordered new Belleville washers expecting to replace them in the process. 

First we had to remove the air cylinder that drives the drawbar release on top of the spindle to get access to the drawbar.  

From there we had to put a wrench to the top of the draw bar so we could break free the tool gripper in the spindle. 

The tool gripper was replaced and gave us another 250 pounds of force. We still needed to come up with 450 pounds of pressure so we had to replace and tighten the belleville washers on the drawbar itself.

With the air cylinder off and the tool gripper out, we had to remove a riser block and a nut to pull the drawbar out. 

 

 

 

Once removed, we had to replace the washers. That involved loosening the set screws and removing the nut on the right side of the drawbar. The washers also had to be greased before being reassembled. We used 70-8535 Gleitmetall Metal Lubricating Paste in a bag with the washers. Then we shook and rolled the bag to evenly coat the washers with the grease. (Also a great way to cover chicken wings with dry rub or sauce when making them at home) 

 

When putting the washers back on, they went on three up three down.

When it came to how tight the washer had to be, we had paperwork from a spindle rebuild a few years earlier done by Atlanta Precision Spindles. This included the distance from the top of the drawbar to the top of the nut. It became a guess as to how much it needed to be tightened with the new washers.  

We initially tightened it to our starting number from the last time we replaced the washers. This wasn’t enough and we had to tighten it another .100 before putting everything back together and checking the pressure again. This gained us another 400 pounds of force so we took everything back out and tightened it another .050. This put us right at 1,600 pounds of pressure.

Once we had everything back together and running again, the tools were sitting .003 higher or deeper in the spindle. The parts were also more consistent. It had only been about a year since the last time we checked the drawbar pressure in our machines. The other machines were still right in range but the machine with the heaviest use was half of what it needed to be. While the parts were still in tolerance, we noticed the finish and numbers were slowly getting worse. Checking the drawbar was an afterthought but became a quick solution to our problems and a relatively easy fix. It was also a cheaper option to replace the washers versus calling a company to come out for maintenance. We purchased 200 washers for $75.00 and the grease for $100.00. 200 washers is enough for two repairs and the grease will last for about four repairs. Instead of paying for a company to come replace the drawbar, it took about two hours of labor and $62.50 for parts to last for a year.  

Make sure to follow us on Facebook  and Instagram for all things machining and to keep up on “Maintenance Monday”. You can also send an email to info@cuddebackmachining.com or go to our website at https://shop.cuddebackmachining.com/